SENAT
Report n° 230 (2006-2007) by M. Christian GAUDIN, Senator (for the parliament office for the evaluation of scientific and technological choices)
Disponible au format Acrobat (12 Moctets)
II. THE POLES: THEIR KEY ROLE IN UNDERSTANDING CLIMATE CHANGE
Climate change has become a major scientific and political concern, but few are aware of the extent to which polar research has contributed and will continue to contribute in the coming years to predicting - and perhaps preventing - climate change. There are three major domains: ice coring, oceanography and the dynamics of the large frozen zones (the North Pole, Greenland and Antarctica).
A. UNDERSTANDING PAST CLIMATES TO UNDERSTAND THE FUTURE CLIMATE
The great inlandsis of Greenland and Antarctica are extraordinary in that they constitute climatic archives.
Fallen snow accumulates and slowly turns into ice due to the temperature and the snow's ever greater weight, because it becomes increasingly dense. During this process, it definitively traps a few air particles and dust from the surrounding environment. In this manner, invaluable information on the world's climate can be found in the form of successive layers of ice. The "seal" is never broken, because the temperatures are always several tens of degrees below zero.
In Antarctica, this sealing takes place at a depth of 100 m, when the density reaches 0.84. When the accumulation is very low, the snow takes a few thousand years to turn into ice and the imprisoned air is therefore younger than the ice. At Vostok, this difference in age has been estimated at 4 to 7 thousand years.
To analyze the ice samples, it is therefore necessary to carry out advanced studies on the photochemistry of the layers of snow, on the chemistry of the snow and on the trapped particles, as well as on the metamorphism of the snow, because over time a transformation occurs which disturbs the signal that is then detected in the ice.
The ice is stratified, with winter and summer layers. In the summer, the layers are less dense, because the particles are larger due to warmer temperatures. The winter layers are denser and are sometimes crusted due to the wind.
These are the first elements which allow for an analysis of the superficial zone and to discover the meteorological conditions of past years.
Over time, all of this ice moves from the top to the bottom of the icecap and from the centre to the coasts.
The speed of this vertical outflow is a few centimeters per year at the surface, depending on precipitation. This speed can vary considerably from site to site. For instance, in Antarctica, even though the two sites are less than 600 km apart, the Vostok ice core - even though its longer (3,650 m) than that of Concordia (3,270) - only goes back 420,000 years, compared to more than 800,000 years for the Concordia core.
This movement is also very slow horizontally: less than 1 m per year on the Plateau and some one hundred metres for the outgoing glaciers. This movement results in a progressive thinning of the successive layers. This distortion can be modeled to enable dating. The weakness of the movements is essential for a long, undisturbed timeline.
In total, the fallen snow in the centre of the continent can take several hundred thousand years to reach the coast. It is this process that makes the Greenlandic and Antarctic icecaps archives of the world's climate.
The science of ice analysis first appeared in the 1950s. The French scientist Claude Lorius explains how the idea came to him to analyze the air bubbles contained in the ice by watching an ice cube in a glass of whiskey: "By observing the bursting [of the air bubbles] when an ice cube melted in a glass of whiskey, I suspected they might represent unique, reliable witnesses of the atmospheric composition, which we then went on to prove over the coming years."
The first deep core samples were taken in Greenland at Camp Century in 1966 and in Antarctica at Byrd in 1968 and Vostok starting in 1970. The first deep sampling (900 m) by the French was undertaken on Dome C in 1978, where the first glaciology studies had been carried out in 1974.
1. Recent ice cores from Greenland
A European programme, the Greenland Ice Core Project (GRIP), carried out a sampling at the top of the inlandsis in 1989; its objective was to sample the glacier's entire depth (3,027 m). A similar American project, the Greenland Ice Sheet Project 2 (GISP 2), produced an ice sample of 3,053 m and a bedrock core of 1.55 m 28 km from the European site. These samplings allowed scientists to reconstruct the climate of the past 105,000 years, beyond which time the samples proved to be of lesser quality.
A new sampling series - this time under Danish direction and with European, American and Japanese partners - was carried out 300 km farther north (North GRIP). The sampling began in 1996 and the bedrock was reached in 2003. The base was closed in 2004.
The recently published results (10 June 2004 in Nature ) show that this 3,085 m sample, the deepest made in this region, goes back farther than the last ice age (115,000 years) . At that time, Greenland's climate was warm and stable.
The cores taken from Greenland will therefore allow scientists to reconstruct the entire climate cycle starting from the last warm period similar to our own.
2. Ice cores from Antarctica
The glacial ice cores from Antarctica, which go back 850,000 years, had a huge impact on our understanding of the world's climate.
The ice cores make it possible to reconstitute :
- The temperature , by using the "isotopic thermometer": in other words, different atoms present in the ice and, in particular, the variations between the isotopes 16 and 18 of oxygen.
- The composition of the atmosphere ; in particular, the presence of greenhouse gases (methane and carbon dioxide) by analyzing air bubbles.
- The atmospheric circulation , by analyzing dust particles present in the ice.
- Inter-seasonal variations, by analyzing salinity. In the winter, the ice's salinity is a tenth of its salinity in the summer, because the winter ice shelf doubles the total surface area of the Antarctic and the icecap is distanced considerably from the sea. It allows dating over some one hundred years.
Of course, the scientists must date these layers of ice. They do this, particularly for the more recent period, by referring to volcanic eruptions and nuclear tests. Because these important events affected the entire planet, they allow scientists - when the events are well known - to precisely date a given layer and serve as reference points.
This is the case with important volcanic irruptions: Laki (1783-1784), Krakatoa (1883), Santamaria (1903), Agung (1963) and Pinatubo (1991).
The same is true for atmospheric nuclear tests from the 1940s to the 1970s.
While these events don't allow for dating in the distant past, they allow scientists to evaluate the accumulation rate of snow and the burial speed, and then to extrapolate for older periods, at least during the Holocene.
The researching of these various elements is shared out within the "Ice Cores - France" group, created in 2005, which allows for the specialization of the four associated laboratories:
- LGGE: gas, dust, chemistry, rheology, heavy metals, dating and geophysics;
- LSCE: water isotopes, permanent gas isotopes;
- CEREGE: beryllium-10;
- EPOC: krypton-81.
This work allows scientists to reconstruct the fundamental elements of climate. The most well-known tool is the relative abundance of oxygen isotopes. Isotopes are different forms of the same atom, varying according to the number of neutrons they contain. Oxygen has two main isotopes: oxygen 16 (8 neutrons) and oxygen 18 (10 neutrons). However, their average abundance in water (99.76% for oxygen 16 and 0.2% for oxygen 18) varies according to the ambient temperature. This was demonstrated by Willi Dansgaard following a storm in Copenhagen on 22 July 1952.
As the temperature cools, oxygen 18 decreases. This fundamental principle is the same for the other isotopes: identification and interpretation of variations according to the average natural abundance.
They increasingly help explain climate changes. Beryllium 10 should allow scientists to reconstruct insolation variations that are believed to be the cause of natural climate changes. Sulphur 33 will explain the influence of large volcanic irruptions (LGGE - the University of San Diego, Science , 5 January 2007).
· The results from the Vostok ice core
The results provided by the Vostok core have allowed for spectacular scientific progress to be made. They were obtained for the most part by French teams from the LGGE (Laboratory of Glaciology and Environmental Geophysics) led by Claude Lorius and the LSCE-CEA (Climate and Environmental Sciences Laboratory) led by Jean Jouzel, in cooperation with Russian and American scientists.
They have been able to reconstitute the Earth's climate over the past 420,000 years .
First considering the history of the climate, it was not until the beginning of the 19th century - when the Swiss Louis Agassiz put forward his hypothesis that moraines are not the result of any downpour, but rather the debris left behind by successive glaciers advancing or falling off - that the idea of a succession of glaciations and warmer periods was generally accepted.
Up until the publication of the Vostok ice core results, the scientific community believed in the existence of only four prior glaciations, which Albrecht Penck named after the Danube's four German tributaries: the Gunz (320,000 to 400,000 years ago), the Mindel (240,000 to 300,000 years ago), the Riß (140,000 to 230,000 years ago) and the Würm (15,000 to 120,000 years ago).
But in fact, between 1.8 million years ago and the beginning of the quaternary period, there were 20 successive glaciations.
The Antarctic ice cores allowed scientists to prove the veracity of the theory put forward by the Serbian mathematician Milutin Milankovitch in 1924, which attributed climatic variations to variations in the position of the Earth in relation to the Sun throughout history, according to various cycles:
- Excentricity (degree of flattening of the ellipse of the Earth's trajectory around the Sun). It varies from 0% (circular orbit) to 6%. Today, this figure is 1.7%, meaning the Earth is closer to the Sun in December than in July. It varies according to two cycles of 400,000 and 100,000 years;
- Obliquity (the degree of the Earth's North-South axial tilt in relation to its orbital path). This figure can vary by up to 2°, according to a 41,000-year cycle. It varies between 22° and 25°. The current obliquity is 23°27', which results in mild seasonal differences;
- The precession of the equinoxes (the changing date on which the equinox occurs in relation to the Earth's orbit). This means that for any given date, the Earth is not to be found in the same location of its annual trajectory around the Sun from one year to the next. It results in a seasonal insolation variation of up to 20% and varies according to two cycles of 19,000 and 23,000 years 9 ( * ) .
Previously, atmospheric forcing was the principal cause of climate change, because four regular glaciation cycles have been discovered.
However, the Milankovitch Theory leaves several important questions unanswered:
- the transition of 40,000 to 100,000-year cycles 800,000 years ago;
- the very great transition that occurred some 400,000 years ago without any major change in insolation;
- the near absence of any glaciations in the northern hemisphere before 1.8 million years ago.
The Vostok results also demonstrated the very close connection that exits between temperature and two greenhouse gases - carbon dioxide (CO 2 ) and methane (CH 4 ) - and therefore their essential role in the amplification of climate changes.
Finally, the results showed that concentrations of greenhouse gases are currently the highest they have ever been for the past 420,000 years, shining light on the impact of man's activities.
· The EPICA ice core results
The Europeans - in particular, the French and Italians - managed to extract the world's oldest ice from Dome C as part of the EPICA programme (European Project for Ice Coring in Antartica).
This project was begun in 1995 and included two sites: Dome C (123° east / 75°06' south) and Kohnen (00°04' east / 75°00' south). 850,000 years of climatic archives were extracted from the ice, which is more than twice as old as those obtained at Vostok and the Fuji Dome in 2003 (350,000 years).
Over the past 850,000 years, the Earth has known eight climatic cycles, with alternating glacial and warmer, interglacial periods.
The Concordia results have largely confirmed the Vostok results:
- perfect data coherency;
- confirmation of the determining role of atmospheric forcing,
- confirmation of the role played by rising greenhouse gases and their correlation with the temperature;
- confirmation that the current concentrations are the highest they've ever been, even though a climate as warm as the present one once existed naturally.
3. Ocean core samples: the transpolar link.
The EPICA programme is coupled with an oceanic programme (EPICAMIS for Marine Isotopic Stages).
The principal interests of oceanic samples are the following:
- they allow scientists to go back several million years ;
- they furnish the "oceanic signal"; in other words, the manner in which the oceans have evolved throughout the various climatic periods (for example, their temperature) ;
- they enable scientists to discover how the large sea currents have changed ;
- finally, they allow scientists to reconstruct the link connecting the two poles and therefore how the global climate mechanism functions in the north and in the south .
However, they are less precise.
The various oceanic, Greenlandic and Antarctic samples are able to be compared thanks to specific events. Some of these events are astronomic. Others are physical and can be explained by the history of the last ice age's gigantic icecaps.
Indeed, between 15,000 and 120,000 years ago, the Earth underwent a period of heavy cooling. This led to the creation of the Laurentide (North America) and Fenno-Scandian (Europe) ice sheets, which together were 80 million km 3 in size and rose to 3,800 and 2,500 m respectively. The sea level was 120 m lower.
In all likelihood, these ice sheets resulted in gigantic calvings in the ocean which had a very great and very rapid effect on the Earth's climate. They led to a very great rise in temperature and then, just as rapidly, a renewed glaciation. They were discovered thanks to the moraines and striations present at the bottom of the Atlantic Ocean. They are known as the Heinrich events, after their discoverer. They are part of the 24 Dansgaard-Oeschger events which marked the Earth's climate in cycles of approximately 1,000 years.
Less marked manifestations have been discovered in Antarctica and in the oceans, allowing for a correlation to be made between these three series of events.
This synthesis was recently completed as part of the EPICA programme, thanks in large part to help from the German scientists of the Alfred Wegener Institute. This synthesis was the subject of an article published in Nature in 2006.
4. The future of glacial core sampling
The question that is now being raised concerns the principal directions which glacial core sampling research should now take. What information do the researchers need in order to understand how climate works?
This questioning has been pursued since 2004 at the instigation of the Americans and British within the framework of the International Partnerships in Ice Core Sciences (IPICS) programme. Four main areas of research have been established:
· Reconstitute the Earth's climate farther back than a million years
The first area of research is concerned with delving farther back into the past. Indeed, oceanic samples demonstrate that up until 800,000 years ago, the great climatic cycles were 40,000 rather than 100,000 years in length .
Explaining this change is essential for at least two reasons:
- Is it connected to varying, long-term levels of greenhouse gases provoking a noticeable difference in the climate's sensitivity to changes in insolation? What role do natural reserves of greenhouse gases play? This would open up a field for analysis of great importance for understanding the future climate.
- Why does a small change in insolation have such an important impact on the climate? This is still not well understood.
To answer these questions, scientists would like to obtain a sufficient series of 40,000-year cycles. They are currently seeking a location in Antarctica where it would be possible to go as far back as 1.2 or 1.5 million years. Ideally, they would carry out two such samplings in different locations so as to maximize their chances of success and to have highly reliable data not subject to local variations.
The scientific community is therefore searching for appropriate sites in the eastern Antarctic.
One site will most likely be Dome A , where the Chinese would like to set themselves up. They already carried out a 110-metre sampling during the last ice-coring project and would like to reach 500 m during the International Polar Year.
France has long collaborated with the main Chinese laboratories in the sector. It is greatly in France's interest to participate in this operation , the details of which have yet to be decided.
· Try to understand our future by studying the Eemian.
It is generally believed that the period most similar to our own in terms of climate was the Eemian, 125,000 years ago, between the Würm and Riâ Ice Ages. The climate was warmer and the sea level was some 6 to 7 metres higher.
Taking into account ice core sampling results from Greenland and estimations of the icecap's mass, as well as of the sea level, it appears that during this period the Greenland icecap had largely melted .
One major question is: did the Greenland icecap completely melt away and what was the volume of any remaining glaciers? This calculation is essential for measuring the impact of today's global warming. How great an effect will it have? What effect will it have on oceanic circulation, sea level and the climate in general?
To try and answer these questions, a new sampling site in Greenland must be found, one that will allow scientists to reach unmixed ice layers that haven't been subjected to melting, layers that are older than those discovered so far and, if possible, older than the last interglacial period. Very old ice has already been found, but such samples have not been usable for climate reconstruction. Several scientific teams, especially the Danes, are convinced that such ice exists and can be found and used.
This is the second most important research area of the IPICS programme, which is aimed at obtaining an ice core that will allow scientists to reconstruct the past 140,000 years of the northern hemisphere's climate .
Much progress has already been made in the identification of sampling sites. The University of Copenhagen has identified two in the northwest of Greenland, in cooperation with the University of Kansas. They used radar soundings to obtain profiles of the icecap. These profiles have revealed different layers that are perfectly identifiable with those of the GRIP and NGRIP ice samples. The first site (NEEM1) is 2,542 m deep with an accumulation rate of 0.23 cm of ice per year; some 80 m would be usable for the Eemian . The second site (NEEM2) is deeper (2,756 m) with a lower accumulation and some 100 m for the Eemian. But at the second site, there are uncertainties concerning the bedrock, which could have the effect of jumbling the bottom of the sample. Therefore, the first site will most likely be chosen.
The sampling should take place during the International Polar Year.
· Better understand climate variability by studying the past 40,000 years
The last 40,000 years cover the transition from the last great ice age to today's climate, during which time sudden changes took place marked by rapid warmings and coolings (Dansgaard-Oeschger events).
This is the best-documented period for climatic reactions to great changes resulting from natural variations.
These climatic changes and the reactions in time and space can help us understand the future climate, with today's climate undergoing an extremely rapid forcing due to the actions of man.
The idea, therefore, is to obtain a series of ice cores providing scientists with the information they need to construct as precise a panorama of these changes as is possible.
Already existing samples from the Antarctic and Greenland would be used, from both coastal regions and from the centre of the inlandsis. This programme should also necessitate new samples: WAIS, Talos Dome, James Ross Island, Neumayer Hinterland (in Antarctica), the Greenland coasts, in the Canadian Arctic islands (Agassiz, Devon, Penny, Prince of Wales) and in Alaska (Mt. Logan).
Certain samplings will be carried out during the International Polar Year, with more to follow.
· Clarify what we already know about the past 2,000 years.
The fourth objective of IPICS is concerned with the past 2,000 years. Data is particularly insufficient and unreliable when one goes back farther than 400 years. There are many uncertainties regarding the functioning of the northern-hemispheric climate, in particular for determining the frequency and amplitude of the Arctic oscillation and for determining whether the climate warming that affected Europe during the Middle Ages was a regional phenomenon or something more widespread. Granted, the various scientific methods that have been used up until now 10 ( * ) have provided scientists with a certain amount of information, but this information remains imprecise and localized.
The objective, therefore, is to collect and study more than 200 ice cores to have as detailed an image as possible of this period. These samplings will be carried out in both the polar regions and on mid-latitude glaciers.
B. THERMOHALINE CIRCULATION
In addition, polar research allows for a better understanding of the ocean's effect on climate, due to the central role played by the polar regions in the world's thermohaline circulation. It also raises the question of the ocean's capacity to serve as a carbon sink.
1. The general circulation system
Henri Poincaré once explained to the members of one of Charcot's expeditions: "The great atmospheric movements, upon which all of meteorology depends, are largely governed by polar phenomena. The Earth is like an immense thermal machine with a hot and a cold source". 11 ( * )
The poles are this cold source. There are several explanations (the polar night, the angle of the sun's rays), but these elements can be partially compensated for by the thinner atmosphere 12 ( * ) and, in certain regions, by the absence of cloud cover.
Indeed, the lack of warming in the polar regions is due to the layer of fresh, white snow that sends 80-90% of the sun's heat back into the atmosphere, resulting in a negative radiation balance during a large portion of the year. This negative balance is compensated for by the positive balance of those regions close to the equator, which together make up the hot source.
These two sources govern the atmospheric and oceanic circulations. In this very schematic manner, the poles play their fundamental role in balancing the world's climate by acting as a cold source.
The oceanic circulation
Red: warm surface currents.
Blue/Purple: cold deep currents (2,000 to 4,000 m).
Yellow dots: areas where cold deep waters form
(Source: IPSL-LSCE-Paleoceanic Teams)
In the oceans, the most important point to consider in the functioning of the general oceanic circulation is that the two poles play an essential role in the creation or disappearance of the great sea currents. This is especially the case with regard to the Arctic's effect on the Gulf Stream.
The Antarctic Ocean plays a singular role, because the circumpolar current is a veritable driving belt; it's the only such current to be largely open to the Earth's three major oceans: the Atlantic, Indian and Pacific Oceans. It absorbs the warm currents and redistributes the cold waters.
For Europe, the most important of these great sea currents is the Gulf Stream. Its presence explains the fact that Europe, up to a very high latitude, enjoys a mild climate that greatly differs from those of the North American and Asian regions subject to the continental influence or cold currents.
Climatic archives tend to show that in the past the oceanic circulation in the Atlantic Ocean was greatly disturbed by global warming during either the Dansgaard-Oeschger events or the Eemian.
Therefore, several researchers believe that, taking into account current global warming, the Gulf Stream will once again be affected and some argue that it could even stop.
Taking into account various studies brought to the attention of your rapporteur during his research that didn't deal precisely with this point, it would seem that the scientific community is divided and that more data is needed to determine whether or not the Gulf Stream has already decreased in speed, how great this decrease could be, and what affect it could have on Europe's climate. The estimations are from 1 to 5 from now until 2150.
2. The importance of the creation of cold, deep waters
A key role played by the polar regions in the oceanic circulation is the formation of cold, deep waters.
Indeed, a few specific polar regions allow for their formation. They must combine climatic conditions (cold, wind) and bathymetric conditions (depth, intermixing of waters). There are two well-identified regions in the Arctic, to the east and west of Greenland. In the Antarctic, the Ross and Weddell Seas are the main regions, but the region of Adélie Land seems to play a more important role than has been thought up to now.
The objective of the Albion programme (Adélie Land Bottom Water Formation and Ice Ocean InteractioNs) is to study the Adélie Land coast. The functioning of this polynya coastal region is not very well understood.
Adélie Land's coastal waters form a 20,000-km 2 polynya which produces 100 km 3 of ice per year, the second largest amount in Antarctica. This polynya could be responsible for up to 25% of the Antarctic's deep water formation. However, its existence is closely linked to the protection offered by the arm of the Mertz glacier, an emissary glacier reaching far out to sea and which could break. This protection, combined with the region's especially violent katabatic winds, prevents the formation of a continuous and homogeneous ice shelf and maintains a favourable polynya by the heat exchanges it provokes upon the formation of deep waters.
This research programme is part of the international SASSI (Synoptic Antarctic Shelf-Slope Interactions) programme studying the Antarctic Plateau.
It is the fruit in France of a wide collaboration, in particular between the oceanographers at LOCEAN (IPSL-UPMC) and paleoclimatologists specialized in sediments (the EPOC laboratory in Bordeaux), as well as satellite observations (LEGOS in Toulouse) and even biologists from Chizé thanks to data they've collected on the environment of "diving" animals, in particular elephant seals.
3. The Antarctic Ocean, a carbon sink
A third important area of oceanographic research has to do with the role played by the polar oceans - the Antarctic Ocean, in particular - in the carbon cycle.
The oceans play a very important part in the carbon cycle, because phytoplankton and the entire trophic chain progressively stock a significant amount of carbon in the oceans' depths. There are also the mechanical exchanges linked to the formation of deep waters.
An ocean's primary production of phytoplankton varies depending on the season. This variation is very great in the Antarctic Ocean, due to the absence or very low level of sunshine during a large part of the year. Nevertheless, the Antarctic Ocean is very productive because it is rich in nutritive salts.
As for mechanical action, the Antarctic Ocean can be seen as a "veritable window to the abyssal waters", in the words of Paul Treguer, from UBO-IUEM in Brest, due to the intermingling of the different waters and their dive towards the ocean bottom.
The scientific community debated the role of the Antarctic Ocean in 2002. This led to a decreased estimation of its contribution as a carbon sink, estimated at 18% of the global oceanic sink . However, south of the polar front, its contribution is very low. Furthermore, the global assessment is liable to reverse itself during the El Niño phenomenon. In addition, the ocean's warming should diminish its capacity to absorb CO 2 .
However, certain researchers believe that it is possible to stimulate the ocean's primary production in order to increase its ability to stock carbon . In particular, by increasing the ocean's iron content, thereby stimulating the production of phytoplankton. First put forward in 1931 by Gran, this idea was again taken up in 1988 by John Martin with his famous line: "Give me a half tanker of iron and I will give you an ice age." The iron content of shallow waters depends both on wind (aeolian contribution) and the contribution of the deep waters and of the continental plates. However, the Antarctic Ocean has a deficit of iron.
This hypothesis has since been the subject of several important experiments. The first was carried out in the Weddell Sea in 1988 in bottles of surface water, then ten out at sea: 2 in the Equatorial Pacific (1993 and 1995), 5 in the Antarctic Ocean (1999, 2000, 2002 and 2004) and 2 in the sub-Arctic Pacific (2001 and 2002). All of these experiments were relatively conclusive, since the addition of iron to iron-poor areas leading to a strong growth in plankton and an absorption of CO 2 .
In 2005, French researchers carried out a larger-scale experiment off the Kerguelen Islands by studying a zone naturally supplied with iron by the erosion of the continental plateau. This study should help scientists understand the impact of additional iron over time and over large areas.
Today, the addition of iron has proved its effectiveness in increasing the ocean's primary production; however, the consequences of such an experiment carried out on a large scale are unknown , because primary production does not lead automatically to a long-term stocking of CO 2 in the sediments and above all can have the inverse effects of sterilization and the reemission of greenhouse gases.
C. THE POLAR REGIONS AT THE HEART OF GLOBAL CLIMATE CHANGE
The increased climate change at the poles - with the high latitudes warming two to three times more rapidly than the temperate regions - is liable to provoke the progressive disappearance of the frozen zones: the Arctic summer ice shelf, the Greenlandic inlandsis and the Antarctic icecap.
1. Will the Arctic ice shelf disappear in the summer?
World public opinion has been greatly aroused about the risk of the Arctic ice shelf progressively disappearing during the summer over the next fifty years .
This concern is the result of a combination of factors , which the researchers presented to your rapporteur:
- The ice shelf's diminishing surface area . Since 1979, the frozen surface has decreased by 9% every ten years.
- The diminishing age of the ice . In the Arctic, a portion of the ice shelf remains frozen from year to year. Permanent in appearance, this ice is in fact mobile and there is hardly any sea ice more than 4 or 5 years old in these regions.
The surface area of this several-year-old ice is also diminishing by 8-10% per year. Simulations based on satellite images show that the average age is constantly decreasing, even that of the perennial ice, which is becoming younger and younger.
- The diminishing thickness . The ice shelf is normally 2.5 to 3 metres thick, on average. However, recent soundings carried out by both American military submarines and scientific ships tend to show a very marked decrease in thickness, of around 40% over the past thirty years.
This important data is confirmed by the eyewitness accounts of professionals working in the Arctic, who are noticing marked changes in their normal working conditions.
However, great uncertainties remain regarding the collected data and the forecasts.
For this reason, the European Commission is supporting the DAMOCLES (Developing Arctic Modeling and Observing Capabilities for Long-term Environmental Studies) scientific programme . This international programme, which will last from 2005 to 2009, gathers together a great many research centres. It is coupled with the American SEARCH programme. It is directed by the French researcher Jean-Claude Gascard. Its objective - thanks to the installation of new autonomous sensors throughout the Arctic basin - is to precisely measure a great many factors: atmospheric pressure, currents, salinity, water and air temperature, etc.
It should allow scientists to model the Arctic environment and predict its future evolution. After 2009, it will most likely take the form of a European observatory to study climate change in the region.
2. Will Greenland melt completely?
The geometry and volume of the icecaps is governed by the balance between the amount of fallen and evacuated snow.
With a surface area of 1.7 million km 2 and an average thickness of 2,000 m, Greenland contains 9% of the Earth's ice.
Its latitude is much lower than that of Antarctica and much of Greenland is located south of the Polar Circle.
Scientists know that historically Greenland has been subjected to brutal climate variations and that its icecap has undoubtedly disappeared - if not entirely, than at least to a great extent.
The icecap's dynamics is very different from that of the Antarctic, because precipitation in Greenland is much greater in proportion to the surface area. There is also much more melting in the summer, resulting in the evacuation of at least half of the precipitation.
Based on observations, it is believed that the Greenlandic icecap is today in disequilibrium. It is losing volume due to melting and the accelerated outflow of its glaciers. What's more, its general profile is becoming more sloping.
Recent studies of Greenland 13 ( * ) have shown a significant weakening of the icecap, between 1992 and 2002, which appears to be accelerating. Over this ten-year period, Greenland has lost 80 km 3 of ice per year, for a total volume of 3 million km 3 . Should more than 20% of the ice be lost, this trend would become irreversible. The melting of Greenland has raised the Earth's sea level by 15 mm over the past 15 years. The point of no return would be reached with a global warming of 3°C, which is probable before the end of the 21 st century.
These evaluations have given rise to important debates within the scientific community for two main reasons:
- There is insufficient data concerning Greenland's natural state during a warm period such as ours , which is why additional core samplings are necessary. We must have more information on the state of the Greenlandic icecap some 120,000 years ago.
- Researchers agree over the general meaning of the current evaluations: Greenland is in the process of shrinking. But the exact magnitude and speed of this loss are hotly debated , due to the present lack of satellite-based means . Indeed, space-based data remains relatively imprecise and incomplete, and there are no long series of data. This situation should soon change thanks to data collected during two combined missions, one calculating Greenland's gravity and therefore mass (GRACE-NASA), the other the volume of its ice, including its sea and coastal ice (Cryosat-ESA) .
Gravimetry consists of measuring terrestrial gravity, which depends on the distribution of masses between the Earth's surface and its centre. The heavier these masses are and the closer they are to the surface, the greater the gravity. Gravity is measured by taking into account natural variations in the gravitational field linked to the diameter 21 km shorter between the poles than at the equator and to surface heterogeneities such as mountains, oceans, ice, etc. This results, therefore, in an ellipsoid indicating a greater force of gravity at the poles than at the equator. For a long time, gravity was only measured by disturbances in the satellites' orbits, but this method was imprecise. Initial progress, with measurements four times as precise, was made possible by the Champ (Challenging Minisatellite Payload for Geoscience and applications) satellite, launched in 2001, which allowed scientists to identify the effects of gravity.
Recently, the GRACE (Gravity Recovery and Climate Experiment) satellites launched by NASA have allowed for a precise measurement of the gravitational field. By comparing measurements made during their successive transits, the satellites will allow scientists to evaluate the increase or decrease in the ice mass, thereby allowing for a more precise assessment.
Cryosat's mission is to monitor the thickness of the continental and sea ice masses, so as to better understand the link between the melting polar ice and the sea level in correlation with climate change.
The Cryosat mission is scheduled to last three years. The satellite will be positioned at an altitude of 700 km, enabling it to make observations up to 88° north and south latitude.
The measurements are made by a highly sophisticated radar altimeter (Synthetic Aperture Interferometric Radar Altimeter or SIRAL), assuring a very precise positioning of the satellite and therefore allowing scientists to monitor any change in the surface's altitude and thereby monitor variations in ice thickness (continental and sea ice).
Sea and shelf ice is relatively thin - a few metres at the most. All the same, this ice great influences the climate because of its effect on oceanic temperature and the circulation of warm and cold waters. Cryosat will also be able to detect and precisely measure variations in ice thickness throughout the year and from one year to the next.
It should therefore be possible to provide a precise answer in 5 to 10 years.
3. Can a diagnosis be made concerning the assessment of Antarctica's mass?
In the case of Antarctica, public opinion was also greatly aroused with the breaking off of gigantic icebergs over the past 10 years.
This phenomenon is still not very well understood. In several cases, it is most likely to be explained by the natural dynamics of the Antarctic icecap, which regularly gives birth to tabular icebergs. In other cases - in particular, in the Peninsula, with the dislocation of the Larsen B ice shelf - it is to be explained rather by climate change.
Generally speaking, it is very difficult to provide an assessment of Antarctica's mass. In 2003, Frédérique Rémy (LEGOS, book cited above) noted an uncertainty of some 20-30%. Numerous mechanisms are still not understood and cannot yet be modelled.
As is also the case for the Arctic, the scientific community believes that satellite observations will greatly help them to understand and measure the present phenomena.
The past can also help us understand the future.
For example, this is the case with the functioning of the Antarctic during the last ice age 18,000 years ago. The climate was 10°C cooler and precipitation was half as great. The eastern and western areas reacted differently.
The west was greater in volume and had a larger surface area than it does today.
Indeed, the volume of the ice and the form of the icecap are greatly influenced by sea level. When the sea level is lower, the ice can rest upon the bedrock, cover a greater area and support a greater volume further up. Its altitude was 80 m higher than it is today, as shown by the Byrd sample.
On the other hand, in the east, the altitude was some 200 m lower, because in this truly continental - rather than archipelagic - zone, the icecap's volume depends essentially on the lower precipitation during the ice age due to the drying-up of the atmosphere. This would suggest that the opposite mechanism is also possible: a thinning and reduction in surface of the western icecap at the same time as the floating ice shelves weaken, as well as a thickening of the eastern icecap due to greater precipitation.
However, due to the continent's isolation and thermic inertia, global warming cannot have a rapid impact on Antarctica. Indeed, during the last deglaciation, most of the melting occurred in the north: the American and Asian icecaps completely disappeared, while Antarctica underwent relatively little change. Only 10 m of the 120-metre rise in sea level was linked to Antarctica. Certain glacial retreats and the fossilized remains of a few penguin-colonies (such as those found near Terra Nova Bay) would indicate that real changes did indeed occur, but these were limited in scope at the continental level.
Therefore, the most likely hypothesis, based on our current knowledge and the observations made, is that the Antarctic Peninsula continues to undergo rapid warming, though this will not affect the general equilibrium of the continental masses anytime in the near future due to the development of the eastern icecap . However, the glaciers of the sub-Antarctic islands are rapidly retreating.
For your rapporteur, three factors must be emphasized:
- The great importance of this research, which is aimed at answering fundamental questions concerning our planet's future, and therefore the need for the French research teams to fully participate;
- The absolute need to help finance the research teams , which must remain at the highest international level, even in the face of increasing competition. The glaciology teams in particular need the means to automatically carry out the traditional analyses, as well as to make progress in new fields of research. They need computing power. They need to be able to welcome young researchers working on their doctorate or post-doctorate. Their on-site means (logistics and sampling system) must also allow them to be ideally positioned in the international programmes. In the end, it is all of these factors together that will provide our scientists with the raw material to analyze and allow them to be the first to publish their results in the main scientific journals ;
- The need to establish, at least at the European level, long-term observatories for the polar regions. The observation programmes must be continued beyond an initial PCRD or ESA mission.
* 9 See Le climat : jeu dangereux, Jean Jouzel and Anne Debroise, Paris, Dunod, collection: Quai des sciences, 2004, 212 pages.
* 10 Dendrochronology, historic documents (grape-harvesting dates, etc.), lacustrine sediments, etc. See Histoire du climat depuis l'an mil , Emmanuel Le Roy-Ladurie, Paris, Flammarion, 1983.
* 11 Cited by Frédérique Rémy in L'Antarctique, la mémoire de la Terre vue de l'espace, CNRS Editions, 2003, p. 20.
* 12 The troposphere is only 7 km thick at the poles, as compared to 20 km at the equator.
* 13 Philippe Huybrechts, Université Libre (Néerlandophone) de Bruxelles, Nature and Geophysical Research Letter.