The expectation of future precipitation change over the Mediterranean region is different from what we observe

Abstract

In this study we assess the role of anthropogenic forcing (greenhouse gases and sulphate aerosols, GS) in recently observed precipitation trends over the Mediterranean region. We investigate whether the observed precipitation trends (1966–2005 and 1979–2008) are consistent with what 22 models project as response of precipitation to GS forcing. Significance is estimated using 9,000-year control runs derived from the CMIP3 archive. The results indicate that externally forced changes are detectable in observed precipitation trends in winter, late summer and in autumn. Natural internal climate variability cannot explain these changes. However, the observed trends (derived from 3 sources) are markedly inconsistent with expected changes due to GS forcing. While the influence of GS signal is detectable in winter and early spring, observed changes are several times larger than the projected response to GS forcing. The most striking inconsistency, however, is the contradiction between projected drying and the observed increase in precipitation in late summer and autumn, irrespective of the data set used. Natural (internal) variability as estimated from the models cannot account for these inconsistencies, which are already present in the large scale circulation patterns (Geopotential height at 500 hPa). The obtained results are robust to the removal of the fingerprint of the North Atlantic Oscillation. The detection of an outright sign mismatch of observed and projected trends in autumn and late summer, leads us to conclude that the recently observed trends can not be used as an illustration of plausible future expected change in the Mediterranean region. These significant shortcomings in our understanding of recent observed changes complicate communication of future expected changes in Mediterranean precipitation.