A lack of water vapour in the atmosphere has caused a global decline in plant growth over the past two decades, resulting in a 59 per cent decline in vegetated areas worldwide.
Studying four global climate datasets, Wenping Yuan at Sun Yat-sen University in Zhuhai, China and his colleagues found that the decline is correlated with a vapour pressure deficit in the atmosphere, which has increased sharply over more than 53 percent of vegetated areas since the late 1990s.
Vapour pressure deficit (VPD) is the difference between the pressure that would be exerted by water vapour when the air is fully saturated and the pressure it actually exerts. When this deficit increases, the pores on the surface of leaves that facilitate gas exchange close up, resulting in lower photosynthesis rates.
The complex dynamics of climate change may be responsible, says Yuan. There has been a decrease in wind speeds over the oceans, which means water vapour doesn’t blow over land as readily, and can lead to this deficit over vegetated areas. The warming planet also plays a role. At a given temperature, the atmosphere can only hold a certain amount of water vapour. As temperatures on land increase, the upper limit on the amount of water vapour the atmosphere can hold increases, so the deficit gets larger, he says.
The team analysed satellite images and found a corresponding drop in global vegetation and leaf coverage, which had previously increased between 1982 and 1998. They also looked at the width of tree rings, which is commonly used as a measure of growth. After 1998, there was a decrease in average ring width at more than 100 of 171 sites around the world.
The team projects that VPD will continue to rise in the decades to come. “This atmospheric drought will last into the end of this century,” says Yuan.
Journal reference: Science Advances, DOI: 10.1126/sciadv.aav8754
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