One of the world’s most influential climate scientists is back at it with a dire new prediction about global warming. But some scientists say he’s missed the mark this time. James Hansen became famous for his groundbreaking testimony at a 1988 Senate hearing on the greenhouse effect, where he warned the world of the dangers of climate change. Now, nearly 40 years later, he and a team of international co-authors have published an incendiary paper suggesting that global warming may proceed even faster than scientists previously predicted, Chelsea Harvey writes. Relying largely on evidence from Earth’s ancient climate history, the paper argues that the planet is far more sensitive to greenhouse gas emissions than other studies have indicated. If Earth’s carbon dioxide concentrations were to double their preindustrial levels — reaching about 560 parts per million in the atmosphere — the planet would warm by about 4.8 degrees Celsius, or 8.6 degrees Fahrenheit, the paper suggests. That’s about 2 C more than the U.N.’s Intergovernmental Panel on Climate Change predicts. Global carbon dioxide concentrations have already risen above 400 parts per million. The outcomes of Hansen’s nightmare scenario would be extreme, including at least a few feet of sea-level rise and catastrophic increases in dangerous heat waves and extreme weather events around the globe. The paper also warns that the target under the Paris climate agreement to keep warming well below 2 C may be approaching faster than expected. The planet could exceed 1.5 C of warming by the end of this decade, it predicts, and 2 C by 2050. Scientific criticisms But some scientists argue that Hansen’s new findings are overblown. They suggest that the paper’s methods, including its reliance on paleoclimate data, are subjective and don’t make a strong enough case for its conclusions. And they take issue with Hansen’s dismissal of other studies, including the IPCC’s own climate projections. The paper, overall, “adds very little to the literature,” Piers Forster, director of the Priestly International Centre for Climate at Leeds University in the U.K. and a lead chapter author of the IPCC’s latest assessment report, said by email. Michael Oppenheimer, director of the Center for Policy Research on Energy and Environment at Princeton University — a scientist who testified alongside Hansen at the seminal 1988 Senate hearing — had a slightly softer take. The paper presents a “worst-worst-case” scenario, he said. It’s possible but probably unlikely. Still, there’s value in examining such scenarios, he added. “I think it’s perfectly legitimate to have a worst-worst-case out there,” he said. “They help people think about what the boundaries of the possible are, and they are necessary for risk management against the climate problem.” Hansen counters that the paper is based on robust evidence and describes its critics’ views as a form of “scientific reticence,” or resistance to new ideas within the scientific community. “It takes a long time for new results to sink into the community,” Hansen said.
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