New calculations show that CO2 could have a much more predicted effect on the climate, thereby making the objectives of the Paris Agreement out of reach.
According to this work carried out by half a dozen countries (United States, United Kingdom, France, Canada, etc.), the CO2 emissions so far associated with a three degree warming could actually raise the temperature by four or even five degrees. Ultimately, these new calculations will feed into the new projections of the IPCC experts expected next year. "Today we have better models, which more precisely represent climate trends", underlines Olivier Boucher, director of the French institute Pierre Simon Laplace who, like all researchers, benefits from the increase in available data and the computing power since the last IPCC projections in 2013.
A question that has more than a century
These findings show that "it will obviously be more difficult to reach the Paris targets, whether 1.5 or 2 degrees" of warming, set in 2015, said Mark Zelinka, of Lawrence Livermore National Laboratory in California, and author principal of the first evaluation of this new generation of climate models, recently published in the journal Geophysical Research Letters.
For more than a century, scientists have been tackling a seemingly simple question: if the amount of CO2 in the atmosphere doubles, by how much will the Earth's surface heat up? But defining exactly this “climatic sensitivity” is difficult, due in particular to multiple variables, such as the influence of the oceans and forests and their role as “carbon sinks”, which currently captures more than half of human emissions.
An optimistic scenario out of reach
For most of the past 10,000 years, the concentration of CO2 in the atmosphere has been approximately 280 parts per million (ppm). But over this period the world population rose from a few million to 7.6 billion and CO2 emissions have grown exponentially since the 19th century and an industrial revolution fuel to fossil fuels, oil, gas and especially coal. As a result, the CO2 concentration is now 412 ppm, an increase of 45%, half of which in the past 30 years.
With only one degree of global warming currently measured compared to the pre-industrial era, the world is facing an upsurge of extreme phenomena, heat waves, droughts, floods or cyclones. The IPCC has developed four scenarios, the most ambitious of which respects the objective of the Paris Agreement to contain global warming "significantly below two degrees" but requires immediately reducing CO2 emissions by around 10% per year. The most pessimistic would see parts of the Earth completely uninhabitable at the end of the century. Most experts already consider the former as out of reach, and the latter unlikely, unless the planet itself massively releases the carbon already captured.
From three to seven degrees of increase
There remain the two median scenarios. "There's a lively debate in the climate modeling community," says Johan Rockstrom, director of the Potsdam Institute for Climate Impact Research (PIK). “You have 12 or 13 models showing a climatic sensitivity which is no longer at 3 but at 5 or 6 degrees for a doubling of CO2. These are not exceptions, which is particularly worrying. ”
Of the 27 new models examined in his study, they are also the ones that best reflect the developments of the past 75 years, strengthening their credibility. "We have to take them seriously, they are state-of-the-art models," emphasizes the researcher. Other models, which will be taken into account by the IPCC, are less pessimistic, even if most of them exceed the forecast of warming hitherto advanced. "The jury has not yet delivered its verdict, but it's worrying," says Johan Rockstrom. “For over 30 years, climate sensitivity has been evaluated between 1.5 and 4.5 degrees. If it goes between 3 and 7 degrees, it would be extremely dangerous. ”
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