Does it matter how warm this past summer was?

Yes, but only in terms of how it fits in the longer-term trend

James Hansen, head of the Goddard Institute for Space Studies, has published a new analysis of the summer just past. Make no mistake about it, June through August was plenty warm. In fact, for the first eight months of the year we were on track to experiencing the warmest year on record.

That may be in question now, thanks to La Niña. But even if temperatures cool down significantly for the rest of the year, a focus on the short-term vicissitudes of climate is misleading. The most recent local seasonal temperature anomaly may not be at all indicative of the long-term trend, Hansen says.

So if the weather near you this fall and winter won’t be the best guides to what’s happening to the climate, what is a good guide? The answer, according to Hansen, is the simple 12-month running mean global temperature. (For an explanation of why that’s a good measure, read this paper by Hansen and colleagues, now in press at Reviews of Geophysics.

In the graphic at the top of this post, the right panel shows what the 12-month running mean looks like, starting in 1880 and using data through June of this year. And as the graph shows, we’re now in record-high territory. As Hansen puts it:

Contrary to a popular misconception, the rate of warming has not declined. Global temperature is rising as fast in the past decade as in the prior two decades, despite year-to-year fluctuations associated with the El Niño-La Niña cycle of tropical ocean temperature. Record high global 12-month running-mean temperature for the period with instrumental data was reached in 2010.

And what does the record of temperature change look like when looked at over the time span of decades, as opposed to year-by-year? Consider the maps below, which portray how much the global temperature of the Earth has varied from long-term base period each of the past four decades.

As Hansen writes:

Global warming on decadal time scales is continuing without letup. Figure 8, showing decadal mean temperature anomalies, effectively illustrates the monotonic and substantial warming that is occurring on decadal time scales.

So if this winter turns out to be particularly cold where you live, and if La Niña does indeed hold down global temperatures a bit for a year or so, come back to this post have a look at these graphics again.