Global assessment of trends in wetting and drying over land

Abstract

Changes in the hydrological conditions of the land surface have substantial impacts on society1, 2. Yet assessments of observed continental dryness trends yield contradicting results3, 4, 5, 6, 7. The concept that dry regions dry out further, whereas wet regions become wetter as the climate warms has been proposed as a simplified summary of expected8, 9, 10 as well as observed10, 11, 12, 13, 14 changes over land, although this concept is mostly based on oceanic data8, 10. Here we present an analysis of more than 300 combinations of various hydrological data sets of historical land dryness changes covering the period from 1948 to 2005. Each combination of data sets is benchmarked against an empirical relationship between evaporation, precipitation and aridity. Those combinations that perform well are used for trend analysis. We find that over about three-quarters of the global land area, robust dryness changes cannot be detected. Only 10.8% of the global land area shows a robust ‘dry gets drier, wet gets wetter’ pattern, compared to 9.5% of global land area with the opposite pattern, that is, dry gets wetter, and wet gets drier. We conclude that aridity changes over land, where the potential for direct socio-economic consequences is highest, have not followed a simple intensification of existing patterns

Drought Trends in the Iberian Peninsula over the Last 112 Years

“…In the last decades, the severity of droughts in the IP has increased, together with an increased tendency for dryness and decrease of vegetation cover, due to the higher atmospheric evaporative demand [28,37]. These agree with the findings of Greve et al [38] that identified the Southwest of the IP as one hot spot of the pattern "dry gets drier." The need for a deeper knowledge of drought frequency, duration, and intensity over the IP during the last century is reinforced by the findings by Hoerling et al [39] using Sea Surface Temperature data that have shown that the period of 1970-2010 over the Mediterranean region was considered drier when compared with 1901-1970.…”

“…Several authors have discussed the role of including ET 0 in the trend analysis of droughts at global [25,31,32,38,56], continental [35,67], and national or regional scales [68,69]. Using distinct datasets and/or different drought indices for various regions of the Mediterranean basin and considering shorter periods, several authors identified colder/wetter and warmer/dryer periods over different regions [22,25,44,70,71].…”

“…Vicente-Serrano et al [28] also pointed to a rise of drought severity in the last 5 decades, together with an increased tendency for dryness and stressed that this increment is independent of the evapotranspiration approach used. Similarly, the global assessment of wetting/drying trends during the period 1948-2005 performed by Greve and coauthors [38] has identified the Southwest of the IP as one hot spot of the pattern "dry gets drier. "…”

The increasing importance of atmospheric demand for ecosystem water and carbon fluxes

“…Looking to the future, it will become even more important to separately resolve VPD and soil moisture effects on ecosystem functioning. VPD is highly sensitive to changes in air temperature and is thus expected to rise globally in the future 1,20 . On the other hand, projected changes in precipitation and soil moisture are less certain, more spatially variable, and smaller in relative magnitude 21 .…”

Plants and Drought in a Changing Climate

“…The canonical description of precipitation changes under increasing CO 2 has been that the "wet get wetter" and the "dry get drier" [19], however this is a poor characterization of the change in pattern of precipitation change over land observed in the historical period [20], or predicted for the future [21][22][23].…”