Drought indices, drought impacts, CO2, and warming : a historical and geologic perspective

Purpose of review: Different types of methods give very conflicting impressions about whether water will become scarcer on land as Earth warms, and in what sense(s). Here I examine how environmental records from past climate changes can be used to clarify the interpretation of these confusing results. Recent findings: Evidence from the last ice age and the historical era agrees that CO2-driven warming causes a runoff response dominated by regional signals of varying sign, and a vegetation response dominated by greening. This result supports comprehensive Earth system model output, while casting doubt on the interpretation of temperature-driven indices that project widespread "drying" with warming. In contrast, evidence from pre-Quaternary warm climates points to exotic features such as wet subtropics and extremely polar-amplified warming which are not found in model simulations, suggesting unknown forcings and/or feedbacks. Summary: The terrestrial eco-hydrologic response to CO2-driven warming in the recent past is consistent with comprehensive models, and not with drought indices. However, in the deeper past it is consistent with neither.