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 anthropogenic 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 versions 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 °C 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.
We thank D. Parker and S. Solomon for comments on the manuscript, and R. Reynolds for reviews. D.W.J.T. and J.M.W. were supported by the NSF Climate Dynamics Program under budget numbers ATM-0132190 and ATM-0613082 (D.W.J.T.) and ATM-0318675 (J.M.W.). J.J.K. was supported by the Joint Defra and MoD Programme, GA01101 (Defra) and CBC/2B/0417_Annex C5 (MoD). P.D.J. was supported by the US Department of Energy (DE-FG02-98ER62601).