WSJ Repost: 'A Less Dire, but More Certain Climate Report'

August 12, 2021

IPCC Climate Change Report Shows Less Cause for Panic — But More Urgency to Act

U.N. panel’s study suggests both lower risk of dramatic increases in temperature and higher risk from inaction.

Low water levels in California’s Lake Oroville reservoir this spring.

PHOTO: PATRICK T. FALLON/AGENCE FRANCE-PRESSE/GETTY IMAGES

 

By Greg Ip

Updated Aug. 11, 2021 9:38 pm ET

 

The conclusion of the latest report by the United Nations advisory body on climate—that temperatures are rising, and some effects are irreversible—wasn’t surprising. Those views are largely in line with its latest report, in 2013.

 

More interesting and potentially more significant is that the Intergovernmental Panel on Climate Change has become more confident in those views.

 

Because climate change is so complex, uncertainty is intrinsic to its study: uncertainty over its underlying drivers, their magnitudes and how they interact; over its consequences, such as on weather patterns; and over the costs and benefits of mitigation. It is why the IPCC, which helps shape its 195 member governments’ mitigation plans, assigns varying levels of confidence to its findings, which are based on reviews of thousands of studies and modeling by numerous independent scientists.

 

The increased confidence the IPCC shows in its findings in its latest report has both encouraging and sobering implications. On the encouraging side, the IPCC has dialed back the probability of (without ruling out) more extreme changes in temperature. Such scenarios have often been used to justify much faster and costlier action to ban or limit fossil fuels. On the sobering side, benign outcomes are also less likely, and uncertainty a less viable excuse for inaction.

 

Extreme scenarios have often been used to justify much faster and costlier action to ban or limit fossil fuels; traffic in Brooklyn, N.Y., on Monday.

PHOTO: SPENCER PLATT/GETTY IMAGES

 

“Climate change is in many ways an issue of ‘tails,’ ” said Zeke Hausfather, director of climate and energy at the Breakthrough Institute, a think tank focused on environmental issues, and a contributing author to the IPCC report. Tails represent rarer, more extreme observations in a set of data.

 

The IPCC now thinks a scenario expected to bring three degrees Celsius of warming is much less likely to end up with five degrees, he said. “If your focus is all about the tails of the distribution, it’s a reason to be a little less worried. But at the same time, if you’re hoping climate may not be that big an issue, it’s a reason to be more worried.”

 

The increased confidence comes from more data and new approaches to modeling. The IPCC asks experts to study several different pathways for future greenhouse-gas emissions. The highest-emission pathway in 2013, dubbed RCP 8.5, assumed no new policies to reduce emissions, and it was often treated as the baseline scenario for policy makers. Outside analysts and climate advocates began referring to it as “business as usual,” and proof that inaction entailed catastrophic consequences: an estimated 4.3 degrees of warming (compared with preindustrial times) by the end of the century, far more than the 1.6 to 2.4 degrees envisioned by lower-emission scenarios.

 

But some scholars have since pointed out that RCP 8.5 assumed growth in gross domestic product and emissions that actual data suggest are now unlikely. This year the IPCC acknowledged those concerns, calling the likelihood of RCP 8.5 low.

 

Roger Pielke Jr. of the University of Colorado Boulder, one of those scholars, said the IPCC still gives RCP 8.5 disproportionate attention, but added: “The scarier…extreme scenarios are less plausible than we once thought they were. That fact alone should be cause for a little bit of optimism.”

Estimated increase in global surface temperatures after doubling in CO2 concentrations, degrees celsius. The probability the actual change in temperature lies within this range is 66%.

Source: Breakthrough Institute based on reports by the Intergovernmental Panel on Climate Change

 

The IPCC has also narrowed the range of warming likely to result from a doubling of atmospheric concentrations of carbon dioxide from preindustrial levels. From 1979 to 2013, the IPCC has generally put this metric, known as “equilibrium climate sensitivity,” at 2.5 to 3 degrees Celsius, with a likely range of 1.5 to 4.5 degrees.

 

This year, its central estimate remains 3 degrees, but the likely range has narrowed significantly, to between 2.5 and 4 degrees, meaning much warmer and cooler outcomes are less probable.

 

Several factors have reduced the IPCC’s uncertainty. This year it decided to combine several approaches to estimating climate sensitivity: the use of ancient paleoclimate evidence such as from glaciers; actual observations of surface and ocean temperatures over the past century; and models based on the earth’s physical properties, Mr. Hausfather said.

 

“All three have been used separately by scientists in different disciplines but never before in a single synthesis,” he said. Combining them yields a narrower range of estimates than any single one, he said. The IPCC has also assigned more weight to models whose results fit the historical data better, Mr. Hausfather said.

 

Still, even as the IPCC sees outlier scenarios as less likely, its confidence has grown in its central projections, and their consequences. “Evidence of observed changes in extremes such as heat waves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened” since 2013, it said this week.

 

James Glynn, a scholar at Columbia University’s Center on Global Energy Policy, credits this increased confidence to advances in the science of attributing weather events to temperature. More powerful computers make it easier to search for such links, while the accumulation of more years at higher temperatures has provided more events and thus more data, he said.

 

The upshot is that the IPCC this week has given the world less reason for panic—and for complacency.

 

Write to Greg Ip at greg.ip@wsj.com

Appeared in the August 12, 2021, print edition as 'A Less Dire, but More Certain, Climate Report.'

 

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