Researchers have now shown a third effect: As CO 2 levels rise, it amps up photosynthesis. Plants in this hotter, CO 2 -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 CO 2 levels were added to find out how they would affect the global water cycle.

While plants everywhere will consume more water in a hotter, CO 2 -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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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.

CO 2 levels rising and rising

For at least 800,000 years the concentration of atmospheric CO 2 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 CO 2 levels were 412 ppm as of September this year, 47 percent higher than pre-industrial levels. The last time CO 2 levels were above 400 ppm was 16 to 25 million years ago, when the planet and its climate were very different.

CO 2 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.