Drought is one of the most important but least understood issues in global environmental change. Although Inter-government Panel on Climate Change (IPCC) projections have shown that extreme drought events will become more frequent and variable as a result of anthropogenic climate change, we still do not understand the factors that control drought.

Drier soil is a good indictor of drought and has been observed in many sites in the northern hemisphere. Most previous studies mainly focused on how moisture in soil responds to amounts of precipitation, but are we missing an important factor that can explain soil moisture variations in general?

The team found that precipitation frequency can explain year-to-year variations in summer soil moisture. The correlation coefficients between soil moisture and precipitation frequency are higher than correlation coefficients between soil moisture and amount of precipitation. This suggests that there is a hitherto ignored factor when considering the parameters that control drought. The researchers also observed that precipitation frequency and summer soil moisture are inter-related. More significant correlations were observed at sites that were neither too wet nor too dry, suggesting that water quickly evaporates in extremely dry areas and runs off in very wet areas.

The findings could explain why estimations based solely on monthly temperature and precipitation amounts might not fully describe climate change-induced soil-moisture variations. The results also highlight how important precipitation frequency and intensity changes are for near-term evolutions of droughts.

With these findings in mind, combined with IPCC projections for precipitation frequency, the team now plans to further investigate and evaluate how susceptible the northern hemisphere will be to future droughts.

The team reported its work in Environmental Research Letters.

For Dr Piao's recent publications, visit www.ecology.pku.edu.cn/en/renyuan_show.asp?oneuser=Piao%20Shilong.