It is estimated that the volume of the Antarctic ice sheet is about 25.4 million km3 (6.1 million cu mi), and the WAIS contains just under 10% of this, or 2.2 million km3 (530,000 cu mi). The weight of the ice has caused the underlying rock to sink by between 0.5 and 1 kilometre (0.31 and 0.62 miles) in a process known as isostatic depression.
Under the force of its own weight, the ice sheet deforms and flows. The interior ice flows slowly over rough bedrock. In some circumstances, ice can flow faster in ice streams,
separated by slow-flowing ice ridges. The inter-stream ridges are
frozen to the bed while the bed beneath the ice streams consists of
water-saturated sediments.
Many of these sediments were deposited before the ice sheet occupied
the region, when much of West Antarctica was covered by the ocean. The
rapid ice-stream flow is a non-linear process still not fully understood; streams can start and stop for unclear reasons.
When ice reaches the coast, it either calves or continues to flow outward onto the water. The result is a large, floating ice shelf affixed to the continent.
Ice mass loss
Indications that the West Antarctic Ice Sheet is losing mass at an increasing rate come from the Amundsen Sea sector, and three glaciers in particular: the Pine Island, Thwaites and Smith Glaciers.
Data reveals they are losing more ice than is being replaced by
snowfall. According to a preliminary analysis, the difference between
the mass lost and mass replaced is about 60%. The melting of these three
glaciers alone is contributing an estimated 0.24 millimetres (0.0094
inches) per year to the rise in the worldwide sea level.
There is growing evidence that this trend is accelerating: there has
been a 75% increase in Antarctic ice mass loss in the ten years
1996–2006, with glacier acceleration a primary cause. As of November 2012 the total mass loss from the West Antarctic Ice Sheet is estimated at 118 ± 9 Gt/y mainly from the Amundsen Sea coast.
Satellite measurements by ESA's CryoSat-2
revealed that the West Antarctic Ice Sheet (WAIS) is losing more than
150 cubic kilometres (36 cubic miles) of ice each year. The loss is
especially pronounced at grounding lines,
the area where the floating ice shelf meets the part resting on
bedrock, and hence affects the ice shelf stability and flow rates.
Potential collapse
Large parts of the WAIS sit on a bed which is both below sea level and sloping downward inland. This slope, and the low isostatic
head, mean that the ice sheet is theoretically unstable: a small
retreat could in theory destabilize the entire WAIS, leading to rapid
disintegration. Current computer models
do not account well for the complicated and uncertain physics necessary
to simulate this process, and observations do not provide guidance, so
predictions as to its rate of retreat remain uncertain. This has been
known for decades.
In January 2006, in a UK government-commissioned report, the head of the British Antarctic Survey, Chris Rapley,
warned that this huge West Antarctic Ice Sheet may be starting to
disintegrate. It has been hypothesised that this disintegration could
raise sea levels by approximately 3.3 metres (11 ft). (If the entire West Antarctic Ice Sheet were to melt, this would contribute 4.8 m (16 ft) to global sea level.) Rapley said a previous (2001) Intergovernmental Panel on Climate Change
(IPCC) report that played down the worries of the ice sheet's stability
should be revised. "I would say it is now an awakened giant. There is
real concern."
Rapley said, "Parts of the Antarctic ice sheet that rest on
bedrock below sea level have begun to discharge ice fast enough to make a
significant contribution to sea level rise.
Understanding the reason for this change is urgent in order to be able
to predict how much ice may ultimately be discharged and over what
timescale. Current computer models do not include the effect of liquid
water on ice sheet sliding and flow, and so provide only conservative
estimates of future behaviour."
Polar ice experts from the US and UK met at the University of Texas at Austin in March, 2007 for the West Antarctic Links to Sea-Level Estimation (WALSE) Workshop.
The experts discussed a new hypothesis that explains the observed
increased melting of the West Antarctic Ice Sheet. They proposed that
changes in air circulation patterns have led to increased upwelling of
warm, deep ocean water along the coast of Antarctica and that this warm
water has increased melting of floating ice shelves at the edge of the ice sheet.
An ocean model has shown how changes in winds can help channel the
water along deep troughs on the sea floor, toward the ice shelves of
outlet glaciers.
The exact cause of the changes in circulation patterns is not known and
they may be due to natural variability. However, this connection
between the atmosphere and upwelling of deep ocean water provides a
mechanism by which human induced climate changes could cause an
accelerated loss of ice from the WAIS.
Recently published data collected from satellites support this
hypothesis, suggesting that the West Antarctic Ice Sheet is beginning to
show signs of instability.
On 12 May 2014, it was announced that two teams of scientists said the
long-feared collapse of the Ice Sheet had begun, kicking off what they
say will be a centuries-long, "unstoppable" process that could raise sea
levels by 1.2 to 3.6 metres (3.9 to 11.8 ft) They estimate that rapid drawdown of Thwaites Glacier will begin in 200 – 1000 years. (Scientific source articles: Rignot et al. 2014 and Joughin et al. 2014.)
In 2016, improved computer modeling revealed that the breakup of
glaciers could lead to a steep rise in sea levels much more quickly than
previously projected. “We’re in danger of handing young people a
situation that’s out of their control,” according to James E. Hansen,
the leader of a number of climate scientists who worked together to
compile the study. In 2018, scientists concluded that high sea levels some 125,000 years ago,
which were 6–9 m (20–30 ft) higher than today, were most likely due to
the absence of the WAIS, and found evidence that the ice sheet collapsed
under climate conditions similar to those of today.
Warming
The
West Antarctic ice sheet (WAIS) has warmed by more than 0.1 °C
(0.18 °F)/decade in the last fifty years, and the warming is the
strongest in winter and spring. Although this is partly offset by fall
cooling in East Antarctica,
this effect was restricted to the 1980s and 1990s. The continent-wide
average surface temperature trend of Antarctica is positive and statistically significant at >0.05 °C (0.090 °F)/decade since 1957. This warming of WAIS is strongest in the Antarctic Peninsula.
In 2012, the temperature records for the ice sheet were reanalyzed with
a conclusion that since 1958, the West Antarctic ice sheet had warmed
by 2.4 °C (4.3 °F), almost double the previous estimate. Some scientists
now fear that the WAIS could now collapse like the Larsen B Ice Shelf did in 2002.
The possible disastrous outcome of a disintegration of the WAIS for global sea levels has been mentioned and assessed in the IPCC Third Assessment Report but was left out in the IPCC Fourth Assessment Report. Jessica O'Reilly, Naomi Oreskes and Michael Oppenheimer discussed the case in a Social Studies of Science
paper 2012. According to them, IPCC authors were less certain about
potential WAIS disintegration not only due to external new science
results. As well pure internal "cultural" reasons, as changes of staff
within the IPCC and externally, made it too difficult to project the
range of possible futures for the WAIS as required. Mike Hulme saw the issue as a showcase to urge for the integration of minority views in the IPCC and other major assessment processes.
Volcanoes
In 2017, geologists from Edinburgh University discovered 91 volcanoes located two kilometres below the icy surface, making it the largest volcanic region on Earth.
Climate variability includes all the variations in the climate that last longer than individual weather events, whereas the term climate change only refers to those variations that persist for a longer period of time, typically decades or more. In the time since the industrial revolution, the climate has increasingly been affected by human activities that are causing global warming and climate change.
The climate system receives nearly all of its energy from the sun. The climate system also radiates energy to outer space. The balance of incoming and outgoing energy, and the passage of the energy through the climate system, determines Earth's energy budget.
When the incoming energy is greater than the outgoing energy, Earth's
energy budget is positive and the climate system is warming. If more
energy goes out, the energy budget is negative and Earth experiences
cooling.
The energy moving through Earth's climate system finds expression in weather, varying on geographic scales and time. Long-term averages and variability of weather in a region constitute the region's climate.
Such changes can be the result of "internal variability", when natural
processes inherent to the various parts of the climate system alter the
distribution of energy. Examples include variability in ocean basins
such as the Pacific decadal oscillation and Atlantic multidecadal oscillation. Climate variability can also result from external forcing,
when events outside of the climate system's components nonetheless
produce changes within the system. Examples include changes in solar
output and volcanism.
Climate variability has consequences for sea level changes, plant life, and mass extinctions; it also affects human societies.
Terminology
Climate variability is the term to describe variations in the mean state and other characteristics of climate (such as chances or possibility of extreme weather,
etc.) "on all spatial and temporal scales beyond that of individual
weather events." Some of the variability does not appear to be caused
systematically and occurs at random times. Such variability is called random variability or noise. On the other hand, periodic variability occurs relatively regularly and in distinct modes of variability or climate patterns.
The term climate change is often used to refer specifically to anthropogenic climate change (also known as global warming).
Anthropogenic climate change is caused by human activity, as opposed to
changes in climate that may have resulted as part of Earth's natural
processes.
In this sense, the term climate change has become synonymous with anthropogenicglobal warming.
Within scientific journals, global warming refers to surface
temperature increases while climate change includes global warming and
everything else that increasing greenhouse gas levels affect.
A related term, climatic change, was proposed by the World Meteorological Organization
(WMO) in 1966 to encompass all forms of climatic variability on
time-scales longer than 10 years, but regardless of cause. During the
1970s, the term climate change replaced climatic change to focus on
anthropogenic causes, as it became clear that human activities had a
potential to drastically alter the climate. Climate change was incorporated in the title of the Intergovernmental Panel on Climate Change (IPCC) and the UN Framework Convention on Climate Change
(UNFCCC). Climate change is now used as both a technical description of
the process, as well as a noun used to describe the problem.
Causes
On the broadest scale, the rate at which energy is received from the Sun and the rate at which it is lost to space determine the equilibrium temperature and climate of Earth. This energy is distributed around the globe by winds, ocean currents, and other mechanisms to affect the climates of different regions.
Factors that can shape climate are called climate forcings or "forcing mechanisms". These include processes such as variations in solar radiation, variations in the Earth's orbit, variations in the albedo or reflectivity of the continents, atmosphere, and oceans, mountain-building and continental drift and changes in greenhouse gas
concentrations. External forcing can be either anthropogenic (e.g.
increased emissions of greenhouse gases and dust) or natural (e.g.,
changes in solar output, the Earth's orbit, volcano eruptions). There are a variety of climate change feedbacks that can either amplify or diminish the initial forcing. There are also key thresholds which when exceeded can produce rapid or irreversible change.
Some parts of the climate system, such as the oceans and ice
caps, respond more slowly in reaction to climate forcings, while others
respond more quickly. An example of fast change is the atmospheric
cooling after a volcanic eruption, when volcanic ash reflects sunlight. Thermal expansion
of ocean water after atmospheric warming is slow, and can take
thousands of years. A combination is also possible, e.g., sudden loss of
albedo in the Arctic Ocean as sea ice melts, followed by more gradual thermal expansion of the water.
Climate variability can also occur due to internal processes.
Internal unforced processes often involve changes in the distribution of
energy in the ocean and atmosphere, for instance, changes in the thermohaline circulation.
Internal variability
Climatic changes due to internal variability sometimes occur in
cycles or oscillations. For other types of natural climatic change, we
cannot predict when it happens; the change is called random or stochastic. From a climate perspective, the weather can be considered random.
If there are little clouds in a particular year, there is an energy
imbalance and extra heat can be absorbed by the oceans. Due to climate inertia,
this signal can be 'stored' in the ocean and be expressed as
variability on longer time scales than the original weather
disturbances. If the weather disturbances are completely random, occurring as white noise,
the inertia of glaciers or oceans can transform this into climate
changes where longer-duration oscillations are also larger oscillations,
a phenomenon called red noise. Many climate changes have a random aspect and a cyclical aspect. This behavior is dubbed stochastic resonance.
Ocean-atmosphere variability
El Niño impacts
La Niña impacts
The ocean and atmosphere can work together to spontaneously generate
internal climate variability that can persist for years to decades at a
time. These variations can affect global average surface temperature by redistributing heat between the deep ocean and the atmosphere and/or by altering the cloud/water vapor/sea ice distribution which can affect the total energy budget of the Earth.
Oscillations and cycles
A climate oscillation or climate cycle is any recurring cyclical oscillation within global or regional climate. They are quasiperiodic (not perfectly periodic), so a Fourier analysis of the data does not have sharp peaks in the spectrum. Many oscillations on different time-scales have been found or hypothesized:
the El Niño Southern Oscillation (ENSO) – A large scale pattern of warmer (El Niño) and colder (La Niña) tropical sea surface temperatures in the Pacific Ocean with worldwide effects. It is a self-sustaining oscillation, whose mechanisms are well-studied.
ENSO is the most prominent known source of inter-annual variability in
weather and climate around the world. The cycle occurs every two to
seven years, with El Niño lasting nine months to two years within the
longer term cycle.
the Madden–Julian oscillation
(MJO) – An eastward moving pattern of increased rainfall over the
tropics with a period of 30 to 60 days, observed mainly over the Indian
and Pacific Oceans.
the Quasi-biennial oscillation – a well-understood oscillation in wind patterns in the stratosphere around the equator. Over a period of 28 months the dominant wind changes from easterly to westerly and back.
the Pacific decadal oscillation
– The dominant pattern of sea surface variability in the North Pacific
on a decadal scale. During a "warm", or "positive", phase, the west
Pacific becomes cool and part of the eastern ocean warms; during a
"cool" or "negative" phase, the opposite pattern occurs. It is thought
not as a single phenomenon, but instead a combination of different
physical processes.
the Atlantic Multidecadal Oscillation
– A pattern of variability in the North Atlantic of about 55 to 70
years, with effects on rainfall, droughts and hurricane frequency and
intensity.
the Arctic oscillation (AO) and Antarctic oscillation
(AAO) – The annular modes are naturally occurring, hemispheric-wide
patterns of climate variability. On timescales of weeks to months they
explain 20-30% of the variability in their respective hemispheres. The
Northern Annular Mode or Arctic Oscillation (AO) in the Northern Hemisphere, and the Southern Annular Mode or Antarctic oscillation
(AAO) in the southern hemisphere. The annular modes have a strong
influence on the temperature and precipitation of mid-to-high latitude
land masses, such as Europe and Australia, by altering the average paths
of storms. The NAO can be considered a regional index of the AO/NAM. They are defined as the first EOF of sea level pressure or geopotential height from 20°N to 90°N (NAM) or 20°S to 90°S (SAM).
A schematic of modern thermohaline circulation.
Tens of millions of years ago, continental-plate movement formed a
land-free gap around Antarctica, allowing the formation of the ACC, which keeps warm waters away from Antarctica.
The oceanic aspects of climate variability can generate variability
on centennial timescales due to the ocean having hundreds of times more
mass than in the atmosphere, and thus very high thermal inertia.
For example, alterations to ocean processes such as thermohaline
circulation play a key role in redistributing heat in the world's
oceans.
Ocean currents transport a lot of energy from the warm tropical
regions to the colder polar regions. Changes occurring around the last
ice age (in technical terms, the last glacial) show that the circulation is the North Atlantic
can change suddenly and substantially, leading to global climate
changes, even though the total amount of energy coming into the climate
system didn't change much. These large changes may have come from so
called Heinrich events
where internal instability of ice sheets caused huge ice bergs to be
released into the ocean. When the ice sheet melts, the resulting water
is very low in salt and cold, driving changes in circulation.
glaciation 2.3 billion years ago triggered by the evolution of oxygenic photosynthesis, which depleted the atmosphere of the greenhouse gas carbon dioxide and introduced free oxygen
global cooling over the past 40 million years driven by the expansion of grass-grazer ecosystems
External climate forcing
Greenhouse gases
CO 2 concentrations over the last 800,000 years as measured from ice cores (blue/green) and directly (black)
Whereas greenhouse gases
released by the biosphere is often seen as a feedback or internal
climate process, greenhouse gases emitted from volcanoes are typically
classified as external by climatologists. Greenhouse gases, such as CO 2, methane and nitrous oxide, heat the climate system by trapping infrared light. Volcanoes are also part of the extended carbon cycle.
Over very long (geological) time periods, they release carbon dioxide
from the Earth's crust and mantle, counteracting the uptake by
sedimentary rocks and other geological carbon dioxide sinks.
Since the industrial revolution, humanity has been adding to greenhouse gases by emitting CO2 from fossil fuel combustion, changing land use through deforestation, and has further altered the climate with aerosols (particulate matter in the atmosphere), release of trace gases (e.g. nitrogen oxides, carbon monoxide, or methane). Other factors, including land use, ozone depletion, animal husbandry (ruminant animals such as cattle produce methane), and deforestation, also play a role.
The US Geological Survey
estimates are that volcanic emissions are at a much lower level than
the effects of current human activities, which generate 100–300 times
the amount of carbon dioxide emitted by volcanoes. The annual amount put out by human activities may be greater than the amount released by supereruptions, the most recent of which was the Toba eruption in Indonesia 74,000 years ago.
Orbital variations
Milankovitch cycles from 800,000 years ago in the past to 800,000 years in the future.
Slight variations in Earth's motion lead to changes in the seasonal
distribution of sunlight reaching the Earth's surface and how it is
distributed across the globe. There is very little change to the
area-averaged annually averaged sunshine; but there can be strong
changes in the geographical and seasonal distribution. The three types
of kinematic change are variations in Earth's eccentricity, changes in the tilt angle of Earth's axis of rotation, and precession of Earth's axis. Combined together, these produce Milankovitch cycles which affect climate and are notable for their correlation to glacial and interglacial periods, their correlation with the advance and retreat of the Sahara, and for their appearance in the stratigraphic record.
During the glacial cycles, there was a high correlation between CO 2 concentrations and temperatures. Early studies indicated that CO 2 concentrations lagged temperatures, but it has become clear that this isn't always the case. When ocean temperatures increase, the solubility of CO 2 decreases so that it is released from the ocean. The exchange of CO 2 between the air and the ocean can also be impacted by further aspects of climatic change. These and other self-reinforcing processes allow small changes in Earth's motion to have a large effect on climate.
Solar output
Variations in solar activity during the last several centuries based on observations of sunspots and beryllium isotopes. The period of extraordinarily few sunspots in the late 17th century was the Maunder minimum.
The Sun is the predominant source of energy input to the Earth's climate system. Other sources include geothermal
energy from the Earth's core, tidal energy from the Moon and heat from
the decay of radioactive compounds. Both long term variations in solar
intensity are known to affect global climate. Solar output varies on shorter time scales, including the 11-year solar cycle and longer-term modulations. Correlation between sunspots and climate and tenuous at best.
Three to four billion years ago, the Sun emitted only 75% as much power as it does today.
If the atmospheric composition had been the same as today, liquid water
should not have existed on the Earth's surface. However, there is
evidence for the presence of water on the early Earth, in the Hadean and Archean eons, leading to what is known as the faint young Sun paradox.
Hypothesized solutions to this paradox include a vastly different
atmosphere, with much higher concentrations of greenhouse gases than
currently exist.
Over the following approximately 4 billion years, the energy output of
the Sun increased. Over the next five billion years, the Sun's ultimate
death as it becomes a red giant and then a white dwarf will have large effects on climate, with the red giant phase possibly ending any life on Earth that survives until that time.
Volcanism
In atmospheric temperature from 1979 to 2010, determined by MSUNASA satellites, effects appear from aerosols released by major volcanic eruptions (El Chichón and Pinatubo). El Niño is a separate event, from ocean variability.
The eruptions
considered to be large enough to affect the Earth's climate on a scale
of more than 1 year are the ones that inject over 100,000 tons of SO2 into the stratosphere. This is due to the optical properties of SO2 and sulfate aerosols, which strongly absorb or scatter solar radiation, creating a global layer of sulfuric acid haze.
On average, such eruptions occur several times per century, and cause
cooling (by partially blocking the transmission of solar radiation to
the Earth's surface) for a period of several years. Although volcanoes
are technically part of the lithosphere, which itself is part of the
climate system, the IPCC explicitly defines volcanism as an external
forcing agent.
At a larger scale—a few times every 50 million to 100 million years—the eruption of large igneous provinces brings large quantities of igneous rock from the mantle and lithosphere to the Earth's surface. Carbon dioxide in the rock is then released into the atmosphere.
Small eruptions, with injections of less than 0.1 Mt of sulfur dioxide
into the stratosphere, affect the atmosphere only subtly, as temperature
changes are comparable with natural variability. However, because
smaller eruptions occur at a much higher frequency, they too
significantly affect Earth's atmosphere.
Plate tectonics
Over the course of millions of years, the motion of tectonic plates
reconfigures global land and ocean areas and generates topography. This
can affect both global and local patterns of climate and
atmosphere-ocean circulation.
The position of the continents determines the geometry of the
oceans and therefore influences patterns of ocean circulation. The
locations of the seas are important in controlling the transfer of heat
and moisture across the globe, and therefore, in determining global
climate. A recent example of tectonic control on ocean circulation is
the formation of the Isthmus of Panama about 5 million years ago, which shut off direct mixing between the Atlantic and Pacific Oceans. This strongly affected the ocean dynamics of what is now the Gulf Stream and may have led to Northern Hemisphere ice cover. During the Carboniferous period, about 300 to 360 million years ago, plate tectonics may have triggered large-scale storage of carbon and increased glaciation. Geologic evidence points to a "megamonsoonal" circulation pattern during the time of the supercontinentPangaea, and climate modeling suggests that the existence of the supercontinent was conducive to the establishment of monsoons.
The size of continents is also important. Because of the
stabilizing effect of the oceans on temperature, yearly temperature
variations are generally lower in coastal areas than they are inland. A
larger supercontinent will therefore have more area in which climate is
strongly seasonal than will several smaller continents or islands.
Other mechanisms
It has been postulated that ionized particles known as cosmic rays
could impact cloud cover and thereby the climate. As the sun shields
the Earth from these particles, changes in solar activity were
hypothesized to influence climate indirectly as well. To test the
hypothesis, CERN designed the CLOUD experiment, which showed the effect of cosmic rays is too weak to influence climate noticeably.
Evidence exists that the Chicxulub asteroid impact
some 66 million years ago had severely affected the Earth's climate.
Large quantities of sulfate aerosols were kicked up into the atmosphere,
decreasing global temperatures by up to 26 °C and producing
sub-freezing temperatures for a period of 3–16 years. The recovery time
for this event took more than 30 years. The large-scale use of nuclear weapons
has also been investigated for its impact on the climate. The
hypothesis is that soot released by large-scale fires blocks a
significant fraction of sunlight for as much as a year, leading to a
sharp drop in temperatures for a few years. This possible event is
described as nuclear winter.
Humans' use of land
impact how much sunlight the surface reflects and the concentration of
dust. Cloud formation is not only influenced by how much water is in the
air and the temperature, but also by the amount of aerosols in the air such as dust. Globally, more dust is available if there are many regions with dry soils, little vegetation and strong winds.
Climate changes that occurred after the widespread deployment of
measuring devices, can be observed directly. Reasonably complete global
records of surface temperature are available beginning from the mid-late
19th century. Further observations are done by satellite and derived indirectly from historical documents. Satellite cloud and precipitation data has been available since the 1970s. Historical climatology
is the study of historical changes in climate and their effect on human
history and development. The primary sources include written records
such as sagas, chronicles, maps and local history literature as well as pictorial representations such as paintings, drawings and even rock art.
Climate variability in the recent past may be detected by corresponding changes in settlement and agricultural patterns. Archaeological evidence, oral history and historical documents can offer insights into past changes in the climate. Changes in climate have been linked to the rise and also the collapse of various civilizations.
Proxy measurements
Variations in CO2, temperature and dust from the Vostok ice core over the last 450,000 years.
Various archives of past climate are present in rocks, trees and
fossils. From these archive, indirect measures of climate, so-called
proxies, can be derived. Quantification of climatological variation of
precipitation in prior centuries and epochs is less complete but
approximated using proxies such as marine sediments, ice cores, cave
stalagmites, and tree rings.
Stress, too little precipitation or unsuitable temperatures, can alter
the growth rate of trees, which allows scientists to infer climate
trends by analyzing the growth rate of tree rings. This branch of
science studying this called dendroclimatology. Glaciers leave behind moraines
that contain a wealth of material—including organic matter, quartz, and
potassium that may be dated—recording the periods in which a glacier
advanced and retreated.
Analysis of ice in cores drilled from an ice sheet such as the Antarctic ice sheet,
can be used to show a link between temperature and global sea level
variations. The air trapped in bubbles in the ice can also reveal the CO2
variations of the atmosphere from the distant past, well before modern
environmental influences. The study of these ice cores has been a
significant indicator of the changes in CO2 over many
millennia, and continues to provide valuable information about the
differences between ancient and modern atmospheric conditions. The 18O/16O ratio in calcite and ice core samples used to deduce ocean temperature in the distant past is an example of a temperature proxy method.
The remnants of plants, and specifically pollen, are also used to
study climatic change. Plant distributions vary under different climate
conditions. Different groups of plants have pollen with distinctive
shapes and surface textures, and since the outer surface of pollen is
composed of a very resilient material, they resist decay. Changes in the
type of pollen found in different layers of sediment indicate changes
in plant communities. These changes are often a sign of a changing
climate. As an example, pollen studies have been used to track changing vegetation patterns throughout the Quaternary glaciations and especially since the last glacial maximum. Remains of beetles
are common in freshwater and land sediments. Different species of
beetles tend to be found under different climatic conditions. Given the
extensive lineage of beetles whose genetic makeup has not altered
significantly over the millennia, knowledge of the present climatic
range of the different species, and the age of the sediments in which
remains are found, past climatic conditions may be inferred.
Analysis and uncertainties
One difficulty in detecting climate cycles is that the Earth's
climate has been changing in non-cyclic ways over most
paleoclimatological timescales. For instance, we are now in a period of anthropogenicglobal warming. In a larger timeframe, the Earth is emerging from the latest ice age, cooling from the Holocene climatic optimum and warming from the "Little Ice Age",
which means that climate has been constantly changing over the last
15,000 years or so. During warm periods, temperature fluctuations are
often of a lesser amplitude. The Pleistocene period, dominated by repeated glaciations, developed out of more stable conditions in the Miocene and Pliocene climate.
Holocene climate has been relatively stable. All of these changes
complicate the task of looking for cyclical behavior in the climate.
Positive feedback, negative feedback, and ecological inertia
from the land-ocean-atmosphere system often attenuate or reverse
smaller effects, whether from orbital forcings, solar variations or
changes in concentrations of greenhouse gases. Certain feedbacks
involving processes such as clouds are also uncertain; for contrails, natural cirrus clouds, oceanic dimethyl sulfide and a land-based equivalent, competing theories exist concerning effects on climatic temperatures, for example contrasting the Iris hypothesis and CLAW hypothesis.
Consequences of climate variability
Life
Top:Arid ice age climateMiddle:Atlantic Period, warm and wetBottom: Potential vegetation in climate now if not for human effects like agriculture.
Vegetation
A change in the type, distribution and coverage of vegetation may
occur given a change in the climate. Some changes in climate may result
in increased precipitation and warmth, resulting in improved plant
growth and the subsequent sequestration of airborne CO2. The effects are expected to affect the rate of many natural cycles like plant litter decomposition rates.
A gradual increase in warmth in a region will lead to earlier flowering
and fruiting times, driving a change in the timing of life cycles of
dependent organisms. Conversely, cold will cause plant bio-cycles to
lag.
Larger, faster or more radical changes, however, may result in vegetation stress, rapid plant loss and desertification in certain circumstances. An example of this occurred during the Carboniferous Rainforest Collapse
(CRC), an extinction event 300 million years ago. At this time vast
rainforests covered the equatorial region of Europe and America. Climate
change devastated these tropical rainforests, abruptly fragmenting the
habitat into isolated 'islands' and causing the extinction of many plant
and animal species.
Wildlife
One of the most important ways animals can deal with climatic change is migration to warmer or colder regions. On a longer timescale, evolution makes ecosystems including animals better adapted to a new climate. Rapid or large climate change can cause mass extinctions when creatures are stretched too far to be able to adapt.
Humanity
Collapses of past civilizations such as the Maya may be related to cycles of precipitation, especially drought, that in this example also correlates to the Western Hemisphere Warm Pool. Around 70 000 years ago the Toba supervolcano eruption created an especially cold period during the ice age, leading to a possible genetic bottleneck in human populations.
Changes in the cryosphere
Glaciers and ice sheets
Glaciers are considered among the most sensitive indicators of a changing climate. Their size is determined by a mass balance
between snow input and melt output. As temperatures increase, glaciers
retreat unless snow precipitation increases to make up for the
additional melt. Glaciers grow and shrink due both to natural
variability and external forcings. Variability in temperature,
precipitation and hydrology can strongly determine the evolution of a
glacier in a particular season.
The most significant climate processes since the middle to late Pliocene (approximately 3 million years ago) are the glacial and interglacial cycles. The present interglacial period (the Holocene) has lasted about 11,700 years. Shaped by orbital variations, responses such as the rise and fall of continental ice sheets and significant sea-level changes helped create the climate. Other changes, including Heinrich events, Dansgaard–Oeschger events and the Younger Dryas, however, illustrate how glacial variations may also influence climate without the orbital forcing.
Sea level change
During the Last Glacial Maximum,
some 25,000 years ago, sea levels were roughly 130 m lower than today.
The deglaciation afterwards was characterized by rapid sea level change. In the early Pliocene, global temperatures were 1–2˚C warmer than the present temperature, yet sea level was 15–25 meters higher than today.
Sea ice
Sea ice plays an important role in Earth's climate as it affects the total amount of sunlight that is reflected away from the Earth.
In the past, the Earth's oceans have been almost entirely covered by
sea ice on a number of occasions, when the Earth was in a so-called Snowball Earth state, and completely ice-free in periods of warm climate.hen there is a lot of sea ice present globally, especially in the tropics and subtropics, the climate is more sensitive to forcings as the ice–albedo feedback is very strong.
Through geologic and historical time
Various climate forcings are typically in flux throughout geologic time, and some processes of the Earth's temperature may be self-regulating. For example, during the Snowball Earth period, large glacial ice sheets spanned to Earth's equator, covering nearly its entire surface, and very high albedo created extremely low temperatures, while the accumulation of snow and ice likely removed carbon dioxide through atmospheric deposition. However, the absence of plant cover to absorb atmospheric CO2
emitted by volcanoes meant that the greenhouse gas could accumulate in
the atmosphere. There was also an absence of exposed silicate rocks,
which use CO2 when they undergo weathering. This created a warming that later melted the ice and brought Earth's temperature back up.
Paleo-Eocene Thermal maximum
Climate changes over the past 65 million years, using proxy data including Oxygen-18 ratios from foraminifera.
The Paleocene–Eocene Thermal Maximum (PETM) was a time period with more than 5–8 °C global average temperature rise across the event. This climate event occurred at the time boundary of the Paleocene and Eocene geological epochs. During the event large amounts of methane was released, a potent greenhouse gas.
The PETM represents a "case study" for modern climate change as in the
greenhouse gases were released in a geologically relatively short amount
of time. During the PETM, a mass extinction of organisms in the deep ocean took place.
The Cenozoic
Throughout the Cenozoic, multiple climate forcings led to warming and cooling of the atmosphere, which led to the early formation of the Antarctic ice sheet,
subsequent melting, and its later reglaciation. The temperature changes
occurred somewhat suddenly, at carbon dioxide concentrations of about
600–760 ppm and temperatures approximately 4 °C warmer than today.
During the Pleistocene, cycles of glaciations and interglacials occurred
on cycles of roughly 100,000 years, but may stay longer within an
interglacial when orbital eccentricity approaches zero, as during the current interglacial. Previous interglacials such as the Eemian phase created temperatures higher than today, higher sea levels, and some partial melting of the West Antarctic ice sheet.
Climatological temperatures substantially affect cloud cover and
precipitation. At lower temperatures, air can hold less water vapour,
which can lead to decreased precipitation. During the Last Glacial Maximum of 18,000 years ago, thermal-driven evaporation from the oceans onto continental landmasses was low, causing large areas of extreme desert, including polar deserts (cold but with low rates of cloud cover and precipitation). In contrast, the world's climate was cloudier and wetter than today near the start of the warm Atlantic Period of 8000 years ago.
The Holocene
Temperature change over the past 12 000 years, from various sources. The thick black curve is an average.
The Holocene is characterized by a long-term cooling starting after the Holocene Optimum, when temperatures were probably only just below current temperatures (second decade of the 21st century), and a strong African Monsoon created grassland conditions in the Sahara during the Neolithic Subpluvial. Since that time, several cooling events have occurred, including:
Certain effects have occurred during these cycles. For example, during the Medieval Warm Period, the American Midwest was in drought, including the Sand Hills of Nebraska which were active sand dunes. The black death plague of Yersinia pestis also occurred during Medieval temperature fluctuations, and may be related to changing climates.
Solar activity may have contributed to part of the modern warming
that peaked in the 1930s. However, solar cycles fail to account for
warming observed since the 1980s to the present day. Events such as the opening of the Northwest Passage and recent record low ice minima of the modern Arctic shrinkage
have not taken place for at least several centuries, as early explorers
were all unable to make an Arctic crossing, even in summer. Shifts in biomes and habitat ranges are also unprecedented, occurring at rates that do not coincide with known climate oscillations.
Modern climate change and global warming
As a consequence of humans emitting greenhouse gases, global surface temperatures
have started rising. Global warming is an aspect of modern climate
change, a term that also includes the observed changes in precipitation,
storm tracks and cloudiness. As a consequence, glaciers worldwide have
been found to be shrinking significantly. Land ice sheets in both Antarctica and Greenland have been losing mass since 2002 and have seen an acceleration of ice mass loss since 2009.
Global sea levels have been rising as a consequence of thermal
expansion and ice melt. The decline in Arctic sea ice, both in extent
and thickness, over the last several decades is further evidence for
rapid climate change.
Variability between regions
Examples of regional climate variability
Land-ocean. Surface air temperatures over land masses have been increasing faster than those over the ocean, the ocean absorbing about 90% of excess heat.
Hemispheres. The Hemispheres' average temperature changes have diverged because of the North's greater percentage of landmass, and due to global ocean currents.
Latitude bands. Three latitude bands that respectively cover
30, 40 and 30 percent of the global surface area show mutually distinct
temperature growth patterns in recent decades.
Altitude. A warming stripes graphic (blues denote cool, reds
denote warm) shows how the greenhouse effect traps heat in the lower
atmosphere so that the upper atmosphere, receiving less reflected
energy, cools. Volcanos cause upper-atmosphere temperature spikes.
Global versus regional. For geographical and statistical reasons, larger year-to-year variations are expected for localized geographic regions (e.g., the Caribbean) than for global averages.
Relative deviation. Though northern America has warmed more
than its tropics, the tropics have more clearly departed from normal
historical variability (colored bands: 1σ, 2σ standard deviations).
In addition to global climate variability and global climate change
over time, numerous climatic variations occur contemporaneously across
different physical regions.
The oceans' absorption of about 90% of excess heat has helped to
cause land surface temperatures to grow more rapidly than sea surface
temperatures.
The Northern Hemisphere, having a larger landmass-to-ocean ratio than
the Southern Hemisphere, shows greater average temperature increases.
Variations across different latitude bands also reflect this divergence
in average temperature increase, with the temperature increase of
northern extratropics exceeding that of the tropics, which in turn exceeds that of the southern extratropics.
Upper regions of the atmosphere have been cooling
contemporaneously with a warming in the lower atmosphere, confirming the
action of the greenhouse effect and ozone depletion.
Observed regional climatic variations confirm predictions
concerning ongoing changes, for example, by contrasting (smoother)
year-to-year global variations with (more volatile) year-to-year
variations in localized regions.
Conversely, comparing different regions' warming patterns to their
respective historical variabilities, allows the raw magnitudes of
temperature changes to be placed in the perspective of what is normal
variability for each region.
Regional variability observations permit study of regionalized climate tipping points such as rainforest loss, ice sheet and sea ice melt, and permafrost thawing. Such distinctions underlie research into a possible global cascade of tipping points.
A desert is a barren area of landscape where little precipitation
occurs and, consequently, living conditions are hostile for plant and
animal life. The lack of vegetation exposes the unprotected surface of
the ground to the processes of denudation. About one-third of the land surface of the world is arid or semi-arid. This includes much of the polar regions, where little precipitation occurs, and which are sometimes called polar deserts
or "cold deserts". Deserts can be classified by the amount of
precipitation that falls, by the temperature that prevails, by the
causes of desertification or by their geographical location.
Deserts are formed by weathering
processes as large variations in temperature between day and night put
strains on the rocks, which consequently break in pieces. Although rain
seldom occurs in deserts, there are occasional downpours that can result
in flash floods. Rain falling on hot rocks can cause them to shatter,
and the resulting fragments and rubble strewn over the desert floor are
further eroded by the wind. This picks up particles of sand and dust and
wafts them aloft in sand or dust storms.
Wind-blown sand grains striking any solid object in their path can
abrade the surface. Rocks are smoothed down, and the wind sorts sand
into uniform deposits. The grains end up as level sheets of sand or are
piled high in billowing sand dunes. Other deserts are flat, stony plains where all the fine material has been blown away and the surface consists of a mosaic of smooth stones. These areas are known as desert pavements, and little further erosion takes place. Other desert features include rock outcrops, exposed bedrock and clays once deposited by flowing water. Temporary lakes may form and salt pans may be left when waters evaporate. There may be underground sources of water, in the form of springs and seepages from aquifers. Where these are found, oases can occur.
Plants and animals living in the desert need special adaptations
to survive in the harsh environment. Plants tend to be tough and wiry
with small or no leaves, water-resistant cuticles, and often spines to deter herbivory. Some annual plants germinate,
bloom and die in the course of a few weeks after rainfall, while other
long-lived plants survive for years and have deep root systems able to
tap underground moisture. Animals need to keep cool and find enough food
and water to survive. Many are nocturnal,
and stay in the shade or underground during the heat of the day. They
tend to be efficient at conserving water, extracting most of their needs
from their food and concentrating their urine. Some animals remain in a state of dormancy for long periods, ready to become active again during the rare rainfall. They then reproduce rapidly while conditions are favorable before returning to dormancy.
People have struggled to live in deserts and the surrounding semi-arid lands for millennia. Nomads
have moved their flocks and herds to wherever grazing is available, and
oases have provided opportunities for a more settled way of life. The
cultivation of semi-arid regions encourages erosion of soil and is one
of the causes of increased desertification. Desert farming is possible with the aid of irrigation, and the Imperial Valley
in California provides an example of how previously barren land can be
made productive by the import of water from an outside source. Many trade routes have been forged across deserts, especially across the Sahara Desert, and traditionally were used by caravans of camels carrying salt, gold, ivory and other goods. Large numbers of slaves
were also taken northwards across the Sahara. Some mineral extraction
also takes place in deserts, and the uninterrupted sunlight gives
potential for the capture of large quantities of solar energy.
Etymology
English desert and its Romancecognates (including Italian and Portuguesedeserto, Frenchdésert and Spanishdesierto) all come from the ecclesiastical Latindēsertum (originally "an abandoned place"), a participle of dēserere, "to abandon".
The correlation between aridity and sparse population is complex and
dynamic, varying by culture, era, and technologies; thus the use of the
word desert can cause confusion. In English before the 20th century, desert was often used in the sense of "unpopulated area", without specific reference to aridity; but today the word is most often used in its climate-science sense (an area of low precipitation). Phrases such as "desert island" and "Great American Desert", or Shakespeare's "deserts of Bohemia" (The Winter's Tale) in previous centuries did not necessarily imply sand or aridity; their focus was the sparse population.
Physical geography
A desert is a region of land that is very dry because it receives low amounts of precipitation
(usually in the form of rain, but it may be snow, mist or fog), often
has little coverage by plants, and in which streams dry up unless they
are supplied by water from outside the area. Deserts generally receive less than 250 mm (10 in) of precipitation each year. The potential evapotranspiration may be large but (in the absence of available water) the actual evapotranspiration may be close to zero. Semi-deserts are regions which receive between 250 and 500 mm (10 and 20 in) and when clad in grass, these are known as steppes.
Deserts have been defined and classified in a number of ways,
generally combining total precipitation, number of days on which this
falls, temperature, and humidity, and sometimes additional factors. For example, Phoenix, Arizona,
receives less than 250 mm (9.8 in) of precipitation per year, and is
immediately recognized as being located in a desert because of its
aridity-adapted plants. The North Slope of Alaska's Brooks Range also receives less than 250 mm (9.8 in) of precipitation per year and is often classified as a cold desert. Other regions of the world have cold deserts, including areas of the Himalayas and other high-altitude areas in other parts of the world. Polar deserts cover much of the ice-free areas of the Arctic and Antarctic.
A non-technical definition is that deserts are those parts of the
Earth's surface that have insufficient vegetation cover to support a
human population.
Potential evapotranspiration supplements the measurement of
precipitation in providing a scientific measurement-based definition of a
desert. The water budget of an area can be calculated using the formula
P − PE ± S, wherein P is precipitation, PE is potential evapotranspiration rates and S is the amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric evaporation and through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. As an example, Tucson, Arizona
receives about 300 mm (12 in) of rain per year, however about 2,500 mm
(98 in) of water could evaporate over the course of a year.
In other words, about eight times more water could evaporate from the
region than actually falls as rain. Rates of evapotranspiration in cold
regions such as Alaska are much lower because of the lack of heat to aid
in the evaporation process.
Deserts are sometimes classified as "hot" or "cold", "semiarid" or "coastal".
The characteristics of hot deserts include high temperatures in summer;
greater evaporation than precipitation, usually exacerbated by high
temperatures, strong winds and lack of cloud cover; considerable
variation in the occurrence of precipitation, its intensity and
distribution; and low humidity. Winter temperatures vary considerably
between different deserts and are often related to the location of the
desert on the continental landmass and the latitude. Daily variations in
temperature can be as great as 22 °C (40 °F) or more, with heat loss by
radiation at night being increased by the clear skies.
Cold desert: snow surface at Dome C Station, Antarctica
Cold deserts, sometimes known as temperate deserts, occur at higher
latitudes than hot deserts, and the aridity is caused by the dryness of
the air. Some cold deserts are far from the ocean and others are
separated by mountain ranges from the sea, and in both cases, there is
insufficient moisture in the air to cause much precipitation. The
largest of these deserts are found in Central Asia. Others occur on the
eastern side of the Rocky Mountains, the eastern side of the southern Andes and in southern Australia.
Polar deserts are a particular class of cold desert. The air is very
cold and carries little moisture so little precipitation occurs and what
does fall, usually snow, is carried along in the often strong wind and
may form blizzards, drifts and dunes similar to those caused by dust and
sand in other desert regions. In Antarctica,
for example, the annual precipitation is about 50 mm (2 in) on the
central plateau and some ten times that amount on some major peninsulas.
Based on precipitation alone, hyperarid
deserts receive less than 25 mm (1 in) of rainfall a year; they have no
annual seasonal cycle of precipitation and experience twelve-month
periods with no rainfall at all.
Arid deserts receive between 25 and 200 mm (1 and 8 in) in a year and
semiarid deserts between 200 and 500 mm (8 and 20 in). However, such
factors as the temperature, humidity, rate of evaporation and
evapotranspiration, and the moisture storage capacity of the ground have
a marked effect on the degree of aridity and the plant and animal life
that can be sustained. Rain falling in the cold season may be more
effective at promoting plant growth, and defining the boundaries of
deserts and the semiarid regions that surround them on the grounds of
precipitation alone is problematic.
A semi-arid desert or a steppe is a version of the arid desert
with much more rainfall, vegetation and higher humidity. These regions
feature a semi-arid climate and are less extreme than regular deserts.
Like arid deserts, temperatures can vary greatly in semi deserts. They
share some characteristics of a true desert and are usually located at
the edge of deserts and continental dry areas. They usually receive
precipitation from 250 mm (10 in) to 500 mm (20 in) but this can vary
due to evapotranspiration and soil nutrition. Semi deserts can be found
in the Tabernas Desert (and some of the Spanish Plateau), The Sahel, The Eurasian Steppe, most of Central Asia, the Western US, most of Northern Mexico, portions of South America (especially in Argentina) and the Australian Outback. They usually feature BSh (hot steppe) or BSk (temperate steppe) in the Köppen climate classification.
Coastal deserts are mostly found on the western edges of
continental land masses in regions where cold currents approach the land
or cold water upwellings rise from the ocean depths. The cool winds
crossing this water pick up little moisture and the coastal regions have
low temperatures and very low rainfall, the main precipitation being in
the form of fog and dew. The range of temperatures on a daily and
annual scale is relatively low, being 11 °C (20 °F) and 5 °C (9 °F)
respectively in the Atacama Desert. Deserts of this type are often long and narrow and bounded to the east by mountain ranges. They occur in Namibia, Chile, southern California and Baja California. Other coastal deserts influenced by cold currents are found in Western Australia, the Arabian Peninsula and Horn of Africa, and the western fringes of the Sahara.
In 1961, Peveril Meigs
divided desert regions on Earth into three categories according to the
amount of precipitation they received. In this now widely accepted
system, extremely arid lands have at least twelve consecutive months
without precipitation, arid lands have less than 250 mm (10 in) of
annual precipitation, and semiarid lands have a mean annual
precipitation of between 250 and 500 mm (10–20 in). Both extremely arid
and arid lands are considered to be deserts while semiarid lands are
generally referred to as steppes when they are grasslands.
Deserts are also classified, according to their geographical location
and dominant weather pattern, as trade wind, mid-latitude, rain shadow,
coastal, monsoon, or polar deserts. Trade wind deserts occur either side of the horse latitudes
at 30° to 35° North and South. These belts are associated with the
subtropical anticyclone and the large-scale descent of dry air moving
from high-altitudes toward the poles. The Sahara Desert is of this type.
Mid-latitude deserts occur between 30° and 50° North and South. They
are mostly in areas remote from the sea where most of the moisture has
already precipitated from the prevailing winds. They include the Tengger and Sonoran Deserts.
Monsoon deserts are similar. They occur in regions where large
temperature differences occur between sea and land. Moist warm air rises
over the land, deposits its water content and circulates back to sea.
Further inland, areas receive very little precipitation. The Thar Desert near the India/Pakistan border is of this type.
In some parts of the world, deserts are created by a rain shadow effect. Orographic lift
occurs as air masses rise to pass over high ground. In the process they
cool and lose much of their moisture by precipitation on the windward slope of the mountain range. When they descend on the leeward side, they warm and their capacity to hold moisture increases so an area with relatively little precipitation occurs. The Taklamakan Desert is an example, lying in the rain shadow of the Himalayas and receiving less than 38 mm (1.5 in) precipitation annually.
Other areas are arid by virtue of being a very long way from the nearest available sources of moisture.
Montane deserts are arid places with a very high altitude; the most prominent example is found north of the Himalayas, in the Kunlun Mountains and the Tibetan Plateau. Many locations within this category have elevations exceeding 3,000 m (9,800 ft) and the thermal regime can be hemiboreal.
These places owe their profound aridity (the average annual
precipitation is often less than 40 mm or 1.5 in) to being very far from
the nearest available sources of moisture and are often in the lee
of mountain ranges. Montane deserts are normally cold, or may be
scorchingly hot by day and very cold by night as is true of the
northeastern slopes of Mount Kilimanjaro.
Polar deserts such as McMurdo Dry Valleys remain ice-free because of the dry katabatic winds that flow downhill from the surrounding mountains. Former desert areas presently in non-arid environments, such as the Sandhills in Nebraska, are known as paleodeserts. In the Köppen climate classification system, deserts are classed as BWh (hot desert) or BWk (temperate desert). In the Thornthwaite climate classification system, deserts would be classified as arid megathermal climates.
Deserts usually have a large diurnal
and seasonal temperature range, with high daytime temperatures falling
sharply at night. The diurnal range may be as much as 20 to 30 °C (36 to
54 °F) and the rock surface experiences even greater temperature
differentials. During the day the sky is usually clear and most of the sun's
radiation reaches the ground, but as soon as the sun sets, the desert
cools quickly by radiating heat into space. In hot deserts, the
temperature during daytime can exceed 45 °C (113 °F) in summer and
plunge below freezing point at night during winter.
One square centimeter (0.16 sq in) of windblown sand from the Gobi Desert
Such large temperature variations have a destructive effect on the
exposed rocky surfaces. The repeated fluctuations put a strain on
exposed rock and the flanks of mountains crack and shatter. Fragmented
strata slide down into the valleys where they continue to break into
pieces due to the relentless sun by day and chill by night. Successive
strata are exposed to further weathering. The relief of the internal
pressure that has built up in rocks that have been underground for aeons
can cause them to shatter. Exfoliation
also occurs when the outer surfaces of rocks split off in flat flakes.
This is believed to be caused by the stresses put on the rock by
repeated thermal expansions and contractions which induces fracturing parallel to the original surface.
Chemical weathering processes probably play a more important role in
deserts than was previously thought. The necessary moisture may be
present in the form of dew or mist. Ground water may be drawn to the
surface by evaporation and the formation of salt crystals may dislodge
rock particles as sand or disintegrate rocks by exfoliation. Shallow
caves are sometimes formed at the base of cliffs by this means.
As the desert mountains decay, large areas of shattered rock and
rubble occur. The process continues and the end products are either dust
or sand. Dust is formed from solidified clay or volcanic deposits
whereas sand results from the fragmentation of harder granites, limestone and sandstone.
There is a certain critical size (about 0.5 mm) below which further
temperature-induced weathering of rocks does not occur and this provides
a minimum size for sand grains.
As the mountains are eroded, more and more sand is created. At
high wind speeds, sand grains are picked up off the surface and blown
along, a process known as saltation. The whirling airborne grains act as a sand blasting mechanism which grinds away solid objects in its path as the kinetic energy of the wind is transferred to the ground. The sand eventually ends up deposited in level areas known as sand-fields or sand-seas, or piled up in dunes.
Dust storms and sandstorms
Dust storm about to engulf a military camp in Iraq, 2005
Sand and dust storms are natural events that occur in arid regions
where the land is not protected by a covering of vegetation. Dust storms
usually start in desert margins rather than the deserts themselves
where the finer materials have already been blown away. As a steady wind
begins to blow, fine particles lying on the exposed ground begin to
vibrate. At greater wind speeds, some particles are lifted into the air
stream. When they land, they strike other particles which may be jerked
into the air in their turn, starting a chain reaction. Once ejected, these particles move in one of three possible ways, depending on their size, shape and density; suspension, saltation
or creep. Suspension is only possible for particles less than 0.1 mm
(0.004 in) in diameter. In a dust storm, these fine particles are lifted
up and wafted aloft to heights of up to 6 km (3.7 mi). They reduce
visibility and can remain in the atmosphere for days on end, conveyed by
the trade winds for distances of up to 6,000 km (3,700 mi).
Denser clouds of dust can be formed in stronger winds, moving across
the land with a billowing leading edge. The sunlight can be obliterated
and it may become as dark as night at ground level.
In a study of a dust storm in China in 2001, it was estimated that 6.5
million tons of dust were involved, covering an area of 134,000,000 km2 (52,000,000 sq mi). The mean particle size was 1.44 μm.
A much smaller scale, short-lived phenomenon can occur in calm
conditions when hot air near the ground rises quickly through a small
pocket of cooler, low-pressure air above forming a whirling column of
particles, a dust devil.
Sandstorms occur with much less frequency than dust storms. They are
often preceded by severe dust storms and occur when the wind velocity
increases to a point where it can lift heavier particles. These grains
of sand, up to about 0.5 mm (0.020 in) in diameter are jerked into the
air but soon fall back to earth, ejecting other particles in the
process. Their weight prevents them from being airborne for long and
most only travel a distance of a few meters (yards). The sand streams
along above the surface of the ground like a fluid, often rising to
heights of about 30 cm (12 in).
In a really severe steady blow, 2 m (6 ft 7 in) is about as high as the
sand stream can rise as the largest sand grains do not become airborne
at all. They are transported by creep, being rolled along the desert
floor or performing short jumps.
During a sandstorm, the wind-blown sand particles become electrically charged. Such electric fields,
which range in size up to 80 kV/m, can produce sparks and cause
interference with telecommunications equipment. They are also unpleasant
for humans and can cause headaches and nausea.
The electric fields are caused by the collision between airborne
particles and by the impacts of saltating sand grains landing on the
ground. The mechanism is little understood but the particles usually
have a negative charge when their diameter is under 250 μm and a
positive one when they are over 500 μm.
Major deserts
The world's largest non-polar deserts
Deserts take up about one third of the Earth's land surface.
Bottomlands may be salt-covered flats. Eolian processes
are major factors in shaping desert landscapes. Polar deserts (also
seen as "cold deserts") have similar features, except the main form of
precipitation is snow rather than rain. Antarctica is the world's largest cold desert (composed of about 98% thick continentalice sheet and 2% barren rock). Some of the barren rock is to be found in the so-called Dry Valleys of Antarctica that almost never get snow, which can have ice-encrusted saline lakes that suggest evaporation far greater than the rare snowfall due to the strong katabatic winds that even evaporate ice.
Deserts, both hot and cold, play a part in moderating the Earth's
temperature. This is because they reflect more of the incoming light and
their albedo is higher than that of forests or the sea.
Features
Aerial view of Makhtesh Ramon, an erosion cirque of a type unique to the Negev
Many people think of deserts as consisting of extensive areas of
billowing sand dunes because that is the way they are often depicted on
TV and in films, but deserts do not always look like this.
Across the world, around 20% of desert is sand, varying from only 2% in
North America to 30% in Australia and over 45% in Central Asia. Where sand does occur, it is usually in large quantities in the form of sand sheets or extensive areas of dunes.
A sand sheet is a near-level, firm expanse of partially
consolidated particles in a layer that varies from a few centimeters to a
few meters thick. The structure of the sheet consists of thin
horizontal layers of coarse silt and very fine to medium grain sand,
separated by layers of coarse sand and pea-gravel which are a single
grain thick. These larger particles anchor the other particles in place
and may also be packed together on the surface so as to form a miniature
desert pavement.
Small ripples form on the sand sheet when the wind exceeds 24 km/h
(15 mph). They form perpendicular to the wind direction and gradually
move across the surface as the wind continues to blow. The distance
between their crests corresponds to the average length of jumps made by
particles during saltation. The ripples are ephemeral and a change in
wind direction causes them to reorganise.
Diagram showing barchan dune formation, with the wind blowing from the left
Sand dunes are accumulations of windblown sand piled up in mounds or
ridges. They form downwind of copious sources of dry, loose sand and
occur when topographic and climatic conditions cause airborne particles
to settle. As the wind blows, saltation and creep take place on the
windward side of the dune and individual grains of sand move uphill.
When they reach the crest, they cascade down the far side. The upwind
slope typically has a gradient of 10° to 20° while the lee slope is
around 32°, the angle at which loose dry sand will slip. As this
wind-induced movement of sand grains takes place, the dune moves slowly
across the surface of the ground.
Dunes are sometimes solitary, but they are more often grouped together
in dune fields. When these are extensive, they are known as sand seas or
ergs.
The shape of the dune depends on the characteristics of the prevailing wind. Barchan
dunes are produced by strong winds blowing across a level surface and
are crescent-shaped with the concave side away from the wind. When there
are two directions from which winds regularly blow, a series of long,
linear dunes known as seif
dunes may form. These also occur parallel to a strong wind that blows
in one general direction. Transverse dunes run at a right angle to the
prevailing wind direction. Star dunes are formed by variable winds, and
have several ridges and slip faces radiating from a central point. They
tend to grow vertically; they can reach a height of 500 m (1,600 ft),
making them the tallest type of dune. Rounded mounds of sand without a
slip face are the rare dome dunes, found on the upwind edges of sand
seas.
In deserts where large amounts of limestone mountains surround a closed basin, such as at White Sands National Park in south-central New Mexico, occasional storm runoff transports dissolved limestone and gypsum into a low-lying pan within the basin where the water evaporates, depositing the gypsum and forming crystals known as selenite.
The crystals left behind by this process are eroded by the wind and
deposited as vast white dune fields that resemble snow-covered
landscapes. These types of dune are rare, and only form in closed arid
basins that retain the highly soluble gypsum that would otherwise be
washed into the sea.
A large part of the surface area of the world's deserts consists of
flat, stone-covered plains dominated by wind erosion. In "eolian
deflation", the wind continually removes fine-grained material, which
becomes wind-blown sand. This exposes coarser-grained material, mainly pebbles with some larger stones or cobbles, leaving a desert pavement, an area of land overlaid by closely packed smooth stones forming a tessellated
mosaic. Different theories exist as to how exactly the pavement is
formed. It may be that after the sand and dust is blown away by the wind
the stones jiggle themselves into place; alternatively, stones
previously below ground may in some way work themselves to the surface.
Very little further erosion takes place after the formation of a
pavement, and the ground becomes stable. Evaporation brings moisture to
the surface by capillary action and calcium salts may be precipitated,
binding particles together to form a desert conglomerate.
In time, bacteria that live on the surface of the stones accumulate a
film of minerals and clay particles, forming a shiny brown coating known
as desert varnish.
Other non-sandy deserts consist of exposed outcrops of bedrock, dry soils or aridisols, and a variety of landforms affected by flowing water, such as alluvial fans, sinks or playas, temporary or permanent lakes, and oases. A hamada is a type of desert landscape consisting of a high rocky plateau where the sand has been removed by aeolian processes.
Other landforms include plains largely covered by gravels and angular
boulders, from which the finer particles have been stripped by the wind.
These are called "reg" in the western Sahara, "serir" in the eastern
Sahara, "gibber plains" in Australia and "saï" in central Asia. The Tassili Plateau
in Algeria is an impressive jumble of eroded sandstone outcrops,
canyons, blocks, pinnacles, fissures, slabs and ravines. In some places
the wind has carved holes or arches, and in others, it has created
mushroom-like pillars narrower at the base than the top. On the Colorado Plateau, it is water that has been the prevailing eroding force. Here, rivers, such as the Colorado, have cut their way over the millennia through the high desert floor, creating canyons that are over a mile (6,000 feet or 1,800 meters) deep in places, exposing strata that are over two billion years old.
Water
Atacama, the world's driest non-polar desert, part of the Arid Diagonal of South America.
One of the driest places on Earth is the Atacama Desert. It is virtually devoid of life because it is blocked from receiving precipitation by the Andes mountains to the east and the Chilean Coast Range to the west. The cold Humboldt Current and the anticyclone of the Pacific are essential to keep the dry climate of the Atacama. The average precipitation in the Chilean region of Antofagasta
is just 1 mm (0.039 in) per year. Some weather stations in the Atacama
have never received rain. Evidence suggests that the Atacama may not
have had any significant rainfall from 1570 to 1971. It is so arid that
mountains that reach as high as 6,885 m (22,589 ft) are completely free
of glaciers and, in the southern part from 25°S to 27°S, may have been glacier-free throughout the Quaternary, though permafrost extends down to an altitude of 4,400 m (14,400 ft) and is continuous above 5,600 m (18,400 ft). Nevertheless, there is some plant life in the Atacama, in the form of specialist plants that obtain moisture from dew and the fogs that blow in from the Pacific.
Flash flood in the Gobi
When rain falls in deserts, as it occasionally does, it is often with
great violence. The desert surface is evidence of this with dry stream
channels known as arroyos or wadis meandering across its surface. These can experience flash floods,
becoming raging torrents with surprising rapidity after a storm that
may be many kilometers away. Most deserts are in basins with no drainage
to the sea but some are crossed by exotic rivers sourced in mountain
ranges or other high rainfall areas beyond their borders. The River Nile, the Colorado River and the Yellow River
do this, losing much of their water through evaporation as they pass
through the desert and raising groundwater levels nearby. There may also
be underground sources of water in deserts in the form of springs, aquifers, underground rivers or lakes. Where these lie close to the surface, wells can be dug and oases may form where plant and animal life can flourish. The Nubian Sandstone Aquifer System under the Sahara Desert is the largest known accumulation of fossil water. The Great Man-Made River is a scheme launched by Libya's Muammar Gadaffi to tap this aquifer and supply water to coastal cities. Kharga Oasis
in Egypt is 150 km (93 mi) long and is the largest oasis in the Libyan
Desert. A lake occupied this depression in ancient times and thick
deposits of sandy-clay resulted. Wells are dug to extract water from the
porous sandstone that lies underneath. Seepages may occur in the walls of canyons and pools may survive in deep shade near the dried up watercourse below.
Lakes may form in basins where there is sufficient precipitation or meltwater
from glaciers above. They are usually shallow and saline, and wind
blowing over their surface can cause stress, moving the water over
nearby low-lying areas. When the lakes dry up, they leave a crust or hardpan behind. This area of deposited clay, silt or sand is known as a playa. The deserts of North America have more than one hundred playas, many of them relics of Lake Bonneville which covered parts of Utah, Nevada and Idaho during the last ice age when the climate was colder and wetter. These include the Great Salt Lake, Utah Lake, Sevier Lake and many dry lake beds. The smooth flat surfaces of playas have been used for attempted vehicle speed records at Black Rock Desert and Bonneville Speedway and the United States Air Force uses Rogers Dry Lake in the Mojave Desert as runways for aircraft and the space shuttle.
Ecology and biogeography
Deserts and semi-deserts are home to ecosystems with low or very low biomass and primary productivity in arid or semi-arid climates. They are mostly found in subtropical high-pressure belts and major continental rain shadows. Primary productivity depends on low densities of small photoautotrophs that sustain a sparse trophic network. Plant growth is limited by rainfall,
temperature extremes and desiccating winds. Deserts have strong
temporal variability in the availability of resources due to the total
amount of annual rainfall and the size of individual rainfall events.
Resources are often ephemeral or episodic, and this triggers sporadic
animal movements and ‘pulse and reserve’ or ‘boom-bust’ ecosystem
dynamics. Erosion and sedimentation are high due to the sparse
vegetation cover and the activities of large mammals and people. Plants
and animals in deserts are mostly adapted to extreme and prolonged water deficits, but their reproductive phenology often responds to short episodes of surplus. Competitive interactions are weak.
Flora
Xerophytes: Cardón cacti in the Baja California Desert, Cataviña region, Mexico
Plants face severe challenges in arid environments. Problems they
need to solve include how to obtain enough water, how to avoid being
eaten and how to reproduce. Photosynthesis
is the key to plant growth. It can only take place during the day as
energy from the sun is required, but during the day, many deserts become
very hot. Opening stomata to allow in the carbon dioxide necessary for the process causes evapotranspiration, and conservation of water is a top priority for desert vegetation. Some plants have resolved this problem by adopting crassulacean acid metabolism, allowing them to open their stomata during the night to allow CO2 to enter, and close them during the day, or by using C4 carbon fixation.
Many desert plants have reduced the size of their leaves or
abandoned them altogether. Cacti are desert specialists, and in most
species, the leaves have been dispensed with and the chlorophyll
displaced into the trunks, the cellular structure of which has been
modified to allow them to store water. When rain falls, the water is
rapidly absorbed by the shallow roots and retained to allow them to
survive until the next downpour, which may be months or years away. The giant saguaro cacti of the Sonoran Desert
form "forests", providing shade for other plants and nesting places for
desert birds. Saguaro grows slowly but may live for up to two hundred
years. The surface of the trunk is folded like a concertina, allowing it to expand, and a large specimen can hold eight tons of water after a good downpour.
Cacti are present in both North and South America with a post-Gondwana origin. Other xerophytic plants have developed similar strategies by a process known as convergent evolution.
They limit water loss by reducing the size and number of stomata, by
having waxy coatings and hairy or tiny leaves. Some are deciduous,
shedding their leaves in the driest season, and others curl their leaves
up to reduce transpiration. Others store water in succulent leaves or
stems or in fleshy tubers. Desert plants maximize water uptake by having
shallow roots that spread widely, or by developing long taproots that reach down to deep rock strata for ground water. The saltbush in Australia has succulent leaves and secretes salt crystals, enabling it to live in saline areas. In common with cacti, many have developed spines to ward off browsing animals.
Some desert plants produce seed which lies dormant in the soil until sparked into growth by rainfall. With annuals,
such plants grow with great rapidity and may flower and set seed within
weeks, aiming to complete their development before the last vestige of
water dries up. For perennial plants, reproduction is more likely to be
successful if the seed germinates in a shaded position, but not so close
to the parent plant as to be in competition with it. Some seed will not
germinate until it has been blown about on the desert floor to scarify the seed coat. The seed of the mesquite
tree, which grows in deserts in the Americas, is hard and fails to
sprout even when planted carefully. When it has passed through the gut
of a pronghorn it germinates readily, and the little pile of moist dung provides an excellent start to life well away from the parent tree.
The stems and leaves of some plants lower the surface velocity of
sand-carrying winds and protect the ground from erosion. Even small
fungi and microscopic plant organisms found on the soil surface
(so-called cryptobiotic soil)
can be a vital link in preventing erosion and providing support for
other living organisms. Cold deserts often have high concentrations of
salt in the soil. Grasses and low shrubs are the dominant vegetation
here and the ground may be covered with lichens. Most shrubs have spiny leaves and shed them in the coldest part of the year.
Fauna
Animals adapted to live in deserts are called xerocoles.
There is no evidence that body temperature of mammals and birds is
adaptive to the different climates, either of great heat or cold. In
fact, with a very few exceptions, their basal metabolic rate is determined by body size, irrespective of the climate in which they live.
Many desert animals (and plants) show especially clear evolutionary
adaptations for water conservation or heat tolerance and so are often
studied in comparative physiology, ecophysiology, and evolutionary physiology. One well-studied example is the specializations of mammalian kidneys shown by desert-inhabiting species. Many examples of convergent evolution have been identified in desert organisms, including between cacti and Euphorbia, kangaroo rats and jerboas, Phrynosoma and Moloch lizards.
Deserts present a very challenging environment for animals. Not only
do they require food and water but they also need to keep their body
temperature at a tolerable level. In many ways, birds are the ablest to
do this of the higher animals. They can move to areas of greater food
availability as the desert blooms after local rainfall and can fly to
faraway waterholes. In hot deserts, gliding birds can remove themselves
from the over-heated desert floor by using thermals to soar in the
cooler air at great heights. In order to conserve energy, other desert
birds run rather than fly. The cream-colored courser
flits gracefully across the ground on its long legs, stopping
periodically to snatch up insects. Like other desert birds, it is well-camouflaged by its coloring and can merge into the landscape when stationary. The sandgrouse is an expert at this and nests on the open desert floor dozens of kilometers (miles) away from the waterhole
it needs to visit daily. Some small diurnal birds are found in very
restricted localities where their plumage matches the color of the
underlying surface. The desert lark takes frequent dust baths which ensures that it matches its environment.
Water and carbon dioxide are metabolic end products of oxidation of fats, proteins, and carbohydrates.
Oxidising a gram of carbohydrate produces 0.60 grams of water; a gram
of protein produces 0.41 grams of water; and a gram of fat produces 1.07
grams of water, making it possible for xerocoles to live with little or no access to drinking water. The kangaroo rat for example makes use of this water of metabolism and conserves water both by having a low basal metabolic rate and by remaining underground during the heat of the day, reducing loss of water through its skin and respiratory system when at rest. Herbivorous mammals obtain moisture from the plants they eat. Species such as the addax antelope, dik-dik, Grant's gazelle and oryx are so efficient at doing this that they apparently never need to drink. The camel is a superb example of a mammal adapted to desert life. It minimizes its water loss by producing concentrated urine and dry dung, and is able to lose 40% of its body weight through water loss without dying of dehydration. Carnivores can obtain much of their water needs from the body fluids of their prey. Many other hot desert animals are nocturnal,
seeking out shade during the day or dwelling underground in burrows. At
depths of more than 50 cm (20 in), these remain at between 30 to 32 °C
(86 to 90 °F) regardless of the external temperature. Jerboas, desert rats,
kangaroo rats and other small rodents emerge from their burrows at
night and so do the foxes, coyotes, jackals and snakes that prey on
them. Kangaroos keep cool by increasing their respiration rate, panting,
sweating and moistening the skin of their forelegs with saliva. Mammals living in cold deserts have developed greater insulation through warmer body fur and insulating layers of fat beneath the skin. The arctic weasel
has a metabolic rate that is two or three times as high as would be
expected for an animal of its size. Birds have avoided the problem of
losing heat through their feet by not attempting to maintain them at the
same temperature as the rest of their bodies, a form of adaptive
insulation. The emperor penguin
has dense plumage, a downy under layer, an air insulation layer next to
the skin and various thermoregulatory strategies to maintain its body
temperature in one of the harshest environments on Earth.
The desert iguana (Dipsosaurus dorsalis) is well-adapted to desert life.
Being ectotherms, reptiles
are unable to live in cold deserts but are well-suited to hot ones. In
the heat of the day in the Sahara, the temperature can rise to 50 °C
(122 °F). Reptiles cannot survive at this temperature and lizards will
be prostrated by heat at 45 °C (113 °F). They have few adaptations to
desert life and are unable to cool themselves by sweating so they
shelter during the heat of the day. In the first part of the night, as
the ground radiates the heat absorbed during the day, they emerge and
search for prey. Lizards and snakes are the most numerous in arid regions and certain snakes have developed a novel method of locomotion that enables them to move sidewards and navigate high sand-dunes. These include the horned viper of Africa and the sidewinder of North America, evolutionarily distinct but with similar behavioural patterns because of convergent evolution. Many desert reptiles are ambush predators and often bury themselves in the sand, waiting for prey to come within range.
Amphibians
might seem unlikely desert-dwellers, because of their need to keep
their skins moist and their dependence on water for reproductive
purposes. In fact, the few species that are found in this habitat have
made some remarkable adaptations. Most of them are fossorial, spending
the hot dry months aestivating in deep burrows. While there they shed their skins a number of times and retain the remnants around them as a waterproof cocoon to retain moisture. In the Sonoran Desert, Couch's spadefoot toad
spends most of the year dormant in its burrow. Heavy rain is the
trigger for emergence and the first male to find a suitable pool calls
to attract others. Eggs are laid and the tadpoles grow rapidly as they
must reach metamorphosis
before the water evaporates. As the desert dries out, the adult toads
rebury themselves. The juveniles stay on the surface for a while,
feeding and growing, but soon dig themselves burrows. Few make it to
adulthood. The water holding frog in Australia has a similar life cycle and may aestivate for as long as five years if no rain falls. The Desert rain frog of Namibia is nocturnal and survives because of the damp sea fogs that roll in from the Atlantic.
Tadpole shrimp survive dry periods as eggs, which rapidly hatch and develop after rain.
Invertebrates, particularly arthropods, have successfully made their homes in the desert. Flies, beetles, ants, termites, locusts, millipedes, scorpions and spiders have hard cuticles
which are impervious to water and many of them lay their eggs
underground and their young develop away from the temperature extremes
at the surface. The Saharan silver ant (Cataglyphis bombycina) uses a heat shock protein in a novel way and forages in the open during brief forays in the heat of the day. The long-legged darkling beetle in Namibia stands on its front legs and raises its carapace to catch the morning mist as condensate, funnelling the water into its mouth. Some arthropods make use of the ephemeral pools that form after rain and complete their life cycle in a matter of days. The desert shrimp does this, appearing "miraculously" in new-formed puddles as the dormant eggs hatch. Others, such as brine shrimps, fairy shrimps and tadpole shrimps, are cryptobiotic and can lose up to 92% of their bodyweight, rehydrating as soon as it rains and their temporary pools reappear.
Human relations
Humans have long made use of deserts as places to live, and more recently have started to exploit them for minerals and energy capture. Deserts play a significant role in human culture with an extensive literature.
History
Shepherd near Marrakech leading his flock to new pasture
Middle Paleolithic hunter-gatherers in a desert environment, south of Iran
People have been living in deserts for millennia. Many, such as the Bushmen in the Kalahari, the Aborigines in Australia and various tribes of North American Indians, were originally hunter-gatherers.
They developed skills in the manufacture and use of weapons, animal
tracking, finding water, foraging for edible plants and using the things
they found in their natural environment to supply their everyday needs.
Their self-sufficient skills and knowledge were passed down through the
generations by word of mouth. Other cultures developed a nomadic way of life as herders of sheep, goats, cattle, camels, yaks, llamas or reindeer.
They travelled over large areas with their herds, moving to new
pastures as seasonal and erratic rainfall encouraged new plant growth.
They took with them their tents made of cloth or skins draped over poles
and their diet included milk, blood and sometimes meat.
Salt caravan travelling between Agadez and the Bilma salt mines
The desert nomads were also traders. The Sahara is a very large expanse of land stretching from the Atlantic rim to Egypt. Trade routes were developed linking the Sahel
in the south with the fertile Mediterranean region to the north and
large numbers of camels were used to carry valuable goods across the
desert interior. The Tuareg were traders and the goods transported traditionally included slaves, ivory and gold going northwards and salt going southwards. Berbers with knowledge of the region were employed to guide the caravans between the various oases and wells. Several million slaves may have been taken northwards across the Sahara between the 8th and 18th centuries. Traditional means of overland transport declined with the advent of motor vehicles, shipping and air freight, but caravans still travel along routes between Agadez and Bilma and between Timbuktu and Taoudenni carrying salt from the interior to desert-edge communities.
Round the rims of deserts, where more precipitation occurred and
conditions were more suitable, some groups took to cultivating crops.
This may have happened when drought
caused the death of herd animals, forcing herdsmen to turn to
cultivation. With few inputs, they were at the mercy of the weather and
may have lived at bare subsistence
level. The land they cultivated reduced the area available to nomadic
herders, causing disputes over land. The semi-arid fringes of the desert
have fragile soils which are at risk of erosion when exposed, as
happened in the American Dust Bowl
in the 1930s. The grasses that held the soil in place were ploughed
under, and a series of dry years caused crop failures, while enormous
dust storms blew the topsoil away. Half a million Americans were forced
to leave their land in this catastrophe.
Similar damage is being done today to the semi-arid areas that
rim deserts and about twelve million hectares of land are being turned
to desert each year. Desertification is caused by such factors as drought, climatic shifts, tillage for agriculture, overgrazing
and deforestation. Vegetation plays a major role in determining the
composition of the soil. In many environments, the rate of erosion and
run off increases dramatically with reduced vegetation cover.
Deserts contain substantial mineral resources, sometimes over their
entire surface, giving them their characteristic colors. For example,
the red of many sand deserts comes from laterite minerals. Geological processes in a desert climate can concentrate minerals into valuable deposits. Leaching by ground water can extract ore minerals and redeposit them, according to the water table, in concentrated form. Similarly, evaporation tends to concentrate minerals in desert lakes, creating dry lake beds or playas rich in minerals. Evaporation can concentrate minerals as a variety of evaporite deposits, including gypsum, sodium nitrate, sodium chloride and borates. Evaporites are found in the USA's Great Basin Desert, historically exploited by the "20-mule teams" pulling carts of borax from Death Valley to the nearest railway. A desert especially rich in mineral salts is the Atacama Desert, Chile, where sodium nitrate has been mined for explosives and fertilizer since around 1850. Other desert minerals are copper from Chile, Peru, and Iran, and iron and uranium in Australia. Many other metals, salts and commercially valuable types of rock such as pumice are extracted from deserts around the world.
Oil and gas form on the bottom of shallow seas when
micro-organisms decompose under anoxic conditions and later become
covered with sediment. Many deserts were at one time the sites of
shallow seas and others have had underlying hydrocarbon deposits
transported to them by the movement of tectonic plates.
Some major oilfields such as Ghawar are found under the sands of Saudi Arabia. Geologists believe that other oil deposits were formed by aeolian processes in ancient deserts as may be the case with some of the major American oil fields.
Traditional desert farming systems have long been established in
North Africa, irrigation being the key to success in an area where water
stress is a limiting factor to growth. Techniques that can be used
include drip irrigation,
the use of organic residues or animal manures as fertilisers and other
traditional agricultural management practices. Once fertility has been
built up, further crop production preserves the soil from destruction by
wind and other forms of erosion.
It has been found that plant growth-promoting bacteria play a role in
increasing the resistance of plants to stress conditions and these rhizobacterial
suspensions could be inoculated into the soil in the vicinity of the
plants. A study of these microbes found that desert farming hampers
desertification by establishing islands of fertility allowing farmers to
achieve increased yields despite the adverse environmental conditions. A field trial in the Sonoran Desert which exposed the roots of different species of tree to rhizobacteria and the nitrogen fixing bacterium Azospirillum brasilense with the aim of restoring degraded lands was only partially successful.
The Judean Desert was farmed in the 7th century BC during the Iron Age to supply food for desert forts.
Native Americans in the south western United States became
agriculturalists around 600 AD when seeds and technologies became
available from Mexico. They used terracing techniques and grew gardens
beside seeps, in moist areas at the foot of dunes, near streams
providing flood irrigation and in areas irrigated by extensive specially
built canals. The Hohokam
tribe constructed over 500 miles (800 km) of large canals and
maintained them for centuries, an impressive feat of engineering. They
grew maize, beans, squash and peppers.
A modern example of desert farming is the Imperial Valley in California, which has high temperatures and average rainfall of just 3 in (76 mm) per year.
The economy is heavily based on agriculture and the land is irrigated
through a network of canals and pipelines sourced entirely from the Colorado River via the All-American Canal.
The soil is deep and fertile, being part of the river's flood plains,
and what would otherwise have been desert has been transformed into one
of the most productive farming regions in California. Other water from
the river is piped to urban communities but all this has been at the
expense of the river, which below the extraction sites no longer has any
above-ground flow during most of the year. Another problem of growing
crops in this way is the build-up of salinity in the soil caused by the
evaporation of river water.
The greening of the desert remains an aspiration and was at one time
viewed as a future means for increasing food production for the world's
growing population. This prospect has proved false as it disregarded the
environmental damage caused elsewhere by the diversion of water for
desert project irrigation.
Solar energy capture
Desertec proposed using the Saharan and Arabian deserts to produce solar energy to power Europe and the Middle East.
The potential for generating solar energy from the Sahara Desert is huge, the highest found on the globe. Professor David Faiman of Ben-Gurion University has stated that the technology now exists to supply all of the world's electricity needs from 10% of the Sahara Desert. Desertec Industrial Initiative
was a consortium seeking $560 billion to invest in North African solar
and wind installations over the next forty years to supply electricity
to Europe via cable lines running under the Mediterranean Sea.
European interest in the Sahara Desert stems from its two aspects: the
almost continual daytime sunshine and plenty of unused land. The Sahara
receives more sunshine per acre than any part of Europe. The Sahara
Desert also has the empty space totalling hundreds of square miles
required to house fields of mirrors for solar plants.
The Negev Desert, Israel, and the surrounding area, including the Arava Valley, receive plenty of sunshine and are generally not arable. This has resulted in the construction of many solar plants. David Faiman has proposed that "giant" solar plants in the Negev could supply all of Israel's needs for electricity.
The Arabs were probably the first organized force to conduct
successful battles in the desert. By knowing back routes and the
locations of oases and by utilizing camels, Muslim Arab forces were able
to successfully overcome both Roman and Persian forces in the period
600 to 700 AD during the expansion of the Islamic caliphate.
Many centuries later, both world wars saw fighting in the desert. In the First World War, the OttomanTurks
were engaged with the British regular army in a campaign that spanned
the Arabian peninsula. The Turks were defeated by the British, who had
the backing of irregular Arab forces that were seeking to revolt against the Turks in the Hejaz, made famous in T.E. Lawrence's book Seven Pillars of Wisdom.
In the Second World War, the Western Desert Campaign began in Italian Libya.
Warfare in the desert offered great scope for tacticians to use the
large open spaces without the distractions of casualties among civilian
populations. Tanks and armoured vehicles were able to travel large distances unimpeded and land mines
were laid in large numbers. However, the size and harshness of the
terrain meant that all supplies needed to be brought in from great
distances. The victors in a battle would advance and their supply chain would necessarily become longer, while the defeated army could retreat, regroup and resupply. For these reasons, the front line moved back and forth through hundreds of kilometers as each side lost and regained momentum. Its most easterly point was at El Alamein in Egypt, where the Allies decisively defeated the Axis forces in 1942.
In culture
Marco Polo arriving in a desert land with camels. 14th-century miniature from Il milione.
The desert is generally thought of as a barren and empty landscape.
It has been portrayed by writers, film-makers, philosophers, artists and
critics as a place of extremes, a metaphor for anything from death, war or religion to the primitive past or the desolate future.
There is an extensive literature on the subject of deserts. An early historical account is that of Marco Polo (c. 1254–1324), who travelled through Central Asia to China, crossing a number of deserts in his twenty four year trek.
Some accounts give vivid descriptions of desert conditions, though
often accounts of journeys across deserts are interwoven with
reflection, as is the case in Charles Montagu Doughty's major work, Travels in Arabia Deserta (1888). Antoine de Saint-Exupéry described both his flying and the desert in Wind, Sand and Stars and Gertrude Bell
travelled extensively in the Arabian desert in the early part of the
20th century, becoming an expert on the subject, writing books and
advising the British government on dealing with the Arabs. Another woman explorer was Freya Stark who travelled alone in the Middle East, visiting Turkey, Arabia, Yemen, Syria, Persia and Afghanistan, writing over twenty books on her experiences. The German naturalist Uwe George spent several years living in deserts, recording his experiences and research in his book, In the Deserts of this Earth.
The American poet Robert Frost expressed his bleak thoughts in his poem, Desert Places,
which ends with the stanza "They cannot scare me with their empty
spaces / Between stars – on stars where no human race is. / I have it in
me so much nearer home / To scare myself with my own desert places."
Deserts on other planets
View of the Martian desert seen by the probe Spirit in 2004.
Mars is the only other planet in the Solar System besides Earth on which deserts have been identified.
Despite its low surface atmospheric pressure (only 1/100 of that of the
Earth), the patterns of atmospheric circulation on Mars have formed a
sea of circumpolar sand more than 5 million km2 (1.9 million
sq mi) in the area, larger than most deserts on Earth. The Martian
deserts principally consist of dunes in the form of half-moons in flat
areas near the permanent polar ice caps in the north of the planet. The
smaller dune fields occupy the bottom of many of the craters situated in
the Martian polar regions. Examination of the surface of rocks by laser beamed from the Mars Exploration Rover have shown a surface film that resembles the desert varnish found on Earth although it might just be surface dust. The surface of Titan, a moon of Saturn, also has a desert-like surface with dune seas.
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