An abrupt drop in Northern Hemisphere sea surface temperature around 1970

Surprise sea temperature drop in the 1970s Global-mean temperatures did not rise continuously through the twentieth century. Rather, they rose from the start of the century to the 1940s, decreased slightly during the middle part of the century, and rose rapidly from the mid-1970s to the 2000s. The mid-century cooling has been commonly interpreted as the response to a peak in sulphate aerosol production and/or natural climate oscillations. David Thompson and colleagues now show that an abrupt change in sea-surface temperatures around 1970 accounts for a substantial amount of the Northern Hemisphere cooling. The same authors previously identified an anomaly in measurements of sea-surface temperature centred at 1945 ( http://go.nature.com/16G48A ), which was found to be an artefact caused by changes in data-collection methodology. That discovery is leading to some of the largest corrections in the global-mean temperature time series made in recent years. The underlying physical mechanism for the newly identified change in the 1970s remains uncertain, but it was too rapid to have been caused by aerosols or multidecadal variability.