Even as greenhouse gas emissions ratchet steadily upward, for 15 years global mean temperatures have mysteriously failed to keep pace. They haven't stopped climbing, but the rate of global warming has slowed. Now, researchers have identified a possible culprit: a cooling trend in tropical Pacific sea-surface temperatures. That trend, the authors note, is just part of a natural cycle, so the reprieve is only temporary.
On average, our atmosphere warmed by about 0.17⁰ C per decade from 1970 to 1998, but by about 0.04⁰ C per decade from 1998 to 2012. Some scientists have suggested that the slowdown may be due to a buildup of aerosols in the atmosphere, to volcanic eruptions, or to a pronounced lull in solar activity in 2008 and 2009. The ocean also plays a part; a study published in Nature Climate Change in 2011 noted that the deep ocean stores much of the “missing heat.” Scientists have wondered whether ocean surface temperatures in the eastern equatorial Pacific—part of a decade-scale cycle of changes in rainfall, temperature, and atmosphere circulation known as the El Niño-Southern Oscillation (ENSO)—also play a significant role.
To find out, climatologist Yu Kosaka of the Scripps Institution of Oceanography in San Diego, California, and climatologist Shang-Ping Xie, also at Scripps, took a closer look at how naturally varying sea-surface temperatures in the eastern equatorial Pacific might affect global atmospheric temperatures. They turned to a system of models known as POGA (Pacific Ocean-Global Atmosphere), previously used to study links between Pacific Ocean surface temperatures, ENSO, and global atmosphere.
Kosaka and Xie ran three different sets of experiments to try to tease out this impact. In one, they ran a model that incorporated only atmospheric changes, such as the observed increased greenhouse gas concentrations. In a second, they included the sea-surface temperatures but held greenhouse gas concentrations steady at 1990 levels. And in the third set, known as POGA-H, they included changes in both ocean temperatures and greenhouse gases.
When the researchers compared the results of the models with observed temperature trends from 2002 to 2012, POGA-H gave the best match. By comparing the temperatures from that model with temperatures from the model that included only atmospheric changes, the researchers were able to suss out the effect of the ocean temperatures. They found that recent cooling in the tropical Pacific—which covers only about 8.2% of Earth’s surface—was responsible for lowering global mean temperatures by 0.15⁰ C , relative to the 1990s, the team reports online today in Nature. In the Northern Hemisphere that hiatus was most pronounced in winter, when ENSO has the greatest impact on the transfer of heat from the tropics to the poles.
“Our study does not tell us when the climate will go out from the hiatus,” Kosaka says, “but now we know that in the timescale of several decades, the climate will continue to warm due to increase in greenhouse gas concentrations in the atmosphere.” Indeed, noted meteorologist Richard Allan of the University of Reading in the United Kingdom in a statement to the U.K. Science Media Centre, these findings highlight that the braking is “likely to be a temporary respite.”
Kosaka and Xie present a convincing case that tropical eastern Pacific cooling is behind the recent hiatus, says climatologist Nat Johnson of the University of Hawaii, Manoa. The study also highlights that focusing on short-term trends of global-mean temperatures can be “a bit misleading”, he says. Not only can natural variability mask the true trend in global temperatures, but regional trends may also differ sharply from global trends. For example, during the hiatus, northwest North America has experienced less warming, but the southern United States got no such break. So a combination of greenhouse-gas warming and cooling in the eastern Pacific can lead to rapid short-term warming in some regions “even as global mean temperatures temporarily flatten.”