by Jean Jouzel
Lindzen defends, among other things, the idea known as “iris”:1 in tropical regions warming would provoke a decrease in the area of convective zones whose summit plays the role of greenhouse effect and thus an increase in those where the air dried by that convection drops, giving rise to a negative retroaction. Observations have not verified the validity of that hypothesis. It is, however, confirmed that the main source of uncertainty comes from retroactions linked to clouds. But we must also emphasize that this simple exercise of a doubling of CO2 does not take into account all the sources of uncertainty, in particular those resulting from interactions between the carbon cycle and the warming that tends to decrease the intensity of absorption of carbon both on the continents and in the oceans.
To look into the future, the climatologist needs, in addition to models, to know the way in which the composition of the atmosphere and its greenhouse effect, as well as the other radiative forcings, those in particular connected to aerosols, will evolve. And here he turns to economists who have reflected on the evolution of CO2, CH4, N2O, and other greenhouse gas emissions and that of aerosols from now to the end of the century. They define families of scenarios with different trajectories, obviously none of which aims to predict our future, rather the whole of which covers all possibilities. With a gamut, moreover extremely wide, taking into account the demography, the type of development, the way in which energy is produced and used, and so forth. Let’s take the case of CO2. The emitting scenario (Figure 13.1), A2, corresponds to a rapid increase based largely on the use of fossil fuel; in 2100, emissions would be multiplied by four compared to the beginning of the century, or 7 gtC, reaching nearly 30 gtC. The least emitter, B1, established on a society of services encouraging clean energy, is conveyed first by an increase followed by a decrease that brings CO2 emission down to around 5 gtC in 2100. Not surprisingly, the most emitting scenario leads to the largest increase in concentrations that, in 2100, would be multiplied, compared to their preindustrial value of 280 ppmv, by around three; in terms of radiative forcing, taking into account the other greenhouse gases and aerosols, we would be at nearly 9 Wm–2 compared to 1750, an additional radiative forcing of around 4%.
Although it shows the least amount of carbon, scenario B1 leads to concentrations close to 550 ppmv, an increase of nearly 90%. Furthermore, these calculations do not take into account the interactions between the carbon cycle and the climate, which are likely to increase those concentrations from around 100 ppmv to 300 ppmv in an extreme case. More disturbing is the fact that this scenario does not lead to a stabilization of the greenhouse effect by 2100. Indeed, it is not sufficient to maintain emissions at their current level—which on average is more or less the case in this scenario—to reach the goal of stabilization of concentrations. This is implacable logic, since each year vegetation and oceans absorb only around half of what is emitted, indeed a bit less; the other half accumulates in the atmosphere. In fact economists do not seriously envision that our emissions can diminish rapidly or greatly, a goal that is indispensable from a climatological point of view, which we will see below.
Figure 13.1. Scenarios from the IPCC: emissions (left) and carbon dioxide concentrations (right). Source: IPCC report, 1996.
However, this range of scenarios enables modelers to make climatic projections that cover all of the climatic possibilities if we do not stabilize the greenhouse effect—unless something unexpected occurs (we will return to this). Let’s look first at the average warming that will be reached by the end of the twenty-first century: the projections presented in 2007, which, for the first time, took into account the interactions between the climate and the carbon cycle, cover a wide gamut between 1.1 and 6.4°C, little different from those of the 2001 report. About half of the large range of these projections comes from the behavior of our societies, which must expect greater warming as they will emit more greenhouse gases, and the other half comes from uncertainty linked to the behavior of the climatic system. However, in 2007 we had more confidence than we did in 2001 in the central levels of the different scenarios, from 1.8°C for the most conserving to 4°C for the most emitting.
Let’s take the case of an intermediary scenario, such as A1B characterized by emissions of CO2 that have returned to 13 gtC in 2100 after having culminated at 17 gtC in 2050; the climate of the period 2080–99 will be warmer by close to 2.8°C compared to 1980–99 and by 3.3°C compared to the pre-industrial climate (Figure 13.2). We can easily see that this would be a true climatic upheaval, since the average warming that our planet would experience from now to the end of the century would be around half of that which characterized the passing from the last glacial period to current conditions, a path that has taken more than five thousand years.
This is especially true since the warming, which as we have emphasized has become more rapid, would be accompanied by other important climatic changes. Thus it is probable that heat waves and events of heavy precipitation will continue to become more frequent; the summer of 2003, which in France was more than 3°C warmer than the average summer of the twentieth century, would be considered a normal summer in the second part of the twenty-first century because France, like other continental regions in general, will warm more than the average. It is probable that tropical cyclones will become more intense with stronger winds and precipitations. Outside the tropics, it is predicted that the trajectories of storms will be more pole-ward, inducing changes in the distribution of winds, precipitation, and temperatures. The amount of precipitation will increase in the high latitudes and will decrease in the subtropical regions and as far as the Mediterranean basin, with less rain in the summer over a large part of France, up to 30% less rain in the south.
Beyond this very general overview, everything that involves the snow and ice will be greatly affected.
What Will Become of Our Glaciers?
Given the general retreat of glaciers during the second half of the twentieth century, with the exception of those of Scandinavia and New Zealand, it is legitimate to raise questions about the survival of some of them in a climate that would warm up notably during the twenty-first century. From the 2007 IPCC report we learn that the annual loss through ablation—essentially summer melting—is great, from 30 to 40 centimeters per degree Celsius, and that it could not be compensated for by a potential increase in precipitation that on average does not exceed 1 to 2%. With a few rare exceptions, the glaciers will thus continue to retreat, especially since the pollution lightly blackens the surface and from that decreases the albedo, as much as 10% for some very polluted regions of the Northern Hemisphere. Beyond these general considerations, it is clear that the future of a glacier depends on the regional characteristics of the evolution of the climate and of those of each glacier—its surface area, volume, orientation, altitude, and geometry of flow. They must be addressed on a case-by-case basis.
Figure 13.2. Evolution of the average global temperature for the twentieth century and projections for scenarios A2, A1B, and B1. The rectangles on the right indicate the warming predicted for a group of six scenarios ordered by increasing levels of emissions and the uncertainties associated with them. Source: S. Solomon et al., Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge: Cambridge University Press, 2007).
Once again we will rely on the extremely well-documented work by Bernard Francou and Christian Vincent, who undertook this inquiry, and take a few examples from it. They draw attention to the fact that for a given warming the annual melting depends enormously on the altitude: in the French Alps, warming has a determinant effect in the lower part of the glacier (up to 2,400 meters) and less or not at all in its upper part (around 3,600 meters and above). The glaciers that are most sensitive to the climate are the maritime glaciers of Scandinavia and New Zealand. With moderate warming, the altitude of the line of equilibrium should rise to around 300 meters in the Alps, which would condemn glaciers like Saint-Sorlin, except for a bit of re
sidual ice not at the summit but in a valley located at 2,900 meters, of Gébroulaz, and that of Arolla in the Valais. That warming would leave little chance of survival for the Argentière Glacier; those of Nigardsreen in Norway and Franz-Josef in New Zealand, which are distinguished by strong advances at the end of the twentieth century, in a hundred years will be severely reduced in the case of the former and a bit less so in the case of the latter. The Mer de Glace should resist better due to the high altitude of its basin of accumulation. If we add shorter snow seasons to that, mountain massifs around the world will be quite different from what they are today. And this is also true for those we have not explicitly cited in North America, South America, Africa, and Asia.
Figure 13.3. The top two figures correspond to the warming predicted for Europe between the periods 2080–99 and 1980–99 in the case of the median scenario A1B, in winter (left) and in summer (right). The bottom two figures indicate variations in the amount of corresponding precipitations. Source: S. Solomon et al., Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge: Cambridge University Press, 2007).
An Arctic Ocean without Ice?
The contrast is astounding when we compare the warming projected in the Arctic on the one hand and in Antarctica on the other. Of the same magnitude as the average warming in the south—around 3°C in the A1B scenario—it is amplified by a factor of two, even three, in the high latitudes of the Northern Hemisphere where regionally it could reach 10°C. These high values are in line with those that have been observed in the Great North for a few decades and result from the processes of amplification that we have already discussed. They are accompanied logically by a decrease in snow cover and a melting of frozen ground with an increase of 30 to 40% of thickness in the active zone, which thaws then freezes every year, and by substantial increases of methane emissions in those regions.
One of the great questions raised by warming concerns the sea ice and its disappearance in the summer; that disappearance would enable some maritime routes to open—initially just partially then eventually extending to the entire Arctic Ocean. Part of the year it would then be free of ice. Here, too, the positive retroactions come into play. Thus when sea ice disappears, the ocean surfaces that replace it are more absorbent, which tends to increase melting. The 2007 IPCC report predicted a moderate decrease in this winter ice and an almost complete disappearance of it in the summer in 2100. This decrease would be less great around Antarctica than in the Arctic. The report also noted a disparity in the results obtained by different sea ice models. The results of the American model from the NCAR, which projects a disappearance of the summer ice by 2040, were emphasized as soon as the summer observations of 2007 that demonstrated an extremely abrupt decrease in the extent of ice in the Arctic were published. In retrospect (Figure 12.6), none of the models used had foreseen such a decrease, and some researchers, followed by Al Gore, went so far as to predict the definitive disappearance of the ice during the summer in the next ten years or even sooner. The rise in the minimum ice cover in 2008 and then in 2009 calls for caution in this type of prediction, but the very low value observed in 2012 gives some credit to it.
Surprises under the Frozen Ground
When you walk along the prestigious Nevsky Avenue in Saint Petersburg, you discover in the souvenir shops many objects sculpted out of mammoth ivory from the remains of those mammals that disappeared ten thousand years ago and were rediscovered in the frozen ground of Siberia. Riches of the frozen ground, mammoths were not the only things found. In particular, that ground contains methane hydrates that are particularly sought after and desired by oil and mineral companies because they are a potential source of fuel that can replace other fossil fuels (oil, coal).
What are these hydrates (also called clathrates) of methane? Are they as mysterious as suggested by the title of the book by Gérard Lambert and his colleagues, Le méthane et le destin de la Terre. Les hydrates de méthane: Rêve ou cauchemar? In this book the authors describe the state of the art of these strange substances. As their name indicates, their formation necessitates at least two basic ingredients: H2O molecules and methane molecules, CH4. But the intimate blending of these molecules requires very special conditions because H2O molecules must be organized in a sort of cage within which those of methane are imprisoned, hence the name clathrate, from the Greek klathron, which means closure. Two factors are necessary for the sequestration of the methane and for the endurance of its hydrates: high pressure and low temperatures, which explains their presence in the frozen ground.
For that which involves the constituents of the cage, the ground generally contains water, which freezes when the temperature becomes negative. In the polar regions this very deep ground remains permanently frozen—we call this permafrost, whose thickness can reach more than a kilometer in Siberia. Near the surface—up to several dozen meters depending on the site—the active layer melts in the summer and freezes again in the winter. The permafrost, not covered with glaciers or ice caps, is essentially concentrated in the Arctic where it covers a huge surface.
This ground, as in the case of frozen peat, is rich in organic matter. Methane, the principal component of natural gas, is formed by methanogenic bacteria that live in an anaerobic environment (that is, deprived of oxygen) from the process of decaying and of digestion of organic matter, whether of vegetal or animal origin. Thus in the permafrost, the source of methane can come from the presence in situ of methanogenic bacteria. But the analysis of this methane indicates that for the most part it comes from the migration of sources as deep as five thousand meters where it is formed at high temperature—we then speak of thermal methane—when buried vegetal remains begin to carbonize under the effect of heat to form peat, lignite, and coal and to free methane. Are the hydrates of methane in the permafrost a dream or a threat for the future?
They are a dream because their use is a potential source of energy whose combustion emits less CO2 and fewer atmospheric pollutants than that of oil or coal. Estimates show a huge reservoir of methane hydrates on the Earth, but the difficulty and costs of extracting it are on par with the amount of the reserve, and most hydrates are found at the bottom of oceans; frozen ground contains only a small proportion of them.
But they are also a threat for the future insofar as climate warming is concerned. As indicated in the 2007 IPCC report, the area occupied by the permafrost in the Northern Hemisphere could decrease by 20 to 35% by 2050, essentially in the more southern zones. Furthermore, the thickness of the active layer, the one that melts each summer and freezes the following winter, will increase in many places. These two phenomena will contribute to a surplus of methane emissions into the atmosphere. However, the IPCC report remains extremely cautious compared to the hypothesis of an increase in the sources of carbon dioxide in Arctic regions.
A More Rapid and Higher Sea-Level Rise than Predicted
We have deliberately left aside aspects connected to a future rise in sea level. Granted, only part of the rising is connected to glaciers, ice caps and inland ice sheets; the other results from thermal expansion. Both are important in the evolution of the sea level in the mid- and long term, but the largest uncertainty lies in the contribution of ice.
What are the facts regarding the dilatation of the oceans? It is directly connected to how much warming occurs. From the least- to the most-emitting scenario, the average value of thermal expansion in the twenty-first century increases from 17 to 29 centimeters with, for a given scenario, a range of variation of more than a factor of two.
The contribution of the cryosphere is analyzed separately for the glaciers and the polar ice caps, which have contributed between 10 and 12 centimeters to the rise in sea level. As for Greenland and Antarctica, the difficulty lies in correctly evaluating the difference in the surface mass and the flow of ice that is emitted into the ocean, which depends on the dynamics of the glacier. Models as well as data indicate that the warming of the
high latitudes of the Northern Hemisphere is accelerating the melting on Greenland in the coastal regions. The increase in snowfall that results from an intensification of the hydrological cycle is not able to compensate for this increased melting (the warmer it is, the more likely a cloud is to contain water vapor and to provide precipitation). The IPCC estimates that over the entire twentieth century, Greenland contributed an average 3- to 6-centimeter rise in sea level. Antarctica, on the other hand, would be too cold to be affected in a significant way by melting in coastal regions; on the contrary, it could increase in volume thanks to that increase of snowfall and thus very partially stall the rise in sea level by 6 to 8 centimeters on average. The result of all of these contributions is an average rise in sea level of 28 to 43 centimeters, which becomes even greater since we know that warming will increase, which means that, taking into account the different uncertainties of the parameters, the rise could be approximately 18 to 59 centimeters.
But these estimates do not fully take into account the acceleration observed in the flow in the coastal regions in Greenland, as well as in West Antarctica. This acceleration has not been formally attributed to human activity. But the IPCC’s preliminary estimates show that if it continues alongside warming it could cause an additional 10- to 20-centimeter sea level rise from now to the end of the century. Some researchers argue that the rise in sea level is underestimated and that it could exceed one meter by 2100.2 What has been observed during the last few years should caution us not to reject those alarmist estimates without another form of analysis; following the rhythm observed for 2005 and 2006, Antarctica and Greenland would contribute to a rise of about 10 centimeters by 2100, not taking into account the IPCC estimates. This contribution could be even greater if the flow of the emitting glaciers continues to accelerate with climate warming.