Evaporative climate change in the British Isles

Evaporation measurements made at 16 sites in the British Isles were analysed for evidence of long-term changes. Half of the series were from sites in Ireland measured with Class A evaporation pans between 1963 and 2005 and half in England and Scotland measured with the British Meteorological Office (MO) sunken evaporation tank between 1885 and 1968. Four of the Irish series showed significant linear trends, three of increasing and one of decreasing evaporation. These significant changes ranged between -0.1 and +0.1 mm year(-1) equivalent to annual changes between -0.22 and +0.15%. Five of the UK series showed statistically significant linear trends, three of them decrease and two increase: These ranged in size between -3.7 and +2.1 mm year(-1), equivalent to annual changes of -1.05 to +0.40% of the mean. Curvilinear time trends accounted for twice the amount of inter-annual variation in evaporation as did the linear trends. Differences in sunshine duration (SD), used as a proxy for global radiation, were found to be the major factor explaining spatial as well as temporal changes in evaporation in the British Isles. The pooled Irish data, expressed as normalized anomalies, showed a small and significant linear increase in evaporation over the last 40 years. Similarly parameterized the UK measurements showed no significant trend up till 1968; the one UK series that did extend till 2004 indicated a marked increase during the last 20 years. The long-term changes found in annual evaporation were similar to those in air temperatures both in Ireland and England. The relevance of these findings to the hypothesis of an acceleration in the hydrological cycle is discussed. Copyright (C) 2007 Royal Meteorological Society.