Plants will demand more water
in the future making less water available for people
November 4, 2019
Dartmouth College
With climate change, plants of
the future will consume more water than in the present day, leading to less
water available for people living in North America and Eurasia, according to a
new study. The research suggests a drier future despite anticipated precipitation
increases for places like the United States and Europe, populous regions
already facing water stresses.
With climate change, plants of
the future will consume more water than in the present day, leading to less
water available for people living in North America and Eurasia, according to a
Dartmouth-led study in Nature Geoscience. The research suggests a drier
future despite anticipated precipitation increases for places like the United
States and Europe, populous regions already facing water stresses.
The study challenges an
expectation in climate science that plants will make the world wetter in the
future. Scientists have long thought that as carbon dioxide concentrations
increase in the atmosphere, plants will reduce their water consumption, leaving
more freshwater available in our soils and streams. This is because as more
carbon dioxide accumulates in our atmosphere plants can photosynthesize the
same amount while partly closing the pores (stomata) on their leaves. Closed
stomata means less plant water loss to the atmosphere, increasing water in the land.
The new findings reveal that this story of plants making the land wetter is
limited to the tropics and the extremely high latitudes, where freshwater
availability is already high and competing demands on it are low. For much of
the mid-latitudes, the study finds, projected plant responses to climate change
will not make the land wetter but drier, which has massive implications for
millions of people.
"Approximately 60 percent
of the global water flux from the land to the atmosphere goes through plants, called
transpiration. Plants are like the atmosphere's straw, dominating how water
flows from the land to the atmosphere. So vegetation is a massive determinant
of what water is left on land for people," explained lead author Justin S.
Mankin, an assistant professor of geography at Dartmouth and adjunct research
scientist at Lamont-Doherty Earth Observatory at Columbia University. "The
question we're asking here is, how do the combined effects of carbon dioxide
and warming change the size of that straw?"
Using climate models, the
study examines how freshwater availability may be affected by projected changes
in the way precipitation is divided among plants, rivers and soils. For the
study, the research team used a novel accounting of this precipitation partitioning,
developed earlier by Mankin and colleagues to calculate the future runoff loss
to future vegetation in a warmer, carbon dioxide-enriched climate.
The new study's findings
revealed how the interaction of three key effects of climate change's impacts
on plants will reduce regional freshwater availability. First, as carbon
dioxide increases in the atmosphere, plants require less water to
photosynthesize, wetting the land. Yet, second, as the planet warms, growing
seasons become longer and warmer: plants have more time to grow and consume
water, drying the land. Finally, as carbon dioxide concentrations increase,
plants are likely to grow more, as photosynthesis becomes amplified. For some
regions, these latter two impacts, extended growing seasons and amplified
photosynthesis, will outpace the closing stomata, meaning more vegetation will
consume more water for a longer amount of time, drying the land. 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 also underscores the importance of improving
how climate models represent ecosystems and their response to climate change.
The world relies on freshwater
for human consumption, agriculture, hydropower, and industry. Yet, for many
places, there's a fundamental disconnect between when precipitation falls and
when people use this water, as is the case with California, which gets more
than half of its precipitation in the winter, but peak demands are in the
summer. "Throughout the world, we engineer solutions to move water from
point A to point B to overcome this spatiotemporal disconnect between water
supply and its demand. Allocating water is politically contentious,
capital-intensive and requires really long-term planning, all of which affects
some of the most vulnerable populations. Our research shows that we can't
expect plants to be a universal panacea for future water availability. 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," added Mankin.
Researchers from
Lamont-Doherty Earth Observatory of Columbia University, Richard Seager, Jason
E. Smerdon, Benjamin I. Cook, who is also affiliated with NASA Goddard
Institute for Space Studies, and A. Park Williams, contributed to this study.
Story Source:
Materials provided by Dartmouth College. Note:
Content may be edited for style and length.
Journal Reference:
Justin S. Mankin, Richard
Seager, Jason E. Smerdon, Benjamin I. Cook & A. Park Williams. Mid-latitude
freshwater availability reduced by projected vegetation responses to climate
change. Nature Geoscience, 2019 DOI: 10.1038/s41561-019-0480-x
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