The global warming slowdown is real — but that’s no reason to question climate science

This Aug. 12, 2013, satellite image captures the vast Pacific Ocean, whose variability may explain a temporary slowdown in the rate of global warming. (AFP photo / HO / NASA Goes Project)

It was the biggest climate change talking point of 2013 and beyond — the idea that global warming had slowed down, paused or, as some put it, hit a “speed bump.”

The argument was widely used by climate doubters to undermine the mega-release of the U.N.’s fifth Intergovernmental Panel on Climate Change report. And no wonder, for it came, to a substantial extent, from scientists themselves. The IPCC itself acknowledged that the rate of warming from 1998 to 2012 had been somewhat slower than the rate of warming since 1951.

Ever since, climate scientists have been trying to explain why the world has seen a somewhat slower rate of warming in recent years — and publishing multiple papers on the topic.

Now, though, a new study in the journal Science suggests that  that the global warming “pause” may soon run its course, and, anyway, it seems to have been caused by natural variability in the climate system. Thus, the slowdown, such as it was, certainly is no reason not to worry about a longer-term climate trend driven by humanity’s greenhouse gas emissions.

“Our study adds additional weight to the notion that this is part of a short-term excursion that is likely to reverse in the years ahead,” says Michael Mann of Penn State University, a co-author of the new study, whose lead author is Byron Steinman of the University of Minnesota, Duluth.

The research focuses on a number of sources of natural variation in the Earth’s climate system, including the widely known wobble called thePacific Decadal (or Multidecadal) Oscillation, which is characterized by swings between relatively warm tropical Pacific Ocean temperatures and colder ones. The oscillation is similar to the El Niño-La Niña cycle but plays out on a much longer timescale involving decades, rather than individual years.

The study found that the slowdown in global warming overall is closely tied to a “sharply decreasing” trend in this oscillation — in which the tropical Pacific has seen a pattern of “heat burial,” characterized by relatively colder temperatures at the sea surface but more warmth at depth. Other research has also suggested that variations in the behavior of the vast Pacific can explain the near-term global temperature trend that some have labeled a “pause.”

“Cool Pacific temperatures have played a key role in modulating atmospheric temperature increases in the past 10 years, only partially offset by modest warming in the Atlantic,” explains the UK Met Office Hadley Center’s Ben Booth in an accompanying commentary published with the study.

There certainly isn’t any good news here; if anything, the researchers expect this current behavior to snap back soon enough, increasing global warming. “Given the pattern of past historical variation, this trend will likely reverse with internal variability, instead adding to anthropogenic warming in the coming decades,” notes the study.

“Our findings do support the notion that the pause is likely to end,” says Mann. “And perhaps 2014 does herald that at some level.” It was, after all, the hottest year on record.

Other researchers, meanwhile, have cited other forms of natural variability to help explain the so-called pause, such as an uptick in volcanic eruptions, whose atmospheric plumes can lead to a cooling effect by scattering sunlight away from the planet.

The upshot of it all, for those following the climate debate, may be this: We argue, day in and day out, about the meaning of each new piece of science that has some bearing upon what is really, in the end, a policy debate. Thus, when the “pause” showed up, it quickly became a political tool even before its scientific meaning was understood.

But that’s just a bad way of doing things, especially in light of how complex the climate system is — driven by both human-caused factors and also natural variability that isn’t completely understood. There will always be surprises, and they’ll have to be studied. But none of these are likely to undermine the central conclusion of climate research, which is that the globe is warming and we’re causing it.

Yes, the rate will change. Yes, the oceans will cycle, the volcanoes belch, the sun fluctuate. But throughout it all, we’ll still have a problem to deal with.

Chris Mooney reports on science and the environment.