According to a report in Nature Geosciences led by Dartmouth. Despite expected increases in rainfall for areas like the United States and Europe, populated regions still facing water stresses the research suggests a drier future.
The research addresses a climate science expectation that plants in the future will make the world weatherier.

Scientists have long assumed that plants would reduce their water consumption as carbon dioxide concentrations rise in the environment, leaving more fresh water in our soils and streams.

This is because in our environment plants, as more carbon dioxide accumulates, they will photosynthesize the same amount while partially covering the pores (stomata) on their leaves. Closed stomata means less atmospheric plant water loss, increasing the amount of water in the soil.

The new findings show that this tale of plants making the land climate is limited to tropics and extremely high latitudes, where the availability of freshwater is already high and there are low competitive demands.
The study finds that predicted plant responses to climate change will not make the land weatherier but drier, which has massive implications for millions of people, for many of the mid-latitudes.

“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, assistant geography professor at Dartmouth and research assistant scientist at Columbia University’s Lamont-Doherty Earth Observatory. “The question we ask here is, how does the combined effects of carbon dioxide and warming change the straw’s size?”

The research explores how the supply of freshwater can be influenced by expected changes in the way precipitation is distributed among plants, rivers and soils using climate models.

The research team used a novel accounting of this precipitation partitioning for the analysis, established earlier by Mankin and colleagues to quantify the potential loss of runoff in a warmer climate enriched with carbon dioxide to future vegetation.

The outcomes of the new study showed that regional freshwater availability will be decreased by the interaction of three main effects of climate change impacts on plants. First as carbon dioxide in the atmosphere rises, plants need less water to photosynthesize, wetting the land.

Second, however, as the planet warms, growing seasons are getting longer and warmer: plants have more time to grow and consume water, drying up the land. Eventually, as the amount of carbon dioxide rises, plants are likely to grow more as photosynthesis becomes amplified.

For certain regions these two latter two impacts prolonged growing seasons and intensified photosynthesis will outstrip the closing stomata, indicating that more vegetation will consume more water for a longer period of time, drying the land.

As a result, plants can leave less water in soils and streams for many of the mid-latitudes, even if there is additional rainfall and vegetation with its use of water is more efficient. The outcome also highlights the importance of enhancing how ecosystems are represented by climate models and their response to climate change.

For human consumption, agriculture, hydropower, and industry the world relies on freshwater. Nonetheless, there is a fundamental disconnect for many areas between when precipitation falls and when people use this water, as is the case for California, which receives more than half of its rainfall in the winter, but there are high demands in the summer.

“We are engineering solutions worldwide to move water from point A to point B to solve this spatio-temporal gap between water supply and water demand. Water distribution is politically contentious, capital-intensive, and requires real long-term planning, impacting some of the most vulnerable populations. Our research shows that for future water availability, we cannot expect plants to be a universal panacea. So being able to clearly assess where and why we should anticipate future changes in water availability is crucial to making sure we can be prepared, “Mankin added.s



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