Nature's Climate Feedback blog points out that the post-World War II cooling is a mirage:
The 1945 temperature drop is nothing else than the result of the sudden but uncorrected change from warm US measurements to cooler UK measurement…
That’s a rather trivial explanation for a long-standing conundrum, so why has it taken so long to find out? Because identifying the glitch was less simple than it might appear, says David Thompson of the State University of Colorado in Boulder. The now digitized logbooks of neither US nor British ships contain any information on how the sea surface temperature measurements were taken, he says. Only when consulting maritime historians it occurred to him where to search for the source of the faintly suspected bias. …
But it may not be the last uncorrected instrument bias in the record. The increasing number of measurements from automated buoys, which in the 1970s begun to replace ship-based measurements, has potentially led to an underestimation of recent sea surface temperature warming.
In other words, older temperature measurements had various biases, and correcting those biases shows that anthropogenic climate change was more intense in the 20th century than previously known. I expect that everyone who got worked up over a 0.15 degree Celsius change in US temperature estimates will be equally vocal about this one.
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I followed this with enormous interest, because I have been arguing some of the specifics of measurement at an on-line forum, and was using plots of the HadCRUT3v dataset, with the land based CRUTEM3v and sea based HadSST2 datasets upon which it is based. I recently noticed (but unfortunately did not comment upon it -- damn!) the sharp 1945-46 SST drop by comparison with land temperatures... which was a problem as I was trying to explain why the SST generally reacts more slowly to forcings than the land.
I don't think it is going to have a great deal of effect on the inferred warming from anthropogenic sources. The letter to Nature on the subject doesn't anticipate that kind of change. I suspect the main effect of this welcome discovery will probably be to give a lower anomaly value as you go back before 1945, smoothing out the hump. But the rate of warming from 1970 onwards, which is when greenhouse emissions really kick off, is not going to be altered... because anomalies are all about a change in temperature. The baseline used for anomalies is a mean over the period 1961-1990 (usually) and by then all the sea measurements will be consistent again, it seems. So the shape of the graph in recent times won't change all that much.
I think.