Showing posts with label GRACE. Show all posts

Written by fatih al-farahat in , , , , ,


Greenland’s Jakobshavn could be poised to speed up. So could even bigger glaciers farther north — with significant implications for global sea level

Greenland’s Jakobshavn could be poised to speed up. So could even bigger glaciers farther north — with significant implications for global sea level

by Tom Yulsman, CE Journal, October 23, 2010

The Jakobshavn Glacier in Greenland already flows so fast you can stand on a ridge above it and watch it move. In fact, it’s the fastest moving glacier on Earth, flowing from the land to the sea at more than 14 kilometers per year.

But we may not have seen anything yet. The map above shows why it could be primed to move even faster, and thereby dump even more ice into the sea.
And if the same process plays out with glaciers farther north — and there is growing evidence that it may do just that (see the map and discussion lower in this post) — there would be significant implications for global sea level.
The image above is a kind of topographic map showing the elevation of the bedrock below and around the Jakobshavn Glacier — as if Greenland’s overlying ice had been removed. Warm colors show areas of the bedrock that are above sea level. The green and blue colors show areas where it is below sea level. The fjord through which Jakobshavn flows out to sea on Greenland’s western coast is visible as the green and blue trough at the left side of the image. As is evident from the map, the fjord actually continues inland for another 50 miles, and to a depth of 1.6 km below sea level. Why is this significant?
The calving front of the glacier marked on the map shows the current edge of the glacier. This is where icebergs calve from Jakobshaven and float out to sea. Since 2001, the calving front has retreated about 6 miles (10 kilometers). During July 6 and 7 this past summer, it pulled back an entire mile. Now, the calving front is poised right at the western edge of the fjord’s continuation into the ice sheet.
“If the calving front retreats from where it is now, it can draw out a lot of ice,” says Konrad Steffen, director of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado.
The effect will be like pulling the plug from the drain of a bathtub. It will mean that much more ice will be able to drain from the Greenland Ice Sheet through Jakobshavn into the sea.
“You can actually pull out a lot of ice with a fjord below sea level like this,” Steffen says. “And there are about four or five others like this.”
The image at left is from a presentation Steffen gave last Thursday at the Center for Environmental Journalism. Like the image at the top of this post, it is a topographic map of bedrock. This is the topography of Greenland with all the ice removed.
The blue areas on the map show regions where the enormous weight of the ice sheet — which is almost 2 miles thick at its maximum — pushes the bedrock below sea level. And the numbers mark the major fjords through which glaciers drain into the sea.
Because the ice must pass through such narrow outlets, these fjords are the bathtub plugs. But if the calving fronts of the glaciers were to retreat far enough into the interior, the plugs would be removed, and the interior of the ice sheet could then drain much more quickly.
Jakobshavn Glacier on the southwestern coast is marked number 4. And as the map shows, there are several more farther north. What’s happening to them?
The graphic below addresses this question. It is based on data collected by NASA’s Grace satellites. (The satellites, which fly in formation together and use gravity measurements to detect changes in mass below them, are seen in the illustration to the right.)
In the map on the left, pink and blue colors indicate where Greenland was losing ice mass between 2003 and 2007. Most of the loss was concentrated on the southeast coast. As the map to the right indicates, by this past March loss of ice had extended all the way up to the northwestern coast — where at least two of the big fjords of concern are located. One of these is the Petermann Glacier (marked number 2 on the bedrock topography map).
In early August, an ice island four times the size of Manhattan Island calved from the front of the Petermann glacier. With this, the glacier lost about one-quarter of its 43-mile long floating ice-shelf. “The freshwater stored in this ice island could keep the Delaware or Hudson rivers flowing for more than two years. It could also keep all U.S. public tap water flowing for 120 days,” says Andreas Muenchow of the University of Delaware’s College of Earth,Ocean and Environment (quoted in a press release).
Petermann and the other more northerly glaciers “go farther inland, are much broader, and go much lower below sea level than Jakobshavn,” Steffen says. (In fact, whereas Jakobshavn is about 7.5 miles across, Petermann is about 60 miles wide.) Right now, these larger rivers of ice are not moving as quickly as Jakobshavn. But if their ice fronts were to retreat more rapidly, and the glaciers were to speed up enough, they could “drain most of northern Greenland in decades or a century.”
And that could have a very significant impact on global sea level. How significant? For insight into that question, please see my previous post, which was also based on Steffen’s presentation to our group.

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Is Greenland losing ice? (psst, the answer is yes, at an accelerating rate) by Robert Way, Skeptical Science

Is Greenland losing ice? (psst, the answer is yes, at an accelerating rate)

by Robert Way, Skeptical Science, September 28, 2010
Confusion caused by anecdotes of structures being buried by accumulating snow on Greenland's ice sheet leads some skeptics to believe Greenland is Gaining Ice. As always, the best way to tease out the truth here by following the research of scientists investigating Greenland's ice mass balance.
In general, the best available science tells us that Greenland is losing ice extensively (Figure 1) and that these losses have drastically increased since the year 2000.


Figure 1. Estimated Greenland Ice Sheet mass balance changes since 1950 using three different methods (Jiang 2010). Mass balance measurement techniques are discussed here.

The evidence suggested by a multitude of different measurement techniques suggests that not only is Greenland losing ice but that these ice losses are accelerating at a rapid pace (Velicogna 2009). Further evidence suggests that although ice losses have up to this point primarily occurred in the South and Southwest portions of Greenland, these losses are now spreading to the Northwest sector of the ice sheet (Khan et al. 2010).

Although there have been some gains at high altitudes, significant ice losses are occurring at low altitudes (Wouters 2008) along the coastline where glaciers are calving ice into the oceans far quicker than ice is being accumulated at the top of the ice sheet (Rignot & Kanagaratnam 2007).

In conclusion, Greenland is losing ice extensively along its margins where fast flowing ice streams are pushing more ice into the ocean than is gained in the center of the ice sheet. For more information on how ice sheets lose mass, a more comprehensive discussion is available here.
This post is the Basic version (written by Robert Way) of the skeptic argument "Greenland is gaining ice." Robert has also written a number of richly informative posts on Greenland ice loss. A good starting point is Why do glaciers lose mass. Part two in this series was How do we measure Antarctic ice loss. Part Three responded to disinformation from Steve Goddard about Antarctic ice loss.

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S. B. Luthcke et al., 2009 Fall AGU, Greenland ice sheet (GrIS) mass changes (through July 25, 2009) from NASA/GSFC GRACE Mascon solutions


Greenland Ice Sheet Mass Changes from NASA GSFC GRACE Mascon Solutions

∗S. B. Luthcke, D. D. Rowlands, J. J. McCarthy, A. Arendt, T. Sabaka, J. P. Boy, F. G. Lemoine, "Recent Changes of the Earth's Land Ice from GRACE," presented at 2009 Fall AGU, H13G-02 (693337), Dec. 14, 2009.

The mass changes of the Greenland Ice Sheet (GrIS) are computed from the Gravity Recovery and Climate Experiment (GRACE) inter-satellite range-rate observations for the period April 5, 2003, through July 25, 2009. The mass of the GrIS has been computed at 10-day intervals and 200-km spatial resolution from a regional high-resolution mascon solution (Luthcke and others, 2008 and 2006). The animation shows the change in mass referenced from April 5, 2003. The spatial variation in surface mass is shown in centimeters equivalent height of water. The time variation of the GrIS mass is shown in the x-y plot insert with units of gigatons.

∗Corresponding author: Scott B. Luthcke, NASA GSFC, Planetary Geodynamics Laboratory, Code 698.
e-mail: Scott.B.Luthcke@nasa.gov; Tel. (301) 614-6112.

This animation shows mass changes over Greenland with a colorbar, graph and clock overlay.    This animation shows mass changes over Greenland with a colorbar, graph and clock overlay.
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This animation shows mass changes over Greenland without the chart, colorbar or seasonal clock.    This animation shows mass changes over Greenland without the chart, colorbar or seasonal clock.
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Overlay of the graph, clock and colorbar with an alpha channel.    Overlay of the graph, clock and colorbar with an alpha channel.

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A high resolution still image of ice sheet mass changes over Greenland on 07/25/2005.    A high resolution still image of ice sheet mass changes over Greenland on 07/25/2005.

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A high resolution still image of ice sheet mass changes over Greenland on 07/25/2009.    A high resolution still image of ice sheet mass changes over Greenland on 07/25/2009.

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This colorbar used for the above animation. The minimum data value is -322.933 while the maximum data value is 101.653.    This colorbar used for the above animation. The minimum data value is -322.933 while the maximum data value is 101.653.

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Animation Number:3663
Completed:2009-12-04
Animators:Cindy Starr (GST) (Lead)
Lori Perkins (NASA/GSFC)
Producer:Paul Reddish (Free Spirit Films)
Scientist:Scott Luthcke (NASA/GSFC)
Platforms/Sensors/Data Sets:Terra and Aqua/MODIS/Blue Marble: Next Generation
Gravity Recovery and Climate Experiment (GRACE)/mascon solution (4/5/2003 - 7/25/2009)
Series:GRACE MASCON Solutions

Science paper:Luthcke, S. B., A. A. Arendt, D. D. Rowlands, J. J. McCarthy and C.F. Larsen. Recent glacier mass changes in the Gulf of Alaska region from GRACE mascon solutions. Journal of Glaciology, Vol. 54, No. 188, 2008. Luthcke, S. B., H. J. Zwally, W. Abdalati, D. D. Rowlands, R. D. Ray, R. S. Nerem, F. G. Lemoine, J. J. McCarthy and D. S. Chinn. Recent Greenland ice mass loss by drainage system from satellite gravity observations. Science, 314, 1286, 2006. [DOI: 10.1126/science.1130776]
Please give credit for this item to: NASA/Goddard Space Flight Center Scientific Visualization Studio.
The Blue Marble data is courtesy of Reto Stockli (NASA/GSFC).
Link to this page: http://svs.gsfc.nasa.gov/vis/a000000/a003600/a003663/

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Recent Changes of the Earth's land ice from GRACE: methods, signals and errors, Scott Luthcke et al., AGU Fall Meeting, San Francisco, December 2009: Abstract H13G-02 (693337)

AGU Fall Meeting, San Francisco, December 2009

Session B.3-b: Cryosphere
 
 
First Author and presenter: Scott Luthcke

Co-Authors: D. D. Rowlands, J. J. McCarthy, T. Sabaka, F. G. Lemoine, and J. P. Boy

Abstract 

The NASA/DLR Gravity Recovery and Climate Experiment (GRACE) mission has been acquiring ultra-precise inter-satellite K-band range and range-rate (KBRR) measurements providing new observations of the complex evolution of the Earth's land ice. The fidelity of the ice mass flux solutions depends on many factors including solution method, parameterization and processing of the GRACE level 1B data including forward modeling of various non-ice mass signals (e.g. hydrology, oceans, atmosphere). In this talk we present the latest GRACE derived ice mass solutions for the Gulf of Alaska glaciers, Greenland and Antarctica. We compare solutions derived from various solution techniques and explore the impact of forward modeling and parameterization on the final solutions. Solution results as well as errors and limitations will be discussed.

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NASA's GRACE data reveal major groundwater loss in California's Central Valley and the Sierra Nevadas

NASA Data Reveal Major Groundwater Loss in California

The twin Grace satellites monitor tiny month-to-month changes in Earth's gravity. GRACE observed trends in groundwater levels: October 2003 – March 2009. Image Credit: University Of California Center For Hydrologic Modeling
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JPL, Pasadena, Calif. – New space observations reveal that since October 2003, the aquifers for California's primary agricultural region – the Central Valley – and its major mountain water source – the Sierra Nevadas – have lost nearly enough water combined to fill Lake Mead, America's largest reservoir. The findings, based on data from the NASA/German Aerospace Center Gravity Recovery and Climate Experiment (GRACE), reflect California's extended drought and increased rates of groundwater being pumped for human uses, such as irrigation.

In research being presented this week at the American Geophysical Union meeting in San Francisco, scientists from NASA and the University of California, Irvine, detailed California's groundwater changes and outlined GRACE-based research on other global aquifers. The twin GRACE satellites monitor tiny month-to-month changes in Earth's gravity field primarily caused by the movement of water in Earth's land, ocean, ice and atmosphere reservoirs. GRACE's ability to directly 'weigh' changes in water content provides new insights into how Earth's water cycle may be changing.

Combined, California's Sacramento and San Joaquin drainage basins have shed more than 30 cubic kilometers of water since late 2003, said professor Jay Famiglietti of the University of California, Irvine. A cubic kilometer is about 264.2 billion gallons, enough to fill 400,000 Olympic-size pools. The bulk of the loss occurred in California's agricultural Central Valley. The Central Valley receives its irrigation from a combination of groundwater pumped from wells and surface water diverted from elsewhere.

"GRACE data reveal groundwater in these basins is being pumped for irrigation at rates that are not sustainable if current trends continue," Famiglietti said. "This is leading to declining water tables, water shortages, decreasing crop sizes and continued land subsidence. The findings have major implications for the U.S. economy, as California's Central Valley is home to one sixth of all U.S. irrigated land, and the state leads the nation in agricultural production and exports."

"By providing data on large-scale groundwater depletion rates, GRACE can help California water managers make informed decisions about allocating water resources," said GRACE Project Scientist Michael Watkins of NASA's Jet Propulsion Laboratory, Pasadena, Calif., which manages the mission for NASA's Science Mission Directorate, Washington.

Preliminary studies show most of the water loss is coming from the more southerly located San Joaquin basin, which gets less precipitation than the Sacramento River basin farther north. Initial results suggest the Sacramento River basin is losing about 2 cubic kilometers of water a year. Surface water losses account for half of this, while groundwater losses in the northern Central Valley add another 0.6 cubic kilometers annually. The San Joaquin Basin is losing 3.5 cubic kilometers a year. Of this, more than 75% is the result of groundwater pumping in the southern Central Valley, primarily to irrigate crops.

Famiglietti said recent California legislation decreasing the allocation of surface waters to the San Joaquin Basin is likely to further increase the region's reliance on groundwater for irrigation. "This suggests the decreasing groundwater storage trends seen by GRACE will continue for the foreseeable future," he said.

The California results come just months after a team of hydrologists led by Matt Rodell of NASA's Goddard Space Flight Center, Greenbelt, Md., found groundwater levels in northwest India have declined by 17.7 cubic kilometers per year over the past decade, a loss due almost entirely to pumping and consumption of groundwater by humans.

"California and India are just two of many regions around the world where GRACE data are being used to study droughts, which can have devastating impacts on societies and cost the U.S. economy $6 to $8 billion annually," said Rodell. Other regions under study include Australia, the Middle East – North Africa region and the southeastern United States, where GRACE clearly captured the evolution of an extended drought that ended this spring. In the Middle East – North Africa region, Rodell is leading an effort to use GRACE and other data to systematically map water- and weather-related variables to help assess regional water resources. Rodell added GRACE may also help predict droughts, since it can identify pre-existing conditions favorable to the start of a drought, such as a deficit of water deep below the ground.

NASA is working with the National Oceanic and Atmospheric Administration and the University of Nebraska-Lincoln to incorporate GRACE data into NOAA's U.S. and North American Drought Monitors, premier tools used to minimize drought impacts. The tools rely heavily on precipitation observations, but are limited by inadequate large-scale observations of soil moisture and groundwater levels. "GRACE is the only satellite system that provides information on these deeper stores of water that are key indicators of long-term drought," Rodell said.

GRACE is a partnership of NASA and the German Aerospace Center (DLR). The University of Texas Center for Space Research, Austin, has overall mission responsibility. JPL developed the satellites. DLR provided the launch, and GeoForschungsZentrum Potsdam, Germany, operates the mission. For more on GRACE, see http://www.csr.utexas.edu/grace/ and  http://grace.jpl.nasa.gov/ . Other media contacts: Margaret Baguio, University of Texas Center for Space Research, 512-471-6922; Jennifer Fitzenberger, University of California, Irvine, 949-824-3969.

JPL is managed for NASA by the California Institute of Technology in Pasadena.
Alan Buis 818-354-0474; Jet Propulsion Laboratory, Pasadena, CA.  Alan.buis@jpl.nasa.gov

Link:  http://www.jpl.nasa.gov/news/news.cfm?release=2009-194&cid=kintera_release_2009-194&msource=19409&tr=y&auid=5707430

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GRACE satellites show Greenland glaciers shrinking continuously in the last few years: the changes are not linear in time but accelerating

Measuring impact of climate change from space: Gravity measurements shed light on key questions



CLICK ON IMAGE TO ENLARGE. Global mass variations from August 2002 to July 2008 as observed by the GRACE satellite mission. Negative signals dominate over Greenland, Alaska and Antarctica, mainly resulting from ice melting. (Credit: University of Stuttgart)


ScienceDaily, December 10, 2009 — What is the impact of climate change on the ice-covered regions of Earth? How does deglaciation affect global sea level changes?

These questions are being addressed by scientists from the Institute of Geodesy at the University of Stuttgart, Germany, and the Department of Spatial Science at the Curtin University of Technology in Perth, Australia. For this purpose, the German-Australian team has been investigating space-borne gravity measurements provided by the GRACE satellite mission.

As a result, they have found out that the Greenland glaciers shrunk continuously in the last few years; above all, they estimated the changes not to be linear in time but accelerating. On average, recent Greenland ice-mass decline caused an annual sea-level rise of about 0.5 millimetres.

For the first time ever, the GRACE satellite mission has allowed the determination of global mass variations -- such as ice melting in the polar areas -- from changes in Earth's gravitational pull. The underlying measurement principle is simple: it is based on the fact that the redistribution of masses on the Earth surface can be mapped in terms of changes of the terrestrial gravity field. Hence, scientists can measure the spatio-temporal variations of Earth's gravitational attraction on a test mass in space, namely the GRACE spacecraft. From these observations they can derive surface mass-variation patterns.

The satellite data clearly reveal that the Greenland area exhibits the main dominant mass shrinkage over the whole globe. It is predominantly caused by the persistent melting of the Greenland glaciers. Presently, the Arctic island loses between 165 and 189 cubic kilometres ice a year. This estimate is considerably higher than the results derived from geometric satellite measurements conducted in the 1990s.

Deglaciation causes melt water influx into the oceans. Furthermore, globally increasing atmospheric temperatures involve thermal expansion of seawater. Both effects substantially contribute to sea-level rise. Most notably, contrary to common reasoning, sea level does not change uniformly -- that is, in terms of a constant layer over the world's oceans. In fact, the global redistribution of masses causes the sea level to vary differently from location to location. As a basic principle, ice melting in the Northern Hemisphere translates into sea-level rise in the Southern oceans. On the other hand, deglaciation over the Antarctica causes sea-level rise in the Northern oceans.

Such effects are alarming, especially considering the fact that millions of people settle down in coastal and near-coastal areas. Based on current findings, extrapolations to the end of the 21st century forecast the Greenland ablation impact on mean sea-level rise to be in the range of five centimetres. This estimate is highly conservative, neglecting both accelerated deglaciation and mass balance over Antarctica, Alaska and further ice-covered regions. Taking progression effects into account, potential sea level rise of 50 centimetres within the next 100 years becomes realistic. Continuing satellite measurements will manifest the reliability of short-, medium- and long-term predictions.

Link:  http://www.sciencedaily.com/releases/2009/12/091209194242.htm

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Richard Black of the BBC: East Antarctic ice sheet may be losing mass

East Antarctic ice sheet may be losing mass

by Richard Black, Environment correspondent, BBC News website, November 22, 2009

Antarctic coast
The mass loss is probably driven by processes occurring on the coast

The East Antarctic ice sheet has been losing mass for the last three years, according to an analysis of data from a gravity-measuring satellite mission.

The scientists involved say they are "surprised" by the finding, because the giant East Antarctic sheet, unlike the west, has been thought to be stable.

Other scientists say ice loss could not yet be pinned on climate change, and uncertainties in the data are large.
The US-based team reports its findings in the journal Nature Geoscience.

The data comes from NASA's Gravity Recovery and Climate Experiment (GRACE) mission.
It energises me as a scientist, but I'm not convinced that as yet it should energise anyone else
Professor Richard Alley
GRACE has previously shown that the smaller West Antarctic and Greenland ice sheets are losing mass.

These two bodies of ice contain enough water to raise sea levels by about 6-7 m (20 ft.) each if they melted completely.

Melting the East Antarctic sheet would raise sea levels by much more -- about 50-60 m.

But scientists have generally discounted the possibility of it happening because the region is so cold.

The GRACE measurements suggest there was no net ice loss between 2002 and 2006.
Antarctica (BBC)

But since then, East Antarctica has been losing 57 billion tonnes (Gt) per year.

"We felt surprised to see this change in East Antarctica," study leader Jianli Chen from the Centre for Space Research at the University of Texas in Austin told BBC News.

The loss still looks small by contrast with West Antarctica, which is losing 132 Gt per year, and with Greenland, where a recent analysis combining GRACE data with other measurements indicated an annual figure of 273 Gt.

Previous GRACE analyses -- and those from other satellites -- had given an inconclusive picture for the giant ice body.

The twin GRACE satellites fly in close formation, detecting minute changes in the Earth's gravity through the marginal changes this causes in their relative positions.

Eastern energy
Measuring Antarctic ice loss is a tricky issue because the continent itself is rising and deforming.
East Antarctic ice - graphic
Its ice cover was significantly thicker during the last Ice Age; as the ice melted, the weight pressing down on the rock abated, and the rock is "isostatically rebounding."

Readings from satellite missions have to be adjusted to allow for this rebound -- and that is one source of uncertainty when trying to assess the significance of the new research, according to Richard Alley, one of the world's leading glaciologists.

"The first thing is that lots of this is dependent on the isostatic [rebound] model, and (recent work has) cast some doubt on the istostatic models that people are using," commented the Penn State University researcher (who was not involved in the paper).

"And then you get into the age-old question of 'is it climate or is it weather?"

"So it energises me as a scientist, but I'm not convinced that as yet it should energise anyone else."

Rising potential
The GRACE data gives a picture of where ice is being lost across the continent; and these areas are mainly on the coast.

It is not clear what physical processes could be driving any loss of mass here, although it is not simply melting due to high air temperatures, because temperatures are well below zero.

One clue could lie in research published last year by Leigh Stearns and colleagues, showing that lakes under the ice sheet can periodically overflow, with the liquid water then acting as a lubricant to speed glaciers on their way towards the sea.
Artist's impression of Grace satellite in orbit
The Grace satellites provide a twin eye on gravity at the Earth's surface

Commenting on the new research, Dr Stearns told BBC News: "In these coastal regions the ice loss could be driven by some interaction with the oceans or some weather patterns, or it could be a sub-glacial lake that drained and caused some thinning -- so it might not be climate-related.

"It's easy to jump to the conclusion that it's exceptional because it's the first time we've recorded it, but we do need a baseline of how things have been in the past so we do need to be cautious," said the University of Kansas researcher.

"Nevertheless, it awakens us to the fact that the East Antarctic sheet is more dynamic than we thought, and we do need to pay attention to it because its potential for sea level rise is so much greater than in West Antarctica or Greenland."

Dr Chen said that one of his team was currently conducting airborne surveys of one of the regions where mass loss had been detected, hoping to shed some light on the mechanisms involved.

Link:  http://news.bbc.co.uk/2/hi/science/nature/8371773.stm

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J. L. Chen, C. R. Wilson, D. Blankenship & B. D. Tapley, Nature Geosci., Accelerated Antarctic ice loss from satellite gravity measurements

Nature Geoscience, published online 22 November 2009; doi: 10.1038/ngeo694 

Accelerated Antarctic ice loss from satellite gravity measurements

J. L. Chen* (Center for Space Research, University of Texas at Austin, Austin, TX 78759, U.S.A.), C. R. Wilson (Center for Space Research, University of Texas at Austin, Austin, TX 78759; Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712, U.S.A.), D. Blankenship (Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712, U.S.A.) and B. D. Tapley (Center for Space Research, University of Texas at Austin, Austin, TX 78759, U.S.A.)

Abstract

Accurate quantification of Antarctic ice-sheet mass balance and its contribution to global sea-level rise remains challenging, because in situ measurements over both space and time are sparse. Satellite remote-sensing data of ice elevations and ice motion show significant ice loss in the range of -31 to -196 Gt yr-1 in West Antarctica in recent years1, 2, 3, 4, whereas East Antarctica seems to remain in balance or slightly gain mass1, 2, 4, with estimated rates of mass change in the range of -4 to 22 Gt yr-1. The Gravity Recovery and Climate Experiment5 (GRACE) offers the opportunity of quantifying polar ice-sheet mass balance from a different perspective6, 7. Here we use an extended record of GRACE data spanning the period April 2002 to January 2009 to quantify the rates of Antarctic ice loss. In agreement with an independent earlier assessment4, we estimate a total loss of 190plusminus77 Gt yr-1, with 132plusminus26 Gt yr-1 coming from West Antarctica. However, in contrast with previous GRACE estimates, our data suggest that East Antarctica is losing mass, mostly in coastal regions, at a rate of -57plusminus52 Gt yr-1, apparently caused by increased ice loss since the year 2006.

*Correspondence e-mail: chen@csr.utexas.edu

Link to abstract:  http://www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo694.html

Hat tip: Paolo Morelli

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BBC's Richard Black: Greenland ice loss 'accelerating'

Greenland ice loss 'accelerating'

by Richard Black, Environment correspondent, BBC News website, November 12, 2009

Ilulissat glacier (Image: BBC)
The Ilulissat glacier has retreated by approximately 15 km over the past decade

The Greenland ice sheet is losing its mass faster than in previous years and making an increasing contribution to sea level rise, a study has confirmed.

Published in the journal Science, it has also given scientists a clearer view of why the sheet is shrinking.

The team used weather data, satellite readings and models of ice sheet behaviour to analyse the annual loss of 273 thousand million tonnes of ice.

Melting of the entire sheet would raise sea levels globally by about 7 m (20 ft).

For the period 2000-2008, melting Greenland ice raised sea levels by an average of about 0.46 mm per year.
If you multiply these numbers up it puts us well beyond the IPCC estimates for 2100
Professor Roger Barry

Since 2006, that has increased to 0.75 mm per year.

"Since 2000, there's clearly been an accelerating loss of mass [from the ice sheet]," said lead researcher Michiel van den Broeke from Utrecht University in the Netherlands.

"But we've had three very warm summers, and that's enhanced the melt considerably.

"If this is going to continue, I cannot tell -- but we do of course expect the climate to become warmer in the future."

In total, sea levels are rising by about 3 mm per year, principally because seawater is expanding as it warms.

Sea change
Changes to the Greenland sheet and its much larger counterpart in Antarctica are subjects commanding a lot of interest within the scientific community because of the potential they have to raise sea levels to an extent that would flood many of the world's major cities.

The 2007 Intergovernmental Panel on Climate Change (IPCC) report projected a sea level rise of 28-43 cm during this century.

But it acknowledged this was almost certainly an underestimate because understanding of how ice behaves was not good enough to make reliable projections.

By combining different sources of data in the way it has, and by quantifying the causes of mass loss, the new study has taken a big step forwards, according to Roger Barry, director of the World Data Center for Glaciology at the University of Colorado in Boulder, CO, U.S.A.

"I think it's a very significant paper; the results in it are certainly very significant and new," he said.

"It does show that the [ice loss] trend has accelerated, and the reported contribution to sea level rise also shows a significant acceleration - so if you multiply these numbers up it puts us well beyond the IPCC estimates for 2100."

Professor Barry was an editor on the section of the IPCC report dealing with the polar regions.

On reflection
An ice sheet can lose mass because of increased melting on the surface, because glaciers flow more quickly into the ocean, or because there is less precipitation in the winter so less bulk is added inland.

The new research shows that in Greenland, about half the loss comes from faster flow to the oceans, and the other half from changes on the ice sheet itself -- principally surface melting.
Artist's impression of Grace satellite in orbit
The Grace satellites provide a twin eye on gravity at the Earth's surface

Another analysis of satellite data, published in September, showed that of 111 fast-moving Greenland glaciers studied, 81 were thinning at twice the rate of the slow-moving ice beside them.

This indicates that the glaciers are accelerating and taking more ice into the surrounding sea.

Melting on the ice sheet's surface acts as a feedback mechanism, Dr van den Broeke explained, because the liquid water absorbs more and reflects less of the incoming solar radiation - resulting in a heating of the ice.

"Over the last 10 years, it's quite simple; warming over Greenland has caused the melting to increase, and that's set off this albedo feedback process," he told BBC News.

"Quite likely the oceans have also warmed, and it's likely that explains the [acceleration of] outlet glaciers because they're warmed from below."

Data provided over just the last few years by the Grace satellite mission -- used in this study -- is giving researchers a closer view of regional variations across the territory.

Grace's twin satellites map gravity at the Earth's surface in unprecedented detail; and it is now possible to tease out from the data that most of the mass is being lost in the southeast, southwest and northwest at low elevations where the air will generally be warmer than at high altitudes.

Professor Barry cautioned that the Grace mission, which has produced valuable data about Antarctica as well as Greenland, has only a further two years to run, and that no replacement is currently scheduled.

e-mail:  Richard.Black-INTERNET@bbc.co.uk

Link to article:  http://news.bbc.co.uk/2/hi/science/nature/8357537.stm

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Michiel van den Broeke et al., Science 326, Partitioning Recent Greenland Mass Loss

Science (13 November 2009): Vol. 326, No. 5955, pp. 984-986; DOI: 10.1126/science.1178176
 

Reports

Partitioning Recent Greenland Mass Loss

Michiel van den Broeke,1,* Jonathan Bamber,2 Janneke Ettema,1 Eric Rignot,3,4 Ernst Schrama,5 Willem Jan van de Berg,1 Erik van Meijgaard,6 Isabella Velicogna,3,4 and Bert Wouters5,6

Abstract

Mass budget calculations, validated with satellite gravity observations [from the Gravity Recovery and Climate Experiment (GRACE) satellites], enable us to quantify the individual components of recent Greenland mass loss. The total 2000–2008 mass loss of ~1500 gigatons, equivalent to 0.46 mm per year of global sea level rise, is equally split between surface processes (runoff and precipitation) and ice dynamics. Without the moderating effects of increased snowfall and refreezing, post-1996 Greenland ice sheet mass losses would have been 100% higher. Since 2006, high summer melt rates have increased Greenland ice sheet mass loss to 273 gigatons per year (0.75 mm per year of equivalent sea level rise). The seasonal cycle in surface mass balance fully accounts for detrended GRACE mass variations, confirming insignificant subannual variation in ice sheet discharge.


1 Institute for Marine and Atmospheric Research, Utrecht University, Netherlands
2 Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol, U.K.
3 Department of Earth System Science, University of California, Irvine, CA, U.S.A.
4 Jet Propulsion Laboratory, Pasadena, CA, U.S.A.
5 Delft Institute of Earth Observation and Space Systems, Delft University of Technology, Delft, Netherlands
6 Royal Netherlands Meteorological Institute, De Bilt, Netherlands

*Correspondence e-mail: m.r.vandenbroeke@uu.nl

Link to abstract:  http://www.sciencemag.org/cgi/content/abstract/sci;326/5955/984

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Andy Reisinger: Polar ice keeps melting – at a faster and faster rate

Polar ice keeps melting – at a faster and faster rate


by Andy Reisinger, degrees of change blog, October 12, 2009

A key rea­son for con­cern about cli­mate change is that it could lead to grad­ual melt­ing of the polar ice sheets in Green­land and Antarc­tica, result­ing in an inex­orable rise in global sea lev­els. A new set of high-precision mea­sure­ments (paper in press with the jour­nal ‘Geo­phys­i­cal Research Let­ters’) now con­firms that the ice sheets are not only melt­ing but that their melt rate has more than dou­bled over the past seven years. This sug­gests that sea lev­els could rise a lot faster than indi­cated in ear­lier stud­ies. Increases in sea level by more than 1m dur­ing the 21st cen­tury now have to be seri­ously considered.

Numer­ous stud­ies since the early 2000s indi­cated that sev­eral glac­i­ers that drain the polar ice sheets into the ocean have accel­er­ated as a result of local warm­ing. These obser­va­tions trig­gered con­cerns that the polar ice sheets could loose ice more quickly and sea level could rise more rapidly than cli­mate mod­els had sug­gested. But it was unclear how rep­re­sen­ta­tive these rather strik­ing changes were – was it just a tem­po­rary rip­ple run­ning through some glac­i­ers or were the entire ice sheets loos­ing ice at an increas­ing rate?

High-precision mea­sure­ments of the change in the total mass of the polar ice sheets, using a pair of orbit­ing satel­lites (the Grav­ity Recov­ery and Cli­mate Exper­i­ment), have now revealed that both ice sheets are indeed loos­ing mass, and at an accel­er­at­ing pace. The satel­lite data cap­ture the changes in total ice mass and hence give a truly com­pre­hen­sive pic­ture of changes in the polar ice sheets rather than only spot mea­sure­ments for par­tic­u­lar glaciers.

The assess­ment by the Inter­gov­ern­men­tal Panel on Cli­mate Change (IPCC) pub­lished in 2007 found that over the period 1993 to 2003, Green­land and Antarc­tica had both lost about 75 bil­lion tonnes of ice every year. If this loss of ice con­tin­ued at that rate for a cen­tury, this would result in sea level rise of 4.2cm. The new satel­lite data now reveal that by 2002/2003, the Green­land ice sheet was already loos­ing as much as 137 bil­lion tonnes of ice per year, and that this rate more than dou­bled again to 286 bil­lion tonnes of ice per year dur­ing 2007–2009. Mean­while, Antarctica’s ice loss had increased from 75 to 104 bil­lion tonnes per year dur­ing 2002–2006, and more than dou­bled to 246 bil­lion tonnes per year by 2006–2009. These most recent rates of ice loss would raise global sea lev­els by 15cm if they con­tin­ued at cur­rent rates over a century.

The trou­ble is of course that there is lit­tle rea­son to assume that the loss of ice will con­tinue only at cur­rent rates while the atmos­phere and oceans con­tinue to warm. It would be extremely sur­pris­ing if the observed melt­ing processes did not increase fur­ther, imply­ing a sig­nif­i­cant risk that sea lev­els would increase by more – and poten­tially a lot more – than the 0.6m that the IPCC had pro­jected for the 21st century.

Unfor­tu­nately, we still do not suf­fi­ciently under­stand all the processes that take place within and under­neath the polar glac­i­ers to under­stand which aspects of warm­ing are respon­si­ble for this accel­er­a­tion. Can­di­dates for the accel­er­ated ice loss include warm­ing of the atmos­phere, increases in local ocean tem­per­a­tures, and the cre­ation of melt­wa­ter on top of the ice sheet that is drain­ing into its base and acts as lubri­cant for the glac­ier flow. Until those processes are bet­ter under­stood, extrap­o­la­tions of the recent trends have to be treated with caution.

How­ever, these recent data are con­sis­tent with a raft of other obser­va­tions and model stud­ies that sug­gest that we now have to seri­ously con­sider that sea lev­els could rise by more than 1m dur­ing the 21st cen­tury as a result of cli­mate change. A report ear­lier this year by Pro­fes­sor Will Stef­fen at ANU for the Aus­tralian Depart­ment of Cli­mate Change advised that sea level rise could lie some­where between 0.5 m and per­haps up to 1.5 m by 2100. It’ll be a busy time for car­tog­ra­phers as we are in the process of re-designing the world’s coastlines.

Link to blog post:  http://sciblogs.co.nz/degrees-of-change/2009/10/12/polar-ice-keeps-melting-%E2%80%93-at-a-faster-and-faster-rate/

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I. Velicogna, GRL 36, Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE

Geophysical Research Letters, 36 (2009) L19503; doi: 10.1029/2009GL040222. 

Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE

I. Velicogna (Department of Earth System Science, University of California, Irvine, CA; and Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, U.S.A.)

Received 28 July 2009; accepted 3 September 2009; published 13 October 2009.

Abstract

We use monthly measurements of time-variable gravity from the GRACE (Gravity Recovery and Climate Experiment) satellite gravity mission to determine the ice mass-loss for the Greenland and Antarctic Ice Sheets during the period between April 2002 and February 2009. We find that during this time period the mass loss of the ice sheets is not a constant, but accelerating with time, i.e., that the GRACE observations are better represented by a quadratic trend than by a linear one, implying that the ice sheets contribution to sea level becomes larger with time. In Greenland, the mass loss increased from 137 Gt/yr in 2002–2003 to 286 Gt/yr in 2007–2009, i.e., an acceleration of −30 ± 11 Gt/yr2 in 2002–2009. In Antarctica, the mass loss increased from 104 Gt/yr in 2002–2006 to 246 Gt/yr in 2006–2009, i.e., an acceleration of −26 ± 14 Gt/yr2 in 2002–2009. The observed acceleration in ice sheet mass loss helps reconcile GRACE ice mass estimates obtained for different time periods.

Velicogna, I. (2009). Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE, Geophysical Research Letters, 36, L19503; doi: 10.1029/2009GL040222.

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V.M. Tiwari, J. Wahr, S. Swensen, GRL 2009: Dwindling groundwater resources in northern India, from satellite gravity observations

Geophysical Research Letters, 36 (2009) L18401; doi: 10.1029/2009GL039401.

Dwindling groundwater resources in northern India, from satellite gravity observations

V. M. Tiwari (National Geophysical Research Institute, CSIR, Hyderabad, India; Department of Physics, University of Colorado, Boulder, CO, U.S.A.), J. Wahr (Department of Physics and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, U.S.A.), and S. Swenson (Advanced Study Program, National Center for Atmospheric Research, Boulder, CO, U.S.A.)

Received 2 June 2009; accepted 28 July 2009; published 17 September 2009. 

Abstract

Northern India and its surroundings, home to roughly 600 million people, is probably the most heavily irrigated region in the world. Temporal changes in Earth's gravity field in this region as recorded by the GRACE satellite mission, reveal a steady, large-scale mass loss that we attribute to excessive extraction of groundwater. Combining the GRACE data with hydrological models to remove natural variability, we conclude the region lost groundwater at a rate of 54 ± 9 km3/yr between April, 2002 (the start of the GRACE mission) and June, 2008. This is probably the largest rate of groundwater loss in any comparable-sized region on Earth. Its likely contribution to sea level rise is roughly equivalent to that from melting Alaskan glaciers. This trend, if sustained, will lead to a major water crisis in this region when this non-renewable resource is exhausted.

Tiwari, V. M., J. Wahr, & S. Swenson (2009), Dwindling groundwater resources in northern India, from satellite gravity observations, Geophys. Res. Lett., 36, L18401; doi: 10.1029/2009GL039401.

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James Famiglietti, GRACE: India's groundwater disappearing at alarming rate

India's groundwater disappearing at alarming rate

by Richard A. Kerr, ScienceNOW Daily News, 10 August 2009

Picture of water

Going down. Several centimeters' worth of water (pink) disappears each year from beneath the northern Indian subcontinent. Credit: Adapted from V. M. Tiwari et al., National Geophysical Research Institute.

Farming is a thirsty business on the Indian subcontinent. But how thirsty, exactly? For the first time, satellite remote sensing of a 2000-kilometer swath running from eastern Pakistan across northern India and into Bangladesh has put a solid number on how quickly the region is depleting its groundwater. The number "is big," says hydrologist James Famiglietti of the University of California, Irvine--big as in 54 cubic kilometers of groundwater lost per year from the world's most intensively irrigated region hosting 600 million people. "I don't think anybody knew how quickly it was being depleted over that large an area."

The big picture of Indian groundwater comes from the Gravity Recovery and Climate Experiment (GRACE) satellite mission, launched in March 2002 as a joint effort by the U.S. National Aeronautics and Space Administration and the German Aerospace Center. Actually two satellites orbiting in tandem 220 kilometers apart, GRACE measures subtle variations in the pull of Earth's gravity by using microwaves to precisely gauge the changing distance between the two spacecraft.

As the lead spacecraft passes over a patch of anomalously strong gravity, it accelerates ahead of the trailing spacecraft. Once past the anomaly, the lead satellite slows back down. Then the trailing spacecraft accelerates and again closes on the leader. By making repeated passes over the same spot, GRACE measures changes in Earth's gravity, which are mainly due to water moving on and under the surface. Most famously, GRACE has recorded the shrinking of ice sheets; it has also detected shifting ocean currents, the desiccation of droughts, and the draining of large lakes.

Outside of wasting ice sheets, the world's largest broad-scale decline in gravity during GRACE's first 6 years came across a 2.7-million-square-kilometer, east-west swath centered on New Delhi. That's according to a study in press in Geophysical Research Letters by geophysicists Virendra Tiwari of the National Geophysical Research Institute in Hyderabad, India; John Wahr of the University of Colorado, Boulder; and Sean Swenson of the National Center for Atmospheric Research in Boulder. Adjusted for natural variations due to changing precipitation and evaporation, the decline in gravity that GRACE determined equates to a net loss of 54 plus or minus 9 cubic kilometers of groundwater per year, the group reports. That would produce a fall in the water table of about 10 centimeters per year averaged over the entire region.

A falling water table across the northern Indian subcontinent comes as no great surprise. The GRACE region of sharp groundwater depletion coincides with the world's most intensely irrigated land: Fifty percent to more than 75% of the land is equipped for irrigation with pumped groundwater or reservoir water. And then there are those 600 million people drawing heavily on groundwater. But, the group calculates, the GRACE-determined depletion rate implies that groundwater was being pumped out 70% faster in this decade than the Central Ground Water Board of India estimated it was in the mid-1990s. The apparent surge in withdrawal would have been large enough to turn a once-stable water table into a falling one that demands ever-deeper wells and bigger pumps and may draw in salty or polluted water.

GRACE "has shown us we can do a pretty reasonable job from space" gauging groundwater depletion, says Famiglietti. "We can help regional water managers by giving them a holistic view of a whole system." Still, across the subcontinent, no one knows how far down the water goes. They just know, as Famiglietti notes, that "it's not bottomless."

Link: http://sciencenow.sciencemag.org/cgi/content/full/2009/810/1

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Gravity Anomaly Maps and The Geoid

Gravity Recover and Climate Experiment (GRACE)

Gravity Anomaly Maps and The Geoid

From NASA's Earth Observatory:

The Earth’s gravity field is depicted in two principal ways: gravity anomaly maps and maps of the Earth’s geoid.

Gravity anomaly maps (see globe below) show how much the Earth’s actual gravity field differs from the gravity field of a uniform, featureless Earth surface. The anomalies highlight variations in the strength of the gravitational force over the surface of the Earth. Gravity anomalies are often due to unusual concentrations of mass in a region. For example, the presence of mountain ranges will usually cause the gravitational force to be more than it would be on a featureless planet — positive gravity anomaly. Conversely, the presence of ocean trenches or even the depression of the landmass that was caused by the presence of glaciers millennia ago can cause negative gravity anomalies.

Earth's gravity field as seen by GRACE

These “gravity anomaly” maps show where models of the Earth’s gravity field based on GRACE data differ from a simplified mathematical model that assumes the Earth is perfectly smooth and featureless. Areas colored yellow, orange, or red are areas where the actual gravity field is larger than the featureless-Earth model predicts—such as the Himalayan Mountains in Central Asia (top left of the left-hand globe)—while the progressively darker shades of blue indicate places where the gravity field is less—such as the area around Hudson Bay in Canada (top center of right-hand globe).

The geoid is a hypothetical Earth surface that represents the mean sea level in the absence of winds, currents, and most tides. The geoid is a useful reference surface. It defines the horizontal everywhere and gravity acts perpendicular to it. A carpenter’s level aligns itself along the geoid and a carpenter’s plumb bob points down the vertical or perpendicular to the geoid. Water will not flow in aqueducts if the pipes are perfectly aligned along the geoid. Surveyors use knowledge of the geoid and the horizontal when they lay out highways and boundaries.

Producing a precise model of the geoid has proven to be a challenge. Until recently, there was no single source for producing a geoid map. Data from several dozen satellites, along with surface measurements over land and from ships at sea, had to be combined to produce a model of the gravitational field. Traditionally, the models have done a fairly good job reproducing large-scale features of the gravity field, but have fallen short when it comes to reproducing finer-scale features or accurately describing time-variable gravity effects like those associated with the hydrologic cycle.

GRACE provides, for the first time, global coverage of the Earth’s gravity field every 30 days from a single source. GRACE is already able to measure the gravity field with a level of precision that is at least 100 times greater than any existing measurement, and continued improvements are expected as the mission progresses. The finer details of the geoid that have evaded scientists for so long are on the verge of being revealed. GRACE also gives us our best opportunity to date to study time-variable gravity effects. As the mission progresses and more data are added to the model, the resolution of the geoid will improve even further.

The Earth’s gravity signal changes day-to-day, even minute by minute. The image above shows how the average variability in Earth’s gravity field in August 2002 compared to the yearly average of 2001. The red and pink areas show where the variation measured in August 2002 is the most different from the variation measured for the year 2001, while the blue and purple areas show where the variation measured in August 2002 is just about the same as the variation measured for the year 2001. The variability has to be accounted for using models in order to produce a mean gravity field that is useful for hydrologic applications. (Image credit: Paul Thompson / UT-CSR)

As the geoid map becomes more detailed, the accuracy of satellite altimetry, synthetic aperture radar interferometry, and digital terrain models covering large land and ice areas — all used in remote sensing applications and cartography — will improve. These techniques provide critical input to many scientific models used in oceanography, hydrology, geology, and related disciplines, and will be used for a variety of applications including:

  • measuring the changing mass of polar ice caps
  • measuring changes in water resources on land
  • understanding shallow and deep ocean current transport
  • understanding sea level change resulting from ocean temperature and water mass changes
  • understanding atmosphere-ocean mass exchange
  • understanding the forces that generate Earth’s geomagnetic field, and
  • understanding internal Earth forces that move tectonic plates and result in earthquakes and volcanic eruptions.

This enhanced knowledge should lead to a better understanding of the forces that drive El Niño and La Niña, more accurate seasonal forecasts of Earth’s weather patterns, an ability to track the changing distribution of water resources in critically important land aquifers, and improved forecasting of natural hazards.





Link to NASA's Earth Observatory page: http://earthobservatory.nasa.gov/Features/GRACE/page3.php

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AGU Fall 2008 Meeting: Greenland ice sheet mass-balance, outlet glaciers, ablation zones, Jakobshavn Isbrae, Helheim Glacier, GRACE, MODIS, etc.

AGU Fall 2008 Meeting: Greenland ice sheet mass-balance, outlet glaciers, ablation zones, Jakobshavn Isbrae, Helheim Glacier, GRACE, MODIS, etc.

Dear Readers,

I have posted about 25 abstracts from the American Geophysical Union's Fall 2008 meeting. These concern the topics mentioned in the title to this post.

You can read this collection of abstracts here. (I posted them on a separate blog in the interests of saving space in this one.)

Or, you can go to AGU's website and put in your own search terms to pull up abstracts -- I simply used "Greenland."

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Jason-1 and GRACE satellite data show accelerated melting of continental icepacks is major reason for rise in sea level between 2003 And 2008

Accelerated melting of continental icepacks is major reason for rise in sea level between 2003 and 2008

ScienceDaily, December 2, 2008 — Researchers at the Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (1) (CNRS/Université Toulouse 3/CNES/IRD) and at a subsidiary of CNES (CLS) (2) have discovered that the accelerated melting of continental icepacks is the major reason for the rise in sea level over the 2003 to 2008 period, something which has minimized the effect of thermal expansion of seawater.

This question was resolved thanks to data from the French-American Satellite Jason-1, from two satellites of the GRACE space gravimetry mission and from the buoys of the Argo system. These results have been published online on the website of the journal Global and Planetary Change.

Between 1993 and 2003, the global mean sea level, measured very accurately by the French-American Topex/Poséidon satellites and their successor Jason-1, showed a relatively constant progression of 3 mm/yr. The last GIEC report, published in 2007, showed that more than half of this rise (approximately 1.5 mm/yr) was due to sea water expansion as it warmed up (steric contribution), while 1.2 mm/yr resulted from the reduction in mass of polar ice sheets and mountain glaciers. Since 2003 however, the situation has changed; a quite rapid rise (2.5 mm/yr) in sea water levels is still observed but, over the same period, the warming of the oceans is showing a plateau, only accounting for a rise of 0.4 mm/yr.

Thermal expansion was calculated using two independent methods:

  • The Argo network of buoys transmits water temperature and salinity profiles across all of the world's oceans. Since 2003, the analysis of all relevant data in the topmost 900 meters of sea water resulted in a steric contribution of about 0.4 mm/yr.
  • This value was independently confirmed by measurements from space by calculating the difference between the water level observed by the altimeters on Topex/Poséidon and Jason-1 and the increase in the ocean volume as witnessed by GRACE. The satellites indicate a steric contribution of 0.3 mm/yr, which is very similar to the value from the Argo buoys.

Consequently, it is above all the increase in the mass of sea water rather than its heat content that is behind the rise in sea level that has been observed since 2003. The increase in the mass of the oceans is equivalent to a rise of 1.9 mm/yr of the mean sea level. What is the source of this extra water in the oceans? Melting continental ice sheets. Data from GRACE has made it possible to measure changes in the mass of the two polar ice sheets in Antarctica and Greenland. These were responsible for a 1 mm/yr increase in sea level (i.e. twice as much as in the previous decade). For mountain glaciers, the most recent estimates from glaciologists show a contribution of 1.1 mm/yr (also higher than during previous years).

Thus, losses from glacial masses can easily account for why the mass of sea water is increasing and are responsible for 80 % of the average rise in sea level in recent years. Given the accelerated melting of glaciers and polar ice sheets, if the steric contribution returned to the values of the 1990s, a rise in sea level of around 4 mm/yr could not be excluded.

Journal reference:

  1. Cazenave et al., Sea level budget over 2003-2008: A reevaluation from GRACE space gravimetry, satellite altimetry and Argo. Global and Planetary Change, 2008; DOI: 10.1016/j.gloplacha.2008.10.004
Adapted from materials provided by CNRS.

Link to article: http://www.sciencedaily.com/releases/2008/11/081129094609.htm