Spend any amount of time reading climate arguments on the Internet, and you'll undoubtedly hear some version of the following argument: the Earth hasn't warmed in 17 years, and none of the climate models predicted that. Although there are a lot of problems with that statement (including the fact that it has warmed a bit), it's probably safe to say that the warming hasn't been as intense as many scientists expected.
Of course, to a scientist, unmet expectations are an opportunity, so a variety of papers have looked into why this has happened. They've found that, while volcanic eruptions seem to have contributed to the relatively slow rise in temperatures, a major player has been the El Niño/Southern Oscillation (ENSO), which has been stuck in a cool, La Niña state for most of the last decade. And, since climate models aren't expected to accurately forecast each El Niño, there would be no reason to expect that they would match the actual atmospheric record.
At least not intentionally. But some researchers have found that, simply by chance, a few of the models do produce an accurate ENSO pattern. And when those models are examined in detail, it turns out they match the existing temperature record pretty well.
The issue the new paper examines comes down to the difference between long-term climate trends and intermediate-term variations. In the long-term, the state of the climate is set by things like solar activity, orbital mechanics, and greenhouse gas levels, among other things. But on shorter time scales, things like volcanic activity and ocean cycles can have a profound effect on temperatures.
Coupled climate models that include both the atmosphere and the oceans accurately reproduce the behavior of the major ocean cycles, including the ENSO. But, since the onset of changes in the ocean is chaotic, the models generally don't get the timing rightthe model may show an El Niño starting three years earlier than it does in reality.
If you're interested in how the models behave over a specific part of the historic record, that mismatch can be a problem, but there are several approaches to dealing with it. You can, for example, subtract out the influence of things like volcanoes and ocean circulation to see what the climate is doing without them. Or, rather than letting your model generate its own ENSO, you can force it to replay historic events in order to see what those do to the temperatures.
The new paper adds an additional approach to handling the problem: simply run a bunch of models and pick those that, by accident, accurately reproduced the ocean's chaotic behavior. The authors started with the CMIP5 collection of climate models and selected the 18 models that include an ocean simulation that's sophisticated enough to provide data on the state of ENSO and other ocean behavior. They started these 18 models in 1880 and used historical forcings (solar activity, greenhouse gas concentrations, etc.) up until 2005, then switched to a standard emission scenario until stopping the models in 2012.
If you look at the four models that were the worst at reproducing ENSO behavior, then you'd think climate modelers were incompetent, as these models all showed rapid warming from 1990 onward. But, if you picked the four that had the best match to real-world ENSO data, then you see exactly what reality produced: a relatively slow rate of warming starting at about the beginning of the century.
The match isn't perfect, as the models leave out other forcings, like volcanoes, and they don't get all the details of the ENSO exactly right. But it's certainly another piece of evidence that ENSO activity has been critical for the recent behavior of our climate system.
Ars talked to climate scientists Gavin Schmidt and Michael Mann, who both emphasized that the new paper is in keeping with a variety of other studies that have come out in the recent past, with Mann saying, "This looks like a thoughtful and careful analysis that adds further weight to other recent studies (including our own recent GRL Frontier article) confirming that the temperature trends of the past decade do not, as some have claimed, contradict model-predicted global warming. The so-called 'speed bump' in global warming is consistent with the expected random fluctuations associated with natural, internal climate variability."
But if the paper's a solid bit of evidence that's in keeping with other results, it seems likely that it was meant to be a bit more than that: ammunition in the arguments that keep erupting on the Internet. In a rather unusual situation for a climate paper, one of the authors is a psychologist. And not just any psychologist, but Stephan Lewandowsky, best known for whipping climate contrarians (and journal editors) into a frenzy with his "Recursive Fury" paper. And the last author is Naomi Oreskes, a historian of science best known for her book Merchants of Doubt, which chronicles the role of think tanks in fostering doubt over the reality of climate change.
Regardless of why these two researchers decided to step into the realm of climate science, their paper clearly shows that some climate models have accurately predicted the recent slowdown in warming. But only because they accidentally got El Niño right.
)
Facebook
Twitter