A long-held scientific assumption has just been turned on its head.

For many years, scientists have thought that as atmospheric carbon dioxide concentrations increase as a result of climate change, plants will reduce their water consumption and leave more fresh water available in soils and streams.

However, new research from Dartmouth indicates that instead of reducing the amount of water consumed, in most parts of the earth climate change will actually cause plants to consume more water, leaving people and plants competing for an increasingly small amount of fresh water.

While it is true that climate change will make the land wetter in the tropics and in extremely high latitudes, where fresh water is already highly available and competing demands on it low, for much of the mid-latitudes, projected plant responses to climate change will actually make the land drier, which has massive implications for the billions of people who live in the earth’s more temperate zones.

The Dartmouth study’s findings revealed how the interaction of three key effects of climate change’s impacts on plants will decrease freshwater availability in many regions. It goes something like this:

  • As carbon dioxide increases in the atmosphere, plants will require less water to photosynthesize, making the land wetter.
  • But as the planet warms, growing seasons get longer and warmer; therefore, plants have more time to grow and consume water, drying the land.
  • As carbon dioxide concentrations increase, plants are likely to grow more; therefore, photosynthesis will be amplified.

For some regions, the extended growing seasons and amplified photosynthesis will outpace the biological effects on plants that would cause increasing wetness. More vegetation will consume more water for a longer amount of time, making the land drier. 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 gets more efficient in its water usage.

Many places in the world have a fundamental disconnect between where precipitation falls and where it is used. California, for example, gets more than half its precipitation in the winter, but peak demand for fresh water happens in the summer. Allocating water is a potentially contentious issue—in fact, some scientists and futurists have expressed concern that water resources could in the future be a cause for wars between nations—it’s important to plan for decreased freshwater availability in the long term for political stability. This will require a lot of money, but if it’s not done, the people most affected will be the world’s most vulnerable populations.

“Our research shows that we can’t expect plants to be a universal panacea for future water availability,” said study lead author Justin S. Mankin of Dartmouth. “So, being able to assess clearly where and why we should anticipate water availability changes to occur in the future is crucial to ensuring that we can be prepared.”

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