Climate change skeptics are pouncing on a perceived “hiatus” in global warming outlined in an upcoming report from a United Nations panel. But many climate scientists say that a recent pause in the acceleration of global warming shouldn't be taken as a sign that things are looking up.
The Intergovernmental Panel on Climate Change -- a group of scientists and officials from more than 110 countries -- is set to release a working draft of its next major report this Friday. Some skeptics of human-driven climate change have seized on one particular aspect of the report: according to the latest data, the rate of global warming has slowed since 1998, with the average surface global temperature staying steady in these first years of the 21st century. The rate of warming seen between 1998 and 2012 is flatter than the trend seen since 1951.
"Most climate scientists wouldn't say that the 15-year period is a good reason to question the overall quality of models," the panel's vice chairman Jean-Pascale van Ypersele told the BBC.
Some scientists that say the discrepancy between the predicted rate of warming and the actual one highlights flaws in the panel’s process.
"All other things being equal, adding more greenhouse gases to the atmosphere will have a warming effect on the planet," Georgia Institute of Technology climatologist Judith Curry told the Los Angeles Times. "However, all things are never equal, and what we are seeing is natural climate variability dominating over human impact."
World leaders will likely be looking for the panel to provide a fuller explanation for the slowdown in temperature rise. (Note that the flattening out of the rate of warming in the past 15 years doesn't mean that the Earth was cooling during that period. Nine out of the 10 hottest years on record since 1880 came between 1998 and 2012.)
What’s behind the recent dip in the warming rate? One of the leading current theories is that the oceans are absorbing much of the excess heat trapped by greenhouse gases. Scientists will need more data on deep sea temperatures to prove this hypothesis. Whatever the reason behind the slower observed rate of warming, the observations have prompted the panel to closely scrutinize its preferred models of the atmosphere.
“Much is being made of a fairly small revision,” Pennsylvania State University climate scientist Michael Mann wrote in an email. “The IPCC has slightly lowered the lower range of the estimated climate sensitivity (how much warming you get for a doubling of [carbon dioxide] concentrations).”
In the current report, the Intergovernmental Panel on Climate Change estimates that a doubling of the CO2 concentrations in the Earth’s atmosphere will result in a warming between 1.5 to 4.5 degrees Celsius (2.7 to 8.1 degrees Fahrenheit), in place of the 2 - 4.5 degrees C range cited in its 2007 estimate. The most-likely estimate for climate sensitivity, however, remains close to a value of 3 degrees C, Mann said.
Even with the revised estimate of sensitivity, we're still likely to pass a 2 degree Celsius warming of the globe within decades, he said.
The panel's draft document also estimates that global average temperatures will likely rise between .3 degrees to 4.8 degrees Celsius by 2100, according to Bloomberg News. The previous report estimated the rise would be between 1.1 and 6.4 degrees Celsius.
In contrast to the downgraded temperature rise forecasts, the panel's new report projects even higher sea rises by the end of this century. In 2007, the group predicted ocean levels would increase between 18 and 59 centimeters (7 to 23 inches). Now they’re projecting a sea level rise between 26 and 81 centimeters (10 to 32 inches). The panel is also raising the probability that humans are driving climate change to 95 percent, from 90 percent.
“By most reasonable measures -- last year having been the warmest on record for the U.S., the last decade having been the warmest on the record for globe, and Arctic sea ice declining far faster than any climate models predicted it [to] -- we are either on, or ahead of schedule, with regard to model predictions of the overall impacts of human-caused climate change,” Mann said.