"The Primary Lesson Is the Confirmation of Preceding Reports"

The winter of 2006-2007 was particularly mild in France and in Europe, is this a sign of Global Warming?

There isn’t an isolated sign in climatology.  It’s through the course of years that we can arrive at statistics that make sense.  If a winter is mild, it takes place repeatedly over a number of years, then we could use the term sign, but one year is insufficient.  However, when we look at the totality of the signs everywhere on the planet showing that the climate is changing, collectively these signs make sense.

The Intergovernmental Panel on Climate Change (IPCC), composed of 2500 climatologists from 130 countries under the aegis of the UN, of which you are a member, made public the 2nd of February, a report regarding the state of our understanding of climate change.  What do you think the primary lesson of the report is?

For me, the primary lesson is the confirmation of preceding reports.  The climate change alerts were given by scientists since the middle of the 1980’s.  The first IPCC report in 1990, produced the “substance” for the Rio Earth Summit in 1992, which itself gave birth to the United Nations Framework Convention on Climate Change, then after the Kyoto Protocol. At this time the models were still very simple, and climate change still hadn’t been observed.  CO2 emissions greatly increased during the 1960’s, and it took several decades for the climatic system to respond to the modifications.  At the end of the 1980’s, we couldn’t yet see any climate change which differed from normal climatic fluctuations. Since then, report after report, there has been a confirmation of the first forecasts, on the one hand, according to models much more complex and complete than those from the first generation, and on the other hand, because we have begun to see indications that the climate is changing like the models had predicted. 

What are those indications?

Of course there is an increase in the temperature that we cannot understand without considering the presence of CO2 in the atmosphere due to human activity.  There are major changes happening in the Arctic – in fact, the models had predicted since the beginning that the artic climates were more sensitive to temperature change than the rest of the planet.  We have started seeing modifications in the hydric cycles, in precipitation, and looking at the planetary level these modifications are coherent with the diagrams of the models.  Lastly, there is the rise in ocean levels, presently 3mm each year, which is beyond what we had noticed during the entire 20th century.

What role does man play in Global Warming? 

The IPCC report tried to statistically analyze the probability of man’s role in the warming that we’ve noticed in the last few decades.  We went from a probability of 66% six years ago to 90% today.  Those are rough estimates of our level of confidence, but that definitely indicates that the observation of temperature change, of precipitation change, ocean level increase, glacier melting, reduction in snow cover, the postponement of blossoming… all of this builds a coherent picture that we cannot explain otherwise than by the result of human activity.

Is there from now on a general consensus about this question?  We see some questioning the reliability of the models…

The models aren’t completely reliable tools. I think anyone who uses models doesn’t have complete confidence in their model.  The problem is to know when to reverse the burden of proof. All of the elements united by science today, by the models, are coherent enough and significant enough that we ask people that say, “nothing is happening”, to provide the burden of proof of their claims. 

Figure: “The colored part represents the change [in temperature] simulated by about 15 models (…) over 3 reference scenarios, which are scenarios where greenhouse gas emissions corresponds to different socio-economic perspectives, but where, in each case, no particular effort is made to reduce emissions –these are the blue, green and red curves that take us to around a warming of 4°C.  The yellow scenario, below, corresponds to the warming that cannot be avoided even if the concentration of CO2 in the atmosphere is stabilized.  The grey bars to the right, attempt to take into account the fact that there are greenhouse gas emission scenarios that are even somewhat pessimistic.  Beyond the processes that are normally taken into account in the models, there are possible amplification phenomena.  Vegetation can release a part of its CO2 into the atmosphere, if it is poorly adapted to the new climate.  This leads to a split much higher that goes up to 6°C“ (quotes by Hervé Le Treut). Source: IPCC report for policymakers

Interview by Jean-Luc Prigent and Gilles Prigent for the Newsteam agency.

Translation by Christopher Le Coq

Hervé Le Treut is the director of the Laboratoire de Météorologie Dynamique, a laboratory of the Ecole Polytechnique, the Ecole Normale Superieure, the University of Paris VI, and of CNRS.