Drought Indices, Drought Impacts, CO2, and Warming: a Historical and Geologic Perspective

Abstract

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. 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. 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.