Rising CO2 levels and a warmer earth means plants will grow bigger and have longer to suck the land dry. That’s bad news for human water supplies.

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.

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Source: nationalgeographic.com