Fingerprints of Precipitation Changes from CMIP5 Models over Land and Ocean

Analysis of climate model data can provide useful information about the origin of changes in the hydrological cycle, even in regions for which long time series of observations are currently unavailable. Our study builds on previous attribution studies by examining where human induced changes in both oceanic and land precipitation on annual and seasonal time scales are expected to have already emerged.

By comparing changes in precipitation over land and ocean since 1950 simulated by the CMIP5 (Coupled Model Intercomparison Project, phase 5) climate models in which natural and anthropogenic forcings have been included, we find that clear global-scale and regional-scale changes due to human influence are expected to have occurred over both land and ocean. These include moistening over northern high latitude land and ocean throughout all seasons and over the northern subtropical oceans during boreal winter. However we show that this signal of human influence is much more indistinct when considered over the relatively small area of land for which there are adequate observations to make assessments of multi-decadal scale trends. These results imply that extensive and significant changes in precipitation over the land and ocean have already happened, even though, because of inadequacies in observations in some parts of the world, it has not yet been possible to identify conclusively such a human fingerprint on the global water cycle. In some regions and seasons, observed trends have been artificially amplified by the sparse and changing observational coverage, underscoring the difficulties of interpreting the apparent magnitude of observed changes in precipitation.