Global warming hiatus tied to cooler temps in Pacific

The recent pause in global warming has resulted from cooling in the tropical Pacific Ocean, new simulations find. The colder ocean temperatures are a consequence of natural fluctuations in climate; global temperatures will start rising again when warmer conditions return to the Pacific, researchers propose August 28 in Nature.

Exactly when that will happen is anyone’s guess. “We can’t predict the next move of the equatorial Pacific,” says study coauthor Shang-Ping Xie of the Scripps Institution of Oceanography in La Jolla, Calif.

Since 1998, average global surface temperatures have remained relatively steady despite growing carbon dioxide emissions. Skeptics point to the warming hiatus as evidence that humans aren’t contributing to climate change. Yet previous decades of warming continue to have clear regional effects: Arctic sea ice has rapidly dwindled while other parts of the world have experienced record-setting droughts and heat waves.

A possible explanation for global temperatures’ stagnation is a surge in air pollution, caused by increasing coal consumption in China, a team led by Robert Kaufmann of Boston University argued in 2011. The pollution’s sunlight-reflecting aerosols have counteracted greenhouse gas warming, the researchers concluded (SN: 7/30/11, p. 17).

Other scientists say the planet has continued to warm, just not at the surface. In 2011, Gerald Meehl of the National Center for Atmospheric Research in Boulder, Colo., and colleagues suggested that the deep ocean soaked up the missing heat. Their work suggested that the break in surface warming might also be tied to La Niña events, when the eastern tropical Pacific is at least half a degree Celsius below average for several months. Since 1998, there have been six La Niñas, which can lead to global cooling.

Now, Xie and Scripps colleague Yu Kosaka have taken a closer look at the tropical Pacific. In simulations of global temperatures that included the cooler Pacific sea surface, the models replicated the current warming pause. The models without the cooler Pacific predicted that global average temperatures should be climbing.

But the work doesn’t explain why the tropical Pacific has cooled, says atmospheric scientist Susan Solomon of MIT. “Did the sea surface temperatures cool on their own or were they forced to do so by, for example, changes in volcanic or pollution aerosols or something else?” she asks.

Climate scientist Gavin Schmidt of NASA’s Goddard Institute for Space Studies in New York City says that several factors probably play some role in the global warming pause. His “informal prediction” is that the long-term surface warming trend will resume once the Pacific warms up again.

Although surface temperatures are stalled for now, the hiatus doesn’t dramatically alter previous estimates of how much warming the planet will experience in the 21st century, researchers from the United Kingdom’s Met Office concluded in three reports released earlier this month. The authors suggest that the warming previously forecasted for 2050 will be delayed by only a few years.