The first line of evidence they considered was modern-day warming. Sherwood and Mark Webb, a climate scientist at the United Kingdom's Met Office, agreed to lead the effort. "Work on the ends, rather than on 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. Many participants were dissatisfied with the IPCC process and wanted to look at how physical mechanisms might set the boundaries of the sensitivity range. The WCRP study arose out of a 2015 workshop at Schloss Ringberg, a castle in the Bavarian Alps. "We now have enough independent lines of evidence that we don't need to use the climate models as their own line." 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. Yet these high-end models struggle to accurately recreate the climate of the 20th century, undermining their credibility. They run hot, some predicting warming of more than 5☌ for a CO 2 doubling, apparently because of the way they render clouds, especially over the Southern Ocean. Recent models suggest the range might even go higher. Those numbers-based in part on a model Hansen had developed-stuck around far longer than anyone imagined: The latest IPCC report, from 2013, gave the same range. That summer, at a meeting in Woods Hole, Massachusetts, led by Jule Charney, scientists produced a paper, known ever since as the Charney report, that predicted between 1.5☌ and 4.5☌ warming for a CO 2 doubling. Models have historically been used to estimate sensitivity, beginning in 1979, with the world's first comprehensive assessment of CO 2-driven climate change. The study dispels uncertainty introduced by the latest climate models. "The decreasing uncertainty could potentially motivate more jurisdictions to act." A clearer picture of those consequences could do much to spur local governments to cut emissions and adapt to warming, says Diana Reckien, a climate planning expert at the University of Twente.
The estimate will also inform projections for sea-level rise, economic damage, and much else. The WCRP sensitivity estimate is designed to be used by the United Nations's Intergovernmental Panel on Climate Change (IPCC) when it publishes its next major report in 2021 or 2022. "For folks hoping for something better, those hopes are less grounded in reality," says David Victor, a climate policy researcher at the University of California, San Diego, who was not part of the study. The report underscores the risks of that course: It rules out the milder levels of warming sometimes invoked by those who would avoid emissions cuts. Humanity has already emitted enough CO 2 to be halfway to the doubling point of 560 parts per million, and many emissions scenarios have the planet reaching that threshold by 2060. Whoever shepherded this deserves our gratitude." "It is an impressive, comprehensive study, and I am not just saying that because I agree with the result. The new study is the payoff of decades of advances in climate science, says James Hansen, the famed retired NASA climate scientist who helped craft the first sensitivity range in 1979. "This is the number that really controls how bad global warming is going to be." They support a likely warming range of between 2.6☌ and 3.9☌, says Steven Sherwood, one of the study's lead authors and a climate scientist at the University of New South Wales. The assessment, conducted under the World Climate Research Programme (WCRP) and publishing this week in Reviews of Geophysics, relies on three strands of evidence: trends indicated by contemporary warming, the latest understanding of the feedback effects that can slow or accelerate climate change, and lessons from ancient climates. Now, in a landmark effort, a team of 25 scientists has significantly narrowed the bounds on this critical factor, known as climate sensitivity. It seems like such a simple question: How hot is Earth going to get? Yet for 40 years, climate scientists have repeated the same unsatisfying answer: If humans double atmospheric carbon dioxide (CO 2) from preindustrial levels, the planet will eventually warm between 1.5☌ and 4.5☌-a temperature range that encompasses everything from a merely troubling rise to a catastrophic one.