So what would the world's temperatures, and the planet, look like in 2100 if greenhouse gas emissions continued as is? And if they were cut by 70 percent?
Researchers at a well-known climate center asked those questions and used a computer model to conclude that it'd be catastrophic if unchecked, but manageable if the world could reduce gases by that much.
"This research indicates that we can no longer avoid significant warming during this century," Warren Washington, the study's lead author, said in a statement. "But if the world were to implement this level of emission cuts, we could stabilize the threat of climate change and avoid catastrophe."
The computer simulation by the National Center for Atmospheric Research in Boulder, Colo., projected that unchecked levels would mean an Earth 4 degrees warmer. But a 70 percent emissions cut would limit the increase to about 2 degrees, the study found.
Other projections if emissions were cut by 70 percent:
- Sea level rise due to thermal expansion from warming waters would be about 5.5 inches instead of 8.7 inches. But either scenario still projects significant sea level rise from melting ice sheets and glaciers.
- Arctic summer sea ice would shrink by about a quarter and stabilize by 2100, as opposed to shrinking at least three-quarters and continuing to melt.
- Arctic warming would be reduced by almost half, helping preserve fisheries, sea birds, polar bears and other wildlife.
- Significant regional changes in precipitation would be cut in half. The U.S. Southwest would not be as dry, and the U.S. Northeast and Canada would not see as much rain as under an unchecked scenario.
The study, which is being published next week in the journal Geophysical Research Letters, is one of the first to use computer models to quantify how much of the effects global warming can be avoided, compared to a world if nothing is done about the problem.
Global emissions still rising
While the study looked at what would happen with dramatic cuts in future emissions, history has shown that reductions are much easier to talk about than to make. The controversial 1997 Kyoto Protocol called for industrialized countries to cut emissions but since then levels worldwide have gone up 25 percent. In the U.S., where emissions are up 6 percent in the last decade, Congress is fiercely arguing over a plan to reduce pollution.
"If we follow on the path that (President Barack) Obama has outlined of cutting emissions by 70 or 80 percent and the rest of the world does it, then we can make a big difference on the climate by the end of the century," Washington told The Associated Press.
But if the United States and Europe cut back on carbon dioxide and China, India and other developing countries do not, then the world is heading toward a harsher, hotter future, Washington said.
The study mapped areas that would benefit the most by emission cuts, comparing what would happen with less carbon dioxide and what would happen if greenhouse gases continue to grow.
Alaska, mountain West vary most
In the United States, the difference between the two scenarios is starkest for temperatures in Alaska and the mountain West, which would see temperatures rise a couple degrees less with emission cuts. Reduced carbon dioxide would also significantly lessen predicted future droughts on the Pacific coast and flooding in the Northeast.
Much of Europe, Russia, China and Australia would see the biggest temperature benefits from reductions in greenhouse gases, while the Mediterranean, Caribbean and North Africa region would benefit the most in predicted changes in rainfall from less global warming.
If the world cuts back on fossil fuels, "it isn't going to be as bad," Washington said.
Washington stressed that the researchers were not studying how cuts could be achieved or advocating a particular policy. "Our goal is to provide policymakers with appropriate research so they can make informed decisions," he said.
The study was funded by the U.S. Department of Energy and the National Science Foundation, which also funds the climate center.