A new study suggests some rather dire news: even if emissions of carbon dioxide stop immediately, the carbon dioxide which is already present in the atmosphere could continue to drive global warming for centuries.
In order to arrive at this prediction, the scientists created a simulation of an earth which had had 1,800 billion tons of carbon dioxide released into its atmosphere, but in which emissions had come to a sudden and complete standstill.
This scenario is often used by scientists when they want to judge how much heat-trapping power that carbon dioxide retains over time.
In this particular simulation, they found that after 20 years the earth’s landmasses and oceans had absorbed about 40 percent of the carbon dioxide. And, by the time the simulation reached the millenium mark, they had absorbed 80 percent of the gas.
On its own, such a decline in carbon dioxide should have created global cooling; however, this is not what the scientists saw. After 100 years of cooling, the earth then warmed up by 0.37 degrees Celsius (0.66 degrees Fahrenheit) during the next four centuries because the oceans absorbed declining amounts of heat.
Although the rise in temperature was small, the scientists note that even a small rise is quite significant. In fact, the Earth has only warmed by 0.85 degrees Celsius (1.5 degrees Fahrenheit) since industrial times began, they say. Further, according to the Intergovernmental Panel on Climate Change, it is estimated that a temperature rise of only 2 degrees Celsius (3.6 degrees Fahrenheit) higher than pre-industrial levels could have serious effects on the climate system. To avoid this outcome, it is necessary to keep total cumulative emissions below 1,000 billion tons, they say. About half of this has already been emitted.
The significance of their findings, the researchers say, is that this 2 degree point may be reached with much less carbon than was originally thought. “If our results are correct,” says study co-author Thomas Fröhlicher, “the total carbon emissions required to stay below 2 degrees of global warming would have to be three-quarters of previous estimates, only 750 billion tons instead of 1,000 billions tons of carbon.” This means, he says, that future carbon emissions would need to be kept below 250 billion tons, rather than the 500 billion tons which was previously believed to be the point where the 2 degree mark would be breached.
Their work also contradicts the currently held consensus that global warming would hold steady or lessen if carbon emissions were suddenly stopped. Previous work, however, did not take into consideration the fact that the oceans would gradually become less able to absorb heat, especially the oceans at the poles, according to Fröhlicher. While temperatures would drop, he says, the declining ability of the oceans to take up heat would offset the drop.
Fröhlicher and his team also showed in their work that the polar oceans have the greatest effect on average global temperature via a mechanism called “ocean-heat uptake efficacy.” This mechanism plays an important role, says Fröhlicher, but previous models did not take this factor into account.
According to Fröhlicher, this study illustrates just how difficult it may be to reverse climate change. Even if we are able to completely stop emissions, global warming could continue for centuries.
The results of the study were published on November 24, 2013 by Nature Climate Change.
By Nancy Schimelpfening