Evidence of how humans may have influenced global drought conditions over the 20th century is presented in this week’s Nature. A drought-severity record reveals three distinct trends, and suggests that greenhouse gasses and, potentially, aerosols generated by human activity may affect drought risk.

Climate change driven by human activity is thought to alter the global hydroclimate, which determines the risk of persistent droughts or increased rainfall. Attempts to understand the effects of human activity on global drought risk have been complicated by regional differences in hydroclimate variability and a lack of detailed observational data.

To address these issues, Kate Marvel and colleagues analysed ‘drought atlases’ derived from tree-ring data, which provide a picture of regional changes in soil moisture, combined with other climate models and observations, to identify potential drivers of soil moisture changes. Their records indicate that drought increased between 1900 and 1949, lessened between 1950 and 1975, and has accelerated since then.

The drying trend in the first half of the 20th century corresponds to increasing levels of greenhouse gas emissions. The reversal of this trend from 1950 to 1975 coincides with an increase in the production of aerosols, which have been shown to affect rainfall and alter cloud cover. However, the association between aerosol concentrations and drought risk requires further investigation, the authors note. The increased drought towards the end of the 20th century seems to be linked to greenhouse gas emissions, although the underlying cause remains to be determined and the link is not statistically robust.