About the Lack of Warming…
by John Nielsen-Gammon, Climate Abyss: Weather and Climate Issues, April 23, 2012
It’s common knowledge among those who follow such things that global temperatures have not gone up very much in the past several years. This has caused many to believe that the recent lack of warming contradicts what climate models say should happen in response to the increasing Tyndall gases. This, in turn, has provoked the counterargument that the Earth is still warming, just on a longer time scale, or that the recent period is too short to yield statistically significant results.
These counterarguments are not compelling. Fundamentally, any change in global temperature, even if it’s just from one year to another, must have a cause. Saying that we need to look at longer time scales denies the need to find the cause of the actual global temperature changes (or lack thereof) at shorter time scales.
Such causes have been sought, and a few papers have proposed various combinations of cloud cover, volcanic aerosols, the El Niño/Southern Oscillation (ENSO), deep ocean heat uptake, and so forth. A recent paper I like by Foster and Rahmsdorf (discussed here and here) takes a statistical approach to attempt to eliminate the effect of the other known forcing mechanisms, and what’s left over is a fairly steady warming. Others have noted, more casually, that 2011 was the warmest La Niña year on record.
I decided to take a simple approach at looking at the effect of ENSO. Using GISTemp Land/Ocean Index values and Niño 3.4 values, I computed 12-month running averages of Niño 3.4 and compared them to the average GISTemp values at lags of 0, 3, and 6 months. Foster and Rahmsdorf used a diferent ENSO index and found optimal lags between 2 and 5 months. So one would guess that a 3-month lag would fit the data best in my case, and indeed it did.
The normal threshold for El Niño or La Niña, as applied by the Climate Prediction Center, is for five consecutive months of at least 0.5 C above or below normal in a key region of the tropical Pacific. For working with annual data, I decided to call an annual average above 0.5 C an El Niño and an annual average below -0.5 C a La Niña. Then I plotted it up, color-coding each year for whether it was El Niño, La Niña, or neither (neutral). Here’s the result:
GISTemp global temperatures, 1951-2011
We see the latter half of the mid-century flat period, followed by the warming since 1970 and the relatively flat recent few years. We also see a few years that were exceptionally cold and whose timing fits with the known injection of aerosols into the stratosphere by the mighty volcanic eruptions of Agung and Pinatubo. It’s easy to see that both of these eruptions caused global temperatures to drop by about 0.3 C temporarily before recovering as the aerosols settled out of the stratosphere over the following 2-3 years. Finally, we see that, as is well known, La Niña years tend to be globally cold years and El Niño years tend to be globally warm, with a global lag of three months as mentioned earlier. And, we see that in a head-to-head match between El Niño and Pinatubo, Pinatubo wins.
To dig deeper, I’ll zoom in on the period since Agung. This isolates the period of nearly steady warming since 1970 and lets us focus a bit more on what has happened since 1998 or so. Here’s the chart:
GISTemp global temperatures from 1967 to present
Somehow, it no longer appears that global temperatures have leveled off in the past decade. That is because, with the color coding according to the phase of ENSO, the eye is able to compare apples to apples: the upward long-term trend during El Niño years (red triangles) is plain, the upward long-term trend during neutral years (green squares) is plain, and the upward long-term trend during La Niña years (blue diamonds) is plain.
Stare hard enough, though, and you see that they have leveled off. The last ten data points have little or no trend. But we see that the lack of trend is at least partly due to the El Niño year near the beginning of the 10-year period and the two La Niña years near the end.
Let’s get quantitative about this. In this case, with the temperature rise being nearly linear, it helps to add trendlines. I’ve excluded the three Pinatubo years from the regressions. Here’s the result:
GISTemp global temperatures, with trends for El Niño, neutral, and La Niña years computed separately. Pinatubo years are excluded.
There aren’t that many full-blown El Niño events, but they seem to be following a steady upward trend. There are more La Niña events, and they too clearly follow a steady upward trend. Finally, the many neutral years also so no sign of departing from a steady upward trend. There’s enough scatter in the neutral years that if one had considered the period 1977-1987, or the period 1987-1997, one might be tempted to say that the neutral years had little or no warming. But the past decade fits nicely with the long-term upward trend of 0.16 C/decade shown by all three time series.
The spacing between the lines is a good measure of the impact of El Niño and La Niña. All else being equal, an El Niño year will average about 0.2 C warmer globally than a La Niña year. Each new La Niña year will be about as warm as an El Niño year 13 years prior.
So we see a couple of recent La Niñas have caused the recent global temperature trend to level off. But be honest: doesn’t it seem likely that, barring another major volcanic eruption, the next El Niño will cause global temperatures to break their previous record? Doesn’t it appear that whatever has caused global temperatures to rise over the past four decades is still going strong?
So about that lack of warming: Yes, it’s real. You can thank La Niña.
As for whether this means that Tyndall gases are no longer having an impact: Nice try.
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