SCIENCE:

New study says climate change 'irreversible' for 1,000 years after emissions stop

The world will keep warming and sea levels will rise for 1,000 years after carbon dioxide emissions stop, according to a new study that concludes that climate change is largely irreversible.

"I think people have imagined if we stopped emitting carbon dioxide, climate change would reverse in 100 years or 200 years," said Susan Solomon, an atmospheric chemist with the National Oceanic and Atmospheric Administration. "That's not true."

Solomon is the lead author of the new research, published today in the journal Proceedings of the National Academy of Sciences.

According to the scientists, "irreversible" climate change includes effects that will continue for a millennium after humans stop adding carbon dioxide to the atmosphere. Rising global temperatures and sea level, along with drier conditions in some regions, fit that bill, they said.

That's due in large part to the behavior of the world's oceans, Solomon said. The seas, slow to warm, have absorbed a lot of heat from the atmosphere, along with about 80 percent of CO2 emissions. But eventually, that will change, and oceans will start pumping heat and CO2 back into the atmosphere.

The effects could be dramatic. If the atmospheric carbon dioxide level reaches 450 to 600 parts per million by the end of the century, sea levels would rise up to 1 meter, or about 3 feet. Rainfall would drop in the Mediterranean, southern Africa and the southwestern United States, enough to prompt "dust-bowl conditions," the scientists said.

To put that in perspective, the current CO2 level in the atmosphere is 385 ppm. And according to the Intergovernmental Panel on Climate Change, limiting CO2 to 450 ppm would require worldwide emission cuts of 50 to 80 percent by 2050.

'The more we add, the worse we're going to make it'

"I think you have to think of this stuff more like nuclear waste than acid rain," Solomon said of carbon dioxide. "The more we add, the worse we're going to make it."

Ken Caldeira, a climate scientist at Stanford University's Carnegie Institution, said the paper is "really important and basically right."

Other scientists, including Caldeira, have examined how long carbon dioxide is likely to linger in the atmosphere once emissions stop. But the new research has "carried it forward" by looking at what the potential effects of that delayed warming would be, Caldeira said.

Kevin Trenberth, a climatologist at the National Center for Atmospheric Research, said it's an important message for the wider public to grasp. "This aspect is one that is poorly appreciated by policymakers and the general public: Many aspects of the changes that are slowly coming are not really or practically reversible," he said.

Trenberth also said that he believes the paper underestimates how high temperatures and sea levels would rise, calling it "quite conservative." But he also said he believes that the paper errs in its description of rainfall changes, arguing that climate change would likely bring more intense weather events, including longer droughts, but would not necessarily change the total amount of precipitation.

Roger Pielke Jr., a climate policy specialist at the University of Colorado, said the study would not likely "change the nature of the debate" about cutting emissions, at least in the short term.

"Decisions are going to be made about mitigation based on short-term costs and benefits of those actions," he said. "In the very long term, if things turn out to be as bad as projected ... then we'll have technologies to do that."

Solomon, the study's lead author, disagreed.

'Nobody knows how to put a cost on that'

If climate change is irreversible, "that seems to me more reason to do something about it, so you're not committing to something you can't back out of," she said. The scientist said her study doesn't mean fighting global warming is hopeless, but that acting quickly to cut emissions is important.

"One way you can think about it is, if we slow it down, we have more time to find solutions," she said.

That means taking action even in the face of some level of scientific uncertainty, said Trenberth.

"The policy relevance is clear: We need to act sooner, even if there is some doubt about exactly what will happen," he said. "By the time the public and policymakers really realize the changes are here, it is far too late to do anything about it. In fact, as the authors point out, it is already too late for some effects."

Joe Romm, a senior fellow at the Center for American Progress, said the study points out flaws in the economic models used to determine the costs of various strategies to limit emissions.

"One of the points that the authors make at the end is that economic models don't get this irreversibility issue," he said. "We have all these debates about costs and benefits, but you can't undo these changes for 1,000 years or more. Nobody knows how to put a cost on that."

The study's other authors include Gian-Kasper Plattner and Reto Knutti of ETH Zurich in Switzerland and Pierre Friedlingstein of Institut Pierre Simon Laplace in Gif-sur-Yvette, France.