Thirsty future ahead as climate change explodes plant growth

By the end of the century plants could consume substantially more water, leaving less for people across North America, Europe, and Central Asia—even if it rains and snows more, a new study reports today in the journal Nature Geoscience.

Plants are the primary regulators of the water cycle, responsible for 60 percent of the flow of water from the land to the atmosphere. Research now shows how climate change is altering this vital cycle in several different ways.

“Plants are like the atmosphere’s straw, dominating how water flows from the land to the atmosphere,” says climate geographer Justin Mankin of Dartmouth College and lead author of the study.

Without massive reductions in carbon dioxide emissions in the coming decades, the global average temperature will rise between 4 and 6 degrees Celsius, with a near doubling of atmospheric CO₂ by century’s end. Those hotter, CO₂-rich future conditions are akin to turning up the heat and pumping CO₂ into a greenhouse. The likely result, assuming no other limiting factors such as lack of nutrients, is an explosion of plant life. But that will leave considerably less water for people to use, said Mankin in an interview. (Read about an effort to protect the world's water.)

Climate change affects the growth of plants in three ways. First, as CO₂ levels increase, plants need less water to do photosynthesis. This well-documented effect was long thought to mean that there would be more fresh water available in soils and streams. But a second effect counters that: A warming world means longer and warmer growing seasons, which gives plants more time to grow and consume water, drying the land.

Researchers have now shown a third effect: As CO₂ levels rise, it amps up photosynthesis. Plants in this hotter, CO₂-rich environment grow bigger, with more leaves. That means when it rains there will be far more wet leaves creating more surface area for more evaporation to occur. Computer modelling shows that such enhanced leaf evaporation has a large effect on runoff and soil moisture, says Mankin.

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, and more that accurately replicate past conditions. Future climate variables such as surface air temperature and CO₂ levels were added to find out how they would affect the global water cycle.

While plants everywhere will consume more water in a hotter, CO₂-rich world, northern and tropical regions are projected to have enough precipitation to offset the additional plant growth, Mankin says.

The study’s take-home message: The combined effects of increased CO₂ and warmer temperatures will increase water consumption by vegetation. That will lead to water declines in rivers and streams in the mid-latitudes, including North America, Europe, and Central Asia.

Bad news on water

It’s long been debated whether the effects of high CO₂ levels on plants means more water availability on the land, says Peter Gleick, a world-renowned water expert and former president of the Pacific Institute, which works on global water issues.

“By more accurately modelling growth of biomass overall, including leaf canopy,” the study reaches “a robust, opposite, and ‘bad news’ conclusion: rising levels of CO₂ and the related climate changes will worsen, not improve, water availability,” says Gleick, who was not involved with the research.

This result is “almost certainly bad news for the western U.S.,” he says.

Previous climate research has found an 80 percent likelihood of a 35-year or longer "megadrought" striking the Southwest and central Great Plains by 2100 with business-as-usual CO₂ emissions. Moderate reductions in emissions will only reduce this risk to 60 percent. And this megadrought model does not include the new findings about how changes in vegetation could worsen conditions, says Gleick.

The atmosphere is already more CO₂ rich and the climate is warmer. There is evidence from satellites showing significant increases in vegetation in the past 40 years, says Mankin. While growing seasons are also getting longer, it is difficult to say this recent greening of the Earth is entirely due to climate change because there have been so many human alterations to the landscape over the last 100 years, he says.

CO₂ levels rising and rising

For at least 800,000 years the concentration of atmospheric CO₂ levels ranged between 180 and 290 parts per million (ppm). In the last 10,000 years they stayed around 280 ppm until the Industrial Revolution sparked widespread use of coal.

Today’s measurements show CO₂ levels were 412 ppm as of September this year, 47 percent higher than pre-industrial levels. The last time CO₂ levels were above 400 ppm was 16 to 25 million years ago, when the planet and its climate were very different.

CO₂ levels are increasing at a rate of 2 ppm per year. With continued use of coal, gas, and oil that could double to 560 ppm by 2100. Under those conditions the modelling shows that droughts happening much faster, lasting longer, and becoming more severe across the mid-latitudes—even when there is normal rainfall, Mankin says.

Water scarcity is already a major issue, with four billion people suffering from severe water scarcity at least one month a year, according to a 2016 study. Any future reduction in water supply is very bad news indeed. That’s true even in a wealthy country like the U.S., he says, where there are already people from Detroit to the Southwest suffering from water stress.