News•March 27, 2013

Why the Globe Hasn’t Warmed Much for the Past Decade

Even the quickest glance at a graph of global temperatures makes it clear that the planet was warming sharply during the 1980s and 1990s. But while the 2000s were the hottest decade on record, the rate of warming slowed considerably after the turn of the current century — even while human emissions of heat-trapping greenhouse gas emissions have continued to grow. The question that has lingered is where’s all the extra heat going?

Global mean land-ocean temperature index, 1880 to present.
Click image to enlarge. Credit: NASA

The answer, according to a new paper in Geophysical Research Letters, is that a lot of it is being stored in the deep ocean, more than a half-mile down. “We normally think about global warming as what we experience on the Earth's surface,” said co-author Kevin Trenberth, of the National Center for Atmospheric Research, in an interview. If extra heat is temporarily stored elsewhere thanks to natural climate variations, we won't necessarily notice it.

But sooner or later it will inevitably emerge, which means that the current slowdown in warming may well be balanced by a period of rapid warming in a few years — nobody knows how many — from now. Scientists have always said that global warming would proceed in fits and starts, not in a smooth upward trend in temperatures. This study offers one specific explanation of why that happens.

The natural variation in this case appears to be changes in wind patterns associated with the Pacific Decadal Oscillation, or PDO, a gradual see-sawing of ocean surface temperatures and wind patterns that goes through warm and cold phases lasting several decades. (The more familiar El Nino/La Nina oscillation, by contrast, see-saws every few years).

According to Trenberth and his colleagues, deep ocean temperatures began to rise significantly starting in about 2000, at about the same time as trade winds in the Pacific were changing in strength, in turn affecting ocean currents, all very plausibly as a result of a shift in the PDO.

Watch 62 Years of Global Warming in 13 Seconds

A progression of changing global surface temperature anomalies from 1950 through 2012.
Double-click video for full screen. Credit: NASA

Ordinarily, heat trapped by greenhouse gases would warm the ocean’s surface water, but since warm water floats on top of colder water, the heat would have a hard time percolating to the depths. “You need something to push it down,” Trenberth said. That something could easily be strong prevailing winds, which can literally stir things up — or in this case, down.

Nobody can actually see this process in action; instead, Trenberth and his colleagues used sophisticated ocean-circulation models and fed in observed data about sea-surface temperatures, winds, currents and even changes in sea level, all of which affect how heat moves around. In the end, changes in the wind turned out to have the most profound effect. It’s still a circumstantial case, but, said Trenberth, “we find it very plausible that this is a real effect.”

Adding to their confidence is the fact that a similar mechanism, only in reverse, explains why 1998 remains one of the hottest years on record. “You can point to the PDO, which took extra heat out of the ocean,” Trenberth said. That pushed global warming along faster than it would naturally have happened.

Indeed, Trenberth, speculates that the PDO could also explain why temperatures rose so quickly during the 1980s and 1990s. “You can argue that the PDO was pulling heat from the ocean during that time, which is just when global warming took off. So it may well be that this natural variability has been modulating the way we see global warming for decades.”

In other words, the PDO is affecting how the ocean takes in the extra heat from manmade global warming, and is helping to influence the rate at which the extra heat gets released back into the atmosphere as well.

If that’s the case, then global temperatures are poised for another rapid rise when the PDO see-saws out of its current phase and begins pulling heat back out of the ocean — something that’s inevitable sooner or later, although nobody knows precisely when it might happen. When it does, the question will no longer be where all the extra heat has gone, but where’s all the extra heat coming from.

But the answer is likely to be exactly the same.