Le « Climate Change Science Compendium 2009 » publié par le GIEC fait le point sur les plus récentes connaissances scientifiques et observations sur le changement climatique. L’augmentation des émissions de CO2 dépasse à l’heure actuelle les scénarios les plus pessimistes que le GIEC avait établies, passant d’une croissance annuelle de 1,1% entre 1990 et 1999 à 3,5% entre 2000 et 2007. Les prévisions sur l’ampleur du réchauffement et de ses conséquences sont toutes revues à la hausse. Les scientifiques s’attendent désormais une élévation de la température moyenne comprise entre 1,4 °C et 4,3°C. Le rapport souligne en particulier le recul généralisé des glaciers, et ses conséquences à terme sur la ressource en eau pour un sixième de la population de la terre ; des sécheresses plus fortes en Australie, Afrique du Nord et au sud de l’Europe ; une élévation du niveau des mers désormais estimée entre 0,5 et 1,4 mètres contre 18 à 64 cm précédemment. Extraits.

GIEC, Climate Change Science Compendium 2009, septembre 2009 (extraits)

EMISSIONS

CO2 emissions from fossil-fuel burning and industrial processes have been accelerating at a global scale, with their growth rate increasing from 1.1 per cent per year for 1990-1999 to 3.5 per cent per year for 2000-2007. The emissions growth rate since 2000 was greater than for the most fossil-fuel intensive of the Intergovernmental Panel on Climate Change emissions scenarios developed in the late 1990s. Global emissions growth since 2000 was driven by an increase in the energy intensity of gross domestic product and the carbon intensity of energy, as well as growing populations and per capita gross domestic product.

Nearly constant or slightly increasing trends in the carbon intensity of energy have been recently observed in both developed and developing regions due to increasing reliance on coal. No region is decarbonizing its energy supply. The emission growth rates are highest in rapidly developing economies, particularly China. Together, the developing and least-developed economies-80 per cent of the world’s population-accounted for 73 per cent of the global growth in 2004’s emissions, but only 41 per cent of total global emissions and only 23 per cent of cumulative emissions since 1750

Implications

Concentrations of atmospheric CO2 are increasing rapidly, because of three processes. Two of these processes concern emissions. Growth of the world economy in the early 2000s, combined with an increase in its carbon intensity, has led to rapid growth in fossil fuel CO2 emissions :

Comparing the 1990s with 2000-2006, the emissions growth rate increased from 1.3 per cent to 3.3 per cent per year. The third process is indicated by increasing evidence of a decline in the efficiency of CO2 sinks in oceans and on land in absorbing anthropogenic emissions. The decline in sink efficiency is consistent with results of climate-carbon cycle models, but the magnitude of the observed signal appears larger than that estimated. All of these changes characterize a carbon cycle that is generating stronger-than-expected and sooner-than-expected climate forcing.

The climate forcing arriving sooner-than-expected includes faster sealevel rise, ocean acidification, melting of Arctic sea-ice cover, warming of polar land masses, freshening in ocean currents, and shifts in circulation patterns in the atmosphere and the oceans/

Focusing on two illustrative irreversible consequences in Earth Systems, dry-season rainfall reductions in some regions and ongoing sea-level rise, researchers looked at the systems’response to increases in atmospheric CO2 concentrations.

Based on an assumption that CO2 will peak between 450 and 600 parts per million (ppm) in the next two or three decades and then cease, they warn of ‘dustbowl’ conditions, or a 10 per cent increase in aridity, dominating the dry seasons in southern Africa, eastern South America, and southwestern North America. The projections get worse : An increase in dry season aridity approaching 20 per cent would dominate in western Australia, northern Africa, and southern Europe.

All these regions would experience wet-season precipitation somewhat drier or similar to current conditions, while southeast Asia would experience up to 5 per cent wetter rainy seasons and 10 per cent more arid dry seasons.

Tipping elements

Nine tipping elements considered as Earth System components vulnerable to possible abrupt change. The time frames and threshold temperature increases presented here will likely be modified as new data and information track characteristics and rates of change :

  Indian summer monsoon-The regional atmospheric brown cloud is one of the many climate change-related factors that could disrupt the monsoon. Possible time-frame : One year ;temperature increase : unknown.

  Sahara and West African monsoon-Small changes to the monsoon have triggered abrupt wetting and drying of the Sahara in the past. Some models suggest an abrupt return to wet times. Possible time-frame : 10 years ; temperature increase : 3-5°C.

  Arctic summer sea-ice-As sea ice melts it exposes darker ocean, which absorbs more heat that ice does, causing further warming. Possible time-frame : 10 years ; temperature increase : 0.2-2°C.

  Amazon rainforest-Losing critical mass of the rainforest is likely to reduce internal hydrological cycling, triggering further dieback. Possible time-frame : 50 years ; temperature increase : 3-4°C.

  Boreal forests-Longer growing seasons and dry periods increase vulnerability to fires and pests. Possible time-frame : 50 years ; temperature increase : 3-5°C.

  Atlantic Ocean thermohaline circulation- Regional ice melt will freshen North Atlantic water. This could shut down the ocean circulation system, including the Gulf Stream, which is driven by the sinking of dense saline water in this region. Possible time-frame : 100 years ; temperature increase : 3-5°C.

  El Niño-Southern Oscillation (ENSO)-El Niño already switches on and off regularly. Climate change models suggest ENSO will enter a near-permanent switch-on. Possible timeframe : 100 years ; temperature increase : 3-6°C.

  Greenland ice sheet-As ice melts, the height of surface ice decreases, so the surface is exposed to warmer temperatures at lower altitudes which accelerates melting that could lead to ice-sheet break up. Possible time-frame : 300 years ; temperature increase : 1-2°C.

  West Antarctic ice sheet-Ice sheet is frozen to submarine mountains, so high potential for sudden release and collapse as oceans warm. Possible time-frame : 300 years ; temperature increase : 3-5°C.

TEMPERATURES

With estimations of 1-5 degrees Celsius as the range of GMT increase over 1750 levels as the threshold for tipping elements and 0-5 degrees Celsius over 1990 levels as reasons for concern, some researchers are realizing that we have already committed ourselves to significant environmental changes. The observed increase in GHG concentration since 1750 has most likely committed the world to a warming of 1.4-4.3 degrees Celsius, above pre-industrial surface temperatures.

The equilibrium warming above pre-industrial temperatures that the world will observe is 2.4 degrees Celsius-even if GHG concentrations had been fixed at their 2005 concentration levels and without any other anthropogenic forcing such as the cooling effect of aerosols.

The range of 1.4 to 4.3 degrees Celsius in the committed warming overlaps and surpasses the currently perceived threshold range of 1-3 degrees Celsius for dangerous anthropogenic interference with many of the climate-tipping elements such as the summer Arctic sea ice, Himalayan glaciers, and the Greenland Ice Sheet .

Researchers suggest that 0.6 degrees Celsius of the warming we committed to before 2005 has been realized so far. Most of the rest of the 1.6 degrees Celsius total we have committed to will develop in the next 50 years and on through the 21st century. The accompanying sea-level rise can continue for more than several centuries. Lastly, even the most aggressive CO2 mitigation steps as envisioned now can only limit further additions to the committed warming, but not reduce the already committed GHGs warming of 2.4 degrees Celsius

Perhaps experiences in everyday life have led us to believe that slow processes such as climate changes pose small risks, on the assumption that a choice can always be made to quickly reduce emissions and reverse any harm within a few years or decades.

This assumption is incorrect for carbon dioxide emissions, because of the longevity of their effects in the atmosphere and because of ocean warming. Irreversible climate changes due to carbon dioxide emissions have already taken place. Continuing carbon dioxide emissions in the future means further irreversible effects on the planet, with attendant long legacies for choices made by contemporary society

GLACES

Glaciers and ice caps in polar, temperate, and high altitude tropical regions are experiencing near-universal retreat and volume loss. Diminishing glacier and ice cap volumes not only dominate current sea-level rise but also threaten the well-being of approximately one-sixth of the world’s population who depend on glacier ice and seasonal snow for their water resources during dry seasons

Data from the World Glacier Monitoring Service track 30 reference glaciers in nine mountain ranges and document strongly accelerating loss of glacier mass. Since the year 2000, the mean loss rate of these 30 reference glaciers has increased to about twice the loss rates observed during the two decades between 1980 and 1999 .

The previous record loss in 1998 has been exceeded already three times-in 2003, 2004 and 2006-and the new record loss in 2006 is almost double that of the previous record loss in 1998. The mean annual loss for the decade 1996-2005 is more than twice the value measured between 1986 and 1995 and more than four times that of the period 1976-1985.

Certain regions such as southern Alaska suffer from significantly higher losses. Positive feedback mechanisms such as albedo change due to dark dust and collapse around glacier peripheries now appear to play an increasingly important role, enhancing mass loss beyond pure climate forcing.

ARCTIQUE

In 2007 the sea ice in the Arctic Ocean shrank to its smallest extent on record, 24 per cent less than the previous record set in 2005 and 34 per cent less than the ‘climatology’ which is the average minimum extent over 1970-2000 . The minimum sea-ice cover of 2007 extended over 452 million hectares of the Arctic Ocean. This is clear evidence of a phenomenon of importance on a planetary scale forced by global warming and caused mainly by an Earth System energy imbalance due to greenhouse gas concentrations increasing in the atmosphere.

The nature of the Arctic sea-ice cover has changed drastically over the last few decades, with much more extensive proportions of first- and second-year ice. In 1988, 21 per cent of the sea-ice cover was 7+ years old and 31 per cent was 5+ years old. In 2007, only 5 per cent was 7+ years old and 10 per cent was 5+ years old. Within the central Arctic Basin in 1987, 57 per cent of the ice pack was 5+ years old and at least 14 per cent was 9+ years old. The Arctic Basin in 2007 had only 7 per cent of ice coverage 5+ years old and no ice that was 9 years or older.

The continuing sparse ice extent recorded in September 2008 spurred further analysis : At current rates of coverage, more than 60 per cent of the Arctic Ocean area is open to increased shortwave and longwave radiation at the end of summer and temperatures in the Arctic autumn now reach 5-6 degrees Celsius above the climatological norm

Grenland Ice

The recent marked retreat, thinning, and acceleration of most of Greenland’s outlet glaciers south of 70° N has increased concerns over contributions from the Greenland Ice Sheet to future sea-level rise (Howat et al. 2005). These rapid changes seem to be parallel to the warming trend in Greenland, the mechanisms are poorly understood.

Another study focused on western Greenland’s Jakobshavn Isbrae responsible for draining 7 per cent of the ice sheet’s area, which switched from slow thickening to rapid thinning in 1997 and suddenly doubled its velocity. Here, the change in glacier dynamics is also attributed to destabilization of the glacier terminus, but the researchers are able to attribute that to warmer ocean water delivered to the fjord. However, these researchers were also able to detect short-term and less significant fluctuations in Jakobshavn Isbrae’s behaviour that could be attributed to meltwater drainage events.

They present hydrographic data documenting a sudden increase in subsurface ocean temperature along the entire west coast of Greenland in the 1990s that reached Jakobshavn Isbrae’s fjord in 1997. The researchers trace the warm flow back to the east of Greenland where the subpolar gyre that rotates counter clockwise south of Iceland scoops warmer water from an extension of the Gulf Stream and directs it back west and south around the tip of southern Greenland. In the early 90s the North Atlantic Oscillation atmospheric pattern switched phase and drove the subpolar gyre closer to the Greenland shore, accelerating the flow of the warm water around the tip and up the western shore, where it eventually reached the Jakobshavn Isbrae fjord.

ANTARCTIQUE

Parts of Antarctica are also losing ice, particularly from the West Antarctic Ice Sheet. Researchers estimate that loss of ice from West Antarctica increased by 60 per cent in the decade to 2006. Ice loss from the Antarctic Peninsula, which extends from West Antarctica towards South America, increased by 140 per cent. The processes affecting the peninsula involve accelerating glacier flows caused by both warmer air and higher ocean temperatures. An additional factor in West Antarctica and the Antarctic Peninsula that could undermine the integrity of the great ice sheets is the recent disappearance of a number of ice shelves that build along those shores. These shelves are immense-the Ross ice shelf is the largest and is a bit smaller than Spain. The shelves are already floating on the ocean so their loss does not add to sea-level rise. But they are attached to the ice sheets and act as buttresses. When they go, the ice sheets may accelerate out into the ocean and that does displace the water.

Recent findings show that significant warming extends well to the south of the Antarctic Peninsula to cover most of West Antarctica, an area of warming much larger than previously reported. West Antarctic warming exceeded 0.1°C per decade over the past 50 years, and has been most marked during winter and spring.

The whole continent’s average nearsurface temperature trend is warming, although this is offset somewhat by East Antarctic cooling in autumn. These trends appear unrelated to changes in the westerlies ; instead, analysis attributes the warming to regional changes in atmospheric circulation and associated changes in sea surface temperature and sea ice.

In contrast to the dramatic decrease in Arctic sea ice cover, the total area of Antarctic sea ice has been increasing since the 1970s. At its maximum near the end of the Southern Hemisphere’s winter in September, Antarctic sea ice covers an area of 15 million hectares or more.

OCEANS

Since at least the 19th century, sea-level changes have been measured directly by tide gauge records and, since the 1990s, by satellite altimetry. Sea-level changes over longer periods of time, thousands to millions of years, are inferred from geologic evidence. The average rate of global mean sea-level rise over the 20th century was about 1.7 millimetres (mm) per year. In the period 1993-2003 global mean sea-level rose about 3.1 millimetres (mm) per year, and since 2003 the rate of rise has been about 2.5 mm per year. The relative importance of the three factors contributing to global average sea-level rise has varied during this time.

Estimates of how much sea-level-global mean sea level and regional levels-will rise over particular periods of time have been vigorously discussed since IPCC AR4 could affirm only 18-59 centimetres (cm) rise over the 21st century. The discussions focus on the dynamic ice changes that were excluded from AR4 estimates because no consensus could be reached based on published literature available at that time.

Since the publication of the IPCC AR4, climatological modelling, without dynamic effects explicitly included, suggests that 21st century sea-level could rise to 0.5 to 1.4 metres above the 1990 level.

By considering rates of discharge from melt and from iceberg fluxes required to drain ice though existing marine outlets, it can be shown that a combined sea-level rise in excess of 1.15 metres from Greenland and Antarctica by 2100 is physically very unlikely. Similarly, glaciers and ice caps are realistically limited to no more than about 0.55 metres by 2100. Introduction of realistic future melt and discharge values into the same analysis suggests that plausible values of total global average sea-level rise, including all land-ice sources plus thermal expansion, may reach 0.8 to 2.0 metres by 2100, although no preferred value was established within this range .

ACIDIFICATION

Ongoing ocean acidification may harm a wide range of marine organisms and the food webs that depend on them, eventually degrading entire marine ecosystems.

The oceanic uptake of anthropogenic CO2 occurs through a series of well-known chemical reactions that increase aqueous CO2, lower seawater pH, and lower carbonate ion levels. To the beginning of the 21st century, anthropogenic CO2 has reduced average surface ocean acidity to 8.1 pH units from a pre-industrial value of 8.2 pH units on a logarithmic scale, a 30 per cent increase in acidity (Caldeira and Wickett 2003). Acidification decreases the concentration of carbonate (CO3), decreasing the saturation state of the CaCO3 mineral calcite in the upper ocean that many marine organisms need to metabolize the shells and skeletons that support their functions.

Projected increase in anthropogenic CO2 emissions will accelerate these chemical changes to rates unprecedented in the recent geological record. At current emission rates, atmospheric CO2 concentrations will increase from 385 parts per million (ppm) in 2008 to 450-650 ppm by 2060, which would decrease average ocean surface acidity to an average of 7.9-7.8 pH units and reduce the saturation states of calcite and aragonite, two more CaCO3 minerals, by 25 per cent-further shrinking optimal regions for biological carbonate formation.

ECOSYSTEMES

Under the highest IPCC scenario-the one that most closely matches current trends-12-39 per cent of the planet’s terrestrial surface could experience novel climate conditions and 10-48 per cent could suffer disappearing climates by the end of this century. Dispersal limitations-imposed by fragmented habitats and physical obstructions, including those built by humans-increase the risk that species will experience the loss of existing climates or the emergence of novel climates. There is a close correspondence between regions with globally disappearing climates and previously identified biodiversity hotspots. While most changes are predicted to occur at high latitudes and high altitudes, many tropical species are incapable of tolerating anything beyond mild temperature variations : Even slight warming may threaten them. And ecosystem niche gaps left by migrating species in tropical lowland ecosystems may endanger those species that are able to adapt to changes within an ecosystem at a particular location, but not to the absence of a key player in that ecosystem

VARIATIONS CLIMATIQUES

Changes in the tropics are becoming more apparent. Several lines of evidence show that over the past few decades the tropical belt, which roughly encompasses equatorial regions, is expanding. This expansion influences all latitudinally determined climatologies, including the intertropical convergence zone, the subtropical dry zones, and the westerlies that dominate weather at subpolar latitudes. The observed rate of expansion already exceeds climate model projections for expansion during the 21st century. This expansion of the tropics not only has a cascading effect on large scale circulation systems but also on precipitation patterns that determine natural ecosystems, agricultural productivities, and water resources for urban and industrial demands. This expansion of the hot and humid tropical zone leads to poleward displacement of the subtropical zones, areas occupied by most of the world’s deserts.

PRECIPITATIONS

In many regions of the world, water is already scarce and, given increased pressures from agriculture and urban expansion, is likely to become more so as global climate change advances. Shortages of water for agriculture and for basic human needs are threatening communities around the world : Southeastern Australia has been short of water for nearly a decade and southwestern North America may have already transitioned to a perennial drought crisis climate.

According to projections, areas expected to be affected by persistent drought and water scarcity in coming years include the southern and northern parts of Africa, the Mediterranean, much of the Middle East, a broad band in Central Asia and the Indian subcontinent, southern and eastern Australia, northern Mexico, and the southwestern United States-a distribution similar to current water-stressed regions. Regional studies are following up on these projections and others from drought-threatened regions.

EVOLUTIONS REGIONALES

SAHEL Debate continues about whether the Sahel, one of the world’s most vulnerable regions to climate variability, is at a tipping point. Some projections suggest that the Sahel region of West Africa could see a sudden revival of rains if global warming and changes in ocean temperatures in the North Atlantic combine to trigger a strengthening of the West African monsoon. This tipping point has been crossed in the past : Between 9,000 and 5,000 years ago, large parts of the Sahel were verdant after an exceptionally dry period around 10,500 years ago. Evidence published in 2008 suggested that even if this revival occurs it may not be as abrupt as some suggest. A study of pollen and lake sediments in the Sahara investigated how the Sahel went from wet to dry conditions over a 1,000 year period that began 6,000 years ago. Other studies suggest this shift happened within a few decades. The search for a reliable means of predicting future precipitation patterns in the Sahel region of Africa continues, with onestudy suggesting that links to sea surface temperatures that held in the 20th century might not apply in the 21st century.

MEDITERRANEE New research indicates that by the end of the 21st century the Mediterranean region will experience more severe increases in aridity than previously estimated by models. This aridity will render the entire region, but particularly the southern Mediterranean, vulnerable to water stress and desertification. Using the highest resolution projections published for the entire Mediterranean basin, researchers project a substantial northward expansion of dry and semi-arid regime lands across the Iberian, Italian, Hellenic, and Turkish peninsulas. These results imply a corresponding retreat of temperate oceanic and continental climate regimes and a likely shift in vegetation cover, with huge implications for agriculture in the region. This study adds to the body of work that corresponds to and projects from the region’s ongoing observations of warming and drying trends.

PERMAFROST

The consequences of persistent climate warming of Arctic and subarctic terrestrial ecosystems, and associated processes, are ominous. The releases of carbon dioxide (CO2), methane (CH4), and more recently, nitrous oxide (N2O) in these regions have accelerated in recent decades Arctic permafrost soils store staggering amounts of carbon. Including all northern circumpolar regions, these ecosystems are estimated to hold twice as much carbon than is currently held in the atmosphere in the form of CO2. If Arctic warming accelerates as expected, the global implications of ensuing feedbacks could cross one of the most dangerous of the tipping elements discussed in Chapter One Earth’s Systems. Current warming in the Arctic is already causing increased emissions of CO2 and CH4 and feedbacks may have already begun