Has global warming slowed down recently?

Global warming certainly hasn’t stopped. The past decade was clearly the warmest decade on record — and 2014 was the hottest year on record.

That said, the rate of temperature increases in the last 15 years does appear to be slightly slower than it was in the 20 years before that. And that’s despite the fact that greenhouse gases are currently piling up in the atmosphere at a record pace. This “slowdown” has gotten a lot of attention in some circles.

So what’s happening? This may simply be due to natural variability. Average temperatures can bounce around a bit from year to year due to a variety of factors (like volcanic eruptions, fluctuations in solar activity, or El Niño and La Niña cycles that shift heat in and out of the ocean).

Scientists have been puzzling over the precise reason for this most recent slowdown in global warming, especially since it appears to be at odds with some climate-model predictions. There are a few possible theories here:

1) One is that there’s been more volcanic activity and more aerosol pollution from countries like China than climate modelers originally assumed. Both of those things can have a short-term cooling effect on the planet by reflecting more solar radiation back into space. Indeed, a 2014 study in Nature Climate Change found that climate models are far better able to reproduce recent temperature trends once those factors are counted properly.

2) Another theory is that much of the extra heat being trapped by greenhouse gases is going into the ocean. The oceans are vast and have long absorbed more than 90 percent of the extra heat that greenhouse gases trap on Earth. So it’s possible they’ve been absorbing even more of that heat lately, and hence slowing the rise of temperatures on the surface. Indeed, measurements show that the oceans are heating up as quickly as ever.

3) Another 2014 paper in Nature Climate Change suggested Pacific trade winds may be responsible for the shifting of heat into the deeper layers of the ocean. If the oceans are indeed the reason for the apparent slowdown, then all that extra heat could eventually return to the Earth’s surface in the years ahead, leading to much hotter temperatures.

4) A different explanation — and one favored by climate skeptics — is that the Earth’s climate is a bit less sensitive to increases in greenhouse gas emissions than scientists and models have assumed. If so, that would mean future global warming could be less severe than projected. This is possible, but far from certain, since scientists have a variety of ways to estimate climate sensitivity, and recent trends are only one piece of evidence.