Global warming ‘hiatus’ just an artifact, study finds

One of the biggest mysteries of modern climate science may never have really existed, updated climate analyses suggest.

Following decades of warming and a hot 1998, Earth’s average surface temperature seemingly plateaued. This warming hiatus, as it came to be known, had climate researchers scrambling for an explanation. Now measurements and analysis by the National Oceanic and Atmospheric Administration suggest that the apparent hiatus was not an actual climate trend. Instead, it was an artifact of incomplete and biased data.

After correcting for biases in ocean temperature measurements and filling gaps in land temperature data, the researchers report online June 4 in Science that recent global warming is more than twice as fast as once thought.

“The Earth is continuing to warm at a rate similar or higher than the rate over the last half of the 20th century,” says study coauthor Jay Lawrimore, a climate scientist at NOAA’s National Centers for Environmental Information in Asheville, N.C.

The results demonstrate that the hiatus was overblown, says David Ridley, an atmospheric scientist at MIT. This means climate simulations “haven’t been as bad at predicting the temperature trend as people had begun to think,” he says.

Decades of rapidly rising global temperatures culminated in 1998, the hottest year on record at the time. In the next 15 years, the warming trend appeared to slow to a crawl. The Intergovernmental Panel on Climate Change estimated in 2013 that the surface warming rate from 1998 through 2012 was around one-third to one-half of the average rate from 1951 through 2012.

While global temperatures have risen for more than a century, the rise hasn’t been steady. Natural cycles cause temperatures to wobble from year to year. Many studies looked for a natural cause of the apparent warming hiatus, ranging from changing wind patterns (SN: 3/22/14, p. 12) to small volcanic eruptions (SN: 8/13/11, p. 5). But none fully explained the difference between the expected rapid warming and the observed slower trend.

Lawrimore and colleagues at NOAA instead looked for gaps and biases in the observational record. Long-term climate trends are stitched together from short-term weather measurements at thousands of different locations. Some temperature-taking methods record slightly warmer or cooler temperatures than others, which can produce a fake trend when one method replaces another (SN Online: 1/16/15).

The NOAA researchers found some small biases that, when fixed, made the recent trend seem even flatter. But these were far outweighed by other fixes that increased the rate of recent warming.

The biggest bias concerned ocean surface temperatures. Before World War II, sailors would scoop a bucket of seawater and take the water’s temperature. Evaporation would cool the water as the bucket sat on deck, leading to artificially cool temperature readings. Later, the preferred method shifted to measuring the temperature of seawater pumped in to cool the ship’s hot engine, giving artificially warm measurements.

In recent years, researchers have switched over to using more accurate ocean-monitoring buoys. Temperature readings from these buoys are on average 0.12 degrees Celsius cooler than those from ships.

The researchers analyzed and updated the sea surface temperature measurements to remove the biases. Then the team added land surface measurements to the ones the IPCC had used. The new measurements included ones from the Arctic, where temperatures are rising twice as fast as at lower latitudes (SN: 3/12/15, p. 13).

The team’s new analysis boosts the observed warming from 1998 through 2012 from 0.039 to 0.086 degrees per decade. Shifting the timeframe forward to 2000 through 2014, the hottest year on record (SN Online: 1/16/15), the warming trend increases to 0.116 degrees per decade, akin to the rate of the second half of the 20th century.

While the global warming hiatus may never have existed, the research hunting for a natural explanation still has value, says Shang-Ping Xie, a climate scientist at the Scripps Institution of Oceanography in La Jolla, Calif. Those studies have provided a better understanding of the forces that drive Earth’s global and regional climate. Regional climate is especially important, he says, because “no one lives in the global average.”

The new results also don’t rule out the idea that natural forces can temporarily slow or halt global warming, says Joshua Willis, a climate scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Natural variability caused a lull in global warming from the 1940s through the 1970s, he points out. “Natural variations are here to stay,” he says. “But we might reach a rate of warming that swamps those natural changes and you can’t get the cooling anymore. By 2040 or 2050, I don’t think we’ll be seeing hiatuses.”

Editor’s Note: This story was updated on June 4, 2015. The lines on the graph were originally mislabeled.