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Post-World War II cooling a mirage

The 20th century warming trend is not a linear affair. The iconic climate curve, a combination of observed land and ocean temperatures, has quite a few ups and downs, most of which climate scientists can easily associate with natural phenomena such as large volcanic eruptions or El Nino events.

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But one such peak has confused them a hell of a lot. The sharp drop in 1945 by around 0.3 °C – no less than 40% of the century-long upward trend in global mean temperature – seemed inexplicable There was no major eruption at the time, nor is anything known of a massive El Nino that could have caused the abrupt drop in sea surface temperatures. The nuclear explosions over Hiroshima and Nagasaki are estimated to have had little effect on global mean temperature. Besides, the drop is only apparent in ocean data, but not in land measurements.

Now scientists have found – not without relief – that they have been fooled by a mirage.

The mysterious post-war ocean cooling is a glitch, a US-British team reports in a paper in this week’s Nature. What most climate researchers were convinced was real is in fact “the result of uncorrected instrumental biases in the sea surface temperature record,” they write. Here is an editor’s summary.


How come? Almost all sea temperature measurements during the Second World War were from US ships. The US crews measured the temperature of the water before it was used to cool the ships engine. When the war was over, British ships resumed their own measurements, but unlike the Americans they measured the temperature of water collected with ordinary buckets. Wind blowing past the buckets as they were hauled on board slightly cooled the water samples. The 1945 temperature drop is nothing else than the result of the sudden but uncorrected change from warm US measurements to cooler UK measurements, the team found.

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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. Our news story here has more.

Scientists can now correct for the overlooked discontinuity, which will alter the character of mid-twentieth century temperature variability. In a News and Views article here (subscription required) Chris Forest and Richard Reynolds lay out why this will not affect conclusions about an overall 20th century warming trend.

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.

Quirin Schiermeier

You can vote or comment on the importance of the new paper in the Journal Club of Nature Reports Climate Change.

Comments

  1. Report this comment

    ArndB said:

    The Pacific SST matter from 1940 to 1945 has been discussed in a paper: “Reliability of sea-surface temperature data taken during war time in the Pacific”, PACON Conference Paper: August 8-9,1997 “The Chinese University of Hong Kong“; in: PACON 97 Proceedings October 97, pp. 240-250; http://www.oceanclimate.de/English/Pacific_SST_1997.pdf

    The sudden SST drop should therefore serve a reminder that the Western Northern Pacific from the Aleutian, Hawaii, and Indonesia had been subject to dramatic naval activities and navel battles over several pre-1945 years. Just to illustrate the situation in 1945, here is a short excerpt from http://www.seaclimate.com/ (Chapter 4_11): “ QUOTE After taking over Okinawa, the US Third fleet had deployed some 26 aircraft carriers, 64 escort carriers and 14,000 combat aircraft for a final attack on Japan . The Japanese loss of combat aircraft was 37,000 (army and navy); the USA lost 8,700 in the battle.

    The material loss in the battle was gigantic. Japan lost more than 500 warships (including 150 submarines) with a total tonnage of about 2,000,000, the figure in merchant tonnage was about 8,000,000 of which 5 Mio (1,150 ships) were sunk by US-submarines and 1.5 Mio by airplanes. UNQUOTE

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    boris said:

    We don’t have a model for global temperature. We don’t have empirical data for analysis.

  3. Report this comment

    Thoughts from Kansas said:

    New Research shows that 20th century warming has been underestimated

    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&#821…

  4. Report this comment

    Thoughts from Kansas said:

    New Research shows that 20th century warming has been underestimated

    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&#821…

  5. Report this comment

    mphelps said:

    This issue has been discussed before. See http://www.climateaudit.org/?p=3114

    The interesting point is that there appears to be evidence that buckets were still widely used in 1970, and were phased out after that point. This would suggest that current measurements are comparable with those made in WW2, and therefore a redistribution of warming over the whole post war period not just 1945-1960.

  6. Report this comment

    ArndB said:

    >>The idea of an abrupt changeover seems a little weird to me.>>, expressed

    Steve McIntyre; Changing Adjustments to 19th Century SST; (Sunday, June 19th, 2005 at 11:56 am) concerning SST measurements taken in the 1940s. This time period was unique in many respect and deserves particular attention, as it is not only a matter of the means of measurements, but are due to the circumstances during WWII (concerning the Atlantic see: http://www.oceanclimate.de/English/Atlantic_SST_1998.pdf ). Any considerations of more recent historical SST adjustment should have started at this time period, (see: Steve McIntyre, The Team and Pearl Harbor , http://www.climateaudit.org/?p=1276 ), and when shipping resumed after 1945 one should not only take into account the possible differences between bucket and engine-inlet, but also the continuously increasing of size, depth, and speed of the merchant ships. Correction of historical SST will be tricky, if not speculative forever.

  7. Report this comment

    Alfred Jones said:

    Why should McIntyre be mentioned? His blog (not a peer-reviewed study) talks at length about the effect of corrections to measurements of sea temperatures, something that has also been discussed in peer reviewed literature and in conferences for years. Isn’t what is new about this study is the showing that the 1945 temperature shift does not look like short term internal variability, and its association with the changes in use of canvas buckets/engine intake observations?

  8. Report this comment

    mphelps said:

    MacIntyre discusses precisely the assumption that “that there was an abrupt and universal change in SST measurement methods away from buckets to engine inlets in 1941, coinciding with the U.S. entry into World War II. As a result, Folland et al introduced an abrupt adjustment of 0.3 deg C to all SST measurements prior to 1941 (with the amount of the adjustment attenuated in the 19th century because of a hypothesized use of wooden rather than canvas buckets.)” A reader the wrote in and said “it looks to me like the WWII records were dominated by engine-warmed intake data, perhaps because the chaos meant much of the bucket data did not get recorded, and after WWII it was business as usual with mostly bucket data resuming.” MacIntyre responded by calculating the effect on SST “assuming that 75% of all measurements from 1942-1945 were done by engine inlets, falling back to business as usual 10% in 1946 where it remained until 1970 when we have a measurement point – 90% of measurements in 1970 were still being made by buckets as indicated by the information in Kent et al 2007- and that the 90% phased down to 0 in 2000 linearly.” The resulting graph does remove much of the 45-70 dip. But what is interesting, as I indicated above,is the discussion of the continued use of buckets post 1970. If you read Climate Audit (why is it not on the Blogroll?) you can find an ongoing discussion of this, including the possibility that there are further complications because of the introduction of insulated buckets. This whole discussion seems worthy of mention.

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