News of a supposed global warming hiatus a few years back caused all sorts of trouble. Otherwise respectable reporters hinted that climate researchers had some explaining to do, and scientists obliged with accounts ranging from ocean heating cycles to air pollution. Now, researchers at the National Oceanographic and Atmospheric Administration say they've got a new take: The hiatus didn't actually happen.
To be clear, no serious climate observer ever thought there was a climate hiatus, per se. Although data in 2013 Intergovernmental Panel on Climate Change report suggested that the rate of warming had slowed in the 15 years prior, thermometers had still been rising. Yet the apparent slowdown remained troubling to climatologists, and they produced possible explanations for the phenomenon in droves. Last year, researchers suggested currents in the Atlantic might be to blame. A study predating the 2013 IPCC report suggested pollution in China may have led to a slowdown. Earlier this year, decade-long cycles in ocean temperatures were declared the cause, though new explanations keep coming.
It's possible that the instruments used to measure surface temperature around the world might have been biased, so that it only looked like there was a global warming slowdown.
Climate change deniers, meanwhile, had a field day.
They needn't have gone to the trouble, argues a team led by NOAA scientist Thomas Karl. "While these analyses and theories have considerable merit in helping to understand the global climate system, other important aspects of the 'hiatus' related to observational biases in global surface temperature data have not received similar attention," Karl and colleagues write today in Science.
In other words, Karl and his team are saying, it's possible that the instruments used to measure surface temperature around the world might have been biased, so that it only looked like there was a global warming slowdown. Such systems are, after all, constantly changing. For instance, there are now more temperature measurements made from ocean buoys, as opposed to ships, than there used to be. There have also been changes in the technology used to measure ocean temperatures, and there are more land-based measurements compared with the past.
Each one of those effects, the team points out, would lead scientists to underestimate actual temperatures in a way that resembles a slowdown in global warming, since new instruments and methods were phased in over time. Indeed, when the researchers corrected for measurement bias, they found that the planet warmed by about 0.13 degrees Celsius between 1998 and 2012, more than twice what researchers estimated in the IPCC report.
"In summary, newly corrected and updated global surface temperature data from NOAA’s [National Centers for Environmental Information] do not support the notion of a global warming 'hiatus,'" the researchers write. Nor has there been any discernible decline in the rate of warming between the second half of the 20th century and the first 15 years of the 21st. "The IPCC’s statement of two years ago—that the global surface temperature 'has shown a much smaller increasing linear trend over the past 15 years than over the past 30 to 60 years'—is no longer valid."
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