Now that 2015 has blown away previous record-hot years, the global warming deniers are scrambling to blame it on anything but global warming. Their favorite candidate is something that does in fact make Earth’s surface get hotter, something that really did contribute to 2015’s record heat: el Niño.
But how much? A post at Carbon Brief addresses just that question. Their conclusion is that el Niño contributed only about 10% of the record.
Much of their conclusion is based on soliciting opinions from experts in the field. One of those, Gavin Schmidt at NASA’s Goddard Institute for Space Studies, estimates el Niño caused 0.07 deg.C extra warmth in the NASA data. He based his estimate on compensating the data for its el Niño influence, yielding “el Niño-corrected” or “adjusted” data:
It’s an effort I’ve been involved in for a few years now. I’ve compensated for multiple factors, including el Niño, atmospheric aerosols (from volcanic eruptions), and fluctuations in solar output.
Of course the adjustments don’t compensate for these factors perfectly, so lately I’ve pondered some improvements. For one thing, it’s possible that el Niño has an impact which lasts longer than the el Niño itself. We know there’s a lag between el Niño and its effect, but perhaps there’s more than one lag at work. Therefore I’ve allowed for el Niño to have an impact at two different lags (I’ve tried three but that didn’t seem to improve things beyond two).
For another thing, the impact of el Niño may not be linear. I’ve suspected that perhaps a strong el Niño has more effect than a weak one, stronger even than the strength of the el Niño itself. This intuition was based on the fact that models which include an el Niño effect don’t seem to match all of its influence — they don’t seem strong enough when the el Niño is strong.
Finally, there may be a seasonal pattern to the influence of el Niño. Back in 2013 I discussed the research of Kosaka and Xie, and emphasized that one important result was their discovery that the influence of el Niño depends on season. I even closed that post by saying “the regression approach of Foster & Rahmstorf … might be improved substantively simply by allowing for a seasonal pattern in the influence of el Niño..”
So, I’ve added these elements to the mix of factors by which el Niño can influence global temperature. The best model I’ve found so far (there’s a lot more to test) involves a linear el Niño effect which lags only 2 months behind the el Niño itself, a nonlinear el Niño effect which lags 10 months, and a seasonal effectiveness of the el Niño impact. In agreement with the research of Kosaka and Xie, the el Niño impact is strongest in northern-hemisphere winter and weakest in northern-hemisphere summer.
With all those in play, with volcanic aerosols and solar fluctuations to boot, the model compares to the observations thus:
It’s particularly interesting to look at annual averages, of the observations, the model, and the adjusted data:
The model doesn’t just match the observations well, it accounts for over 70% of the variance of the data from a steady linear increase since 1970.
We can also plot the impact of el Niño on each year’s temperature:
My result indicates that el Niño led to 0.08 deg.C warmer temperature in 2015. That’s hardly enough to explain the record heat, which was mainly due to global warming. Note, however, that el Niño caused fully 0.2 deg.C warming in 1998, so the record heat of that year — which the deniers love to point to as the “end” of global warming — really was due to el Niño.
We can even look at the el Niño influence on a month-by-month basis:
This illustrates that el Niño is the main reason that the last few months of 2015 were so much hotter than the preceding months of 2015.
For 2015 as a whole, el Niño contributed in a small way to its extreme heat. But the main factor was the continuing trend. That’s due to man-made greenhouse gases, and it’s called global warming.
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