The importance of unresolved biases in twentieth-century sea-surface temperature observations

Biases in sea-surface temperature observations lead to larger uncertainties in our understanding of mid-to-late 20th century climate variability than previously thought. A new analysis of sea-surface temperature (SST) observations indicates notable uncertainty in observed decadal climate variability in the second half of the 20th century, particularly during the decades following World War II. The uncertainties are revealed by exploring SST data binned separately for the two predominant measurement types: “engine-room intake” (ERI) and “bucket” measurements. ERI measurements indicate large decreases in global-mean SSTs from 1950 to 1975, whereas “bucket” measurements indicate increases in SST over this period before bias adjustments are applied but decreases after they are applied. The trends in the bias adjustments applied to the “bucket” data are larger than the global-mean trends during the period 1950-1975, and thus the global-mean trends during this period derive largely from the adjustments themselves. This is critical, since the adjustments are based on incomplete information about the underlying measurement methods, and are thus subject to considerable uncertainty. The uncertainty in decadal-scale variability is particularly pronounced over the North Pacific, where the sign of low-frequency variability through the 1950s-1970s is different for each measurement type. The uncertainty highlighted here has important – but in our view widely overlooked – implications for the interpretation of observed decadal climate variability over both the Pacific and Atlantic basins during the mid-to-late 20th century.