Even if the world can cap carbon dioxide emissions tied to global warming, expect to see droughts and sea level rise that span centuries, not just decades, according to a new study sponsored by the U.S. government.
"People have imagined that if we stopped emitting carbon dioxide the climate would go back to normal in 100 years, 200 years; that's not true," lead author Susan Solomon told reporters.
Instead, the team concluded, warming tied to higher CO2 "is largely irreversible for 1,000 years after emissions stop."
"Climate change is slow, but it is unstoppable" said Solomon, a researcher at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo.
All the more reason to act quickly, so the long-term situation doesn't get even worse, Solomon said.
Waiting could compound problems
Gerald Meehl, a senior scientist at the National Center for Atmospheric Research, added that "the real concern is that the longer we wait to do something, the higher the level of irreversible climate change to which we'll have to adapt." Meehl was not part of Solomon's research team.
The findings were announced as President Barack Obama ordered reviews that could lead to automobiles that emit less CO2 and get higher mileage.
Climate change has been driven by gases in the atmosphere that trap heat from solar radiation and raise the planet's temperature — the "greenhouse effect." Carbon dioxide has been the most important of those gases because it remains in the air for hundreds of years. While other gases are responsible for nearly half of the warming, they degrade more rapidly, Solomon said.
Before the industrial revolution the air contained about 280 parts per million of carbon dioxide. That has risen to 385 ppm today, and politicians and scientists have debated at what level it could be stabilized.
The peer-reviewed study concludes that if CO2 is allowed to peak at 450-600 parts per million, the results would include persistent decreases in dry-season rainfall that are comparable to the 1930s U.S. Dust Bowl in zones including the U.S. southwest, southern Europe, Africa, eastern South America and western Australia.
The study, which relied on computer models and historical temperatures, was published in Tuesday's edition of Proceedings of the National Academy of Sciences.
Warming and the seas
Warmer climate also is causing expansion of the ocean and that factor alone is likely to lock in a 1.3 to 3.2 foot sea level rise by the year 3000 if CO2 peaks at 600 ppm, and double that if it peaks at 1,000 ppm, the researchers calculated.
"Additional contributions to sea level rise from the melting of glaciers and polar ice sheets are too uncertain to quantify in the same way," Solomon said in a statement. "They could be even larger but we just don’t have the same level of knowledge about those terms. We presented the minimum sea level rise that we can expect from well-understood physics, and we were surprised that it was so large."
Solomon noted that while global warming has been slowed by the oceans, which absorb carbon, that positive effect will wane over time and eventually oceans will actually warm the planet by giving off their accumulated heat to the air.
Alan Robock, of the Center for Environmental Prediction at Rutgers University, agreed with the report's assessment.
"It's not like air pollution where if we turn off a smokestack, in a few days the air is clear," said Robock, who was not part of Solomon's research team. "It means we have to try even harder to reduce emissions," he said.
Solomon's report "is quite important, not alarmist, and very important for the current debates on climate policy," added Jonathan Overpeck, a climate researcher at the University of Arizona.
'Quite conservative' figures
While scientists have been aware of the long-term aspects of climate change, the new report highlights and provides more specifics on them, said Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research.
"This aspect is one that is poorly appreciated by policymakers and the general public and it is real," said Trenberth, who was not part of the research group.
"The temperature changes and the sea level changes are, if anything underestimated and quite conservative, especially for sea level," he said.
While he agreed that the rainfall changes mentioned in the paper are under way, Trenberth disagreed with some details of that part of the report.
"Even so, there would be changes in snow (to rain), snow pack and water resources, and irreversible consequences even if not quite the way the authors describe," he said. "The policy relevance is clear: We need to act sooner ... because 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."
Geoengineering to remove CO2 from the atmosphere was not considered in the study. "Ideas about taking the carbon dioxide away after the world puts it in have been proposed, but right now those are very speculative," Solomon stated.
Co-authors of the paper were Gian-Kaspar Plattner and Reto Knutti of the Swiss Federal Institute of Technology in Zurich and Pierre Friedlingstein of the National Institute for Scientific Research, Gif sur Yvette, France.