Past changes in drought over decadal to millennial time scales have an important role in evaluating current droughts and the potential for future changes under global warming. Many severe droughts have plagued the planet over the 20th century causing problems for agriculture and water supply, and in some cases leading to migration and famine. For example, the Sahel drought of the 1970-80s was associated with famine and about 600,000 deaths. The U.S Dustbowl drought of the 1930s led to land abandonment and continuation of a depressed economy. More recently, a host of severe droughts and heat waves has affected every continent, including heat waves in Europe, fires in Russia, long-term drought in northern China, the “Big Dry” in southeast Australia, long-term drought in the Western US and a series of droughts in the Amazon and Argentina. Underlying this, short-term droughts continue to affect every corner of the world as they have always done in the past and will continue to do in the future. Currently, the Horn of Africa is suffering from a multi-year drought and escalating food insecurity made worse by internal conflicts. A key question is are recent droughts getting longer and more severe? How do they compare to events in living memory and over the last millennium, and how do they compare to climate model projections of the future?

We present an overview of the droughts in recent decades and put them in the context the past and the potential impacts of global warming. The variability of drought is derived from reconstructions based on paleoclimate data for the past millennium and land surface modeling in the instrumental era. For some regions, 20th century droughts pale into insignificance compared to the long-term droughts of the past 1000 years. In other regions, recent changes may be outside of the range of variability of the long-term record and hint at the influence of global warming on increased drought occurrence and severity. A detectable signal is, however, generally confounded by uncertainties in the data, particularly for precipitation, but also the combined impact of changes in evapotranspiration, its driving radiative and aerodynamic controls and the models used to calculate the land surface hydrological response. We present the latest updates to our reconstructions of drought over recent decades and an improved estimate of the uncertainties. We also evaluate the impact of changes in radiation, humidity and windspeed on the water budget and characteristics of drought. Changes in the drivers of atmospheric demand will be a key factor in future changes in drought and the feedbacks between the land and atmosphere that intensify droughts and heatwaves, although near-term changes (5-10 years) will be dominated by slow moving variability of the oceans. Future climate projections over the 21st century indicate that some regions will get drier on average, such as the US southwest, the Mediterranean and southern Africa, and others regions will get wetter. However, warming that will induce greater atmospheric demand globally via increases in radiation and atmospheric water holding capacity will exacerbate this, even if decreases in windspeed offset this to some extent. Overall, the interplay of changes in precipitation characteristics, and the increase in atmospheric demand tends to increase the occurrence and severity of drought in most regions of the world, even when mean precipitation is expected to increase.