How much, exactly, will greenhouse gases heat the planet?
For more than 40 years, scientists have expressed the response as a range of possible temperature increases, between 1.5 and 4.5 degrees Celsius, that will result from carbon dioxide levels doubling since pre-industrial times. Now, a team of researchers has dramatically narrowed the temperature range, adjusting it to between 2.6 and 4.1 degrees Celsius.
Steven Sherwood, a climate scientist at the University of New South Wales in Sydney, Australia, and author of the new report, said the group’s research suggested that these temperature changes, known as “climate sensitivity” because they reflect how sensitive the planet is on the rise in carbon dioxide levels, it is now unlikely below the lower end of the range. The research also suggests that “alarmingly high sensitivities” of 5 degrees Celsius or more are less likely, although “they are not impossible,” said Dr. Sherwood.
However, what remains is a series of effects that spell global disaster if emissions are not drastically reduced in the coming years.
Masahiro Watanabe, a professor at the University of Tokyo’s Atmospheric and Oceans Research Institute and author of the report, said determining a precise range of temperatures was of critical importance to international efforts to tackle global warming, such as the climate deal. from Paris, and to mitigate the effects of climate change.
“Reducing uncertainty is relevant not only for climate science but also for the society responsible for making sound decisions,” he said.
The new document, published Wednesday in the Reviews of Geophysics magazine, narrowed the temperature range considerably and shifted it toward warmer results. The researchers determined that there was less than a 5 percent chance of a temperature change below two degrees, but a 6 to 18 percent chance of a temperature change greater than 4.5 degrees.
If the effects of carbon dioxide are at the lower end of the range or even below it, climate change will be less affected by emissions and the planet will heat up more slowly. If Earth’s climate is more sensitive to carbon dioxide levels, the expected results are not only more imminent, but also more catastrophic.
Scientists noted that Earth’s temperature is already about 1.2 degrees Celsius above pre-industrial levels, and that if current emission trends continue, doubling of atmospheric carbon dioxide could occur well before the end of this century.
Andrew Dessler, a climate scientist at Texas A&M University, who was not the author of the report but was one of its first external reviewers, called the document “a true tour de force”, adding that “this is probably the most important article I have read it in years.
For many years, those who wanted to minimize the threat of climate change have tried to say that sensitivity is low, so an increase in greenhouse gases would have little effect. And some recent climate models have suggested that warming could be terribly worse.
The value of the document, Dr. Dessler said, lies in how it reduces the likely range of temperatures the world can expect. “There were a number of people who argued that climate sensitivity was much lower, and a smaller number who claimed it was much higher,” he said, “and I think the case for any one of those positions is much weaker now than this document has been published. “
That means those who undermine the severity of climate change and the need to act have a much more difficult case to present now, Dr. Dessler said. “It would be great if skeptics were right,” he said. “But it is pretty clear that the data is not compatible with that contention.”
The document, produced under an international scientific organization, the World Climate Research Program, brought together three broad fields of climate evidence: temperature records from the industrial revolution, records of prehistoric temperatures preserved in things like sediment samples and tree rings, as well as satellite observations and computer models of the climate system. Neither could only determine the range, but the researchers mathematically found ways to reconcile the three disciplines to reach their conclusions.
“This document is really the first to attempt to include all of those disparate sources of observational evidence in a coherent package that really makes sense,” said Gavin A. Schmidt, director of NASA’s Goddard Institute for Space Studies and author of the article. .
Another author of the article, Gabriele Hegerl, a professor of climate system science at the University of Edinburgh, said the way the threads of the research came together was surprising: “We don’t expect these three lines of evidence to fully coincide.” she said, but hoped they would “overlap.” And they did, he said, so “our research is stronger than I initially expected.”
Not everyone is prepared to accept the new results. Nicholas Lewis, a freelance scientist who has been critical of aspects of conventional climate research and who has found flaws in the work of others that led to the retraction last year of a major study on ocean warming, questioned the dependency of the new document in computer models to interpret the lines of evidence, as well as the group definition of climate sensitivity itself. He also suggested that the document ignored some possible complications from cloud changes and convection.
Dr. Schmidt said the new document made all the data and methodology available. “This is a real challenge for people who think experts are wrong to go in, change assumptions, run code, and show us their results,” he said.
A certain degree of uncertainty about global warming is inevitable, said Zeke Hausfather, a scientist at The Breakthrough Institute and author of the article. But the current range “is not a good amount of warming,” he said, noting that removing the extremes still leaves a mid-range that means a climate disaster. “It doesn’t take five degrees of warming to justify climate action,” he said. “Three degrees is bad enough.”
William Collins, a climate scientist at Lawrence Berkeley National Laboratory who was not involved in the study, praised the effort to tie so much research together into one document, but said that major advances in computing and data collection will continue to drive the search for answers. He compared climate sensitivity research to climbing Mount Everest and said: “This is an extremely important base camp. We are not at the pinnacle yet. ”
