by Jean Jouzel
Warming Is a Certainty
Saturday, February 2, 2007: the IPCC Group I report dealing with the scientific elements of climate change had just been approved in Paris. Susan Solomon from the United States, co-president of that group with Qin-Dahe from China, presented the principal results before more than four hundred journalists from around the world. A sentence caught our attention because of its simplicity: “Climate warming is unequivocal.”6 At the end of the 1980s, as we have mentioned, warming was just beginning following a phase of light cooling, which, in the 1950s and 1960s, had followed the warming observed in the first half of the twentieth century.
Since then warming has accelerated. To be convinced of this one need only look at the data that enable us to follow the evolution of the mean temperature of the planet since 1850 (Figure 12.3). Warming has been more than twice as rapid in the last twenty-five years (close to 0.2°C per decade) than during the last hundred years (warming of 0.74°C over the period 1906–2005). Every recent year has been a warm one. With the exception of 1996, eleven of the twelve years in the period 1995–2006 have been warmer than all those that preceded them. And 2007 has just been added to that list. The year 1998 was particularly warm because of an important El Niño phenomenon, which was at the origin of a marked warming over a part of the Pacific Ocean, whereas 2007 was characterized by an inverse event, La Niña, without which it would have probably been a record year. It is a good bet, as Jim Hansen predicted, moreover, that the record of 1998 will be broken before 2010–11 at the latest, which has since been verified, although 2010 is only slightly warmer than 1998.
Figure 12.3. Change in temperature, of the level of the seas, and of the snow cover in the Northern Hemisphere. Source: IPCC, Climate Change 2007: Fourth Assessment Report (Cambridge: Cambridge University Press, 2007).
What is true for the planet is true for France. The national weather service, as we have seen, recently published a study in which the data available for France were examined closely. Once processed, they were used to establish maps of the variation in temperature during the twentieth century (1901–2000). These revealed that the southwestern quarter, north of the Massif Central and part of the Alps, have warmed by more than 1°C. Everywhere else the increase is between 0.8 and 1°C except for a part of the northeastern quarter for which it is between 0.6 and 0.8°C. Except in those regions, warming is greater than that observed, on average, on the planet. And it has been particularly noticeable since 1985.
Beyond mean annual temperatures, many other indicators have led the IPCC to its precise diagnosis of an unambiguous warming. It is also visible in the oceans, whose average temperatures have increased from the surface to depths of at least 3,000 meters. The result is that oceans are expanding. This thermal expansion of the oceans, together with the melting of glaciers, glacial ice caps, and inland ice sheets, has played a role in the recent acceleration of the rise in sea level; the level of 3.1 millimeters per year observed over the period 1993–2003 is almost double what it was in the last century, estimated at 1.7 millimeters per year, or a rise of 17 centimeters since the beginning of the twentieth century. Another observation that testifies to warming, which we will return to, is a generalized melting of snow and ice. It is also interesting to note that the increase in the average quantity of water vapor in the atmosphere is compatible with the warming observed.
The fact that warming has clearly continued since the beginning of the twenty-first century is one of the elements that, in 2007, enabled us to strengthen our assessment vis-à-vis the role of human activity. Also, new data have shown that the variations of the solar activity since 1750 were previously overestimated by a factor of two. In addition, the effort made by the modelers of comparing simulations that take into account natural forcings on the one hand and all forcings on the other has been intensified. That exercise demonstrated that the influence of human activity was visible not only on the scale of average temperatures of the planet (Figure 12.4) but also on each of the continents taken individually, except Antarctica. It was thus with solid proof that the IPCC reported that “most of the observed increase since the mid–20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.” 7 This was almost a certainty. The IPCC added that it is likely that increases in greenhouse gas concentrations alone would have caused more warming than observed because volcanic and anthropogenic aerosols have offset some warming that would otherwise have taken place.
Figure 12.4. Variation in temperature with respect to the period 1901–50, as observed (thick line), and as predicted by a group of models taking into account both anthropogenic and natural forcings (a) and natural forcings alone (b). All the results of the models are represented by the gray area and their average value by the continuous, thinner curve. Source: IPCC, Climate Change 2007: Fourth Assessment Report (Cambridge: Cambridge University Press, 2007).
The Arguments of Skeptics
The scientific community have been presenting solid arguments for quite some time, but they are still the object of skepticism, in particular among various researchers from other disciplines and outside the scientific realm, whether in the world of political policymakers, large corporations, or the man on the street. In many cases, this skepticism has amounted to denying the impact of human activity on the climate or at least minimizing it.
This seems to be the appropriate moment to look at the steps taken by skeptics from various domains. Let us stress that we believe deeply in the virtue of scientific debate, a debate that is all the richer in that some arguments might provide contradictions to what gradually is becoming the dominant belief. Still, the process undertaken by skeptics must show as much scientific rigor in disproving the link between human activity and global warming as has the scientific community in linking the two. In any event, it is up to those defending the dominant point of view to convince the others. We fully agree with the conclusions of the IPCC, to which two of us, Jean Jouzel and Dominique Raynaud, have contributed. We also have a responsibility to respond to the arguments put forward by skeptics, among whom in France we encounter some in Ma vérité sur la planète (2007) by Claude Allègre. In 2010 Allègre, a geochemist, the recipient of the prestigious Crafoord Prize in 1986, and a former minister from 1997 to 2000, attacked what he called “the climatic imposture.”8 His colleague, Vincent Courtillot,9 challenged the validity of certain data used in the IPCC reports. In France, as in other countries, their arguments resonate both with the general public and with some policymakers in the political and economic spheres.
The rise in climate skepticism has increased since 2009. With the partial failure of Copenhagen—we will come back to this—there was a campaign to discredit the way the IPCC functioned, as well as the conclusions published by that group of experts. That campaign began right before Copenhagen with “Climategate,” which entailed the publication of a series of e-mails hacked from the site of English researchers at the University of East Anglia, including Phil Jones, who is well-known for his work on recent climate data. The campaign was strengthened by the revelation of errors identified in the 2007 IPCC report and was pursued in France and in other countries through the publication of various materials on climate skepticism, such as those we have just cited.
We won’t dwell on Climategate, which among other things illustrates, through exchanges that are rather coarse, the healthy divergence of opinion that can exist among climatologists. The results of a parliamentary inquiry exonerated the University of East Anglia researchers who were accused of falsifying data by certain climate skeptics. In any event, we remain absolutely convinced that those e-mail exchanges, which a priori should not have gone beyond the private realm of the recipients, do not challenge the conclusions of the IPCC.
The “errors” of the IPCC are really regrettable and do it a disservice. They do, however, appear to be inconsequential. At the beginning of 2010, there was in fact only one established error. It concerned the assertion according to which the Himalayan
glaciers will have lost 80% of their surface area by 2035. This could have been a simple typo (2035 might have been typed instead of 2350) or have been connected to the use of “gray” documents that had not been peer reviewed, but the origin of this error, recognized by the IPCC, has not been completely clarified. In any case it does not constitute a key point in the conclusions of a nearly three-thousand-page report. Another “error” was the IPCC’s conclusion that 55% of the Netherlands was located below sea level. The correct number is 26%; an additional 29% could be affected by flooding. Thus it should have been noted that for 55% of the territory, there is a risk of flooding. As for other so-called identified errors (a decrease in harvests in Africa, an overestimation of the cost of climate disasters, a response by the Amazon forest to climate warming), skeptics have pointed these out in an effort to challenge certain conclusions, to which clear responses have been made by scientists directly involved.
Beyond these “errors,” the very functioning of the IPCC has been questioned. In this context, it is interesting to note that the IPCC requested an external audit to examine on its functioning and the various procedures it has followed. With the assurance of the InterAcademy Council, the report disclosed at the end of the summer of 2010 found that the IPCC assessment process has been successful overall but also made recommendations that might be put into effect for the next report. These recommendations aim, in particular, to strengthen the IPCC’s governance and the management (through the election of an executive director and the establishment of an executive committee) as well as the review procedures for creating the IPCC reports (with a more important role for the review editors). Other recommendations deal with improving the evaluation of the uncertainties behind the IPCC assessments, as well as communication and transparency.
Of more concern is the rise in skepticism widely conveyed by the media. It is a skepticism that in itself is legitimate but which we believe is not based on true scientific arguments. It is difficult to present an exhaustive inventory of the arguments here—arguments made by the skeptics—but we will attempt to illustrate our position by examining those that are most often put forth to place in doubt the key conclusions of the 2007 IPCC report. We will focus on a few key points, but the interested reader should refer to the RealClimate website, which is sponsored by American scientists such as Gavin Schmidt and Ray Pierrehumbert and presents an in-depth study of the debate. In France, journalists such as Stéphane Foucart at Le Monde and Sylvestre Huet at Libération, who is also the author of L’imposteur c’est lui,10 have revealed errors and manipulations published in the aforementioned work by Claude Allègre.
In the face of false accusations, an open letter, “Scientific Ethics and Climate Science,” which we signed in April 2010 along with six hundred French scientists involved in climate research, demonstrates the scientific integrity of our research community and solicits the beginning of a truly professional and serious debate. It is that same notion of scientific integrity that more than two hundred members of the American Academy of Sciences put forward in a letter titled “Climate Change and the Integrity of Science,” which was published in the journal Science in May 2010.11
As an example of results that have been distorted by climate skeptics, we cite the observation, used by Claude Allègre, according to which “variations in temperature preceded that of CO2 by 800 years.”12 In fact the presentation of those results, of which one of us, Jean Jouzel, is coauthor, is skewed and biased. With Nicolas Caillon and our glaciologist colleagues, we wrote that the increase in the temperature in Antarctica preceded the variations in CO2 by 800 years but also that these began a few thousand years before the melting of enormous quantities of ice accumulated on the continents of the Northern Hemisphere (Figure 12.5).13 The difference is essential because that sequence is completely compatible with a contribution of an increase in CO2 to the deglaciation of the large ice sheets of the Northern Hemisphere. And it takes a bit of bad faith to use those results with a view toward minimizing the role of human activity on the climate because, in any event—we stress this here—the recent rise in the greenhouse effect has a different character. As a result of human activity (no one denies this), the rise in CO2 preceded the global warming currently under way.
Figure 12.5. Sequence of events during a deglaciation, around 250,000 years ago. The vertical line corresponds to the beginning of the deglaciation in the Northern Hemisphere, around 238,000 years ago, as it is indicated by a modification in the content of oxygen 18 in the air and in the concentration of methane. Several thousand years before the deglaciation, the concentration of carbon dioxide began to increase, and then about 800 years later the temperature in Antarctica began to increase; around 238,500 years ago the temperature in Antarctica had already reached its maximum, and that would be true a few hundred years later for carbon dioxide. Source: Adapted from Nicolas Caillon et al., “Timing of Atmospheric CO2 and Antarctic Temperature Changes across Termination-III,” Science 299 (2003): 1728–31.
The ability to precisely measure an average global temperature, such as the curve produced by Phil Jones, one of those used by the IPCC, is itself doubted by skeptics. This curve, of course, was established with the greatest care, which involved correcting certain effects such as those linked to urbanization, the moving of certain stations, or local modifications in the environment. Recent studies have confirmed that both urbanization and changes in land use have had a negligible effect, less than six-thousandths of a degree per decade, on global temperature measurements. The IPCC does not rely on Phil Jones’s curve alone but also on those produced independently by two American teams; the three series are completely coherent since the beginning of the twentieth century, a period used to compare the variations, observed and predicted, of the average temperature of the planet. The results of an independent and new analysis conducted by a team led by the physicist Richard Muller, founder of Berkeley Earth Surface Temperature at University of California, Berkeley, were presented at the end of 2011. This team finds reliable evidence of a rise in world land temperature of approximately 1°C since the mid-1950s. As mentioned by Muller, this confirms that previous studies were done carefully and that potential biases identified by climate change skeptics did not seriously affect their conclusions.
Whereas the vast majority of scientists who are working on the recent evolution of our climate think that “warming is unequivocal,” some climate skeptics have argued that in the last few years we have been in a period of cooling. Indeed, 2008 was colder than 2007—itself a bit colder than 2005—and warming has clearly shown a slowdown since 2001. But for a climatologist the climate must be assessed on the scale of decades at least. The conclusions are clear: the first decade of the twenty-first century was the warmest since the end of the nineteenth century, by a quarter of a degree compared to the 1990s, which were already considered warm. To look at the climate year by year is not without risk to the skeptics’ arguments: 2010 was the hottest year in 130 years. The temperature data compiled by Jim Hansen’s American team (NASA/GISS) support this assertion, even if there is a small difference between Hansen’s data and those of the English team of Phil Jones (HadCRUT). The difference observed between the two series, a tenth of a degree at most, can be attributed to the fact that Hansen used more stations in the Arctic regions. When the same stations are used, that difference is reduced to a few hundredths of a degree, an agreement that demonstrates, if need be, the futile nature of the attacks made against Jones within the framework of Climategate. Let’s add that a slowdown such as that observed since 2001 is absolutely not abnormal in a climate that is warming under the effect of a regular increase in greenhouse effect. To be convinced of this, one need only examine in detail the projections made for different emission scenarios: warming is not regular from one year to the next, and the plateaus of a dozen years, or even more, can be identified in these simulations even if the greenhouse effect is regularly increasing. Finally, the idea, dear to skeptics, of a cooling under way makes no se
nse when we look at other indicators. Thus over the period 2005–8, Greenland and West Antarctica have each year contributed about a millimeter in the rise in sea level, and the sea level rise can only be explained by a warming effect (thermal expansion of the ocean, melting of continental ice). This is far from being anecdotal.
The difference between the warming measured on the surface of the Earth and that observed in the atmosphere from balloons and satellite data, which was claimed to be three times weaker, was another point on which skeptics based their arguments until recently. This difference was due to a skewing linked to the fact that the influence of the cooling of the stratosphere over satellite measurements was not correctly taken into account. Once that problem was fixed, the different sets of data provided results that were completely similar in the period over which they could be compared (1979 to the present), with warming by decade at 0.16 to 0.18°C on the surface and 0.12 to 0.19°C in altitude, leaving skeptics without an argument.