By the end of the century, climate change could drive plants in North America, Europe, South & Central America to consume more water, leaving less for people, according to a new study published in Nature Geoscience.
The world heavily relies upon freshwater for agriculture, energy, human consumption, and industrial use. Water is regulated by the water cycle (transfer of water from source to sink) among different Earth systems/spheres. Vegetation plays a vital role in the cycling of water by the use of transpiration, a phenomenon that helps plants in releasing water to the atmosphere through their leaves. Environmental variables including climate change play a vital role in determining plant growth, function and geographic extent.
Plants and water dependency
Scientists have long hypothesized that a rise in atmospheric carbon dioxide (CO2) concentration will cause plants to decrease the consumption of water, leaving more available freshwater in soils and surface water bodies. This is because more CO2 accumulates in the air and plants can partially close their leave pores (stomata) while performing the same amount of photosynthesis. As plants release water from their stomata (transpiration), this would reduce water loss to the atmosphere, while keeping more water on land (soil and water bodies).
While most scientists in the world conclude that vegetation like plants will contribute towards the betterment of our world, the results of the present study do not come to the same conclusion. In the present study, Justin S. Mankin and his team deduce a drier future in spite of precipitation increase in populous parts of Europe and the USA that are currently water-stressed areas.
Previously supposed hypothesis is limited to regions with extremely high latitudes and the tropics, where freshwater availability is high and competing water demands are low. For mid-latitude regions, plants will leave less water in streams and soils leading to drying of land surface, regardless of any additional rainfall.
As a result, for much of the mid-latitudes, plants will leave less water in soils and streams, even if there is additional rainfall, and vegetation is more efficient with its water usage. The result underscores the importance of improving how climate models represent specific ecosystems and their response to climate change, said Mankin.
Plants derive approximately 60% of the water flux from land to atmosphere, therefore “Plants are like the atmosphere’s straw,” said lead author Professor Justin Mankin.
Mankin and his team used climate models to examine freshwater availability and its future projections among plants, soils, and streams to determine future effects of a warmer, CO2 enriched climate. 16 different climate models with historical data were employed by Mankin’s team that included several variables, including; soil evaporation, soil moisture, leaf evaporation, precipitation, leaf area index.
The study shows how the interaction of 3 major impacts of climate change on plants will decrease freshwater availability on a regional scale. Firstly, Increase in atmospheric CO2 enables plants to use less water for photosynthesis resulting in increased moisture on land. Secondly, as the planet warms, growing seasons will become longer and warmer, intensifying photosynthesis, giving plants more time to grow, consume water and dry out the land. Finally, an increase in atmospheric CO2 will most likely cause plants to grow more because photosynthesis will become more efficient.
For some regions, the latter two impacts i.e. extended growing seasons and increase in photosynthesis activity – will outpace the closing of stomata. This means that more vegetation will consume more water for a longer amount of time, with the net result of drier land.
Mankin’s team used 16 different climate models with historical data for a number of variables including precipitation, leaf evaporation, soil evaporation, leaf area index, soil moisture, etc that accurately replicate past conditions. Future climate variables such as surface air temperature and CO2 levels were also included to determine how these climate variables will affect the global water cycle.
Credit: Justin Mankin
Projected future changes in water availability.
“In a hotter, CO2 rich environment, plants across the globe are expected to use more water, the northern and tropical regions are projected to have enough rainfall to offset the additional plant growth,” Mankin says.
As Climate Warms, Plants May Demand More Water, Cutting Supplies for People.
Dartmouth College (press release) | November 4, 2019 | Columbia University, USA.