Recent slowdown in atmospheric warming thanks to La Niña

Map of Pacific sea surface temperature in December, 2010, showing a cool La Niña across the equatorial Pacific.

NASA Earth Observatory

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 CO₂ 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.