Solomon is a scientist with the National Oceanic and Atmospheric Administration. Her new study looked at the consequences of this long-term effect in terms of sea level rise and drought.

If we continue with business as usual for even a few more decades, she says, those emissions could be enough to create permanent dust-bowl conditions in the U.S. Southwest and around the Mediterranean.

"The sea level rise is a much slower thing, so it will take a long time to happen, but we will lock into it, based on the peak level of [carbon dioxide] we reach in this century," Solomon says.

The idea that changes will be irreversible has consequences for how we should deal with climate change. The global thermostat can't be turned down quickly once it's been turned up, so scientists say we need to proceed with more caution right now.

"These are all ... changes that are starting to happen in at least a minor way already," says Michael Oppenheimer of Princeton University. "So the question becomes, where do we stop it, when does all of this become dangerous?"

The answer, he says, is sooner rather than later. Scientists have been trying to advise politicians about finding an acceptable level of carbon dioxide in the atmosphere. The new study suggests that it's even more important to aim low. If we overshoot, the damage can't be easily undone. Oppenheimer feels more urgency than ever to deal with climate change, but he says that in the end, setting acceptable limits for carbon dioxide is a judgment call.

"That's really a political decision because there's more at issue than just the science. It's the issue of what the science says, plus what's feasible politically, plus what's reasonable economically to do," Oppenheimer says.

But despite this grim prognosis, Solomon says this is not time to declare the problem hopeless and give up.

"I guess if it's irreversible, to me it seems all the more reason you might want to do something about it," she says. "Because committing to something that you can't back out of seems to me like a step that you'd want to take even more carefully than something you thought you could reverse."