Late Twentieth-Century Warming and Variations in Cloud Cover

Abstract

From 1950 to 1987 a strong relationship existed between the El Nino-Southern Oscillation (ENSO) and HadCRUT4 global average temperature anomaly, interrupted occasionally by volcanic erup-tions. After 1987 the relationship diverged, with temperature anomaly increasing more than ex-pected, but was re-established after 1997 at an offset of ~0.48°C higher. The period of increased warming from 1987 to 1997 loosely coincided with the divergence of the global average tempera-ture anomalies over land, which are derived from observation station recordings, and the global average anomalies in sea surface temperatures. Land-based temperatures averaged 0.04°C below sea temperatures for the period 1950 to 1987 but after 1997 averaged 0.41°C above sea tempera-tures. The increase in the global average temperature anomaly and the divergence of land and sea surface temperatures also coincided with two significant changes in global average cloud cover. Total cloud cover decreased during the period from 1987 to 1997 and, for most of the remainder of the period from 1984 to 2009, decreases in low-level cloud were accompanied by increases in middle and upper level cloud. These changes can be found in both global average cloud cover and in each of the six 30°C-latitude bands. The impact of these changes in cloud cover can account for the variations in HadCRUT4 global average temperature anomalies and the divergence between land and sea temperatures.