WIDESPREAD INCREASES IN LOW-FREQUENCY VARIABILITY OF PRECIPITATION OVER THE PAST CENTURY

PRECIPITATION (rain and snow) provides, through its participation in t he global hydrological and energy cycles, most of the heat flux within the atmosphere, For this reason, a knowledge of precipitation variabi lity is important to understanding the behaviour and changes of the Ea rth's climate system, Precipitation measurements have been made at tho usands of sites over the past hundred years or so, and recent efforts have resulted in the compilation of global data sets(1-3) and regional networks(4,17) . The latter have been analysed to reveal high-frequen cy (up to interannual) regional trends in precipitation variability ov er the past century(4). Here I exploit the global data sets to examine the low-frequency (decadal to multi-decadal) variability of precipita tion over a similar time period, In agreement with other analyses(5), I find that the global mean precipitation has not changed. The fluctua tions about the mean, however, have increased significantly (on decada l to multi-decadal timescales), That is, over the past century-during which climate warming has occurred-the global precipitation field has undergone changes on those scales, in which extremes have become more probable, This result is consistent with predictions from model simula tions of global climate-warming scenarios(6-8).