by Peter Webster
The mid-20th century temperature “bump” (peaking circa 1940) is an interesting feature of the temperature record. This “bump” was discussed in an email from Tom Wigley to Phil Jones referring to a WUWT post that discusses a paper by Thompson et al.
The issue of the mid century temperature bump was raised on a previous thread by Girma in the context of an email he sent to Kevin Trenberth (some excerpts provided below):
G: The following is my interpretation of the [global temperature] data.
G: In the last 100 years, the globe had TWO warming phases. The first was from 1910 to 1940 and the second was from 1970 to 2000, and their global warming rate was about 0.15 deg C per decade giving a warming of 0.45 deg C. In the intermediate 30-years period from 1940 to 1970, there was slight global cooling.
G: Based on these observed data, as the global warming rate of the two global warming phases were identical, the effect of human emission of CO2 for 60-years has not increased the global warming rate. Though CO2 is a greenhouse gas, the observed data says it has no effect on the global temperature trend. Observation should win theory all the time.
KT: The warming in the first phase was not global but focused in the North Atlantic. It was related to changes in the ocean. The warming in the southern hemisphere is more steadily upwards: no steadying off or down phase. So the patterns of change also matter.
Looking at the raw surface temperature (unhomogenized), it can be shown that the warming through the thirties and early forties was indeed global and not restricted to the North Atlantic.
The first figure (from Polyakov et al 2003) shows a time series of surface land temperature in the Arctic from 1880-2000. The numbers above the abscissa show the number of stations used in the compilation. The graph shows a gradually rising surface temperature trend over the 105 years of the temperature record but the dominant feature is the nearly 2C (trough to ridge) warming from about 1920 to 1940, followed by a decline bottoming out in the mid-1960s. At that stage the temperature trends upward to almost the same level as the previous peak.
The Polyakov et al paper received a brief mention in IPCC WG1 Chapt 3 :
A slightly longer warm period [compared to the present], almost as warm as the present, was observed from the late 1920s to the early 1950s. Although data coverage was limited in the first half of the 20th century, the spatial pattern of the earlier warm period appears to have been different from that of the current warmth. In particular, the current warmth is partly linked to the Northern Annular Mode (NAM; see Section 3.6.4) and affects a broader region (Polyakov et al., 2003) (Chapter 3.2.2.4)
We can test Trenberth’s contention that the earlier warming “…was focused in the North Atlantic” and that “….The warming in the southern hemisphere is more steadily upwards: no steadying off or down phase…”
The second figure is a preliminary analysis using land station data kindly provided by Phil Jones of CRU (unhomogenized). The figure shows the surface land station data clustered in areas (numbered: these are the World Meteorological Organization regions) for the period 1930-1950 relative to the mean of that period between 30N and 45S. 5-year running averages are shown. A clear warming occurs up to the early and mid-1940s followed by cooling through the 1950s and 1960. Contrary to Trenberth’s statement, the warming that peaks around 1940 is not restricted to the North Atlantic, with the southern hemisphere showing the same signature. Thus, it is safe to say that the “bump” in the land surface temperature pattern was global.
So, a number of questions arise:
- Even though the IPPC noted the mid-20th century warming , why was it not highlighted as something worth investigating? Various unsatisfying explanations (to me at least) for this are given in Ch 9 on attribution..
- Why is the southern hemisphere drop in temperature from 1945+ much the same as the northern hemisphere? The standard argument for the drop is the increase in aerosols with enhanced industrial activity after WW2. But this seems strange as even now the ration of aerosols between hemispheres is 2:1 NH: SH.
It would seem to me that there were large-scale natural oscillations occurring during the earlier part of the 20th century. Clearly, examination of these oscillations, during a time of relatively large data coverage, might help determine the proportion of the warming in the latter half of the 20th century that is due to anthropogenic influences and those associated natural coupled ocean-atmosphere modulations.
While this preliminary analysis focuses on land temperatures, there are many issues surrounding the sea surface temperature data during this period and we will return to this particular issue at another time. For example, Thompson et al (2008) attempted to explain the sea-surface temperature drop after 1945 as an uncorrected measurement bias caused by the change from bucket to engine-intake temperature measurement:
Data sets used to monitor the Earth’s climate indicate that the surface of the Earth warmed from 1910 to 1940, cooled slightly from 1940 to 1970, and then warmed markedly from 1970 onward1. The weak cooling apparent in the middle part of the century has been interpreted in the context of a variety of physical factors, such as atmosphere–ocean interactions and anthro- pogenic emissions of sulphate aerosols2. Here we call attention to a previously overlooked discontinuity in the record at 1945, which is a prominent feature of the cooling trend in the mid- twentieth century. The discontinuity is evident in published ver- sions of the global-mean temperature time series1, but stands out more clearly after the data are filtered for the effects of internal climate variability. We argue that the abrupt temperature drop of 0.3 6C in 1945 is the apparent result of uncorrected instrumental biases in the sea surface temperature record. Corrections for the discontinuity are expected to alter the character of mid-twentieth century temperature variability but not estimates of the century- long trend in global-mean temperatures.
Yet, the surface land data shows a similar drop in phase with the drop found over the ocean areas. I understand that CRU is revising its ocean surface temperature analysis; it will be interesting to see how the mid-20th century ocean temperatures change.