The study led by Princeton University researchers suggests that it might take a lot less carbon than previously thought to reach the global temperature scientists deem unsafe.
The researchers simulated an Earth on which, after 1,800 billion tonnes of carbon entered the atmosphere, all carbon dioxide emissions suddenly stopped.
Scientists commonly use the scenario of emissions screeching to a stop to gauge the heat-trapping staying power of carbon dioxide.
Within a millennium of this simulated shutoff, the carbon itself faded steadily with 40 percent absorbed by Earth's oceans and landmasses within 20 years and 80 percent soaked up at the end of the 1,000 years.
By itself, such a decrease of atmospheric carbon dioxide should lead to cooling. But the heat trapped by the carbon dioxide took a divergent track.
After a century of cooling, the planet warmed by 0.37 degrees Celsius during the next 400 years as the ocean absorbed less and less heat.
While the resulting temperature spike seems slight, a little heat goes a long way here. Earth has warmed by only 0.85 degrees Celsius since pre-industrial times.
The Intergovernmental Panel on Climate Change estimates that global temperatures a mere 2 degrees Celsius higher than pre-industrial levels would dangerously interfere with the climate system, researchers said.
To avoid that point would mean humans have to keep cumulative carbon dioxide emissions below 1,000 billion tones of carbon, about half of which has already been put into the atmosphere since the dawn of industry.
The lingering warming effect the researchers found, however, suggests that the 2-degree point may be reached with much less carbon, said first author Thomas Frolicher.
"If our results are correct, the total carbon emissions required to stay below 2 degrees of warming would have to be three-quarters of previous estimates, only 750 billion tones instead of 1,000 billion tons of carbon," said Frolicher.
"Thus, limiting the warming to 2 degrees would require keeping future cumulative carbon emissions below 250 billion tons, only half of the already emitted amount of 500 billion tons," Frolicher said.
The researchers' work contradicts a scientific consensus that the global temperature would remain constant or decline if emissions were suddenly cut to zero.

The study was published in the journal Nature Climate Change.


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