Slower Warming Trend Explained

Like cold bathwater, unusually cool surface water in the tropical Pacific Ocean can help explain why the average surface air temperature around the globe has slowed its warming trend, a study suggests (Nature 2013, DOI: 10.1038/nature12534).

The findings indicate that the global air temperature flattening observed during the past 15 years reflects Earth’s natural climate variability and is temporary, says climate scientist Andrew E. Dessler of Texas A&M University, who wasn’t involved in the study. Man-made greenhouse-gas emissions will win out in the long haul and continue to drive up global temperatures, he says.

The global air-temperature-flattening trend is expected to be a point of discussion in the Intergovernmental Panel on Climate Change’s next report, which is due for release starting in late September.

Recent research suggests that temporary blips in long-term air temperature trends can be caused by the movement of warm and cool ocean water, which transfers heat to and from the air. One such cycle is the Pacific Decadal Oscillation (PDO). This water circulation pattern is similar to the warming and cooling of the tropical Pacific Ocean’s El Niño and La Niña cycles, but it operates on a longer timescale.

The current Pacific surface cooling is part of the PDO cool period, researchers say. But they haven’t known whether that cooling is contributing to a slowdown of the global air temperature rise.

Seeking answers, Yu Kosaka and Shang-Ping Xie of Scripps Institution of Oceanography ran a climate model with traditional “climate forcers”—including greenhouse gases, solar activity, and aerosols—along with the observed Pacific surface temperatures as an input.

The model results mimic recorded temperatures from 1970 to 2012, including seasonal and decadal variations, the researchers found. The model also reproduced regional climate patterns, including recent U.S. droughts.

The findings suggest PDO’s La Niña-like event is causing the recent air temperature flattening, the authors say, but air temperatures will rise again when the cycle reverses.

The study, Dessler argues, casts doubt on arguments that the climate is less responsive to carbon dioxide than thought or that the temperature flattening stems from reduced solar activity, higher emissions of air-cooling aerosols from coal burning, or volcanic activity.

“This paper is a part of the discussion but not the last word,” says Gavin A. Schmidt of NASA’s Goddard Institute for Space Studies. He notes that the study doesn’t explain whether the Pacific cooling could be itself influenced by other climate forcers.

The study also doesn’t explain where the heat that the air hasn’t taken up is going, adds Kevin E. Trenberth of the National Center for Atmospheric Research. Dessler suspects that the heat is going deeper into the ocean, which would be consistent with how PDO’s cool period is thought to work.