Cooler Pacific Ocean waters level rise in global warming

The flattening over the past 15 years of a rise in the world’s average surface temperature springs from a natural cooling pattern in the eastern Pacific Ocean, climate scientists reported Wednesday.

That leveling off fed part of the skepticism toward warming predictions in recent years, but researchers behind the new report see this “hiatus” as a pause in an inevitable climb.

“Our results strongly confirm the role that ( man- made) emissions are having on the climate,” says climate scientist Shang- Ping Xie, senior author on the Nature journal study. “At one point over the long term, the effect we are seeing in the Pacific will stop. I’m confident the bigger increases in warming will resume.”

For now, the “hiatus” in global warming has left average surface temperatures lodged about 1.4 degrees Fahrenheit above normal for the past century. The top 10 warmest years on record have all come since 1998, as a result, but none looks markedly warmer than another.

Climate scientists have disagreed over the cause of the pause. Explanations range from the atmospheric cooling effects of recent volcanic eruptions to Asia’s increased industrial smog or else natural cooling oscillations in ocean surface waters.

“Our results show that Pacific cooling has indeed pulled down the average global surface increases seen from global warming,” Xie says. Essentially, a persistent La Niña- like weather pattern across the tropical Eastern Pacific, an area covering about 8.2% of the globe, has created a “cool spot,” he says. That has balanced out temperature increases manifested elsewhere with the continuing melting of the Arctic, rising sea levels and record summer heat waves across continents, he says.

“The climate is complicated, and natural variability can mask trends seen over century- long timescales,” says David Easterling of the National Oceanic and Atmospheric Administration’s National Climatic Data Center. Similar plateaus in temperature increases have occurred in seven to nine- year steps in the past, as Easterling and colleagues showed in a 2009 study. “A lot of people have the misconception that global warming means that temperatures should be steadily marching upward, but natural variability plays a big role on the decade scale,” he says.

MIT’s Susan Solomon is more skeptical of the Pacific Ocean cooling as an explanation. “Did the sea surface temperatures cool on their own, or were they forced to do so by ... changes in volcanic or pollution aerosols, or something else? This paper can’t answer that question.”

Xie acknowledges the criticism, saying some evidence suggests global warming has transferred a portion of that heat to the ocean depths.

A report in The Economist magazine questioned whether the hiatus points to overestimates in climate predictions. “I think we have strengthened the case that what we are seeing is a cooling that will naturally end at some point, and the warming will resume,” Xie says.