The studies, published in the journal Nature, come at the tail end of the first meeting of President Obama's Major Economies Forum on Climate and Energy. The meeting drew leaders from 17 of the world's highest carbon emitting countries to begin a dialogue that they hope will lead to a new treaty on global warming at the U.N. climate change conference in Copenhagen in December.
The scientists said that they believe the new studies will provide useful insight for policymakers.
"Climate policy needs an exit strategy. As well as reducing carbon emissions right now, we need a play for phasing out net emissions entirely," researcher Myles Allen, an Oxford University physicist and an author of one of the papers, said in a press conference.
More than 100 nations have endorsed the goal of limiting global temperature increases from carbon emissions to 2 degrees Celsius in order to avoid the most dangerous repercussions. But scientists are not certain of the best way to figure out the relationship between the amount of carbon dioxide spewed into the air and the resulting climate change.
Generally, scientists have considered the proportion of carbon dioxide in the air at any one time, but in the new studies, the researchers instead looked at the total amount of carbon released over many decades.
Allen's study found that once humanity had collectively released about 1 trillion tons, the chance of the planet warming more than 2 degrees Celsius rose to one in four. And according to Allen, we're about halfway to the 1 trillion mark and racing there ever more quickly.
"It took us 250 years to burn the first half trillion, and on current trends we'll burn the next half trillion in less than 40," he said. In fact, we've already burned through one third of it in the past 9 years.
In order to reach the goal of not exceeding 1 trillion tons of carbon, the world will have to leave untouched about 75 percent of its fossil fuel reserves -- including almost all of the coal, according to the researchers.
"It really casts doubt on whether any investment into more fossil fuel exploration is really a good investment," said German research Malte Meinshausen, who led the second study.
It also suggests that reaching the G8 target of halving global emissions by 2050 will still leave a significant risk of a more than 2 degree Celsius climate jump, according to Meinshausen.
"Only a fast switch away from fossil fuels will give us a reasonable chance to avoid considerable warming," he said.
Meanwhile, at the climate change summit, envoys said that they came out of meetings more optimistic about the chances of reaching a deal in Copenhagen, although they acknowledged the challenges ahead in getting countries to agree on their level of commitments.
"I come out of this meeting a bit more optimistic," the chief U.S. negotiator on climate change, Todd Stern, told AFP. But he added: "Believe me, I'm not trying to oversell. I would not downplay or underestimate the difficulty of getting an agreement in Copenhagen."
