Scientists have warned that new climate models show that carbon dioxide (CO2) is a more potent greenhouse gas than previously understood, a finding that could put the goals of the United Nations-sponsored Paris Agreement treaty out of reach. to limit global warming.
Developed in parallel by separate teams in half a dozen countries, the models, which will support revised UN temperature projections next year, suggest that scientists have underestimated the warming potential of CO2 for decades.
The AFP news agency reported that much more information and computing power is available since the current projections of the Intergovernmental Panel on Climate Change (IPCC) were finalized in 2013.
"We have better models now," Dr. Olivier Boucher, director of the Institut Pierre Simon Laplace Climate Modeling Center in Paris, told AFP, adding that "they represent current climate trends with greater precision."
The most influential projections from government-backed teams in the United States, Britain, France, and Canada point to a future in which CO2 concentrations that have long been equated with a three-degree Celsius world would likely warm the planet's surface by four degrees. or five degrees.
"If you think that the new models give a more realistic image, then, of course, it will be more difficult to achieve the Paris targets, whether it is 1.5 or two degrees centigrade," scientist Dr. Zelinka told the same news agency.
Dr. Zelinka, from Lawrence Livermore National Laboratory in California, is the lead author of the first peer-reviewed assessment of the new generation of models, published earlier this month in Geophysical Research Letters.
For more than a century, scientists have questioned a seemingly simple question: If the amount of CO2 in the atmosphere doubles, how much will the Earth's surface warm over time?
The resulting temperature rise is known as the Earth's "weather sensitivity". This number has been difficult to pin down due to a large number of elusive variables.
It is difficult to predict whether the oceans and forests, for example, will continue to absorb more than half of the CO2 emitted by humanity. However, the biggest wild card has always been the clouds.
"How clouds evolve in a warmer climate and whether they will exert a tempering or amplifying effect has long been a major source of uncertainty," explained Imperial College London researcher Dr Joeri Rogelj, IPCC lead author on Budget. global carbon, the amount of greenhouse gases that can be emitted without exceeding a given temperature limit.
AFP detailed that the new models reflect a better understanding of cloud dynamics in at least two ways that reinforce the impact of warming CO2.
Dr. Zelinkad said that new research confirmed that high clouds in the lower layer of Earth's atmosphere increase radiation from the Sun, and global warming accentuates that dynamic.
"Another big uncertainty has been how low-lying clouds, such as stratocumulus covers on the west coast of continents, will change," he said, adding: "That has been the holy grail of climate modelers for a long time."
Recent observations suggest that this type of cloud cover decreases with warming, which means that less energy from the Sun is returned to space by the white surfaces.
For most of the last 10,000 years, during which humanity's numbers increased from a few million to 7.6 billion, the CO2 concentration in the atmosphere was nearly constant at 280 parts per million (ppm).
However, in the early 1800s, when the industrial revolution accelerated, fueled by oil, gas, and especially coal, the number of CO2 molecules in air soared.
Today the concentration is 412 ppm, an increase of 45 percent, half in the last three decades.
Last year alone, human activity injected more than 41 billion tons of CO2 into the atmosphere, about five million tons per hour.
With one degree Celsius of warming so far, the world is grappling with increasingly deadly heat waves, droughts, floods, and tropical cyclones that are made more destructive by rising seas.
In the 1890s, the Swedish chemist Svante Arrhenius calculated that doubling the CO2 would eventually raise the average surface temperature of Tierracinco or six degrees Celsius, although he later revised the figure to 4.0 ° C.
Surprisingly, he recognized that burning fossil fuels could one day drive that change.
In the late 1970s, scientists established a climate sensitivity of 3.0 ° C (plus or minus 1.5 ° C), corresponding to approximately 560ppm of CO2 in the atmosphere. That assessment has remained largely unchanged, until now.
The IPCC, the UN's climate advisory body, poses four scenarios for future warming, depending on how aggressively humanity works to reduce greenhouse gases.
Most ambitious, in line with the Paris Agreement goal of limiting temperature rise to "well below" 2.0 ° C, would require drastically reducing CO2 emissions by more than 10 percent per year, from now on.
At the other extreme, a so-called “business as usual” trajectory of increased use of fossil fuels would leave large areas of the planet uninhabitable by the end of the century.
The first scenario has become an illusion, according to many scientists, while the worst case scenario is unlikely unless the Earth itself begins to release natural reserves of greenhouse gases, for example by melting permafrost.
That leaves two intermediate scenarios, known as RCP4.5 and RCP6.0, which likely reflect our climate future.
According to the IPCC, the former would correspond to 538ppm of CO2 in the atmosphere, while an RCP6.0 pathway would see an increase in CO2 concentration to 670ppm.
A doubling of CO2 levels from 1850 to about 570 ppm falls between the two, and thus takes on a real-world importance that would probably have surprised Arrhenius, the late 19th-century Swedish chemist.
"Right now, there is an enormously heated debate within the climate models community," said Earth system scientist Professor Johan Rockstrom, director of the Potsdam Institute for Climate Impact Research.
"He has 12 or 13 models that show a sensitivity that is no longer 3.0 ° C, but 5.0 ° C or 6.0 ° C with a doubling of CO2," he commented in an interview with AFP, adding: "What is particularly worrying is these are not the outliers ”.
Models from France, the US Department of Energy, Britain's Met Office and Canada show a climate sensitivity of 4.9C, 5.3C, 5.5C and 5.6C respectively, said Dr. Zelinka. ”Take it seriously these models are highly developed and are the latest in technology ”, explained the scientist.
AFP reported that among the 27 new models examined in Dr. Zelinka's study, these were also among the best adapted to climate change in the past 75 years, a further validation of their accuracy.
That said, other models that will be incorporated into the next IPCC Main Assessment Report found significantly smaller increases, although almost all were higher than previous estimates.
Scientists also continue to carefully study the results to look for methodological flaws or inconsistencies.
“Climatic sensitivity has been in the range of 1.5 ° C to 4.5 ° C for over 30 years. If it now moves between 3.0 ° C and 7.0 ° C, that would be tremendously dangerous, ”said Professor Rockstrom.
The models suggest that scientists have consistently underestimated the warming potential of CO2 for decades.
News models reflect a better understanding of cloud dynamics in at least two ways that reinforce the impact of warming CO2.
Last year, human activity injected more than 41 billion tons of CO2 into the atmosphere, about five million tons per hour.
With just one degree Celsius of warming so far, the world is grappling with increasingly deadly heat waves, wildfires, droughts, floods, and tropical cyclones that become more destructive by rising seas.