What do you see when you look at this chart?
A few things stand out. First, the planet has grown considerably warmer since mid-century, by more than half a degree Celsius. Second, even if the overall trend is upward, there's a fair bit of variation year to year. Some of that, as we can see, has to do with El Niño and La Niña cycles, which can shift heat into and out of the ocean.
There's a third aspect of this chart, however, that's getting a barrage of attention lately. The past decade has clearly been the warmest decade on record. But the rate of warming in the last 15 years has been slower than it was in the 20 years before that. And that's despite the fact that greenhouse gases are piling up in the atmosphere at a record pace.
So what should we make of this recent "slowdown" in global warming? Is it just a random blip — the sort of natural variation we've seen before and will likely see again? Or does it tell us anything interesting about climate change?
Here are are a couple of big points to consider:
1) Global warming is still very much with us. The recent slowdown in temperature rise has left climatologists a bit puzzled, but it certainly doesn't disprove global warming. The evidence on the effects of greenhouse gases, after all, stretches back more than a century.
Scientists know that carbon dioxide is a greenhouse gas that traps heat on Earth. And it's simple to see that humans are putting more carbon dioxide into the atmosphere by burning fossil fuels. Researchers at NASA, meanwhile, have found that more solar energy is now entering Earth than is escaping back out into space. Basic physics suggests that this "energy imbalance" should, over time, heat up the planet.
The mystery, then, is why all that extra energy hasn't translated into even higher surface temperatures in recent years. Where is the extra heat going? Clearly something else must be at play here...
2) One theory is that the oceans are responsible for the recent warming slowdown. The oceans are vast and have long absorbed more than 90 percent of the extra energy that greenhouse gases trap on Earth. So it's possible that they've somehow been absorbing even more of that heat lately, and hence slowing the rise of temperatures on the surface.
Recent work by Magdalena Balmaseda, Kevin Trenberth, and Erland Källén has suggested that the warming of the oceans has accelerated in the past 15 years — and that the "missing heat" may be lurking in the deep layers, 700 meters below the surface:
This is still only a theory, but as Fred Pearce writes, many researchers seem to agree that the oceans are playing some role here. Once you take them into account, Richard Allan of the University of Reading in England tells him, “global warming has actually not slowed down.” Similarly, a recent paper in Nature led by Virginie Guemas claims to have found a "robust" link between the recent plateau in temperatures and increased heat uptake by the oceans.
If the oceans are indeed the reason for the pause, that's not comforting news, since that extra heat should eventually rise to the Earth’s surface in the years ahead, leading to much hotter temperatures.
That said, it's not entirely clear why the oceans have been grabbing a bigger share of the heat lately. It's also not certain when, exactly, that heat will return to the surface. Climate models still have trouble capturing the precise mechanisms by which the oceans transfer heat to the surface over short time scales. Which brings us to our third part...
3) Even if you place a lot of weight on the recent slowdown, it doesn't change projections of future warming too dramatically.
Here's a question we can ask. What if the recent slowdown in surface temperatures isn't just a blip, but is actually an extremely important fact about the world? How much should it shift our view of climate change? And the answer turns out to be: A little bit.
That's the issue Alexander Otto and his co-authors explored in a recent paper published in Nature Geoscience. They focused on data from the last decade to come up with new estimates of the "transient climate response," or what will happen in the very short term every time we double the amount of carbon dioxide in the atmosphere.
What Otto and his colleagues found is that, if you look at data from the last decade and extrapolate from there, we can expect the Earth to warm an extra 0.9°C to 2°C in the short term whenever the amount of carbon dioxide in the atmosphere doubles. By contrast, earlier climate models had put that range at between 1°C to 2.5°C. (Note: These are lower numbers than estimates of "climate sensitivity," which measures how the Earth would keep heating up thereafter, as the oceans and ice sheets reach equilibrium.)
That's slightly better news, but the broad picture hasn't change all that much. As Otto told me: "Even if we give a lot of weight to what we’ve seen over the last decade, you’re not getting rid of the problem." And here's how Myles Allen, a co-author, put it: "Taken at face value, our new findings mean that the changes we had previously expected between now and 2050 might take until 2065 to materialize instead."
In a phone interview, Otto also made another point: The temperature trends over the past decade aren't the only data scientists have to work with. When he and his co-authors recalculated the Earth's transient climate response using data from the last 40 years, his results (0.7°C to 2.5°C) lined up more closely with earlier models. (See here for Otto's explanation of the paper.)
What's more, scientists have a variety of other ways to estimate longer-term climate sensitivity, including data from prehistoric periods, measures of the Earth's response to volcanic eruptions, and physical models. As NASA's Gavin Schmidt explains, different approaches tend to given different answers, some higher, and some lower. But they're all worth considering.
4) We're still on pace to blow past that 2°C climate target. Intricate arguments about climate sensitivity often bypass a crucial point. Humanity is on pace to do a lot more than simply double the amount of carbon in the atmosphere by the end of the century (compared with pre-industrial levels). Doubling means going up to 560 parts per million. We’re currently at about 400 ppm and rising fast.
"Even if you adjust the transient climate response downwards, if we keep emitting at the pace we are now, we're going to go significantly past the 2°C climate target," Otto says. "There's no reason for complacency here."