Science Source

Twentieth-century hydroclimate changes consistent with human influence

States that the detection of human influence on global drought has been complicated by large internal variability and the brevity of observational records
Addresses these challenges using reconstructions of the Palmer drought severity index obtained with data from tree rings that span the past millennium
Shows that three distinct periods are identifiable in climate models, observations and reconstructions during the twentieth century
Finds that in recent decades (1981 to present), the signal of greenhouse gas forcing is present but not yet detectable at high confidence
Finds that observations and reconstructions differ significantly from an expected pattern of greenhouse gas forcing around mid-century (1950–1975), coinciding with a global increase in aerosol forcing
Finds that in the first half of the century (1900–1949), however, a signal of greenhouse-gas-forced change is robustly detectable
Multiple observational datasets and reconstructions using data from tree rings confirm that human activities were probably affecting the worldwide risk of droughts as early as the beginning of the twentieth century