Has global cooling begun?

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Part of the challenge in thinking about global warming is the time scale involved. Global warming is a century(ies)-long process. Temporal context counts for a lot trying to decipher what's happening during any 10- or 11-year period.

This is illustrated by the temperature graph at the top of this post. It shows by how much global average surface temperatures have risen above or fallen below the 1951-1980 average each year.

The five-year running average (red trend line) from 1900 to about 1910? Oh, my gosh, global cooling! Maybe, maybe not. Looking at 1880 to 1910, not much of an overall trend there. Just ups and downs.

How about 1940 to around 1950? Oh, my gosh! Global cooling! Eh, not so much compared with the previous 60 years.

The cool spell from about the mid-1950s to around 1970? Global cooling! Eh, not so much compared with the previous 90 years; the trend is still up. And the bottom of the "trough" is warmer that the bottom of the preceding trough.

From around 1970 on? Temperature deviations from the norm continue to rise to a peak in 1998, then flatten for the ensuing period.

So, is there really enough in the last decade's worth of readings to pronounce the end of global warming and the beginning of global cooling. Eh, not so much.

Another challenge? Location, location, location.

People tend to think about global warming in terms of what's happening where they live or in their immediate region. That's natural. But just as global warming requires temporal context, it also requires geographic context -- we're talking the globe here, not just Foxboro, Mass. (It was a pretty spectacular football game if you're a New England fan!)

The global -- there's that word again -- surface-temperature maps that the National Oceanic and Atmospheric Administration published at summer's end (see second image above) show that much of the lower 48 states and up into Canada had a cooler-than-normal summer. It was the 34th coolest summer on record for the continental US as a whole.

But if you lived in Australia, you'd have quite a different take on temperature variations during the June-August period. There, it was much warmer than normal. The June-August period was the third warmest on record globally, NOAA reported.

El Niño undoubtedly played a role. But that's one of the gotchas of global warming. Natural climate swings such as El Niño wind up being superimposed atop the much-longer-term warming trend.

So, global cooling has begun, eh? Not likely.

Added later by editor: Statistics experts reject global cooling claims.

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