It seems like such a simple question: How hot is Earth going to get? However, for 40 years, climate scientists have repeated the same unsatisfactory answer: if humans double atmospheric carbon dioxide (COtwo) From pre-industrial levels, the planet will eventually warm to between 1.5 ° C and 4.5 ° C, a temperature range that encompasses everything from a merely worrying rise to a catastrophic one.
Now, in a historic effort, a team of 25 scientists has significantly reduced the limits of this critical factor, known as climate sensitivity. The assessment, conducted under the World Climate Research Program (WCRP) and published this week in Geophysical Reviews, is based on three pieces of evidence: trends indicated by contemporary warming, the latest understanding of feedback effects that can slow or accelerate climate change, and lessons from ancient climates. They support a likely warming range of between 2.6 ° C and 3.9 ° C, says Steven Sherwood, one of the study's lead authors and a climate scientist at the University of New South Wales. "This is the number that really controls how bad global warming will be."
The new study is the reward for decades of advances in climate science, says James Hansen, the famous retired NASA climate scientist who helped craft the first sensitivity range in 1979. "It is an impressive and comprehensive study, and I am not saying that because I agree with the result Whoever guided this deserves our gratitude.
Humanity has already emitted enough COtwo halfway through the doubling point of 560 parts per million, and many emission scenarios cause the planet to reach that threshold by 2060. The report underscores the risks of that course: it rules out the milder levels of warming sometimes invoked by those that would avoid emission cuts. "For people hoping for something better, those hopes are less based on reality," says David Victor, a climate policy researcher at the University of California, San Diego, who was not part of the study.
The WCRP sensitivity estimate is designed to be used by the United Nations Intergovernmental Panel on Climate Change (IPCC) when it publishes its next major report in 2021 or 2022. The estimate will also report projections for sea level rise. , economic damages and much more. plus. A clearer picture of those consequences could do much to stimulate local governments to reduce emissions and adapt to warming, says Diana Reckien, an expert in climate planning at the University of Twente. "Decreasing uncertainty could motivate more jurisdictions to act."
The study dispels the uncertainty introduced by the latest climate models. Historically, models have been used to estimate sensitivity, beginning in 1979, with the world's first comprehensive CO assessment.twodriven by climate change. That summer, at a meeting in Woods Hole, Massachusetts, led by Jule Charney, the scientists produced a document, known since then as the Charney report, that predicted a warming of between 1.5 ° C and 4.5 ° C for a COtwo duplication. Those numbers, based in part on a model Hansen had developed, stayed much longer than anyone imagined - the latest IPCC report from 2013 gave the same range.
Recent models suggest that the range could even be greater. They heat up, some predict a warming of more than 5 ° C for a COtwo doubling, apparently due to the way they generate clouds, especially over the Southern Ocean. However, these high-end models struggle to accurately recreate the climate of the 20th century, undermining their credibility. Such models play only a supporting role in the new assessment, says Robert Kopp, a climate scientist at Rutgers University, New Brunswick, who was not involved in the effort. "We now have enough independent lines of evidence that we don't need to use climate models as their own line."
The WCRP study grew out of a 2015 workshop in Schloss Ringberg, a castle in the Bavarian Alps. Many participants were dissatisfied with the IPCC process and wanted to see how physical mechanisms could set the limits of the sensitivity range. "Work at the extremes, rather than the middle," says Bjorn Stevens, a cloud scientist at the Max Planck Institute for Meteorology, who edited the WCRP report with Sandrine Bony of the Pierre Simon Laplace Institute. Sherwood and Mark Webb, a climate scientist at the UK Meteorological Office, agreed to lead the effort.
The first line of evidence they considered was modern warming. Since record keeping began in the 19th century, the average surface temperature increased by 1.1 ° C. Continuing that trend into the future would lead to warming at the lower end of the range. But recent observations have shown that the planet is not heating up evenly; in particular, warming has barely touched parts of the eastern Pacific Ocean and the southern Ocean, where cold, deep waters gush and absorb heat. Eventually, paleoclimatic models and records suggest that these waters will heat up, not only by removing a heat sink, but also by stimulating the formation of clouds over them that will trap more heat. Adjusting temperature projections for this fact rules out low-sensitivity estimates, says Kate Marvel, a climate scientist at NASA's Goddard Institute for Space Studies.
A hot number
The limits on Earth's climate sensitivity (to what extent temperatures will eventually rise to double atmospheric carbon dioxide) were not lowered in 40 years. Using new lines of evidence, a major study now says substantial warming is likely.
Graphic: C. Bickel /Science; Data: Meehl et al., Sci. Adv., 6, eaba1981 (2020)
Second, the team investigated individual weather comments. Some of these, such as the heating effect of water vapor, are well known. But clouds, which can cool or heat the planet depending on how they reflect sunlight and trap heat, have long been a wild card. In particular, climate scientists want to understand the covers of the stratocumulus clouds that form on the coasts. If they grow more in response to warming, as some suspect, they could have a cooling effect.
Several years ago, a set of high-resolution cloud models identified two feedbacks that would have the opposite effect, thinning the clouds and worsening warming. In the models, the higher temperatures allowed more dry air to penetrate thin clouds from above, preventing them from thickening. At the same time, higher COtwo Levels trap heat near cloud tops, reducing turbulence that drives more clouds to form. Since then, satellites have observed this dynamic in warmer-than-average parts of the atmosphere. “There is a growing consensus that the [cloud] the feedback is positive, but not great, “says Thorsten Mauritsen, climate scientist at Stockholm University.
Finally, the team analyzed records from two past climates: 20,000 years ago, at the peak of the last ice age, and a warm period 3 million years ago, the last time atmospheric COtwo levels were similar to today. Recent work suggests that climate sensitivity is not a fixed property of the planet, but changes over time. During warm periods, for example, the absence of ice sheets probably increased sensitivity. Old temperature and CO recordstwo The levels allowed the team to specify sensitivities of 2.5 ° C and 3.2 ° C for the cold and hot periods, respectively. “It’s really comprehensive,” says Jessica Tierney, a paleoclimatologist at the University of Arizona, who was not part of the report. Even for the coldest weather state, she says, the possibility of a sensitivity below 2 ° C seems insignificant.
Gathering the three lines of evidence was a huge task. But wiring them for a unified prediction was even more difficult, says Marvel. The team used Bayesian statistics to record their collected data, which allowed the researchers to test how their assumptions influence the results. “The real advantage” of Bayesian statistics, Tierney says, is how it allows uncertainties at each stage to fuel a bottom line. Co-authors often bump heads, says Marvel. “It was such a long and painful process.” The final range represents a 66% confidence interval, which coincides with the traditional “probable” range of the IPCC. The WCRP team also calculated a 90% confidence interval, which ranges from 2.3 ° C to 4.7 ° C, leaving a slight chance of warming above 5 ° C.
Either way, the report has a simple conclusion: Sherwood says: a doubling of COtwo anything but warms over 2 ° C. “Three main lines of evidence are very difficult to reconcile with the lower end of climate sensitivity.”
In recent years, another uncertainty in the future of the climate has also been reduced: it seems unlikely that global emissions will reach the worst scenarios that the IPCC helped create 15 years ago, ruling out some forecasts of extreme warming. “We are light years from where we were in 1979,” says Reto Knutti, co-author and climate scientist at ETH Zurich.
Unfortunately, the years of work required to achieve that certainty came at a cost: 4 decades of additional emissions and global warming, endlessly.
