A new report in the journal Nature Climate Change goes against the results of similar research, suggesting that even if global carbon dioxide emissions came to a sudden halt, the residual CO2 in the atmosphere would continue to contribute to global warming for centuries.
The implications of the Princeton University-led research suggest that the world could reach unsafe temperatures by way of a lot less CO2 than previously believed.
The research is based on a climate simulation in which an Earth, loaded with 1,800 billion tons of carbon in the atmosphere, suddenly stopped taking on new carbon dioxide emissions.
Immediate effects of the sudden carbon shutoff would be a cooling effect. But as time progresses, the model predicted gradual warming starting about 100 years after the hypothetical shutoff. Within 400 years of the shutoff, the model predicts global temperatures to rise 0.37 degrees Celsius (0.66 Fahrenheit). The rise seems small, but the researcher note that since pre-industrial times, global temperature has only risen by 0.85 C (1.5 F).
The Intergovernmental Panel on Climate Change estimates that a 2 C (3.6 F) rise in temperature would lead to a dangerous impact on the global climate system. To avoid this, the IPCC moves to keep cumulative CO2 emissions from exceeding 1 billion tons, about half of which has already been put into the atmosphere since pre-industrial times.
But the lingering warming effect predicted by the model suggests the 2 degree Celsius bench mark could be met with much less carbon, according to scientist Thomas Frölicher, the climate study's lead author.
"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 tons instead of 1,000 billion tons of carbon," Frölicher said. "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."
Frölicher and his colleagues' work goes against many other studies suggesting that if carbon emissions were suddenly shut off, global temperature would remain constant or decline.
The researchers account for the contradiction by noting that previous models did not account for a gradual reduction in the ocean's ability to absorb heat from the atmosphere, notably in the polar oceans. An uptick in the heat intake of the polar regions has a larger effect on the global mean temperature than a similar change at lower latitude.
"The regional uptake of heat plays a central role. Previous models have not really represented that very well," Frölicher said.
"Scientists have thought that the temperature stays constant or declines once emissions stop, but now we show that the possibility of a temperature increase can not be excluded," Frölicher said. "This is illustrative of how difficult it may be to reverse climate change -- we stop the emissions, but still get an increase in the global mean temperature."
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