Matter of degree: Temperature goal an issue in climate talks

In 2010, international negotiators formally adopted a goal of limiting warming to 2 degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial times; Earth has already warmed nearly 1 degree.

Climate scientists say both numbers are a bit arbitrary, but keeping warming to 1.5 degrees could increase the chance of survival for coral reefs, slow the rise in the number of ever-increasing severe weather disasters, and help keep the planet from hitting dreaded but so far unseen tipping points of irreversible environmental damage.

"There is no such thing as a guardrail where we are free from all the impacts of climate change; we've already seen climate change and it's widespread," said Chris Field, a Carnegie Institution scientist who headed an international scientific assessment of climate impacts two years ago.

For some countries, keeping warming to 1.5 degrees may require steep and painful cuts in carbon dioxide emissions from the burning of coal, oil and gas.

Climate negotiators are trying to figure out how much countries must reduce carbon dioxide emissions to keep the planet from heating up to dangerous levels; how to make sure countries do what they promise; and who pays for it all.

"Whether the text will also take into account a very justifiable request from the most vulnerable countries to improve on those efforts, it remains to be seen how that is going to be handled," Figueres said in an interview with The Associated Press.

Others, like James Hansen, a former NASA climate chief turned activist, said 2 degrees is too much warming, calling it "a crazy target to have; you cannot say that's a safe guardrail."