Sixteen years into the mysterious ‘global-warming hiatus’, scientists are piecing together an explanation.
Published on January 16, 2014, in Nature.
The biggest mystery in climate science today may have begun, unbeknownst to anybody at the time, with a subtle weakening of the tropical trade winds blowing across the Pacific Ocean in late 1997. These winds normally push sun-baked water towards Indonesia. When they slackened, the warm water sloshed back towards South America, resulting in a spectacular example of a phenomenon known as El Niño. Average global temperatures hit a record high in 1998 — and then the warming stalled.
For several years, scientists wrote off the stall as noise in the climate system: the natural variations in the atmosphere, oceans and biosphere that drive warm or cool spells around the globe. But the pause has persisted, sparking a minor crisis of confidence in the field. Although there have been jumps and dips, average atmospheric temperatures have risen little since 1998, in seeming defiance of projections of climate models and the ever-increasing emissions of greenhouse gases. Climate sceptics have seized on the temperature trends as evidence that global warming has ground to a halt. Climate scientists, meanwhile, know that heat must still be building up somewhere in the climate system, but they have struggled to explain where it is going, if not into the atmosphere. Some have begun to wonder whether there is something amiss in their models.
Now, as the global-warming hiatus enters its sixteenth year, scientists are at last making headway in the case of the missing heat. Some have pointed to the Sun, volcanoes and even pollution from China as potential culprits, but recent studies suggest that the oceans are key to explaining the anomaly. The latest suspect is the El Niño of 1997–98, which pumped prodigious quantities of heat out of the oceans and into the atmosphere — perhaps enough to tip the equatorial Pacific into a prolonged cold state that has suppressed global temperatures ever since.
“The 1997 to ’98 El Niño event was a trigger for the changes in the Pacific, and I think that’s very probably the beginning of the hiatus,” says Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. According to this theory, the tropical Pacific should snap out of its prolonged cold spell in the coming years.“Eventually,” Trenberth says, “it will switch back in the other direction.”
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