Aridity Projections - Drier types




Aridity Projections - Wetter dryland




Projected changes in dryland types with future climate change, shownas shifts in drier and wetter types from 1981-2010 (observations) to 2071-2100 (RCP 8.5) based on classification of dryland types and changes in the Aridity Index.
Source: WAD3-JRC 2018, modified from Spinoni, J., 2015


Global extent of drylands is increasing due to climate change

The maps are based on the ensemble average of 27 CMIP5 (Coupled Model Intercomparison Project v. 5) climate models under a business-as-usual (RCP 8.5) scenario (see Box: Representative Concentration Pathways on the right). Changes in the Aridity Index for Imminent Future, Near Future and Far Future are each calculated relative to Current conditions Feng and Fu concluded that in the past sixty years, global drylands have significantly expanded, and will continue to expand into the future. They projected that under a high greenhouse gas emission scenario, by the end of the 21st century, global drylands will expand by about 10 % (or 5.8 × 106 km2). In a similar modelling study, it is estimated that global drylands could expand by as much as 23 % and that as much as 80 % of this will occur in developing countries. Obviously, while there are many sources of uncertainties and limitations when predicting changes in Aridity Index (AI), there remains a high degree of confidence that aridity will continue to expand globally. In their Fifth Assessment Report, the United Nations Intergovernmental Panel on Climate Change (IPCC) concluded that the climate system of the planet is warming at a significant rate. Since 1880, the surface temperature of the planet has risen by about 0.8 ℃, and each of the past three decades has been successively warmer than the preceding one. The 10 hottest years ever recorded have all occurred since 1998, and 2016 was the hottest year on record. If there is not a substantial reduction in greenhouse gases, the IPCC posits that there is a 62 % chance that by 2081-2100 global temperatures could be more than 4 ℃ higher than in pre-industrial times. One of the major consequences of a warming climate is the potential for increased global aridity. Aridity is defined here as the degree of dryness of the climate at a given location, expressed as a function of precipitation (P) and potential evapotranspiration (PET), which is the “demand” or “drying power” of the atmosphere to remove water from the land surface. A commonly used index to measure changes in aridity is the ratio P/PET, or the Aridity Index (AI) (see Aridity, page 72). Decreases in AI mean that conditions are becoming drier; in contrast, increases in AI mean conditions are getting wetter. Hence, as air temperature systematically increases globally – accompanied by shifts in other key variables, including precipitation, relative humidity, solar radiation and wind speed – a long-term shift to increased aridity is anticipated. Indeed, recent observational studies have shown that the average AI is decreasing (a drying effect) as the globe warms. Projected expansions of drylands are not homogeneous over the globe. This figure depicts some of the changes in drylands from drier and/or to wetter types for the Far Future (2071–2100) relative to the Current situation (1981-2010). More than 80 % of the 27 ensemble models were in agreement on the major projected expansions of drylands shown. Although slightly different methodologies were used in Feng and Fu’s study, the results presented here are similar. In comparing the two maps, a strong pattern emerges showing how increased aridity is the predominant pattern. Major expansions of drylands are seen over regions of North America, the northern fringe of Africa, the Mediterranean, southern Africa, coastal regions of Australia, the Middle East and central Asia (e.g. Iraq, Iran, Afghanistan) and South America (especially eastern Brazil, southern Argentina and coastal Chile). Over the northern fringe of Africa (including Morocco, Algeria and Tunisia), the arid and hyper-arid climate is projected to expand into semi-arid ones, which is consistent with predictions of the vulnerability of semiarid drylands to climate change. In southern Africa, semi-arid regions may expand northward and eastward, while the arid climate is projected to strongly impact countries such as Namibia and Botswana. Drylands will become wetter in regions of tropical Africa, India and parts of north-western China, indicating a reduction in aridity. There appears to be a poleward shift of northern African drylands indicated by the retreat of drylands from hyperarid (arid) to arid (semi-arid) in the southern Sahara countries along with the expansions of hyper-arid and arid regions in the northern fringe of Africa. However, all of these regions involve relatively small land mass as compared to those that show increasing aridity.

Imminent future - 2011-2040
Projected future changes in the Aridity index (AI) for three 30-year periods (ImminentFuture, Near Future and Far Future) relative to Current (observed data, 1981-2010). Shown are results from scenario RCP 8.5 using 27 CMIP5 climate models.
Source: Reynolds, J. et al

Near future - 2041-2070
Projected future changes in the Aridity index (AI) for three 30-year periods (ImminentFuture, Near Future and Far Future) relative to Current (observed data, 1981-2010). Shown are results from scenario RCP 8.5 using 27 CMIP5 climate models.
Source: Reynolds, J. et al

Far future - 2071-2100
Projected future changes in the Aridity index (AI) for three 30-year periods (ImminentFuture, Near Future and Far Future) relative to Current (observed data, 1981-2010). Shown are results from scenario RCP 8.5 using 27 CMIP5 climate models.
Source: Reynolds, J. et al