If a warming world becomes a drier one, how will the green things respond? Not well, according to a new prediction.

The air of planet Earth has been gradually drying this century. If this goes on, that could be bad news for humankind. In a warming world crop harvests will dwindle, even in well-watered farmlands, and trees could shrink in height.

The prospect of stunted forests and shortages of food in a world hit by global heating, climate change and rapid population growth is ominous. But if US and Canadian scientists are right, it may be a simple consequence of plant response to a rarely-discussed worldwide phenomenon known as vapour pressure deficit, which has been rising for the past 20 years as the world has warmed.

The argument isn’t a simple one. Higher global temperatures mean more evaporation. Higher atmospheric temperatures also mean that the capacity of the atmosphere to hold moisture also rises − the rule of thumb is 7% more vapour per degree Celsius rise. So a warmer world should be a wetter world.

But climate science also predicts that although those regions already rainy will get rainier, the drylands and arid zones will get even dryer as the thermometer soars.

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“As we race to increase production to feed a bigger population, this is a new hurdle. Atmospheric drying could limit yields, even in regions where irrigation or soil moisture is not limiting”

Now there is another factor in the calculations: vapour pressure deficit, or the overall drying of the atmosphere, and how plants react to the problem of dwindling atmospheric moisture.

New research in the journal Global Change Biology analyses 50 years of research and 112 plant species, and 59 physiological traits in those plants. The evidence suggests that atmospheric drying reduces plant yield, as the plants adjust to new conditions.

“When there is a high vapour pressure deficit, our atmosphere pulls water from other sources: animals, plants, etc. An increase in vapour pressure deficit places greater demand on the crop to use more water. In turn, this puts more pressure on farmers to ensure this demand for water is met − either via precipitation or irrigation − so that yields do not decrease,” said Walid Sadok, of the University of Minnesota.

“We believe a climate change-driven increase in atmospheric drying will reduce plant productivity and crop yields both in Minnesota and globally.”

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The paradox is that plants can adjust to a changing world but in this case by becoming more drought-resistant. Which, in the case of wheat, maize and even birch trees, means growing less.

Less productive plants

Findings such as this are tentative, and will in any case be tested by time. But they also illustrate just how much there is yet to learn about the consequence of climate change in a complex, responsive world.

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Other research teams have repeatedly observed that even in the drier regions, plants have so far responded to rising greenhouse gas emissions by an increase in global greenness. But there is nothing simple about the greenhouse effect. And there has been repeated evidence too that forest conservation and more tree plantations may not provide all the answers to the challenge of growth in an ever-warmer world.

The reasoning within the new study is that plant stomata, those tiny holes in foliage through which plants breathe and release water, adjust according to new conditions. The plants become more conservative. They grow shorter, smaller and more resistant to drought, even if there is no drought. And in parallel, they become less able to fix atmospheric carbon dioxide to provide new tissue. So, overall, plant productivity is reduced.

“As we race to increase production to feed a bigger population, this is a new hurdle that will need to be cleared,” said Dr Sadok. “Atmospheric drying could limit yields, even in regions where irrigation or soil moisture is not limiting, such as Minnesota.” − Climate News Network