Dimethyl sulfide (DMS) or methylthiomethane is an organosulfur compound with the formula (CH3)2S. Dimethyl sulfide is a flammable liquid
that boils at 37 °C (99 °F) and has a characteristic disagreeable odor.
It is a component of the smell produced from cooking of certain
vegetables, notably maize, cabbage, beetroot, and seafoods. It is also an indication of bacterial contamination in malt production and brewing. It is a breakdown product of dimethylsulfoniopropionate (DMSP), and is also produced by the bacterial metabolism of methanethiol.
Natural occurrence
DMS originates primarily from DMSP, a major secondary metabolite in some marine algae. DMS is the most abundant biological sulfur compound emitted to the atmosphere. Emission occurs over the oceans by phytoplankton. DMS is also produced naturally by bacterial transformation of dimethyl sulfoxide (DMSO) waste that is disposed of into sewers, where it can cause environmental odor problems.
Marine phytoplankton also produce dimethyl sulfide, and DMS is also produced by bacterial cleavage of extracellular DMSP. DMS has been characterized as the "smell of the sea",
though it would be more accurate to say that DMS is a component of the
smell of the sea, others being chemical derivatives of DMS, such as
oxides, and yet others being algal pheromones such as dictyopterenes.
Dimethyl
sulfide is normally present at very low levels in healthy people,
namely <7nM in blood, <3 nM in urine and 0.13 – 0.65 nM on expired
breath.
At pathologically dangerous concentrations, this is known as dimethylsulfidemia. This condition is associated with blood borne halitosis and dimethylsulfiduria.
In people with chronic liver disease (cirrhosis), high levels of
dimethyl sulfide may be present the breath, leading to an unpleasant
smell (fetor hepaticus).
Smell
Dimethyl sulfide has a characteristic smell commonly described as cabbage-like.
It becomes highly disagreeable at even quite low concentrations. Some
reports claim that DMS has a low olfactory threshold that varies from
0.02 to 0.1 ppm between different persons, but it has been suggested
that the odor attributed to dimethyl sulfide may in fact be due to di-
and polysulfides and thiol impurities, since the odor of dimethyl
sulfide is much less disagreeable after it is freshly washed with
saturated aqueous mercuric chloride.
Dimethyl sulfide is also available as a food additive to impart a
savory flavor; in such use, its concentration is low. Beetroot, asparagus, cabbage, corn and seafoods produce dimethyl sulfide when cooked.
Dimethyl sulfide is also produced by marine planktonic micro-organisms such as the coccolithophores and so is one of the main components responsible for the characteristic odor of sea wateraerosols, which make up a part of sea air. In the Victorian era, before DMS was discovered, the origin of sea air's 'bracing' aroma was attributed to ozone.
Dimethyl sulfide has been used in petroleum refining to pre-sulfide hydrodesulfurization catalysts, although other disulfides or polysulfides are preferred and easier to handle. It is used as a presulfiding agent to control the formation of coke and carbon monoxide in ethylene production. DMS is also used in a range of organic syntheses, including as a reducing agent in ozonolysis reactions. It also has a use as a food flavoring component. It can also be oxidized to dimethyl sulfoxide, (DMSO), which is an important industrial solvent.
The largest single commercial producer of DMS in the world is Gaylord Chemical Corporation. The Chevron Phillips Chemical company is also a major manufacturer of DMS. CP Chem produces this material at their facilities in Borger, Texas, USA and Tessenderlo, Belgium.
Other uses
Dimethyl sulfide is a Lewis base, classified as a soft ligand. It forms complexes with many transition metals.
It serves a displaceable ligand in chloro(dimethyl sulfide)gold(I) and other coordination compounds. Dimethyl sulfide is also used in the ozonolysis of alkenes, reducing the intermediate trioxolane and oxidizing to DMSO.
Safety
Dimethyl sulfide is highly flammable and an eye and skin irritant. It is harmful if swallowed. It has an unpleasant odor at even extremely low concentrations. Its ignition temperature is 205 °C.
The primary causes and the wide-ranging effects
of global warming and resulting climate change. Some effects constitute
feedback mechanisms that intensify climate change and move it toward climate tipping points.
Climate change feedback is important in the understanding of global warming because feedback processes may amplify or diminish the effect of each climate forcing, and so play an important part in determining the climate sensitivity and future climate state. Feedback
in general is the process in which changing one quantity changes a
second quantity, and the change in the second quantity in turn changes
the first. Positive (or reinforcing) feedback amplifies the change in the first quantity while negative (or balancing) feedback reduces it.
The term "forcing" means a change which may "push" the climate system in the direction of warming or cooling. An example of a climate forcing is increased atmospheric concentrations of greenhouse gases.
By definition, forcings are external to the climate system while
feedbacks are internal; in essence, feedbacks represent the internal
processes of the system. Some feedbacks may act in relative isolation to
the rest of the climate system; others may be tightly coupled; hence it may be difficult to tell just how much a particular process contributes.
Forcing can also be driven by socioeconomic factors such as "demand for
biofuels or demand for soy bean production." These drivers work as
forcing mechanisms by the direct and indirect effects they cause from an
individual to a global scale.
Forcings and feedbacks together determine how much and how fast the climate changes. The main positive feedback in global warming is the tendency of warming to increase the amount of water vapor in the atmosphere, which in turn leads to further warming. The main negative feedback comes from the Stefan–Boltzmann law,
the amount of heat radiated from the Earth into space changes with the
fourth power of the temperature of Earth's surface and atmosphere.
Observations and modelling studies indicate that there is a net positive
feedback to warming. Large positive feedbacks can lead to effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change.
Positive
Carbon cycle feedbacks
There have been predictions, and some evidence, that global warming
might cause loss of carbon from terrestrial ecosystems, leading to an
increase of atmospheric CO 2
levels. Several climate models indicate that global warming through the
21st century could be accelerated by the response of the terrestrial
carbon cycle to such warming. All 11 models in the C4MIP study found that a larger fraction of anthropogenic CO2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO2
varied between 20 and 200 ppm for the two extreme models, the majority
of the models lying between 50 and 100 ppm. The higher CO2
levels led to an additional climate warming ranging between 0.1° and
1.5 °C. However, there was still a large uncertainty on the magnitude of
these sensitivities. Eight models attributed most of the changes to the
land, while three attributed it to the ocean. The strongest feedbacks in these cases are due to increased respiration of carbon from soils throughout the high latitude boreal forests of the Northern Hemisphere. One model in particular (HadCM3) indicates a secondary carbon cycle feedback due to the loss of much of the Amazon Rainforest in response to significantly reduced precipitation over tropical South America.
While models disagree on the strength of any terrestrial carbon cycle
feedback, they each suggest any such feedback would accelerate global
warming.
Observations show that soils in the U.K have been losing carbon at the rate of four million tonnes a year for the past 25 years
according to a paper in Nature by Bellamy et al. in September 2005, who
note that these results are unlikely to be explained by land use
changes. Results such as this rely on a dense sampling network and thus
are not available on a global scale. Extrapolating to all of the United
Kingdom, they estimate annual losses of 13 million tons per year. This
is as much as the annual reductions in carbon dioxide emissions achieved
by the UK under the Kyoto Treaty (12.7 million tons of carbon per
year).
It has also been suggested (by Chris Freeman) that the release of dissolved organic carbon (DOC) from peatbogs
into water courses (from which it would in turn enter the atmosphere)
constitutes a positive feedback for global warming. The carbon currently
stored in peatlands (390–455 gigatonnes, one-third of the total
land-based carbon store) is over half the amount of carbon already in
the atmosphere.
DOC levels in water courses are observably rising; Freeman's hypothesis
is that, not elevated temperatures, but elevated levels of atmospheric
CO2 are responsible, through stimulation of primary productivity.
Tree deaths are believed to be increasing as a result of climate change, which is a positive feedback effect.
Methane climate feedbacks in natural ecosystems.
Wetlands and freshwater ecosystems are predicted to be the largest potential contributor to a global methane climate feedback.
Long-term warming changes the balance in the methane-related microbial
community within freshwater ecosystems so they produce more methane
while proportionately less is oxidised to carbon dioxide.
Arctic methane release
Photo
shows what appears to be permafrost thaw ponds in Hudson Bay, Canada,
near Greenland. (2008) Global warming will increase permafrost and
peatland thaw, which can result in collapse of plateau surfaces.
Warming is also the triggering variable for the release of carbon (potentially as methane) in the arctic. Methane released from thawing permafrost such as the frozen peatbogs in Siberia, and from methane clathrate on the sea floor, creates a positive feedback. In April 2019, Turetsky et al. reported permafrost was thawing quicker than predicted.
Recently the understanding of the climate feedback from permafrost
improved, but potential emissions from the subsea permafrost remain
unknown and are - like many other soil carbon feedbacks - still absent from most climate models.
Thawing permafrost peat bogs
Western Siberia is the world's largest peat bog, a one million square kilometer region of permafrost peat bog that was formed 11,000 years ago at the end of the last ice age. The melting of its permafrost is likely to lead to the release, over decades, of large quantities of methane. As much as 70,000 million tonnes
of methane, an extremely effective greenhouse gas, might be released
over the next few decades, creating an additional source of greenhouse
gas emissions. Similar melting has been observed in eastern Siberia.
Lawrence et al. (2008) suggest that a rapid melting of Arctic sea ice
may start a feedback loop that rapidly melts Arctic permafrost,
triggering further warming.
May 31, 2010. NASA published that globally "Greenhouse gases are
escaping the permafrost and entering the atmosphere at an increasing
rate - up to 50 billion tons each year of methane, for example - due to a
global thawing trend. This is particularly troublesome because methane
heats the atmosphere with 25 times the efficiency of carbon dioxide"
(the equivalent of 1250 billion tons of CO2 per year).
In 2019, a report called " Arctic report card " estimated the
current greenhouse gas emissions from Arctic permafrost as almost equal
to the emissions of Russia or Japan or less than 10% of the global
emissions from fossil fuels.
Hydrates
Methane clathrate, also called methane hydrate, is a form of waterice that contains a large amount of methane within its crystal
structure. Extremely large deposits of methane clathrate have been
found under sediments on the sea and ocean floors of Earth. The sudden
release of large amounts of natural gas from methane clathrate deposits,
in a runaway global warming
event, has been hypothesized as a cause of past and possibly future
climate changes. The release of this trapped methane is a potential
major outcome of a rise in temperature; it is thought that this might
increase the global temperature by an additional 5° in itself, as
methane is much more powerful as a greenhouse gas than carbon dioxide.
The theory also predicts this will greatly affect available oxygen
content of the atmosphere. This theory has been proposed to explain the
most severe mass extinction event on earth known as the Permian–Triassic extinction event, and also the Paleocene-Eocene Thermal Maximum climate change event. In 2008, a research expedition for the American Geophysical Union
detected levels of methane up to 100 times above normal in the Siberian
Arctic, likely being released by methane clathrates being released by
holes in a frozen 'lid' of seabed permafrost, around the outfall of the Lena River and the area between the Laptev Sea and East Siberian Sea.
In 2020, the first leak of methane from the sea floor in
Antarctica was discovered. The scientists are not sure what caused it.
The area where it was found had not warmed yet significantly. It is on
the side of a volcano, but it seems that it is not from there. The
methane - eating microbes, eat the methane much fewer that was supposed,
and the researchers think this should be included in climate models.
They also claim that there is much more to discover about the issue in
Antarctica Quart of the marine methane is found in the region of Antarctica
Abrupt increases in atmospheric methane
Literature assessments by the Intergovernmental Panel on Climate Change (IPCC) and the US Climate Change Science Program (CCSP) have considered the possibility of future projected climate change leading to a rapid increase in atmospheric methane. The IPCC Third Assessment Report,
published in 2001, looked at possible rapid increases in methane due
either to reductions in the atmospheric chemical sink or from the
release of buried methane reservoirs. In both cases, it was judged that such a release would be "exceptionally unlikely"
(less than a 1% chance, based on expert judgement).
The CCSP assessment, published in 2008, concluded that an abrupt release of methane into the atmosphere appeared "very unlikely"
(less than 10% probability, based on expert judgement).
The CCSP assessment, however, noted that climate change would "very
likely" (greater than 90% probability, based on expert judgement)
accelerate the pace of persistent emissions from both hydrate sources
and wetlands.
On 10 June 2019 Louise M. Farquharson and her team reported that
their 12-year study into Canadian permafrost had "Observed maximum thaw
depths at our sites are already exceeding those projected to occur by
2090. Between 1990 and 2016, an increase of up to 4 °C has been observed
in terrestrial permafrost and this trend is expected to continue as
Arctic mean annual air temperatures increase at a rate twice that of
lower latitudes."
Determining the extent of new thermokarst development is difficult, but
there is little doubt the problem is widespread. Farquharson and her
team guess that about 231,000 square miles (600,000 square kilometers)
of permafrost, or about 5.5% of the zone that is permafrost year-round,
is vulnerable to rapid surface thawing.
Decomposition
Organic matter stored in permafrost generates heat as it decomposes in response to the permafrost melting.
As the tropics get wetter, as many climate models predict, soils are
likely to experience greater rates of respiration and decomposition,
limiting the carbon storage abilities of tropical soils.
Peat decomposition
Peat, occurring naturally in peat bogs, is a store of carbon significant on a global scale. When peat dries it decomposes, and may additionally burn. Water table adjustment due to global warming may cause significant excursions of carbon from peat bogs. This may be released as methane, which can exacerbate the feedback effect, due to its high global warming potential.
Rainforest drying
Rainforests, most notably tropical rainforests,
are particularly vulnerable to global warming. There are a number of
effects which may occur, but two are particularly concerning. Firstly,
the drier vegetation may cause total collapse of the rainforest ecosystem. For example, the Amazon rainforest would tend to be replaced by caatinga
ecosystems. Further, even tropical rainforests ecosystems which do not
collapse entirely may lose significant proportions of their stored
carbon as a result of drying, due to changes in vegetation.
Forest fires
The IPCC Fourth Assessment Report predicts that many mid-latitude
regions, such as Mediterranean Europe, will experience decreased
rainfall and an increased risk of drought, which in turn would allow
forest fires to occur on larger scale, and more regularly. This releases
more stored carbon into the atmosphere than the carbon cycle can
naturally re-absorb, as well as reducing the overall forest area on the
planet, creating a positive feedback loop. Part of that feedback loop is
more rapid growth of replacement forests and a northward migration of
forests as northern latitudes become more suitable climates for
sustaining forests. There is a question of whether the burning of
renewable fuels such as forests should be counted as contributing to
global warming. Cook & Vizy also found that forest fires were likely in the Amazon Rainforest, eventually resulting in a transition to Caatinga vegetation in the Eastern Amazon region.
Desertification
Desertification is a consequence of global warming in some environments. Desert soils contain little humus,
and support little vegetation. As a result, transition to desert
ecosystems is typically associated with excursions of carbon.
Modelling results
The global warming projections contained in the IPCC's Fourth Assessment Report (AR4) include carbon cycle feedbacks. Authors of AR4, however, noted that scientific understanding of carbon cycle feedbacks was poor.
Projections in AR4 were based on a range of greenhouse gas emissions
scenarios, and suggested warming between the late 20th and late 21st
century of 1.1 to 6.4 °C.
This is the "likely" range (greater than 66% probability), based on the
expert judgement of the IPCC's authors. Authors noted that the lower
end of the "likely" range appeared to be better constrained than the
upper end of the "likely" range, in part due to carbon cycle feedbacks. The American Meteorological Society has commented that more research is needed to model the effects of carbon cycle feedbacks in climate change projections.
Isaken et al. (2010) considered how future methane release from the Arctic might contribute to global warming. Their study suggested that if global methane emissions were to increase by a factor of 2.5 to 5.2 above (then) current emissions, the indirect contribution to radiative forcing
would be about 250% and 400% respectively, of the forcing that can be
directly attributed to methane. This amplification of methane warming is
due to projected changes in atmospheric chemistry.
Schaefer et al. (2011)
considered how carbon released from permafrost might contribute to
global warming. Their study projected changes in permafrost based on a
medium greenhouse gas emissions scenario (SRES
A1B). According to the study, by 2200, the permafrost feedback might
contribute 190 (+/- 64) gigatons of carbon cumulatively to the
atmosphere. Schaefer et al. (2011) commented that this estimate may be low.
Implications for climate policy
Uncertainty
over climate change feedbacks has implications for climate policy. For
instance, uncertainty over carbon cycle feedbacks may affect targets for
reducing greenhouse gas emissions.
Emissions targets are often based on a target stabilization level of
atmospheric greenhouse gas concentrations, or on a target for limiting
global warming to a particular magnitude. Both of these targets
(concentrations or temperatures) require an understanding of future
changes in the carbon cycle. If models incorrectly project future
changes in the carbon cycle, then concentration or temperature targets
could be missed. For example, if models underestimate the amount of
carbon released into the atmosphere due to positive feedbacks (e.g., due
to melting permafrost), then they may also underestimate the extent of
emissions reductions necessary to meet a concentration or temperature
target.
Cloud feedback
Warming is expected to change the distribution and type of clouds.
Seen from below, clouds emit infrared radiation back to the surface, and
so exert a warming effect; seen from above, clouds reflect sunlight and
emit infrared radiation to space, and so exert a cooling effect.
Whether the net effect is warming or cooling depends on details such as
the type
and altitude of the cloud. Low clouds tend to trap more heat at the
surface and therefore have a positive feedback, while high clouds
normally reflect more sunlight from the top so they have a negative feedback. These details were poorly observed before the advent of satellite data and are difficult to represent in climate models.
Global climate models were showing a near-zero to moderately strong
positive net cloud feedback, but the effective climate sensitivity has
increased substantially in the latest generation of global climate
models. Differences in the physical representation of clouds in models
drive this enhanced climate sensitivity relative to the previous
generation of models.
A 2019 simulation predicts that if greenhouse gases reach three
times the current level of atmospheric carbon dioxide that stratocumulus
clouds could abruptly disperse, contributing to additional global
warming.
Gas release
Release of gases of biological origin may be affected by global
warming, but research into such effects is at an early stage. Some of
these gases, such as nitrous oxide released from peat or thawing permafrost, directly affect climate.
Others, such as dimethyl sulfide released from oceans, have indirect effects.
Ice-albedo feedback
Aerial photograph showing a section of sea ice. The lighter blue areas are melt ponds and the darkest areas are open water; both have a lower albedo than the white sea ice. The melting ice contributes to ice-albedo feedback.
When ice melts, land or open water takes its place. Both land and open
water are on average less reflective than ice and thus absorb more solar
radiation. This causes more warming, which in turn causes more melting,
and this cycle continues. During times of global cooling,
additional ice increases the reflectivity which reduces the absorption
of solar radiation which results in more cooling in a continuing cycle. Considered a faster feedback mechanism.
1870–2009 Northern hemisphere sea ice extent
in million square kilometers. Blue shading indicates the pre-satellite
era; data then is less reliable. In particular, the near-constant level
extent in Autumn up to 1940 reflects lack of data rather than a real
lack of variation.
Albedo change is also the main reason why IPCC
predict polar temperatures in the northern hemisphere to rise up to
twice as much as those of the rest of the world, in a process known as polar amplification. In September 2007, the Arctic sea ice area reached about half the size of the average summer minimum area between 1979 and 2000. Also in September 2007, Arctic sea ice retreated far enough for the Northwest Passage to become navigable to shipping for the first time in recorded history. The record losses of 2007 and 2008 may, however, be temporary.
Mark Serreze of the US National Snow and Ice Data Center views 2030 as a "reasonable estimate" for when the summertime Arctic ice cap might be ice-free. The polar amplification of global warming is not predicted to occur in the southern hemisphere. The Antarctic sea ice reached its greatest extent on record since the beginning of observation in 1979,
but the gain in ice in the south is exceeded by the loss in the north.
The trend for global sea ice, northern hemisphere and southern
hemisphere combined is clearly a decline.
Ice loss may have internal feedback processes, as melting of ice over land can cause eustatic sea level rise, potentially causing instability of ice shelves
and inundating coastal ice masses, such as glacier tongues. Further, a
potential feedback cycle exists due to earthquakes caused by isostatic rebound further destabilising ice shelves, glaciers and ice caps.
The ice-albedo in some sub-arctic forests is also changing, as stands of larch (which shed their needles in winter, allowing sunlight to reflect off the snow in spring and fall) are being replaced by spruce trees (which retain their dark needles all year).
Water vapor feedback
If the atmospheres are warmed, the saturation vapor pressure
increases, and the amount of water vapor in the atmosphere will tend to
increase. Since water vapor is a greenhouse gas, the increase in water
vapor content makes the atmosphere warm further; this warming causes the
atmosphere to hold still more water vapor (a positive feedback), and so on until other processes stop the feedback loop. The result is a much larger greenhouse effect than that due to CO2 alone. Although this feedback process causes an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.
Climate models incorporate this feedback. Water vapor feedback is
strongly positive, with most evidence supporting a magnitude of 1.5 to
2.0 W/m2/K, sufficient to roughly double the warming that would otherwise occur. Water vapor feedback is considered a faster feedback mechanism.
Negative
Blackbody radiation
As the temperature of a black body increases, the emission of infrared radiation back into space increases with the fourth power of its absolute temperature according to Stefan–Boltzmann law. This increases the amount of outgoing radiation as the Earth warms. The impact of this negative feedback effect is included in global climate models summarized by the IPCC. This is also called the Planck feedback.
Carbon cycle
Le Chatelier's principle
Following Le Chatelier's principle, the chemical equilibrium of the Earth's carbon cycle will shift in response to anthropogenic CO2 emissions. The primary driver of this is the ocean, which absorbs anthropogenic CO2 via the so-called solubility pump. At present this accounts for only about one third of the current emissions, but ultimately most (~75%) of the CO2
emitted by human activities will dissolve in the ocean over a period of
centuries: "A better approximation of the lifetime of fossil fuel CO2 for public discussion might be 300 years, plus 25% that lasts forever". However, the rate at which the ocean will take it up in the future is less certain, and will be affected by stratification induced by warming and, potentially, changes in the ocean's thermohaline circulation.
Chemical weathering
Chemical weathering over the geological long term acts to remove CO2 from the atmosphere. With current global warming, weathering is increasing, demonstrating significant feedbacks between climate and Earth surface. Biosequestration also captures and stores CO2 by biological processes. The formation of shells by organisms in the ocean, over a very long time, removes CO2 from the oceans. The complete conversion of CO2 to limestone takes thousands to hundreds of thousands of years.
Net primary productivity
Net primary productivity changes in response to increased CO2,
as plants photosynthesis increased in response to increasing
concentrations. However, this effect is swamped by other changes in the
biosphere due to global warming.
Lapse rate
The atmosphere's temperature decreases with height in the troposphere. Since emission of infrared radiation varies with temperature, longwave radiation
escaping to space from the relatively cold upper atmosphere is less
than that emitted toward the ground from the lower atmosphere. Thus, the
strength of the greenhouse effect depends on the atmosphere's rate of
temperature decrease with height. Both theory and climate models
indicate that global warming will reduce the rate of temperature
decrease with height, producing a negative lapse rate feedback
that weakens the greenhouse effect. Measurements of the rate of
temperature change with height are very sensitive to small errors in
observations, making it difficult to establish whether the models agree
with observations.
Feedback loops from the book Al Gore (2006). An inconvenient truth.
Impacts on humans
The graphic at right suggests that the overall effect of climate change upon human numbers and development will be negative.
If this is so, then the century-scale prospects for climate change is
that Earth's biosphere may adjust to a new, but radically different,
equilibrium if large numbers of humans cannot survive future conditions.
Timeline of the five known great glaciations, shown in blue. The periods in between depict greenhouse conditions.
Throughout the history of the Earth, the planet's climate has been fluctuating between two dominant climate states: the greenhouse Earth and the icehouse Earth. These two climate states last for millions of years and should not be confused with glacial and interglacial periods, which occur only during an icehouse period and tend to last less than 1 million years. There are five known great glaciations in Earth's climate history; the main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric carbon dioxide, changes in the Earth's orbit, long-term changes in the solar constant, and oceanic and orogenic changes due to tectonic plate dynamics. Greenhouse and icehouse periods have profoundly shaped the evolution of life on Earth.
Greenhouse Earth
Overview of greenhouse Earth
A "greenhouse Earth" is a period in which there are no continental glaciers whatsoever on the planet, the levels of carbon dioxide and other greenhouse gases (such as water vapor and methane) are high, and sea surface temperatures (SSTs) range from 28 °C (82.4 °F) in the tropics to 0 °C (32 °F) in the polar regions. The Earth has been in a greenhouse state for about 85% of its history.
This state should not be confused with a hypothetical hothouse earth, which is an irreversible tipping point corresponding to the ongoing runaway greenhouse effect on Venus. The IPCC
states that "a 'runaway greenhouse effect'—analogous to [that of]
Venus—appears to have virtually no chance of being induced by anthropogenic activities."
Causes of greenhouse Earth
There are several theories as to how a greenhouse Earth can come about. The geological record shows CO2 and other greenhouse gases are abundant during this time. Tectonic movements were extremely active during the more well-known greenhouse ages (such as 368 million years ago in the Paleozoic Era). Because of continental rifting (continental plates moving away from each other) volcanic activity became more prominent, producing more CO2 and heating up the Earth's atmosphere.
Earth is more commonly placed in a greenhouse state throughout the
epochs, and the Earth has been in this state for approximately 80% of
the past 500 million years, which makes understanding the direct causes
somewhat difficult.
Icehouse Earth
Overview of icehouse Earth
An "icehouse Earth" is a period in which the Earth has at least two ice sheets, Arctic and Antarctic (on both poles); these sheets wax and wane throughout shorter times known as glacial periods (with other ice sheets in addition to the 2 polar ones) and interglacial periods
(without). During an icehouse Earth, greenhouse gases tend to be less
abundant, and temperatures tend to be cooler globally. The Earth is
currently in an icehouse stage, that started 34 Ma with the ongoing Late Cenozoic Ice Age. Inside it, the last glacial, Würm,
recently ended (110 to 12 ka), still has remnants of non-polar ice
sheets (Alps, Himalaya, Patagonia). It will likely be soon followed by
another interglacial, similar to the last one, Eemian
(130 to 115 ka), when there were forests in North Cape and hippopotamus
in the rivers Rhine and Thames.
Then glacials and interglacials, of similar lengths as the recent ones,
will continue to alternate until the end of the 2 pole ice sheets,
meaning the end of the current Icehouse and the start of the next
Greenhouse.
Causes of icehouse Earth
The
causes of an icehouse state are much debated, because not much is
really known about the transitions between greenhouse and icehouse
climates and what could make the climate change. One important aspect is
clearly the decline of CO2 in the atmosphere, possibly due to low volcanic activity.
Other important issues are the movement of the tectonic plates and the opening and closing of oceanic gateways.
These seem to play a crucial part in icehouse Earths because they can
bring cool waters from very deep water circulations that could assist in
creating ice sheets or thermal isolation of areas. Examples of this
occurring are the opening of the Tasmanian gateway 36.5 million years ago that separated Australia and Antarctica and which is believed to have set off the Cenozoic icehouse, and the creation of the Drake Passage 32.8 million years ago by the separation of South America and Antarctica, though it was believed by other scientists that this did not come into effect until around 23 million years ago. The closing of the Isthmus of Panama and the Indonesian seaway approximately 3 or 4 million years ago may have been a major cause for our current icehouse state.
For the icehouse climate, tectonic activity also creates mountains,
which are produced by one continental plate colliding with another one
and continuing forward. The revealed fresh soils act as scrubbers
of carbon dioxide, which can significantly affect the amount of this
greenhouse gas in the atmosphere. An example of this is the collision
between the Indian subcontinent and the Asian continent, which created the Himalayan Mountains about 50 million years ago.
Glacials and interglacials
Within icehouse states, there are "glacial" and "interglacial"
periods that cause ice sheets to build up or retreat. The causes for
these glacial and interglacial periods are mainly variations in the
movement of the earth around the Sun. The astronomical components, discovered by the Serbian geophysicist Milutin Milanković and now known as Milankovitch cycles, include the axial tilt of the Earth, the orbital eccentricity (or shape of the orbit) and the precession (or wobble) of the Earth's rotation.
The tilt of the axis tends to fluctuate between 21.5° to 24.5° and back
every 41,000 years on the vertical axis. This change actually affects
the seasonality upon the earth, since more or less solar radiation
hits certain areas of the planet more often on a higher tilt, while
less of a tilt would create a more even set of seasons worldwide. These
changes can be seen in ice cores, which also contain information showing
that during glacial times (at the maximum extension of the ice sheets),
the atmosphere had lower levels of carbon dioxide. This may be caused
by the increase or redistribution of the acid/base balance with bicarbonate and carbonateions that deals with alkalinity. During an icehouse, only 20% of the time is spent in interglacial, or warmer times. Model simulations suggest that the current interglacial climate state will continue for at least another 100,000 years, due to CO 2 emissions - including complete deglaciation of the Northern Hemisphere.
Snowball earth
A "snowball earth"
is the complete opposite of greenhouse Earth, in which the earth's
surface is completely frozen over; however, a snowball earth technically
does not have continental ice sheets like during the icehouse state.
"The Great Infra-Cambrian Ice Age" has been claimed to be the host of such a world, and in 1964, the scientist W. Brian Harland brought forth his discovery of indications of glaciers in low latitudes (Harland and Rudwick). This became a problem for Harland because of the thought of the "Runaway Snowball Paradox" (a kind of Snowball effect)
that, once the earth enters the route of becoming a snowball earth, it
would never be able to leave that state. However, in 1992 Joseph Kirschvink [de]
brought up a solution to the paradox. Since the continents at this time
were huddled at the low and mid-latitudes, there was less ocean water
available to absorb the higher amount solar energy hitting the tropics,
and at the same time, increased rainfall due to more land mass exposed
to higher solar energy might have caused chemical weathering (removing
CO2 from atmosphere). Both these conditions might have caused a substantial drop in CO2
atmospheric levels resulting in cooling temperatures, increasing ice
albedo (ice reflectivity of incoming solar radiation), further
increasing global cooling (a positive feedback). This might have been
the mechanism of entering Snowball Earth state. Kirschvink explained
that the way to get out of Snowball Earth state could be connected again
to carbon dioxide. A possible explanation is that during Snowball
Earth, volcanic activity would not halt, accumulating atmospheric CO2. At the same time, global ice cover would prevent chemical weathering (in particular hydrolysis), responsible for removal of CO2 from the atmosphere. CO2 was therefore accumulating in the atmosphere. Once the atmosphere accumulation of CO2
would reach a threshold, temperature would rise enough for ice sheets
to start melting. This would in turn reduce ice albedo effect which
would in turn further reduce ice cover, exiting Snowball Earth state. At
the end of Snowball Earth, before reinstating the equilibrium
"thermostat" between volcanic activity and the by then slowly resuming
chemical weathering, CO2 in the atmosphere had accumulated
enough to cause temperatures to peak to as much as 60° Celsius, before
eventually settling down. Around the same geologic period of Snowball
Earth (debated if caused by Snowball Earth or being the cause of
Snowball Earth) the Great Oxygenation Event (GOE) was occurring. The
event known as the Cambrian Explosion followed, which produced the beginnings of multi-cellular life. However some biologists claim that a complete snowball Earth could not have happened since photosynthetic life would not have survived underneath many meters of ice without sunlight. However, sunlight has been observed to penetrate meters of ice in Antarctica. Most scientists
today believe that a "hard" Snowball Earth, one completely covered by
ice, is probably impossible. However, a "slushball earth", with points
of opening near the equator, is possible.
Recent studies may have again complicated the idea of a snowball
earth. In October 2011, a team of French researchers announced that the
carbon dioxide during the last speculated "snowball earth" may have been
lower than originally stated, which provides a challenge in finding out
how Earth was able to get out of its state and if it were a snowball or
slushball.
Transitions
Causes
The Eocene, which occurred between 53 and 49 million years ago, was the Earth's warmest temperature period for 100 million years. However, this "super-greenhouse" eventually became an icehouse by the late Eocene. It is believed that the decline of CO2 caused this change, though it is possible that positive feedbacks contributed to the cooling.
The best record we have for a transition from an icehouse to greenhouse period where that plant life existed during the Permianperiod
that occurred around 300 million years ago. 40 million years ago, a
major transition took place, causing the Earth to change from a moist,
icy planet where rainforests covered the tropics, into a hot, dry and windy location where little could survive. Professor Isabel P. Montañez of University of California, Davis,
who has researched this time period, found the climate to be "highly
unstable" and "marked by dips and rises in carbon dioxide".
Impacts
The
Eocene-Oligocene transition, the latest transition, occurred
approximately 34 million years ago, resulting in a rapid global
temperature decrease, the glaciation of Antarctica and a series of
biotic extinction events. The most dramatic species turnover event
associated with this time period is the Grande Coupure, a period which saw the replacement of European tree-dwelling and leaf-eating mammal species by migratory species from Asia.
Research
The science of paleoclimatology attempts to understand the history of greenhouse and icehouse conditions over geological time. Through the study of ice cores, dendrochronology, ocean and lakesediments (varve), palynology, (paleobotany), isotope analysis (such as Radiometric dating and stable isotope analysis), and other climate proxies, scientists can create models of Earth's past energy budgets and resulting climate. One study has shown that atmospheric carbon dioxide levels during the Permian age rocked back and forth between 250 parts per million (which is close to present-day levels) up to 2,000 parts per million. Studies on lake sediments suggest that the "Hothouse" or "super-Greenhouse" Eocene was in a "permanent El Nino state" after the 10 °C warming of the deep ocean and high latitude surface temperatures shut down the Pacific Ocean's El Nino-Southern Oscillation. A theory was suggested for the Paleocene–Eocene Thermal Maximum on the sudden decrease of the carbon isotopic composition of the global inorganic carbon pool by 2.5 parts per million. A hypothesis posed for this drop of isotopes was the increase of methane hydrates, the trigger for which remains a mystery. This increase of methane in the atmosphere,
which happens to be a potent, but short-lived, greenhouse gas,
increased the global temperatures by 6 °C with the assistance of the
less potent carbon dioxide.
List of Icehouse and Greenhouse Periods
A greenhouse period ran from 4.6 to 2.4 billion years ago.
Huronian Glaciation – an icehouse period that ran from 2.4 billion years ago to 2.1 billion years ago
A greenhouse period ran from 2.1 billion to 720 million years ago.
Cryogenian – an icehouse period that ran from 720 to 635 million years ago, at times the entire Earth was frozen over
A greenhouse period ran from 635 million years ago to 450 million years ago.
A greenhouse period ran from 260 million years ago to 33.9 million years ago
Late Cenozoic Ice Age – the current icehouse period which began 33.9 million years ago
Modern conditions
Currently, the Earth is in an icehouse climate state. About 34 million years ago, ice sheets began to form in Antarctica; the ice sheets in the Arctic did not start forming until 2 million years ago.[8]
Some processes that may have led to our current icehouse may be
connected to the development of the Himalayan Mountains and the opening
of the Drake Passage between South America
and Antarctica but climate model simulations suggest that the early
opening of the Drake Passage played only a limited role, while the later
constriction of the Tethys and Central American Seaways is more
important in explaining the observed Cenozoic cooling.
Scientists have been attempting to compare the past transitions between
icehouse and greenhouse, and vice versa, to understand where our planet
is now heading.
Without the human influence on the greenhouse gas concentration, the Earth would be heading toward a glacial period. Predicted changes in orbital forcing suggest that in absence of human-made global warming, the next glacial period would begin at least 50,000 years from now, but due to the ongoing anthropogenic greenhouse gas emissions, the Earth is heading towards a greenhouse Earth period.
Permanent ice is actually a rare phenomenon in the history of the
Earth, occurring only in coincidence with the icehouse effect, which has
affected about 20% of Earth's history.
