“Aridity” is the ratio of atmospheric water supply — known in lay terms as precipitation — to the demands of “evapotranspiration” — or transpiration and evaporation. As anthropogenic climate change continues to impact the environment, the land will become much drier. In fact, if we don’t step up and accept aggressive global average temperature increase limits, large regions of the planet will face substantial aridification and develop more deserts.
A variety of methods can be used to estimate evaporation from land surfaces and the resulting moisture balance. Applying 27 global climate projection models under different warming scenarios, a team of researchers from the University of East Anglia (UEA) in the UK and Southern University of Science and Technology (SUSTech) in China had the goal of predicting land drying patterns. The team looked to areas of the world where aridity might change when compared to the year-to-year variations they currently experience.
More than a quarter of Earth’s land surface will become “significantly” drier even if humanity manages only to limit global warming to two degrees Celsius, which is the basic goal within the Paris Agreement. The Paris Agreement is a bridge between today’s policies and climate neutrality before the end of the century.
Governments who signed it agreed to:
- A long-term goal of keeping the increase in global average temperature to well below 2°C above pre-industrial levels
- Aim to limit the increase to 1.5°C, since this would significantly reduce risks and the impacts of climate change
- Acknowledge the need for global emissions to peak as soon as possible, recognizing that this will take longer for developing countries
- Undertake rapid reductions thereafter in accordance with the best available science
(Note: the US withdrew from the Agreement under the Trump Administration/#fakepresident.)
But the UEA and SUSTech aridity researchers have concluded that, if we contain average warming to 1.5 C (2.7 degrees Fahrenheit) pre-industrial, a much smaller fraction of the planet’s surface — one-12th, or half the area of the 2 degree threshold — would experience increased aridity. That 1.5 C goal is the more ambitious benchmark within the Paris Agreement. Early action for accomplishing the 1.5 degree Celsius temperature goal has the potential to reduce the likelihood that large regions will face substantial aridification and related impacts.
The Data Behind Aridity & Global Warming
Over the latter half of the twentieth century, temperature-driven increases in summer evapotranspiration appear to have been partially responsible for net declines in summer water availability in arctic and boreal areas. These observations have led to hypotheses that continued increases in average temperatures may cause future evapotranspiration rates to exceed predicted increases in precipitation, thereby exerting increased drying across the landscape.
As a result, projected changes in the 21st century climate are likely to impact water resources substantially. Increases in temperature and changes in precipitation can have profound
effects on regional hydrology, including shrinking wetlands, ice recession, permafrost thaw, and an increase in fire frequency and intensity across the landscape as a result of more droughts and thunderstorms.
Continuation of these trends points to further changes in the hydrologic cycle, with significant implications for the people and ecosystems.
During the recent aridification data study, emergence of significant planet surface drying occurs before global mean warming reaches 1.5 °C and 2 °C above the pre-industrial level. “Our research predicts that aridification would emerge over about 20-30 per cent of the world’s land surface by the time the global mean temperature change reaches 2 degrees Celsius,” said Manoj Joshi from UEA. “But two thirds of the affected regions could avoid significant aridification if warming is limited to 1.5 degrees Celsius.”
The Link between Aridity and Natural Disasters
The planet has already warmed 1 degree since pre-industrial times, and most climate scientists agree that temperature change is significantly responsible for increasing drought or desertification from Brazil to the Mediterranean and Australia. The dryness of the Earth’s surface is a major factor in determining the incidence of natural disasters. Increases in aridity, or aridification, in a region tend to lead to the degradation of land and, eventually, desertification.
“Aridification is a serious threat because it can critically impact areas such as agriculture, water quality, and biodiversity. It can also lead to more droughts and wildfires — similar to those seen raging across California,” said Chang-Eui Park from SusTech. Drought severity has been increasing across the Mediterranean, southern Africa, and the eastern coast of Australia over the course of the 20th century, while semi-arid areas of Mexico, Brazil, southern Africa, and Australia have encountered desertification for some time as the world has warmed.
Potential evapotranspiration (PET) is the amount of water that would be evaporated and transpired if there were sufficient water available. This demand incorporates the energy available for evaporation and the ability of the lower atmosphere to transport evaporated moisture away from the land surface. As the climate continues to warm and the growing season gets longer, scientists expect PET and precipitation will both increase. If the increase in water lost from the landscape through PET is not offset by an equivalent increase in incoming precipitation, many of the world’s areas may experience more severe water-deficits during the growing season.
“Another way of thinking of the emergence of aridification is a shift to continuous moderate drought conditions, on top of which future year-to-year variability can cause more severe drought,” said Park. “For instance, in such a scenario 15 per cent of semi-arid regions would actually experience conditions similar to ‘arid’ climates today.”
According to the researchers’ calculations, if planetary warming reaches 2 degrees Celsius — which would happen sometime between 2052 and 2070 — then 24 to 32% of the earth’s surface would become drier. “The world has already warmed by 1 degree Celsius. But by reducing greenhouse gas emissions into the atmosphere in order to keep global warming under 1.5 degrees Celsius or 2 degrees Celsius could reduce the likelihood of significant aridification emerging in many parts of the world,” Joshi said.
It is likely that as the climate warms and growing season moisture availability becomes more limited, this dynamic will contribute to the already complex interplay between climate and vegetation. Warmer temperatures will tend to drive a shift from tundra to shrub, from shrub to forest, and from coniferous to deciduous forest, although moisture limitations may alter such changes.
“The areas of the world which would most benefit from keeping warming below 1.5 degree Celsius are parts of South East Asia, Southern Europe, Southern Africa, Central America, and Southern Australia — where more than 20% of the world’s population live today,” said Tim Osborn from UEA.
The aridity research findings were published in January, 2018 in the journal Nature Climate Change.
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