Although nine of the 10 warmest years on record have come after the year 2000, global average surface temperature hasn’t increased as rapidly during that time as it did in the 1990s. This hasn’t sent climate scientists scrambling to see if CO 2 might not be a greenhouse gas after all, but identifying the cause of this behavior is a valid and interesting challenge.

Several possibilities have been proposed, including an uptick in reflective aerosols and an increase in heat moving into the deep ocean. One study found that when major sources of year-to-year temperature variability are accounted for—solar cycles, volcanic activity, and El Niño/La Niña—the underlying trend is a steady increase in global temperature.

The El Niño Southern Oscillation—a pattern of warm (El Niño) or cool (La Niña) surface water in the equatorial Pacific—has a sizeable impact on average global temperature. To test the impact of the frequent La Niñas of the 2000s, Yu Kosaka and Shang-Ping Xie of the Scripps Institution of Oceanography in San Diego tried a new approach.

They simulated the last 60 years using two different configurations of a climate model developed by NOAA’s Geophysical Fluid Dynamics Laboratory. One version was simply run using the observed changes in greenhouse gas concentrations, aerosols (both anthropogenic and natural), and the solar cycles. The result (purple line below) matched measured global temperature (black line) pretty well, but it rose higher than the Earth's temperature did in recent years. The other version used these same inputs but also forced the model to use the actual sea surface temperatures in the equatorial Pacific rather than letting the model itself generate them.

That simulation (red line below) generated an almost exact fit with global temperature, including the recent behavior. A little bit of that improved match would be expected—after all, they have replaced some simulated sea surface temperatures with measured ones—but they only manipulated 8.2 percent of the Earth’s surface. The cooler equatorial Pacific sea surface during the recent rash of La Niñas has had a large impact on global climate.

Because they used a climate model, they were able to look closely at regional patterns of climate over the last decade. They found La Niña’s fingerprints in many places. Their model reproduced the cooler winters seen in the American northwest and up into Canada, as well as the warm and dry conditions in the southern US. Globally, it also matched the seasonal disparity of the last decade, with warming in the Northern Hemisphere summer months but cooling in winter.

Also in accordance with reality, energy trapped by greenhouse gases continued to increase in the model, with ocean heat content rising apace. The modeled climate system didn’t cease warming; it just didn’t show up strongly in the atmosphere.

It adds up to a pretty coherent picture pointing to a cluster of La Niñas as the cause of the slowdown in atmospheric warming. But why all the La Niñas? The researchers chalk it up to natural variability—a lot of coin flips have simply come up La Niña lately. If that’s the case, the researchers write, “the hiatus [in atmospheric warming] is temporary, and global warming will return when the tropical Pacific swings back to a warm state.”

Nature, 2013. DOI: 10.1038/nature12534 (About DOIs).