Was Mid-Century Cooling Caused By the Oceans? Don’t Ask

Published: September 23rd, 2010

Michael D. Lemonick

A study in today’s issue of Nature has news outlets buzzing with the implication that a major piece of conventional climate wisdom may be wrong. The wisdom has to do with a period lasting for a couple of decades, from the 1940s through the 1970s, when temperatures stopped rising and actually cooled a bit, particularly in the Northern Hemisphere.

The cause of this so-called mid-century cooling is widely attributed to air pollution, which reached record levels in the industrialized world after World War II, and only dropped after anti-pollution laws went into effect, such as the Clean Air Act in the United States. The tiny aerosol particles spewed out by factories acted as tiny reflectors that bounced some sunlight out into space (the same idea lies behind some geoengineering schemes). This explanation makes good sense, which is why it appears in climate change FAQs at websites like Skeptical Science.

Global land and ocean surface temperatures since 1880, showing the pause in warming between about 1940 and the late 1970s.
Credit: NASA Goddard Institute for Space Studies.

But the new paper offers a different explanation: the cooling may instead have been caused by an abrupt temperature drop of about 0.3°C in the North Atlantic, presumably caused by changes in ocean currents. The authors discovered the anomaly by re-examining measurements made by ocean-going ships. It's not a big surprise that ocean currents can affect global temperatures: the alternating Pacific currents known as El Nino and La Nina routinely boost and retard global temperatures temporarily (the reason some skeptics claim global warming has stopped is that if you start measure from the strong El Nino year of 1998, when temperatures spiked, it looks, superficially, as though that's true). If this is a newly discovered instance, it would clearly add to scientists' knowledge of the mysterious workings of the oceans, which are still far from understood. This cooling episode, for example, seems to coincide with something called the Great Salinity Anomaly, when the North Atlantic became less salty for a while. Changes in salinity affect water density, and that in turn can mess with currents — but for now, nobody really knows what the story is. If the result holds up, it would make clearer how natural variability can speed up or slow down the underlying warming caused by greenhouse gases. It's hard to imagine it would undermine the basic science of climate change, however.

Even so, news outlets, including the BBC, USA Today and Reuters jumped on the story as if the textbooks now have to be thrown out. But over at DotEarth, Andy Revkin took a step back and looked at whether this is as big a deal as it's being made to seem. The climate scientists he polled weren’t entirely convinced. One in particular, Carl Wunsch of MIT, questioned the whole idea of reporting on each new paper as though it were somehow definitive. I called Wunsch — a physical oceanographer — to further explore his views on this study. Here’s some of what he told me:

"I think this is a good, decent paper that the professional community will take seriously and try to understand. If it had been published in a more technical journal, the public wouldn't hear about it. People in the community would, and they'd say "this is kind of interesting, let’s re-analyze the data." Some time would go by, someone would put a grad student on it, there would be follow-up, and eventually it would be clear how important it might be."

But there are, he says, hundreds of papers like this. Why pick this one to feature in the news?

"I think the answer is because Nature is publishing it. [Reporters are] delegating to Nature the decisions about what is interesting and important. I’ve had enough dealing with Nature that I’m convinced this is a pretty shaky way to make a judgment. Getting published there is kind of a lottery. If you get a junior editor on a bad day he or she may reject a paper out of hand."

Prestigious journals like Nature and Science also, says Wunsch, go out of their way to publish papers that will catch the public’s attention.

"The problem in climate science is that things are hyped, then they’re picked up and exaggerated, sometimes by these crazy bloggers, and you get senators and congressmen talking about it on the floor of the House and Senate."

"The wider community is trying to make climate science go faster than it can. Science has a natural rhythm, and you can’t make people think faster or more effectively by saying "you’ve got to know the answer by next week." Things get re-thought, people find bugs in the research. We can hope for breakthroughs, but it will probably take years to realize that they’ve happened. When I get calls from the press, it starts to make me uneasy, since the best they’ll get is a comment from off the top of my head. When I get a request for a comment from a colleague, I take the paper home, skim it, put it aside, read it again, think about it. And even then, sometimes there are papers I didn’t quite appreciate. So these instant off the top of the head reactions don’t do the science any good and don’t do the public any good."

So is this new paper important? Maybe yes, maybe no, but there’s really no point in expecting a useful answer for quite a while.