Wetlands from Bangladesh to Florida submerged. Drought and devastating heat in important granaries such as the Yangtze floodplain in China or Ukraine. Rains that come too often or too hard in India or the U.S. Northeast. The list of potentially devastating impacts from climate change is a long one. But with greenhouse gas emissions continuing to climb and concentrations in the atmosphere rising by roughly two parts per million (ppm) a year, climate catastrophes are looking more and more imminent.

"Today's greenhouse gas levels [387 ppm] would already be plenty high enough to cause over two degrees of warming even if we stabilized concentrations tomorrow," says physicist Myles Allen of the University of Oxford. "Two degrees isn't 'safe,' in that there will be negative impacts for many regions and systems even with two degrees of warming, but anything over two degrees starts to look much more serious."

So how do we keep global average temperatures from warming more than two degrees Celsius? Scientists have begun to turn their attention to answering this critical question now that the potential impacts of climate change have become clear. The solutions offered range from a tax on emissions of carbon dioxide to an end to forest-clearing for agriculture.

Past is prologue?
Today's climate situation could appear relatively benign. After all, digging into the geologic record for climate change reveals that previous periods, such as the Eemian interglacial more than 130,000 years ago, have been nearly this warm. "On a global average it was around 1 degree Celsius warmer," says climate scientist James Hansen of the NASA Goddard Institute for Space Studies (GISS). But "it was still the same planet. It was not that different."

One major difference, of course, was that sea levels were at least four meters higher in the Eemian, and preindustrial climate changes seem to have been driven by small shifts in Earth's orbit, which, in turn affect the amount of sunlight reaching the planet.

Present climate change, driven by increasing greenhouse gases in Earth's atmosphere, has other effects beyond droughts, heat waves, rising sea levels and more intense or more frequent rains. Most notably, the warming climate is rendering the oceans more acidic—with attendant impacts on sea life, such as coral reefs.

And it remains unclear how other factors strengthen or diminish the effects of greenhouse gases trapping more of the sun's heat. "The question is how do the feedbacks kick in from water vapor, clouds, sea ice that amplify or maybe diminish the impact from CO2, methane and the rest," says climate scientist Jon Foley of the University of Minnesota.

Plus, "the human-made forcing is very unusual. The forcings that drove paleoclimate were much slower and weaker," Hansen says. "Now we've got nearly instantaneous change within a century."

And that instantaneous change seems to be accelerating. Climatologist Stephen Schneider of Stanford University argues that "given the current trajectory, there's not a large chance that we won't warm above two degrees Celsius." But "the world is not fine at 1.8, it does not die at 2.2. It's a judgment about when it's gone beyond danger."

A matter of pricing and deforestation
That judgment, of course, varies depending on whether one is living in the Arctic or a coral atoll or in the middle of a large continental landmass like North America or Asia. But it is clear that there are several keys to reining in greenhouse gas emissions. "As long as fossil fuels are the cheapest form of energy, they are going to continue to be burned," Hansen argues, identifying the largest source of greenhouse gas emissions from human activity. "You've got to put a price, a gradually rising price, on carbon emissions."

That price will have to be global, according to various climate scientists. "Any credible plan for avoiding dangerous climate change will have to address the question of what India, China, Russia and the U.S. are going to do with the coal they have underground that we cannot afford for them to release into the atmosphere," Oxford's Allen notes. "If they are not going to use that coal, ever, then who is going to compensate them for the benefits lost? And if they are going to use it, then who is going to pay for its carbon content to be sequestered?"

On top of that, slowing the ongoing clearing of forests from Brazil to Indonesia would address a full 12 percent of greenhouse gas emissions from human activity. "Deforestation continues to be one billion to two billion metric tons of carbon pear year, more than most countries in the world," Minnesota's Foley notes. "These forests are hugely important to biodiversity, watersheds, bioprospecting. These are treasure troves and we are burning them down for cheap land. Maybe we can help the economics be different."

That cheap land, of course, is being cleared to grow food—whether grains and other crops directly or to make pasture for cattle. Such agriculture will bear the brunt of climate change as many of the crops humanity relies on are finely tuned to thrive in the present climate and may wither as temperatures warm. More importantly, from the perspective of feeding a growing population, yields will likely drop without new varieties being developed. "All the grains we grow are beyond their thermal maximum. For every degree Celsius [that mean global temperature] increases, yield drops 10 percent," notes geochemist Wallace Broecker of Columbia University's Earth Institute. "In the meantime, population is going up by 30 percent. With three degrees [Celsius] warming, it's 30 percent down in grain and 30 percent up in population, then you've got a big problem."

A range of win–win scenarios
Fortunately, scientists are now able to answer questions such as the impacts from a given policy or program. For example, researchers can now identify specific sectors or programs that can yield specific results as far as addressing environmental problems, says climate modeler Gavin Schmidt of NASA's GISS. "Switch to plug-in electric vehicles and, even with the current mix of [electricity] generation, you still end up ahead." The same is true for burning wood or coal for cooking or heat in Asia. A switch to electricity for these needs would diminish both deforestation and the Asian brown cloud. "By finding win–win–win scenarios across a range of policy goals you end up bringing more people to the table," Schmidt notes.

And there is a great deal of room for progress in using energy more efficiently. "The U.S. has a vast opportunity to increase efficiency to match Europe or Japan," Foley says.

Mandatory performance standards are needed worldwide, adds Stanford's Schneider: "Nobody needs a kilowatt. What they need is air conditioning or a refrigerator."

But "adaptation policies to deal with climate change in the pipeline we can't prevent" will also be needed, Schneider argues. "Who is most vulnerable to climate change? Poor people in hot countries. Who contributed the least to the problem? Poor people in hot countries."

In order to alleviate the poverty that still afflicts more than one billion people, policymakers need to leave room for development—and its attendant greenhouse gas emissions. "The world is going to have to create a carbon pie eventually and divide it up by country," Columbia's Broecker says. "A total amount of CO2 that each country will be able to add to the atmosphere. If they want to burn more than that then they'd have to take an equivalent amount out again."

And, what is also unclear is exactly what will happen when and if greenhouse gas emissions—and concentrations in the atmosphere—begin to come back down again. "Anthropogenic carbon dioxide is mixed rapidly through the atmosphere, land biosphere and near-surface ocean, such that a certain fraction remains in the atmosphere. This fraction has remained remarkably constant over the past century and we don't really know how it will change if and when we cut emissions, never having done such a thing for an extended period of time," Oxford's Allen notes. But "to solve the problem we need to eliminate net emissions of carbon dioxide entirely."