The great blip of 1945 explained

Climatologists have long puzzled over what caused an abrupt drop in global average temperatures in 1945. To explain the anomaly, which, unlike most other similar rapid falls, is not associated with a volcanic eruption, most invoke an increase in airborne industrial activity following the end of the Second World War, even though the ocean temperature dip begins several years before industrial activity takes off. The idea is the clouds seeded by the extra aersols reflected more sunlight before it could be trapped in the atmosphere. But that was always a guess, and now it seems there was a simpler answer.

A team of scientists say the real reason was good old fashioned measurement error. In other words, there was no actual temperature decline. In a letter in the current issue of Nature, David W. J. Thompson of Colorado State University and his colleagues say the anomaly can best be explained by differences in the way British and American sailors sampled sea water temperatures. Once you correct for the disparity in technique, the drop ;;;;; about 0.3°C ;;;;; disappears.

See the graph at right, which is taken from the latest IPCC assessment summary, for an idea of what the anomalous drop looked like. The black line represents the puzzling observations, which don't match the natural forcings (thick blue line) and natural+anthrogenic forcings (in pink) one would expect from climate models.

From Thompson et al:

...the sudden drop in SSTs [sea surface temperatures] in late 1945 is consistent with the rapid but uncorrected change from engine room intake measurements (US ships) to uninsulated bucket measurements (UK ships) at the end of the Second World War. As the drop derives from the composition of the ICOADS data set, it is present in all records of twentieth-century climate variability that include SST data.

Although air temperature records never showed a comparable drop, when you add the air and sea records together the global average did.

The findings, if true, do nothing to change the subsequent global warming trend. In fact, as an accompanying News and Views in Nature, notes, "Global warming would remain a reality ;;;;; it would just be a bit more than previously thought."

But this could be important because if post-war aerosols had been to blame, global temperature would be even more sensitive to aerosols than models predict, and climatologists would have a valuable clue that might eventually help explain the role of cloud cover in the planet's thermoregulatory systems. In fact, figuring that out is one of the biggest challenges in climate research. Will warmer temperatures lead to more clouds, which could act as a negative feedback and put the brakes on further warming? Probably not, but we don't know.

[Update: As reported in the Independent, some believe this actually supports the aerosol theory, because there still is a drop in global temperatures, a drop more consistent with industrial emissions. The Indy has a good graph on that.]

This is also another example of the importance of nailing down measurement protocols. Not too many years ago, more than a few climate change skeptics were troubled by the apparent failure of some satellite temperature records to show what should have been a more significant warming trend. Only when it became clear that the satellites data had to be recalibrated did that anomaly go away.

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Thompson, D.W., Kennedy, J.J., Wallace, J.M., Jones, P.D. (2008). A large discontinuity in the mid-twentieth century in observed global-mean surface temperature. Nature, 453(7195), 646-649. DOI: 10.1038/nature06982