(The Permian–Triassic extinction event, labelled “P-Tr” here, is the most significant extinction event in this plot for marine genera, https://en.wikipedia.org/wiki/File:Extinction_intensity.svg)
In the years 1980-1990, such risks imagined for the future were those related to the consequences of a major nuclear war, or volcanism. These risks were associated with a sudden cooling said nuclear winter, concept developed as tensions related to the Cold War were still alive, in a study commissioned to 5 leading researchers on the effects of nuclear war. Their conclusion, published in Science in 1983, in an article entitled Global Atmospheric Consequences of Nuclear War, was that aerosols and dust from bombs of 100 megatonnes in 100 cities were enough to cloud the atmosphere as photosynthesis is impossible within hours, and that kills plants causing a chain in ecosystems collapse, threatening the survival of all humans. Based on the impacts of recent volcanic eruptions, the study estimated that the mean temperature would drop from 5 to 15°C. This article has influenced Mikhail Gorbachev and many elected officials who then sought to implement a nuclear disarmament process.
Since the late 1990s, some experts do not rule out a brutal reverse change; a warming caused by massive emissions of greenhouse gases which are not decreasing as fast as that means scenarios and commitments had planned.
Recent developments, fast, melting the edges of the West Antarctic ice and the Greenland ice sheet, the state of the seas and coral, the advance of desertification and deforestation concerned experts and seem to bear the worst of the IPCC scenarios. These phenomena are accelerated for 20 years, exceeding the most pessimistic forecasts of early 1990s The current generation of models is still not powerful enough to properly understand these processes and anticipate their impacts. It therefore ignores the part of a potential serious imbalance start that could lead to an abrupt warming in the twenty-first century, and the short or medium term imbalances that would only be natural adjustments to recent climate change.
Synergistic and aggravating effects are possible and probable. And they may express themselves more intensely and more frequently in a warmer climate. But we do not know today predict the extent and implementation of the precautionary principle is difficult in this area.
Methane: A sharp rise in methane levels in the air, from the melting of clathrates is considered by some experts:
- it would contribute a priori, in a self-perpetuating cycle, to a brutally accelerated warming;
- an increase in forest fires and soil erosion (water turbidity, dead zones, degradation of estuaries …) could jointly worse in terms of content of the air in greenhouse gases;
- models available in 2009 are not yet very accurate, and there is insufficient knowledge of the stock clathrates (estimates vary by a factor of 1-10), and the life cycle of methane (methanotrophic bacteria in particular ..) is also poorly known. But recent models suggest a possible doubling of CH4 emissions at high latitudes (around the poles), or that CH4 has a shorter lifetime than CO2 contributes to global warming 21 times more than CO2;
- no modeling, nor isotope analyzes of polar ice cores, indicate only very large quantities of methane hydrates have been abruptly released in the air for 100000 years. There is no strong evidence of current emissions or for the very near future. It seems to have been a crisis of this type to the Paleocene-Eocene boundary, there are about 56 million years, but the hypothesis has yet to be confirmed.
If the risk of catastrophic release of methane into the atmosphere during the next century seemed until recently (around 2010) very unlikely, current models suggest a doubling of emissions in the next century, but they are still a priori unrealistic, as uncertainties about methane are numerous. A rapid increase can not be excluded nor affirmed. The accelerated melting of methane hydrates is probable and accepted, but its extent is difficult to estimate. Before it remains to deal with the problem of methane from the coalfields, landfills, sewers, degraded estuaries, etc.
The risk of significant eruptions in the Arctic methane has now become important, disturbing. Expert groups have thus expressed in 2012 to launch a red alert. Articles in Nature recall that the clathrates, founding, release significant quantities of methane they prevented the eruption because they formed a tight layer, now disappearing over. The quantities of carbon, including methane contained in these soils (land or sea) have also been significantly revalued. They are based on recent studies from NASA (satellite monitoring of methane concentrations and temperatures around the North Pole), the University of Alaska Fairbanks, supported on visual investigations on land and at sea. The eastern Siberian Arctic zone (east Siberian Shelf area) is identified as an area of particular eruptions attributed to destabilization of clathrates layer by seismic activity in the area. Recovery with methane concentrations observed by satellite is the basis of this rapprochement. An observation campaign in the summer of 2010 and 2011 revealed a significant increase in bubble areas (naval campaign led by Simelitov). These observations are corroborated the recent demonstrations of a brutal start warming: fires, significant temperature increases.
The amounts being rash is still difficult to determine, but there is some consensus that they are a very alarming extent. The severity of these eruptions is related to the extent of the areas, and the difficulty of human actions that could lead to halting these exhausts, or reduce their consequences. The mottles are indeed associated with all of the permafrost (25% of the land surface of the northern hemisphere), with extensive marine areas on hundreds or thousands of kilometers, even restricting themselves to more “urgent” areas.
Self-sustained and accelerated changes in the polar albedo
Paleoclimatology shows that rapid significant changes in albedo and ice volume always have in the past experienced been associated with abrupt climate change, and with a reverse correlation between levels of CO2 and CH4 from the air and global ice volume.
A small rise of the oceans took place, clearly visible in the North Atlantic, in the Carolingian period. Before that, the last major sea level rise has been associated with a general melting glaciers at the end of the last ice age (around – 20 000 years), with an average increase of 10-20 millimeters per year and a major “surge at glacial melt” more than 50 mm/year (on average) during centuries clearly demonstrates the potential of fast ice sheets because the sea level and major changes.
Rising sea level of glaciers and ice sheets has recently accelerated. The balance between gains and losses in mass from the north pole has decreased from almost zero in the early 1990s to a net loss of 100 gigatons per year (GT/year) to over 200 GT/year for more recent observations in 2006. The Antarctic ice has thickened locally, but its overall mass balance is a net loss of about 80 Gt/year in the mid-1990s to almost 130 GT/year the 2000s. The latest estimate (2007) mass balance of “small” glaciers and ice caps is a loss at least three times higher (380-400 GT/year) than the estimated loss for the mid-nineteenth century. Recent observations of the ice sheet show that changes in ice dynamics have been underestimated, and there is nothing to say they can not accelerate further.
There is a strong correlation between periods of strong surface melting and increasing the speed of glaciers, which could be due to the decrease in albedo (the color of the seas and dark polar soil allowing them to absorb more solar heat), warming the soil and increases the drainage of water from the accelerated melting of the glaciers. This drainage lubricant the glacier base by accelerating its descent to the sea. An additional warming could further increase the melt flow rate and mass loss, non-linearly or exponentially. The margins of Antarctica and the Greenland ice sheet show accelerated melting and thinning of the ice sheet, with a speed of glaciers, which more than doubled in recent decades; The acceleration of these glaciers was often immediately following their reduction or loss of floating extensions.
The fonts of the fronts of some glaciers seem so accelerated by basal melting, despite apparently low ocean warming. The interaction with the warm waters of the outskirts of large ice sheets, still poorly modeled, would represent one of the possible causes of sudden change in the climate system. The IPCC AR4 report estimates that earlier projections of sea level for the end of this century have been underestimated by the models.
The role of the thermohaline circulation
A blockage of the “treadmill” of major ocean currents is sometimes mentioned and can not be excluded in the long term. The 2009 estimates nevertheless suggest that this slowdown for the twenty-first century should not exceed 25-30% (which may be sufficient to generate significant local and delayed impacts), and it is “very unlikely” this cycle collapses or is the subject of an abrupt transition before the end of this century (although this possibility can not be entirely excluded specify the authors of the report which added that even without the collapse of this cycle twenty-first century, the possible consequences of this event can be serious, including a southward shift of the belt of tropical rain and a faster rise in North Atlantic sea level, and disruption of marine ecosystems.
- If this treadmill is changed, the heat transfer area between the sea and the atmosphere will also be with a major impact on many aspects of the global climate system and that, on several scales of time scales.
- Many clues indicate a strong link between past changes in thermohaline circulation and past fluctuations of the Atlantic surface temperature.
- Paleoclimatic data show that seems to have been in the past (during the ice age in particular) significant changes over short periods (ten-year), with an overall climate impact very important locally (in the Atlantic but also in the remote regions of the world). And even with projections based on a weakening moderate scenario, it is very likely that a multi-decadal scale warming occurs on most of the European Region and North America.
Translated from Wikipedia