JUST HOW much will the Earth heat up over the next 100 or 200 years? Climate scientists are not able to predict with high certainty. They have estimated that average global temperatures will increase by 1.5 to 4.5 degrees Celsius — 2.7 to 8.1 degrees Fahrenheit — given a doubling of carbon dioxide in the atmosphere. That range of estimates for “climate sensitivity” would mean the difference between relatively small effects and significant consequences for human welfare.
There are skeptics — not out-and-out climate-change deniers — who accept the physics that human-produced greenhouse gases will have some influence on climate but point to the lower estimates to argue that the issue is not urgent. A new paper in the journal Nature suggests they are wrong — that the consequences of climate change are likely to be toward the middle or higher end of the predicted temperature range. “This new research takes away the lower end of climate sensitivity estimates,” said University of New South Wales’s Steven Sherwood, author of the report. “Meaning that global average temperatures will increase by 3 degrees Celsius to 5 degrees Celsius with a doubling of carbon dioxide.” That translates into a rise of 4 degrees Celsius by the year 2100 and perhaps 8 degrees Celsius by 2200, barring a reduction in carbon dioxide output.
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The research relies on insights into the effects of clouds on climate. In finely tuned climate models, much depends on figuring out how evaporated water behaves. As high-level clouds, some will exert a net warming effect by absorbing heat. As low-level clouds, some will exert a net cooling effect by reflecting sunlight back into space. And even lower to the ground, some may prevent low-level clouds from developing by dehydrating the cloud-forming layer. One way or another, cloud activity will feed back into the climate-change process.
The study’s authors compared various models to real-world observations and found that the models that matched the observations predict more upward pressure on temperature. Their results offer one more argument against assuming a relatively benign climate future. That doesn’t mean the future can be forecast, even now, with certainty. It does mean that to take no action, on the hope that nothing too bad is in store, is to place a foolish bet with humanity’s future. It would be much more prudent to spend something now to head off the risks, even if they aren’t known exactly.
Next year, international negotiators will gather in Paris in another attempt to create a working international anti-carbon system. It’s important to invest diplomatic capital in that effort. But leaders cannot rely on that forum to produce the action the world needs. The United States needs to lead the way with a smarter climate policy and then encourage a global response.