When it comes to fighting climate change, pick a number -- any number.
Nearly 200 countries have signed a U.N. treaty pledging to avoid "dangerous" climate change. But lately, it seems, "dangerous" is lost in translation. Fifteen years since that agreement took effect, scientists and governments are still grappling with what carrying out its promise means.
For the European Union, it means limiting Earth's warming to just 2 degrees Celsius hotter by the end of this century than it was before the Industrial Revolution. That's a goal many experts believe is roughly equivalent to capping atmospheric carbon dioxide at 450 parts per million. But a growing number of countries -- mostly vulnerable ones and small island nations like the Maldives -- say that won't prevent rising sea levels from swamping their coasts.
They're calling for an even stricter standard: 350 parts per million, a number endorsed by NASA climatologist James Hansen.
Some scientists mapping out Earth's potential futures say both targets are arbitrary. What's essential, they insist, is that countries start cutting their greenhouse gas emissions soon and stay flexible in case the planet behaves in unexpected ways.
"The best guesses are not carved in stone," says Gavin Schmidt, a climate modeler at NASA's Goddard Institute for Space Studies. "There well may be surprises, pleasant or unpleasant."
Many experts said ecosystems may react in ways that could exacerbate warming. Some studies have suggested that the world's oceans, which absorb a significant chunk of CO2 emissions, will lose some of that ability as temperatures rise. Other analyses predict that warming will thaw large swaths of permafrost, releasing huge amounts of methane that would accelerate climate change.
Acknowledging the difficulty in pinpointing how sensitive Earth may turn out to be, the Intergovernmental Panel on Climate Change projected that the planet would warm anywhere from 1 to 6 degrees Celsius by the end of the century, said Stephen Schneider, a climate scientist at Stanford University's Woods Institute for the Environment.
Scientists say just 'dial it back'
"That's the difference between a little more than twice where we are now and 'Oh my god,'" Schneider said of the IPCC's temperature range.
To put those numbers in context, Earth has warmed about 0.7 degrees Celsius since the beginning of the Industrial Revolution. In that same period, the level of CO2 in the atmosphere rose from about 280 ppm to its current 387 ppm.
Even reaching the 450 ppm target isn't a guarantee -- a sentiment stated bluntly in a letter that 20 prominent climate scientists recently sent to members of Congress, urging an even more stringent emissions goal "to hold the risk of ruinous climatic change to an acceptably low level."
"When you put in all those uncertainties, the idea that you can hopefully say 450 [ppm] will be 2 degrees [Celsius], it'll be the same thing -- that's rather optimistic," NASA's Schmidt said.
"If we're very lucky, 450 might be another 1 degree. If we're very unlucky, it might be another 3 degrees." With that in mind, the scientist said he thinks 350 ppm is a good long-term goal.
"We can see changes in ice sheets happening now at roughly 390," he said, "and the planet hasn't even caught up with 390 yet. ... The prudent thing is to say, 'If we can dial it back again, let's do it.'"
Along those lines, Jason Lowe, head of mitigation advice at the U.K.-based Met Office's Hadley Centre, said the best approach to managing climate may simply be to start cutting emissions as soon as possible. His recent research suggests that governments need to start slashing emissions during the next decade, peaking by 2016 and cutting their output 4 percent every year after that.
"The headline result is that as long as you carry that on, you do have a chance of meeting the 2 degree target," he said.
'No such thing as a safe level'
At the most basic level, Lowe said, it means countries that cut earlier will have less onerous cuts to make in later years. "In some ways, you're lowering the risk," Lowe explained. "Every year of delay will make it harder to cut emissions down the road."
Flexibility is also important, said Schmidt, pointing to lessons gleaned from another U.N. environmental treaty, the 1990 Montreal Protocol. Countries that signed the treaty eventually agreed to phase out the use of ozone-depleting chlorofluorocarbons. But it took years of experience and several rounds of revisions to reach that point.
"It took 4 or 5 goes for [treaty parties] to get to those decisions that had a big impact," Schmidt said. With climate change, he said, governments should have short-term targets "commensurate with the size of the problem" that they can revisit regularly.
"2050 is long enough away that we are going to get a few bites at this cherry," he said.
Despite the wrestling over treaty targets, scientists said one point is clear: The world needs to cut its greenhouse gas output.
Richard Betts, head of climate impacts at the Met Office, said that if current CO2 emissions trends continue, the world could warm an average of 4 degrees Celsius by 2100, with devastating effects.
Some parts of the globe would bear the brunt of that heating, said Betts, quoting new results he is set to present today at a conference organized by Oxford University. The Arctic -- where scientists already predict summer sea ice could disappear by 2040 -- could warm up to 10 degrees Celsius, or roughly 18 degrees Fahrenheit. Temperatures could rise up to 7 degrees in southern and western Africa, decreasing rainfall by 20 percent or more and raising the risk of severe drought.
"Everyone's focused on the 2 degree target, but if we don't reduce emissions in the next decade, there's a fair chance we'll go above 2 degrees," Betts said. "Four degrees is quite possible later this century."
Schneider, the Stanford scientist, put it bluntly.
"We're betting the planet," he said of haggling over emissions targets. "There's no such thing as a safe level. There's a level of very risky, versus mildly risky."