Carbon dioxide indirectly causes up to 50 percent more global warming than originally thought, a finding that raises questions over targets for stabilizing carbon emissions over the long term, a study said yesterday.
In a paper published in the journal Nature Geoscience, British scientists said a tool commonly used in climate modeling may have miscalculated the sensitivity of key natural processes to the warming caused by carbon dioxide.
As a result, calculations for man-made global warming on the basis of carbon emissions may be underpitched by between 30 and 50 percent, they said.
PHOTO: REUTERS
The study was published on the eve of a 12-day UN conference in Copenhagen aimed at providing a durable solution to the greenhouse-gas problem.
The authors said the more-than-expected warming would unfold over a matter of hundreds of years, rather than this century.
The findings do not mean that the predictions for temperatures by 2100 established by the UN’s Intergovernmental Panel on Climate Change (IPCC) should be rewritten, they said.
“We don’t want to be overly alarmist here,” said lead author Dan Lunt of the University of Bristol. “But if people are thinking about stabilizing CO2 at a certain atmospheric level, or putting together a treaty, or having a debate about what the levels should be, it really is important to know what the long-term consequences of those emissions are going to be, because CO2 hangs around for so long.”
Lunt and colleagues tested a widely used climate model on an epoch called the mid-Pliocene warm period, about 3 million years ago, when Earth heated up in response to natural processes.
Cores drilled from ocean sediment provide a good idea about atmospheric carbon levels and temperature at the time.
What the team found, though, was that the CO2 levels in the Pliocene — around 400 parts per million — were not consistent with the warming, which was around 3ºC higher than today.
The difference could only be fully explained by the long-term loss of icesheets and changes in vegetation, the paper says. These changes cause Earth’s surface to absorb more solar radiation, which causes more warming, and so on.
When applied to what awaits us this century, the adjusted model suggests that nothing significantly different will happen compared to what has already been estimated.
“In that time scale, we don’t think the Greenland icesheet is going to melt completely or that East Antarctica will melt. That was what we saw in the model for 3 million years ago, but it is unlikely to take place in the nextcentury,” Lunt said.
Where it poses a dilemma, though, is how to fix a target for stabilizing emissions so that future generations are not hit by this long-term warming mechanism.
One goal is to limit warming since pre-industrial times to 2ºC. Lunt says that today’s level may already be too high in this context.
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