The Trillion Tree Campaign is a project which aims to plant one trillion trees worldwide. It seeks to repopulate the world's trees and combat climate change as a nature-based solution. The project was launched at PlantAhead 2018 in Monaco by Plant-for-the-Planet. In the fall of 2018, the project's official website was published in
order to register, monitor, and donate trees to reforestation projects
around the world.
The campaign is a continuation of the activities of the earlier Billion Tree Campaign, instigated by Wangari Maathai, who founded the Green Belt Movement in Africa in 1977.
As of 30 May 2021, 164 restoration projects participate in the campaign and 13.96 billion (1.396% of the goal) trees have been planted worldwide.
History
Billion Tree Campaign
The Green Belt Movement began its activity in Africa in 1977, eventually planting more than 30 million trees.
The Billion Tree Campaign was inspired by Nobel Peace Prize laureate Wangari Maathai,
founder of the Green Belt Movement. When an executive in the United
States told Maathai their corporation was planning to plant a million
trees, her response was: "That's great, but what we really need is to
plant a billion trees."
The project was launched in 2006 by the United Nations Environment Programme (UNEP) under the patronage of Prince Albert II of Monaco and the World Agroforestry Centre-ICRAF as a response to the challenges of climate change, as well as to a wider array of sustainability challenges from water supply to biodiversity loss,and achieved the initial target of planting a billion trees in 2007. The billionth tree, commonly known as an African olive, was planted in Ethiopia in November 2007.
In 2008, the campaign's objective was raised to 7 billion trees, a goal
which was surpassed three months before its target of the climate change
conference that was held in Copenhagen, Denmark, in December 2009.
The 2-billionth tree took root as part of the United Nations World Food Programme agroforestry initiative. The campaign's target was then raised to seven billion trees.
In 2009, UNEP mobilized action across the globe through the "Twitter
for Trees" campaign. UNEP pledged to plant one tree to feed into the
Billion Tree Campaign for every follower who joined from 5 May 2009 to
World Environment Day on 5 June 2009. The campaign was a success, with
10,300 people following the page by World Environment Day.
The World Organization of the Scout Movement also planted trees under the campaign, in line with its mandate to study and protect nature across several countries.
United Nations Peacekeeping missions
also joined the campaign and planted trees within their field missions
in East Timor, Ivory Coast, Darfur, Lebanon, Haiti, Congo, and Liberia,
among others.
After the campaign
Felix Finkbeiner
addressed the United Nations in a speech to open the International Year
of Forests 2011, saying: "It is now time that we work together. We
combine our forces, old and young, rich and poor; and together, we can
plant a trillion trees. We can start the Trillion Tree Campaign."
In December 2011, after more than 12 billion trees had been planted,
UNEP formally handed management of the program to the youth-led
not-for-profit Plant-for-the-Planet Foundation (an organisation that had been participating in the Billion Tree Campaign since 2007), based in Tutzing, Germany. Momentum has since continued, with 40,000 young ambassadors spreading the message in over 100 countries.
In 2015, researcher Tom Crowther found that about 3 trillion trees exist in the world and later it was also estimated that planting 1.2 trillion more trees would counteract 10 years of anthropogenic CO2 emissions.
In September 2019, the Plant-for-the-Planet app was released
under an open-source license. It allowed users to register planted trees
or to plant trees by donating to different tree-planting organizations
around the world. The foundation does not take any commissions for donations made through the campaign.
One Trillion Tree initiative
The 2020 World Economic Forum,
held in Davos, announced the creation of the One Trillion Tree
initiative platform for governments, businesses, and civil society to
provide support to the UN Decade on Ecosystem Restoration (2020–2030), led by UNEP and FAO. Forum participant Donald Trump, then-president of the United States, announced that the government of the U.S. would commit to the initiative.
In the years 2011—2022, China restored more than 70 million hectares (700,000 km2)
of forests. The nation committed to plant and conserve 70 billion trees
by the year 2030 as part of the Trillion Tree Campaign.
The United States has pledged to plant, grow, and restore around 51
billion trees by the year 2030. Currently, forests in the country absorb
15% of its carbon emissions. This can rise to 27% with responsible
reforestation.
The term above sea level generally refers to the height above mean sea level (AMSL). The term APSL means above present sea level, comparing sea levels in the past with the level today.
Earth's radius at sea level is 6,378.137 km (3,963.191 mi) at the
equator. It is 6,356.752 km (3,949.903 mi) at the poles and
6,371.001 km (3,958.756 mi) on average. This flattened spheroid, combined with local gravity anomalies, defines the geoid of the Earth, which approximates the local mean sea level for locations in the open ocean. The geoid includes a significant depression in the Indian Ocean,
whose surface dips as much as 106 m (348 ft) below the global mean sea
level (excluding minor effects such as tides and currents).
Measurement
Sea level measurements from 23 long tide gauge records in geologically stable environments show a rise of around 200 millimetres (7.9 in) during the 20th century (2 mm/year).
Precise determination of a "mean sea level" is difficult because of the many factors that affect sea level.
Instantaneous sea level varies substantially on several scales of time
and space. This is because the sea is in constant motion, affected by
the tides, wind, atmospheric pressure, local gravitational differences, temperature, salinity, and so forth. The mean sea level at a particular location may be calculated over an extended time period and used as a datum. For example, hourly measurements may be averaged over a full Metonic 19-year lunar cycle to determine the mean sea level at an official tide gauge.
Still-water level or still-water sea level (SWL) is the level of the sea with motions such as wind waves averaged out.
Then MSL implies the SWL further averaged over a period of time such that changes due to, e.g., the tides, also have zero mean.
Global MSL refers to a spatial average over the entire ocean
area, typically using large sets of tide gauges and/or satellite
measurements.
One often measures the values of MSL with respect to the land; hence a change in relative MSL or (relative sea level)
can result from a real change in sea level, or from a change in the
height of the land on which the tide gauge operates, or both.
In the UK, the ordnance datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Before 1921, the vertical datum was MSL at the Victoria Dock, Liverpool.
Since the times of the Russian Empire, in Russia and its other former parts, now independent states, the sea level is measured from the zero level of Kronstadt Sea-Gauge.
In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 0.146 m (5.7 in) above chart datum
and 1.304 m (4 ft 3.3 in) below the average sea level.
In France, the Marégraphe in Marseilles measures continuously the sea
level since 1883 and offers the longest collated data about the sea
level. It is used for a part of continental Europe and the main part of
Africa as the official sea level. Spain uses the reference to measure heights below or above sea level at Alicante, while the European Vertical Reference System is calibrated to the Amsterdam Peil elevation, which dates back to the 1690s.
Satellite altimeters have been making precise measurements of sea level since the launch of TOPEX/Poseidon in 1992. A joint mission of NASA and CNES, TOPEX/Poseidon was followed by Jason-1 in 2001 and the Ocean Surface Topography Mission on the Jason-2 satellite in 2008.
Height above mean sea level (AMSL) is the elevation (on
the ground) or altitude (in the air) of an object, relative to a
reference datum for mean sea level (MSL). It is also used in aviation,
where some heights are recorded and reported with respect to mean sea
level (contrast with flight level), and in the atmospheric sciences, and in land surveying. An alternative is to base height measurements on a reference ellipsoid approximating the entire Earth, which is what systems such as GPS do. In aviation, the reference ellipsoid known as WGS84
is increasingly used to define heights; however, differences up to 100
metres (328 feet) exist between this ellipsoid height and local mean sea
level. Another alternative is to use a geoid-based vertical datum such as NAVD88 and the global EGM96 (part of WGS84). Details vary in different countries.
When referring to geographic features such as mountains, on a topographic map variations in elevation are shown by contour lines.
A mountain's highest point or summit is typically illustrated with the
AMSL height in metres, feet or both. In unusual cases where a land
location is below sea level, such as Death Valley, California, the elevation AMSL is negative.
It is often necessary to compare the local height of the mean sea
surface with a "level" reference surface, or geodetic datum, called the geoid.
In the absence of external forces, the local mean sea level would
coincide with this geoid surface, being an equipotential surface of the
Earth's gravitational field which, in itself, does not conform to a simple sphere or ellipsoid and exhibits gravity anomalies such as those measured by NASA's GRACE satellites.
In reality, the geoid surface is not directly observed, even as a
long-term average, due to ocean currents, air pressure variations,
temperature and salinity variations, etc. The location-dependent but
time-persistent separation between local mean sea level and the geoid is
referred to as (mean) ocean surface topography. It varies globally in a typical range of ±1 m (3 ft).
Several terms are used to describe the changing relationships between sea level and dry land.
"relative" means change relative to a fixed point in the sediment pile.
"eustatic" refers to global changes in sea level relative to a fixed
point, such as the centre of the earth, for example as a result of
melting ice-caps.
"isostatic" refers to changes in the level of the land relative to a
fixed point in the earth, possibly due to thermal buoyancy or tectonic effects, disregarding changes in the volume of water in the oceans.
The melting of glaciers at the end of ice ages results in isostatic post-glacial rebound, when land rises after the weight of ice is removed. Conversely, older volcanic islands experience relative sea level rise, due to isostatic subsidence from the weight of cooling volcanos. The subsidence of land due to the withdrawal of groundwater is another isostatic cause of relative sea level rise.
On planets that lack a liquid ocean, planetologists
can calculate a "mean altitude" by averaging the heights of all points
on the surface. This altitude, sometimes referred to as a "sea level" or
zero-level elevation, serves equivalently as a reference for the height of planetary features.
Local mean sea level (LMSL) is defined as the height of the
sea with respect to a land benchmark, averaged over a period of time
long enough that fluctuations caused by waves and tides
are smoothed out, typically a year or more. One must adjust perceived
changes in LMSL to account for vertical movements of the land, which can
occur at rates similar to sea level changes (millimetres per year).
Some land movements occur because of isostatic adjustment to the melting of ice sheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds. Changes in ground-based ice volume also affect local and regional sea levels by the readjustment of the geoid and true polar wander. Atmospheric pressure, ocean currents and local ocean temperature changes can affect LMSL as well.
Eustatic sea level change (global as opposed to local
change) is due to change in either the volume of water in the world's
oceans or the volume of the oceanic basins.
Two major mechanisms are currently causing eustatic sea level rise.
First, shrinking land ice, such as mountain glaciers and polar ice
sheets, is releasing water into the oceans. Second, as ocean
temperatures rise, the warmer water expands.
Short-term and periodic changes
The Last Glacial Period caused a much lower global sea level.Warming temperatures and melting glaciers are currently raising the sea level.
Many factors can produce short-term changes in sea level, typically
within a few metres, in timeframes ranging from minutes to months:
Between 1901 and 2018, the average sea level rose by 15–25 cm
(6–10 in), with an increase of 2.3 mm (0.091 in) per year since the
1970s. This was faster than the sea level had ever risen over at least the past 3,000 years. The rate accelerated to 4.62 mm (0.182 in)/yr for the decade 2013–2022. Climate change due to human activities is the main cause. Between 1993 and 2018, melting ice sheets and glaciers accounted for 44% of sea level rise, with another 42% resulting from thermal expansion of water.
Sea level rise lags behind changes in the Earth's
temperature by many decades, and sea level rise will therefore continue
to accelerate between now and 2050 in response to warming that has
already happened. What happens after that depends on human greenhouse gas emissions.
If there are very deep cuts in emissions, sea level rise would slow
between 2050 and 2100. It could then reach by 2100 slightly over 30 cm
(1 ft) from now and approximately 60 cm (2 ft) from the 19th century.
With high emissions it would instead accelerate further, and could rise
by 1.0 m (3+1⁄3 ft) or even 1.6 m (5+1⁄3 ft) by 2100.
In the long run, sea level rise would amount to 2–3 m (7–10 ft) over
the next 2000 years if warming stays to its current 1.5 °C (2.7 °F) over
the pre-industrial past. It would be 19–22 metres (62–72 ft) if warming
peaks at 5 °C (9.0 °F).
Rising seas affect every coastal and island population on Earth. This can be through flooding, higher storm surges, king tides, and tsunamis. There are many knock-on effects. They lead to loss of coastal ecosystems like mangroves. Crop yields may reduce because of increasing salt levels in irrigation water. Damage to ports disrupts sea trade.
The sea level rise projected by 2050 will expose places currently
inhabited by tens of millions of people to annual flooding. Without a
sharp reduction in greenhouse gas emissions, this may increase to
hundreds of millions in the latter decades of the century.
Local factors like tidal range or land subsidence will greatly affect the severity of impacts. For instance, sea level rise in the United States is likely to be two to three times greater than the global average by the end of the century. Yet, of the 20 countries with the greatest exposure to sea level rise, twelve are in Asia, including Indonesia, Bangladesh and the Philippines. The resilience and adaptive capacity of ecosystems and countries also varies, which will result in more or less pronounced impacts. The greatest impact on human populations in the near term will occur in the low-lying Caribbean and Pacific islands. Sea level rise will make many of them uninhabitable later this century.
Societies can adapt to sea level rise in multiple ways. Managed retreat, accommodating coastal change, or protecting against sea level rise through hard-construction practices like seawalls are hard approaches. There are also soft approaches such as dune rehabilitation and beach nourishment. Sometimes these adaptation strategies go hand in hand. At other times choices must be made among different strategies. Poorer nations may also struggle to implement the same approaches to adapt to sea level rise as richer states.
Pilots can estimate height above sea level with an altimeter set to a defined barometric pressure.
Generally, the pressure used to set the altimeter is the barometric
pressure that would exist at MSL in the region being flown over. This
pressure is referred to as either QNH or "altimeter" and is transmitted to the pilot by radio from air traffic control (ATC) or an automatic terminal information service
(ATIS). Since the terrain elevation is also referenced to MSL, the
pilot can estimate height above ground by subtracting the terrain
altitude from the altimeter reading. Aviation charts
are divided into boxes and the maximum terrain altitude from MSL in
each box is clearly indicated. Once above the transition altitude, the
altimeter is set to the international standard atmosphere (ISA) pressure at MSL which is 1013.25 hPa or 29.92 inHg.
A
model of present-day mass change due to post-glacial rebound and the
reloading of the ocean basins with seawater. Blue and purple areas
indicate rising due to the removal of the ice sheets. Yellow and red
areas indicate falling as mantle material moved away from these areas in
order to supply the rising areas, and because of the collapse of the forebulges around the ice sheets.This layered beach at Bathurst Inlet, Nunavut
is an example of post-glacial rebound after the last Ice Age. Little
to no tide helped to form its layer-cake look. Isostatic rebound is
still underway here.
Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound and isostatic depression are phases of glacial isostasy (glacial isostatic adjustment, glacioisostasy), the deformation of the Earth's crust in response to changes in ice mass distribution. The direct raising effects of post-glacial rebound are readily apparent in parts of Northern Eurasia, Northern America, Patagonia, and Antarctica. However, through the processes of ocean siphoning and continental levering, the effects of post-glacial rebound on sea level are felt globally far from the locations of current and former ice sheets.
Overview
Changes in the elevation of Lake Superior due to glaciation and post-glacial rebound
During the last glacial period, much of northern Europe, Asia, North America, Greenland and Antarctica were covered by ice sheets, which reached up to three kilometres thick during the glacial maximum about 20,000 years ago. The enormous weight of this ice caused the surface of the Earth's crust to deform and warp downward, forcing the viscoelasticmantle material to flow away from the loaded region. At the end of each glacial period when the glaciers
retreated, the removal of this weight led to slow (and still ongoing)
uplift or rebound of the land and the return flow of mantle material
back under the deglaciated area. Due to the extreme viscosity of the mantle, it will take many thousands of years for the land to reach an equilibrium level.
The uplift has taken place in two distinct stages. The initial uplift following deglaciation was almost immediate due to the elastic response of the crust as the ice load was removed. After this elastic phase, uplift proceeded by slow viscous flow at an exponentially decreasing rate.[citation needed] Today, typical uplift rates are of the order of 1 cm/year or less. In northern Europe, this is clearly shown by the GPS data obtained by the BIFROST GPS network; for example in Finland, the total area of the country is growing by about seven square kilometers per year.
Studies suggest that rebound will continue for at least another 10,000
years. The total uplift from the end of deglaciation depends on the
local ice load and could be several hundred metres near the centre of
rebound.
Recently, the term "post-glacial rebound" is gradually being
replaced by the term "glacial isostatic adjustment". This is in
recognition that the response of the Earth to glacial loading and
unloading is not limited to the upward rebound movement, but also
involves downward land movement, horizontal crustal motion, changes in global sea levels and the Earth's gravity field, induced earthquakes, and changes in the Earth's rotation.
Another alternate term is "glacial isostasy", because the uplift near
the centre of rebound is due to the tendency towards the restoration of
isostatic equilibrium (as in the case of isostasy of mountains).
Unfortunately, that term gives the wrong impression that isostatic
equilibrium is somehow reached, so by appending "adjustment" at the end,
the motion of restoration is emphasized.
Effects
Post-glacial
rebound produces measurable effects on vertical crustal motion, global
sea levels, horizontal crustal motion, gravity field, Earth's rotation,
crustal stress, and earthquakes. Studies of glacial rebound give us
information about the flow law of mantle rocks, which is important to
the study of mantle convection, plate tectonics and the thermal evolution of the Earth. It also gives insight into past ice sheet history, which is important to glaciology, paleoclimate,
and changes in global sea level. Understanding postglacial rebound is
also important to our ability to monitor recent global change.
Vertical crustal motion
The
elastic behavior of the lithosphere and mantle, illustrating subsidence
of the crust with respect to landscape properties as a result of the
downward force of a glacier ("Before"), and the effects that melting and
glacial retreat have on the rebound of the mantle and lithosphere in
("After").Much of modern Finland is former seabed or archipelago: illustrated are sea levels immediately after the last ice age.
Erratic boulders, U-shaped valleys, drumlins, eskers, kettle lakes, bedrockstriations are among the common signatures of the Ice Age.
In addition, post-glacial rebound has caused numerous significant
changes to coastlines and landscapes over the last several thousand
years, and the effects continue to be significant.
In Sweden, Lake Mälaren was formerly an arm of the Baltic Sea, but uplift eventually cut it off and led to its becoming a freshwaterlake in about the 12th century, at the time when Stockholm was founded at its outlet. Marine seashells found in Lake Ontario sediments imply a similar event in prehistoric times. Other pronounced effects can be seen on the island of Öland, Sweden, which has little topographic relief due to the presence of the very level Stora Alvaret. The rising land has caused the Iron Age settlement area to recede from the Baltic Sea,
making the present day villages on the west coast set back unexpectedly
far from the shore. These effects are quite dramatic at the village of
Alby, for example, where the Iron Age inhabitants were known to subsist on substantial coastal fishing.
In several other Nordic ports, like Tornio and Pori (formerly at Ulvila),
the harbour has had to be relocated several times. Place names in the
coastal regions also illustrate the rising land: there are inland places
named 'island', 'skerry', 'rock', 'point' and 'sound'. For example, Oulunsalo "island of Oulujoki" is a peninsula, with inland names such as Koivukari "Birch Rock", Santaniemi "Sandy Cape", and Salmioja "the brook of the Sound".
Map of Post Glacial Rebound effects upon the land-level of Ireland and the British Isles.
In Great Britain, glaciation affected Scotland but not southern England,
and the post-glacial rebound of northern Great Britain (up to 10 cm per
century) is causing a corresponding downward movement of the southern
half of the island (up to 5 cm per century). This will eventually lead
to an increased risk of floods in southern England and south-western Ireland.
Since the glacial isostatic adjustment process causes the land to
move relative to the sea, ancient shorelines are found to lie above
present day sea level in areas that were once glaciated. On the other
hand, places in the peripheral bulge area which was uplifted during
glaciation now begins to subside. Therefore, ancient beaches are found
below present day sea level in the bulge area. The "relative sea level
data", which consists of height and age measurements of the ancient
beaches around the world, tells us that glacial isostatic adjustment
proceeded at a higher rate near the end of deglaciation than today.
The present-day uplift motion in northern Europe is also monitored by a GPS network called BIFROST. Results of GPS data show a peak rate of about 11 mm/year in the north part of the Gulf of Bothnia, but this uplift rate decreases away and becomes negative outside the former ice margin.
In the near field outside the former ice margin, the land sinks
relative to the sea. This is the case along the east coast of the United
States, where ancient beaches are found submerged below present day sea
level and Florida is expected to be submerged in the future. GPS data in North America also confirms that land uplift becomes subsidence outside the former ice margin.
Global sea levels
To
form the ice sheets of the last Ice Age, water from the oceans
evaporated, condensed as snow and was deposited as ice in high
latitudes. Thus global sea level fell during glaciation.
The ice sheets at the last glacial maximum
were so massive that global sea level fell by about 120 metres. Thus
continental shelves were exposed and many islands became connected with
the continents through dry land. This was the case between the British
Isles and Europe (Doggerland), or between Taiwan, the Indonesian islands and Asia (Sundaland). A land bridge also existed between Siberia and Alaska that allowed the migration of people and animals during the last glacial maximum.
The fall in sea level also affects the circulation of ocean
currents and thus has important impact on climate during the glacial
maximum.
During deglaciation, the melted ice water returns to the oceans, thus sea level in the ocean increases
again. However, geological records of sea level changes show that the
redistribution of the melted ice water is not the same everywhere in the
oceans. In other words, depending upon the location, the rise in sea
level at a certain site may be more than that at another site. This is
due to the gravitational attraction between the mass of the melted water
and the other masses, such as remaining ice sheets, glaciers, water
masses and mantle rocks[7] and the changes in centrifugal potential due to Earth's variable rotation.
Horizontal crustal motion
Accompanying vertical motion is the horizontal motion of the crust. The BIFROST GPS network shows that the motion diverges from the centre of rebound. However, the largest horizontal velocity is found near the former ice margin.
The situation in North America is less certain; this is due to
the sparse distribution of GPS stations in northern Canada, which is
rather inaccessible.
Tilt
The
combination of horizontal and vertical motion changes the tilt of the
surface. That is, locations farther north rise faster, an effect that
becomes apparent in lakes. The bottoms of the lakes gradually tilt away
from the direction of the former ice maximum, such that lake shores on
the side of the maximum (typically north) recede and the opposite
(southern) shores sink. This causes the formation of new rapids and rivers. For example, Lake Pielinen
in
Finland, which is large (90 x 30 km) and oriented perpendicularly to the
former ice margin, originally drained through an outlet in the middle
of the lake near Nunnanlahti to Lake Höytiäinen. The change of tilt caused Pielinen to burst through the Uimaharju esker at the southwestern end of the lake, creating a new river (Pielisjoki) that runs to the sea via Lake Pyhäselkä to Lake Saimaa.
The effects are similar to that concerning seashores, but occur above
sea level. Tilting of land will also affect the flow of water in lakes
and rivers in the future, and thus is important for water resource
management planning.
In Sweden Lake Sommen's
outlet in the northwest has a rebound of 2.36 mm/a while in the eastern
Svanaviken it is 2.05 mm/a. This means the lake is being slowly tilted
and the southeastern shores drowned.
Ice, water, and mantle rocks have mass, and as they move around, they exert a gravitational pull on other masses towards them. Thus, the gravity field,
which is sensitive to all mass on the surface and within the Earth, is
affected by the redistribution of ice/melted water on the surface of the
Earth and the flow of mantle rocks within.
Today, more than 6000 years after the last deglaciation terminated, the flow of mantle material back to the glaciated area causes the overall shape of the Earth to become less oblate. This change in the topography of Earth's surface affects the long-wavelength components of the gravity field.
The changing gravity field can be detected by repeated land measurements with absolute gravimeters and recently by the GRACE satellite mission.
The change in long-wavelength components of Earth's gravity field also
perturbs the orbital motion of satellites and has been detected by LAGEOS satellite motion.
Vertical datum
The vertical datum
is a reference surface for altitude measurement and plays vital roles
in many human activities, including land surveying and construction of
buildings and bridges. Since postglacial rebound continuously deforms
the crustal surface and the gravitational field, the vertical datum
needs to be redefined repeatedly through time.
State of stress, intraplate earthquakes and volcanism
According to the theory of plate tectonics,
plate-plate interaction results in earthquakes near plate boundaries.
However, large earthquakes are found in intraplate environments like
eastern Canada (up to M7) and northern Europe (up to M5) which are far
away from present-day plate boundaries. An important intraplate
earthquake was the magnitude 8 New Madrid earthquake that occurred in mid-continental US in the year 1811.
Glacial loads provided more than 30 MPa of vertical stress in
northern Canada and more than 20 MPa in northern Europe during glacial
maximum. This vertical stress is supported by the mantle and the flexure
of the lithosphere.
Since the mantle and the lithosphere continuously respond to the
changing ice and water loads, the state of stress at any location
continuously changes in time. The changes in the orientation of the
state of stress is recorded in the postglacial faults in southeastern Canada.
When the postglacial faults formed at the end of deglaciation 9000
years ago, the horizontal principal stress orientation was almost
perpendicular to the former ice margin, but today the orientation is in
the northeast–southwest, along the direction of seafloor spreading at the Mid-Atlantic Ridge.
This shows that the stress due to postglacial rebound had played an
important role at deglacial time, but has gradually relaxed so that
tectonic stress has become more dominant today.
According to the Mohr–Coulomb theory
of rock failure, large glacial loads generally suppress earthquakes,
but rapid deglaciation promotes earthquakes. According to Wu &
Hasagawa, the rebound stress that is available to trigger earthquakes
today is of the order of 1 MPa.
This stress level is not large enough to rupture intact rocks but is
large enough to reactivate pre-existing faults that are close to
failure. Thus, both postglacial rebound and past tectonics play
important roles in today's intraplate earthquakes in eastern Canada and
southeast US. Generally postglacial rebound stress could have triggered
the intraplate earthquakes in eastern Canada and may have played some
role in triggering earthquakes in the eastern US including the New Madrid earthquakes of 1811.
The situation in northern Europe today is complicated by the current
tectonic activities nearby and by coastal loading and weakening.
Increasing pressure due to the weight of the ice during
glaciation may have suppressed melt generation and volcanic activities
below Iceland and Greenland. On the other hand, decreasing pressure due
to deglaciation can increase the melt production and volcanic activities
by 20-30 times.
Recent global warming
Recent global warming has caused mountain glaciers and the ice sheets in Greenland and Antarctica to melt and global sea level to rise. Therefore, monitoring sea level rise and the mass balance of ice sheets and glaciers allows people to understand more about global warming.
Recent rise in sea levels has been monitored by tide gauges and satellite altimetry (e.g. TOPEX/Poseidon).
As well as the addition of melted ice water from glaciers and ice
sheets, recent sea level changes are affected by the thermal expansion
of sea water due to global warming,
sea level change due to deglaciation of the last glacial maximum
(postglacial sea level change), deformation of the land and ocean floor
and other factors. Thus, to understand global warming from sea level
change, one must be able to separate all these factors, especially
postglacial rebound, since it is one of the leading factors.
Mass changes of ice sheets can be monitored by measuring changes
in the ice surface height, the deformation of the ground below and the
changes in the gravity field over the ice sheet. Thus ICESat, GPS and GRACE satellite mission are useful for such purpose.
However, glacial isostatic adjustment of the ice sheets affect ground
deformation and the gravity field today. Thus understanding glacial
isostatic adjustment is important in monitoring recent global warming.
One of the possible impacts of global warming-triggered rebound
may be more volcanic activity in previously ice-capped areas such as
Iceland and Greenland.
It may also trigger intraplate earthquakes near the ice margins of
Greenland and Antarctica. Unusually rapid (up to 4.1 cm/year) present
glacial isostatic rebound due to recent ice mass losses in the Amundsen
Sea embayment region of Antarctica coupled with low regional mantle
viscosity is predicted to provide a modest stabilizing influence on marine ice sheet instability in West Antarctica, but likely not to a sufficient degree to arrest it.
Applications
The speed and amount of postglacial rebound is determined by two factors: the viscosity or rheology (i.e., the flow) of the mantle, and the ice loading and unloading histories on the surface of Earth.
The viscosity of the mantle is important in understanding mantle convection, plate tectonics,
the dynamical processes in Earth, and the thermal state and thermal
evolution of Earth. However viscosity is difficult to observe because creep
experiments of mantle rocks at natural strain rates would take
thousands of years to observe and the ambient temperature and pressure
conditions are not easy to attain for a long enough time. Thus, the
observations of postglacial rebound provide a natural experiment to
measure mantle rheology. Modelling of glacial isostatic adjustment
addresses the question of how viscosity changes in the radial and lateral directions and whether the flow law is linear, nonlinear, or composite rheology. Mantle viscosity may additionally be estimated using seismic tomography, where seismic velocity is used as a proxy observable.
Ice thickness histories are useful in the study of paleoclimatology, glaciology
and paleo-oceanography. Ice thickness histories are traditionally
deduced from the three types of information: First, the sea level data
at stable sites far away from the centers of deglaciation
give an estimate of how much water entered the oceans or equivalently
how much ice was locked up at glacial maximum. Secondly, the location
and dates of terminal moraines
tell us the areal extent and retreat of past ice sheets. Physics of
glaciers gives us the theoretical profile of ice sheets at equilibrium,
it also says that the thickness and horizontal extent of equilibrium
ice sheets are closely related to the basal condition of the ice
sheets. Thus the volume of ice locked up is proportional to their
instantaneous area. Finally, the heights of ancient beaches in the sea
level data and observed land uplift rates (e.g. from GPS or VLBI) can be used to constrain local ice thickness. A popular ice model deduced this way is the ICE5G model.
Because the response of the Earth to changes in ice height is slow, it
cannot record rapid fluctuation or surges of ice sheets, thus the ice
sheet profiles deduced this way only gives the "average height" over a
thousand years or so.
Glacial isostatic adjustment also plays an important role in understanding recent global warming and climate change.
Discovery
Before the eighteenth century, it was thought, in Sweden, that sea levels were falling. On the initiative of Anders Celsius
a number of marks were made in rock on different locations along the
Swedish coast. In 1765 it was possible to conclude that it was not a
lowering of sea levels but an uneven rise of land. In 1865 Thomas Jamieson
came up with a theory that the rise of land was connected with the ice
age that had been first discovered in 1837. The theory was accepted
after investigations by Gerard De Geer of old shorelines in Scandinavia published in 1890.
Legal implications
In
areas where the rising of land is seen, it is necessary to define the
exact limits of property. In Finland, the "new land" is legally the
property of the owner of the water area, not any land owners on the
shore. Therefore, if the owner of the land wishes to build a pier over
the "new land", they need the permission of the owner of the (former)
water area. The landowner of the shore may redeem the new land at market
price. Usually the owner of the water area is the partition unit of the landowners of the shores, a collective holding corporation.
Formulation: sea-level equation
The sea-level equation (SLE) is a linear integral equation that describes the sea-level variations associated with the PGR.
The basic idea of the SLE dates back to 1888, when Woodward published his pioneering work on the form and position of mean sea level, and only later has been refined by Platzman and Farrell in the context of the study of the ocean tides. In the words of Wu and Peltier, the solution of the SLE yields the space– and time–dependent change of ocean bathymetry which is required to keep the gravitational potential of the sea surface constant for a specific deglaciation chronology and viscoelastic earth model. The SLE theory was then developed by other authors as Mitrovica & Peltier, Mitrovica et al. and Spada & Stocchi. In its simplest form, the SLE reads
where is the sea–level change, is the sea surface variation as seen from Earth's center of mass, and is vertical displacement.
In a more explicit form the SLE can be written as follow:
where is colatitude and is longitude, is time, and are the densities of ice and water, respectively, is the reference surface gravity, is the sea–level Green's function (dependent upon the and viscoelastic load–deformation coefficients - LDCs), is the ice thickness variation, represents the eustatic term (i.e. the ocean–averaged value of ), and
denote spatio-temporal convolutions over the ice- and ocean-covered
regions, and the overbar indicates an average over the surface of the
oceans that ensures mass conservation.
.jpg)
.jpg)
