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Even as climate change sets in, scientists estimate that many plants will require more water than is needed today. That would limit the availability of water for citizens in North America and Eurasia. A study led by scientists at Dartmouth predicts that the decades to come could see a waterless future, even though there have been predicted increases in rainfall at places like the United States and Europe. These are dense areas that are undergoing a lot of stresses, related to water resources.

The study, published in Nature Geoscience, contravenes expectations among scientists that there will be more water in the world, thanks to the plants. It was assumed that with more carbon dioxide in the atmosphere, plants will reduce their water consumption, which would leave more freshwater for the land. With greater carbon dioxide in the air, scientists presumed that plants can photosynthesize the CO2 concentrations even as they partly close the stomata or pores.

As the stomata shut off, they translate into less plant water loss to the atmosphere, which would mean more water to the land. The new study shows that plants leading to more water happens mainly in the tropics and the very high latitudes at places where freshwater is already available, so the competition for water is low. In the mid-latitudes, projected plant responses to climate change would make the land drier, in turn impacting the humans here.

"Approximately 60 per cent 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?"

With climate models, researchers study how freshwater availability gets impacted by projected changes in the manner in which rainfall gets distributed. The interaction of three main factors will have an effect on the impact of climate change on plants and will bring down the amount of freshwater that will be available. Firstly, due to higher levels of carbon dioxide in the air, plants would need much less water to photosynthesize, which would make the land flooded. Secondly, warmer seasons have long days and the planet is getting hotter due to climate change. So plants literally get more time to grow and drink in water which automatically makes the land drier.

Finally, denser carbon dioxide concentrations would lead to more photosynthesis by a larger number of plants. In some areas, due to growing seasons, further photosynthesis would outpace the closing stomata. Ironically, with more greenery, the land would get drier. In the mid-latitude areas, therefore, there will be significantly less water in the land, although rainfall would increase and vegetation would enhance water usage.

It is vital, therefore, to enhance our understanding of how climate models represent ecosystems as well as their reactions to climate change. The paradox of a wetter earth with less water for major regions is one of the ironical consequences of climate change.