by Jean Jouzel
But beyond these uncertainties, all the models (this assertion remains nearly verified now that the number of simulations has been multiplied by more than a factor of ten) forecasted a warming. What is more, this warming is systematically greater than that which would be obtained in the absence of any climatic feedback. This would be the case if our atmosphere were composed of only nitrogen, oxygen, and argon, without any of the greenhouse gases we have mentioned—water vapor, carbon dioxide, methane—and if the albedo of our planet were fixed once and for all. We would then expect a warming of 1.2°C for a CO2 doubling, less than the lowest level of 1.5°C predicted by the models.
Some amplifying mechanisms thus dominate the climate’s response during an increase in the greenhouse effect, and this can be proven. Thus a warming of the atmosphere will, with a certain delay, be transmitted to the layers of the ocean surface. This brings an increase in evaporation, which grows exponentially as a function of the temperature, and consequently of the quantity of water vapor present in the atmosphere. Since water vapor is itself a greenhouse gas, the radiative forcing is amplified. The decrease of sea ice and snow-covered surfaces, which are very reflective vis-à-vis solar radiation, in response to this gradual heating of the oceans constitutes a second amplifying factor because it is then replaced by much more absorbent ocean and continental surfaces.
A large majority of the scientific community has been convinced of the amplitude of the problem and of the necessity of analyzing all its facets. The first results obtained from the Antarctic ice at Vostok also suggest that the climate is an amplifying system. Those results have thus helped raise awareness of the gradual warming of the planet. And that awareness has been further amplified by the fact that the average temperatures measured on the surface of the Earth in the 1980s are clearly rising.
Some scientists believe it is necessary (and important) to make people aware of warming through the media. In the United States, Jim Hansen directs a climate research institute in New York that is part of NASA. In the spring and summer of 1988, a drought that had the potential to be catastrophic affected a large part of North America. Hansen suggested that it was the result of climate warming and that the greenhouse effect had already arrived. He placed a wager—which he won—that warming would increase and that the hottest year up to that point would be surpassed within the following three years. We know Jim Hansen well, as we have collaborated with him. He is a first-rate scientist, and even if some criticize him for being quick to react, he did so with the necessary caution and all scientific honesty. Many others are sounding the alarm. From the closed circle of climate specialists, the issue of the greenhouse effect became known by the general public by the end of the 1980s.
The Establishment of the IPCC
The results of the climate modelers were taken seriously and motivated expert scientists to hold meetings and write reports on climate, one of which, published in 1979 at the initiative of the NSF, had very well-documented conclusions. The World Meteorological Organization (WMO) initiated the World Climate Research Programme (WCRP) and organized the first World Conference on Climate. It concluded that human activity was likely to influence our climate in a significant way. But it was during the following decade that true awareness grew not only within the scientific community but well beyond it as people began to realize the scope of the consequences of this climatic change, both ecologically and economically. In 1985 the Villach conference in Austria on the evaluation of the role of carbon dioxide and other greenhouse gases marked a turning point; the consensus that arose among scientists at the meeting was that as a result of an increase in concentrations of greenhouse gases one could expect, during the first half of the twenty-first century, an increase in the average temperature greater than what had ever been known before.
The scientific community quickly got organized through both the WCRP, which developed rapidly under the direction of the French climatologist Pierre Morel, and the International Council for Science (ICSU), which brought together scientific organizations from around the world. Aware of the strong interactions that exist between the climate and the environment, in 1986 the ICSU launched an ambitious program dedicated to the study of the geosphere and the biosphere: the International Geosphere-Biosphere Program (IGBP). The WCRP and the IGBP were two of the four components of what has henceforth been known as the Global Change program. (The two others are devoted to biodiversity and the human dimension of climate change.) Thanks to these international initiatives, which are now well established by research organizations in many countries and in Europe, an understanding of the complex mechanisms that govern the evolution of the climate has largely been elevated during the last twenty years.
Governments did not delay in getting involved, aware that this was an issue that could not be ignored, especially since the protocol that prohibited the production of components contributing to the destruction of the ozone layer, signed in Montreal in 1987, had already established what could be undertaken on behalf of the global environment. With some distance, the decisions made within that framework appeared completely right and, thanks to them, we can hope for a slowing of the decrease in the ozone layer in the decades to come, then a reversal of the process, and in the second half of the twenty-first century a return to the conditions that prevailed before human activity disturbed the ozone layer. The scientific consensus that was quickly established, the well-identified consequences, and the restricted number of producers largely facilitated the signing and then the implementation of the protocol. Although the issue of climate change presents many similarities to that of stratospheric ozone, the former is much more complex. The existence of a connection between human activity and climate warming then rests essentially on the predictions of models, which we know are still rudimentary, and the consequences of this warming are not well understood. Furthermore, if measures to limit the use of fossil fuels must be created and implemented, it will be difficult for the general public to accept them since economic development and personal comfort are intimately mixed with consumption of energy. The first step thus consists of establishing a diagnosis.
It was with this objective that in 1988, under the joint auspices of the United Nations Environment Programme (UNEP) and the WMO, the IPCC was created. From the beginning the IPCC was interested in assessing the available knowledge in three distinct ways: looking at the scientific aspects of the evolution of the climate (Group I); examining its impacts and analyzing the measures of adaptation (Group II); and assessing the options for mitigating climate change (Group III). Four assessment reports have been published (1990, 1996, 2001, and 2007). Each report is divided into chapters written by a team of a dozen authors from different countries. To produce each chapter the authors solicited contributors working in a specific field. Summaries around fifty pages long of each of the four reports (close to a thousand pages) were written, followed by “summaries for policymakers,” which were much shorter and written in a more accessible style. Everything is brought together by a synthesis report. Once written, each report was commented upon by the scientific community (reviewers) and then by representatives of governmental as well as nongovernmental organizations. It takes more than two years for a text to be written and reviewed, approved by the scientific community, and then proposed to governments. Commentaries from different sources (the scientific community, governmental agencies, nongovernmental organizations) are taken into consideration by the editors, and the texts are then edited. If the editors consider a commentary unfounded, which is sometimes the case, they have to justify the reason or reasons that led them not to take it into consideration. After the publication of the third report, editors were hired to ensure that all of the commentaries are taken into account. As an example, for Group I alone there were 160 principal authors who participated in the fourth report and nearly 600 scientists including editors, contributors, and reviewers.
Then comes the last step before publication: approval by the governments who are members of the IPCC
(more than a hundred countries). The summaries for policymakers were discussed line by line by delegates from member countries and approved, after possible changes, during meetings that representatives from nongovernmental organizations can attend as observers. The general rule is to reach consensus, which is sometimes very difficult despite the efforts of the IPCC Group I co-chairs and the authors present at the final meeting. They try to reach a solution that is acceptable to everyone and reflects the complete reports. The content of the reports is not discussed again, but that of the extended summaries is also subject to approval, and the coherence between the different stages of the reports receives a great deal of attention.
The rest of this chapter and the next chapter look mainly at the work of Group I, which deals with the science of climate change and in particular with the physical aspects of those changes. In the first IPCC report, the role of human activity in the modification of the composition of the atmosphere and the associated increase in the greenhouse effect were fully recognized. The rise in temperature and other climatic parameters was, in each of the successive reports, scrutinized. Two questions came up repeatedly: Has human activity already changed the climate? This question is examined in the present chapter. The second is: What climate for the future? It is discussed in the following chapter.
It is entirely legitimate to wonder whether the warming that we are experiencing on a global scale has a connection with the known increase in the greenhouse effect. In 1990 that warming, estimated at 0.3–0.6°C over the last century, was just getting under way. Without mentioning the glacial periods, we need only turn to the recent past to observe that there is no need for man to intervene for the climate to change in such a notable way, as it did during the twentieth century. For the period that we call the Little Ice Age (from the mid-fifteenth century to the late nineteenth century), this is beyond question. It is supported by much evidence, such as the advance of Alpine glaciers and Flemish painters’ renderings of rivers covered with ice at that time. During that cold period, which reached its height between 1550 and 1770, the temperature was, at least in western Europe, at a minimum one degree colder than it was in the twentieth century. By contrast, the beginning of the last millennium, a period in which the south of Greenland was possibly a land a bit more hospitable than it is today, was relatively warmer. It is difficult in these conditions to assert that this early warming was already connected to human activity.
The first report (1990) of the IPCC responded to the question regarding human activity’s impact on the climate in the following way: “The size of the warming over the last century is broadly consistent with the prediction by climate models, but is also of the same magnitude as natural climate variability. If the sole cause of the observed warming were the human-made greenhouse effect, then the implied climate sensitivity would be near the lower end of the range inferred from models. Thus the observed increase could be largely due to this natural variability: alternatively this variability and other human factors could have offset a still larger human-induced greenhouse warming.”
Scientists noted that the greenhouse effect was increasing and that the climate was warming, but at the time they were not at all in a position to establish a relationship of cause and effect.
The Problem of Aerosols
If we look at the increase in greenhouse gases since the beginning of the industrial era, the models of the evolution of the climate predicted a warming of nearly 1°C, roughly double of that observed. But those predictions did not take into account all the aspects connected to human activity. Thus they ignored the effect of cooling resulting from the presence of microscopic particles, solids or liquids, suspended in the air, which are called aerosols. This was one of the advances of the second report of the IPCC (1996), which focused on them, even though the exact role of those aerosols on climate was at the time and still is not perfectly understood.
Aerosols can have different origins: they can be natural (desert, marine, or volcanic), anthropogenic, connected to the use of fossil fuels, forest fires, or pollution in industrial regions. Their lifetime is very short, on the order of a week, and the geographical distribution of their sources is very heterogeneous. They diffuse and absorb solar radiation and, to a lesser degree, infrared radiation. In general they have a cooling effect, except for carbon soot and for the aerosols issued from biomass fires. From recent measurements, including data obtained from satellites, we can estimate the combined radiative forcing of all of these aerosols at –0.5 Wm–2 with, however, an uncertainty as large as ± 0.4 Wm–2.
This is called the direct radiative forcing of aerosols. But things are even more complicated because these aerosols also act on the microphysical properties inside clouds, some of which play the role of condensation nuclei. Through that indirect effect, aerosols modify the albedo of clouds and thus the radiative forcing associated with them. Once again, there is a cooling effect, of –0.7 Wm–2, but with an even greater uncertainty in the estimates, since those range from –0.3 to –1.8 Wm–2. So the effect of all human activity that involves greenhouse gases, the direct and indirect effect of aerosols, as well as other more marginal contributions, such as those linked to persistent contrails from airplanes, changes in vegetation, and in the deposition of aerosols on snow-covered surfaces, was still not known with great precision in the 2007 report. The effect of such activity was unquestionably dominated by the greenhouse effect, with a positive value of 1.6 Wm–2, but, following the real effect of aerosols, the estimates were between 0.6 and 2.4 Wm–2.
In retrospect, the authors of the second report could have been quite right since the direct radiative forcing was then estimated at –0.5 Wm–2 with an indirect forcing of comparable value. It was in fact sufficient to counteract part of the warming due to an increase in the greenhouse effect and to eliminate the discord between predictions and observations. More convincing for specialists was the inclusion of an entire series of indications corresponding to a series of imprints suggesting that the warming observed was likely not simply of natural origin. These indications rested on geographical, seasonal, and latitudinal comparisons of warming, whose predicted and observed characteristics agreed all the more in that the models took into account the role of the greenhouse effect and of that of aerosols and not only the natural causes of climatic variability such as volcanic eruptions, which could provoke a notable cooling but of short duration, or weak fluctuations in solar activity.
Figure 12.1. Variation between the beginning of the industrial era, around 1750, and 2005, in the principal factors of radiative forcing expressed in watts per square meter (Wm–2). Source: IPCC, Climate Change 2007: Fourth Assessment
None of these elements taken individually constituted proof, but their convergence in 1996 led scientists to conclude that despite uncertainties, “the balance of evidence suggests that there is a discernible human influence on global climate.”3 To suggest, even cautiously, that human activity was beginning to have an influence on the climate placed climate change at the center of the problems that our society would have to confront in matters concerning the environment and gave it an undeniable socioeconomic dimension.
The Climate in the Last Millennium
The assessment was refined between the 1996 and 2001 reports thanks in particular to a better knowledge of the variations in the climate during the last centuries. We quickly understood that it was absolutely crucial to distinguish between the warming of natural origin and that which might already be attributed to an increase in the greenhouse effect linked to human activities. There were many rather homogeneous sources of information for reconstituting that so-called recent climate. We are indebted to Michael Mann and his American colleagues4 for having ingeniously integrated the data drawn from different natural archives—tree rings, coral, polar ice—with historical archives to create a curve describing the evolution of the mean temperature since the year 1000 for the Northern Hemisphere, where climatic series are more numerous. That mean temperature decreased slig
htly from 1000 ad to the end of the nineteenth century: it climbed to a medieval high point between the eleventh and fourteenth centuries and then sank during the Little Ice Age between the fifteenth and the nineteenth centuries. The associated variations in temperatures are weak, however, and the different regions of the Northern Hemisphere were in fact not affected in the same way or at the same time. This curve (Figure 12.2), called the “hockey stick” because of its overall shape, led the IPCC to conclude in 2001 that the warming of the Northern Hemisphere in the twentieth century is likely to have been the largest of any century during the past thousand years. It has since been the object of much criticism, to which the 2007 report mainly responded. That report indicated a greater variability in the temperatures of the Northern Hemisphere than the 2001 report did, with, in particular, colder periods in the twelfth to fourteenth centuries and in the seventeenth to nineteenth centuries. It concluded that average Northern Hemisphere temperatures during the second half of the twentieth century were very likely higher than during any other fifty-year period in the last five hundred years and likely the highest in at least the past 1,300 years. That conclusion left little doubt: recent warming is beyond natural variability.
Figure 12.2. Reconstruction of the temperature of the Northern Hemisphere during the last millennium (known as the “hockey stick” curve) from records derived from the analysis of tree rings, coral, ice cores, and historical archives (thin line), with a smoothed curve in a thick line. Source: IPCC, Climate Change 2001: Third Assessment Report (Cambridge: Cambridge University Press, 2007).
To be able to attribute that warming to human activity we had to separate warming that was the result presumably of that activity from warming that naturally makes the climate vary, primarily volcanoes and solar activity. Volcanic eruptions and variations in solar activity are well documented for the twentieth century, climatic models have made important progress, and the computing power has progressed a great deal, allowing for an increase in the number of simulations. The conclusions are clear. Lengthy simulations, done while separately taking into account the natural and anthropogenic forcings, then by combining them, show that the warming of the last hundred years is most likely not due to natural causes alone. In particular, the noticeable warming of the last fifty years can only be explained if one takes into account the increase in the greenhouse effect. Thus this conclusion in the 2001 report: “Most of the observed increase in global average temperatures since the mid-20th century is likely due to the observed increase in anthropogenic greenhouse gas concentrations.”5 From “perhaps” in 1996 we are now at “probably” in 2001, which in our jargon corresponds to more than two chances out of three.