Because every tenth of a degree of global warming matters not just to the planet but also in the highly charged political debate over climate change, researchers once again have analyzed ocean temperature readings over the past 75 years. Their findings refute the already debunked contention that warming paused from 1998-2012.
Using a global network of buoys, robotic floats and satellites to trace the rise of sea surface temperatures, the study, published Jan. 4 in Science Advances, shows there was no slowdown in the pace of global warming. The scientists concluded that oceans have warmed consistently over the previous 50 years, at about 0.12 degrees Celsius per decade, nearly twice as fast as previous estimates of 0.07 degrees Celsius.
The so-called hiatus was widely reported and used by climate science deniers to bolster their political opposition to cutting greenhouse gas emissions.
But in 2015, a study led by scientists with the National Oceanic and Atmospheric Administration (NOAA) concluded that the supposed pause was based on an inconsistency—between ocean temperature measurements collected by ships over the past 100 years and more accurate data collected by modern instruments designed for climate monitoring.
The NOAA researchers found that modern buoys show more ocean warming than older ship-based systems, even when measuring the same part of the ocean at the same time. The study recalculated global ocean temperatures based on that finding.
Shortly after it was released, the House Committee on Science, Space and Technology led by Rep. Lamar Smith (R-Texas) subpoenaed documents related to the NOAA report. Some Republican committee members accused NOAA of doctoring the climate numbers for political purposes. NOAA released the raw data but refused to turn over other documents. Critics of the probe, including ranking Democratic members of the committee, characterized the investigation as a "witch hunt" aimed at intimidating climate scientists.
The House Science Committee did not respond to requests for comment about the new study.
"Our results verify the NOAA study. They weren't cooking the books," said Zeke Hausfather, a graduate student in UC Berkeley's Energy and Resources Group, the new study's lead author. "It was just due to a bias in the ship records...the slowdown never happened."
Hausfather said that in 1990, 95 percent of the data came from ship observations. Now, 90 percent of the data comes from buoys. (More than 7,000 buoys and robotic Argo floats are now deployed worldwide.) "The rate of warming we see in buoys is much higher than from the ship readings," he said.
The study was carried out by researchers from the University of York in the UK, the University of California, Berkeley, and Berkeley Earth, a non-profit research institute, as well as NASA's Jet Propulsion Laboratory and the California Institute of Technology. The research was funded by Berkeley Earth.
The new results provide more support for NOAA's findings, which suggest that recent warming rates had been underestimated, said Tim Osborn, research director of the University of East Anglia's Climatic Research Unit.
"It's also a nice example of how policy-relevant scientific findings should be evaluated," said Osborn, who was not involved in the latest study. "There is much more potential for progressing our scientific understanding by careful analysis of scientific data than by trying to subpoena emails between scientists."
Osborn said the new research will help explain whether variations in the rate of global warming are due to natural variability or other factors like changes in the way the data is collected. Approaching the issue with a "binary choice of 'slowdown' or 'no slowdown' is counterproductive for science communication," he said.
Accurately assessing ocean temperatures is important because two-thirds of the Earth's surface is water, and the oceans suck up more than 90 percent of the excess heat trapped by human greenhouse gases emissions, according to University of York climate researcher and co-author Kevin Cowtan.
"So if we don't include the oceans, we miss a lot of the heat," Cowtan said. "The oceans give us a clearer picture, as they don't show the weather extremes we see over land.
"Science is hard," he said. "We don't expect to get it right the first time, and so scientists seldom trust the first study to show a result...It's only when multiple groups get the same result by multiple methods that we generally accept a result."
Cowtan said the "manufactured" political controversy around the NOAA paper was a strong motivation for the new research effort.
Another dataset from the Japan Meteorological Agency published in 2014, meanwhile, showed similar results, also concluding that there was no slowdown in the rate of ocean warming in recent years.