Climate-Change Response Demands Urgency


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.


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.


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