Global Urbanisation and Aridity

How will increasing aridity impact the Big Cities of the world?

There are 1 692 cities in the world where population exceeds 300 000 people, referred to here as “Big Cities”. On the maps above and opposite, these Big Cities are shown in relation to global maps that illustrate projected changes in aridity. Changes in aridity is quantified in terms of percentage change in the Aridity Index (AI) (see page 78 on Aridity projections). The histograms show a summary of the distribution of the two types of cities (non-dryland and dryland) for each climate projection. They also include changes in the number of cities that experience increased aridity (lower AI) or increased wetness (higher AI) based on two projected climate scenarios: RCP 4.5 and RCP 8.5. Under the current climate, 35 % of these Big Cities (583) are located in drylands, and 65 % (1 109) in non-dryland (or humid) regions. Since evidence clearly indicates that global aridity will change in the future, will these percentages change or remain the same? Will the climate zone in which each city is presently located become drier or wetter in the future? Analysis shows that in the RCP 8.5 scenario, 71 %, 84 % and 86 % of the 1 109 non-dryland cities become drier in the Imminent Future, Near Future and Far Future scenarios, respectively (as compared to 29 %, 16 % and 14 % of them becoming wetter). This drying trend is occurring where 75 % of the non-dryland city populations presently reside (based on 2015 estimates). However, the trends are similar for both RCP 4.5 and RCP 8.5 scenarios (see page 78 on Aridity projections for definition of RCP). For the 583 dryland cities, the trend is roughly 50:50 (see above graph) – that is, the climate in about half of the cities becomes wetter while about half become drier. Specifically, the climate in 43 %, 51 % and 54 % of the dryland cities will become drier in the Imminent Future, Near Future and Far Future RCP 8.5 scenarios, respectively (as compared to 57 %, 49 % and 46 % becoming wetter). This drying trend is occurring where 50 % of the dryland city populations presently reside (based on 2015 estimates).
Potential consequences
The IPCC concluded with a very high level of confidence that urban areas are extremely vulnerable to climate change. There are numerous issues related to increasing aridity that pose major concerns for urban populations, the urban infrastructure and its associated economies and ecosystems. These include increased heat stress, severe storms, extreme precipitation (and related flooding and landslides), increased air pollution as temperatures rise, drought and water scarcity. Of course, risks are always magnified for people who lack essential infrastructure and services or who reside in exposed areas. One major concern is the heat island effect. As the climate warms, extreme heat events will increase, which will lead to increased heat stress, especially in urban areas. Heat stress is a leading cause of weatherrelated human mortality in many countries. It is well known that human beings have a definitive upper limit to cope with heat and humidity stress and that extended exposure to combinations of high temperatures and humidity above this threshold will lead to hyperthermia and death. It is estimated that this physiological threshold will become important in the distant future as the global mean temperature rises. Pal and Eltahir conducted a case study on this physiological limit with regional climate models under future scenarios of increasing aridity in the Arabian Peninsula. They explored how a combined index of temperature and humidity (which represents the physiological threshold of humans) would increase if carbon emissions continue on current trends and the world warms by 4 ℃ this century. Their study suggests that, if current emissions continue unabated, extreme heat waves could potentially become commonplace after 2070 and the hottest days of today would become a near-daily occurrence in cities such as Abu Dhabi, Dubai and Doha. Although the Arabian Peninsula is a specific regional case study, it suggests that increasing hotspots do not bode well for human habitability of cities in the future unless significant mitigation occurs. The IPCC argues that urban adaptation is critical in order to address the many challenges faced by an urbanising planet. Some examples of adaptation mechanisms include reducing energy and water consumption in urban areas via greening cities and water recycling. Importantly, cities must develop a resilient infrastructure in order to reduce their vulnerability to the many risks associated with climate change.

Summary of trends in the number of Big Cities that will be impacted by changing global aridity. It is evident that cities will become drier underfuture climates and this trend is most pronounced in the Far Future than in Imminent Future and for non-dryland cities. RCP 8.5 results only.
Source: Reynolds, J. et al.

Imminent Future 2011-2040

Near Future 2041-2070

Far Future 2071-2100

Source: : Reynolds, J. et al; source Big Cities: World Urbanization Prospects: The 2014 Revision, Highlights (ST/ESA/SER.A/352). Department of Economic and Social Affairs, Population Division. © 2014 United Nations. Reused with the permission of the United Nations..