Plants’ heat response means fiercer heatwaves

Asia faces more extreme heat by mid-century as some plant species react unexpectedly to rising average temperatures, new research shows.

Tomorrow’s heat waves could be even hotter than climate scientists have so far predicted. Maximum temperatures across the Asian continent from Europe to China could be 3°C to 5°C higher than previous estimates – because the forests and grasslands will respond in a different way.

Australian scientists report in the journal Scientific Reports that they looked at the forecasts made by the Intergovernmental Panel on Climate Change under the notorious “business-as-usual” scenario, in which the world’s nations go on burning ever more fossil fuels, to release ever more greenhouse gases.

The average global temperatures will rise steadily – but this rise will be accompanied by ever greater and more frequent extremes of heat.

But then Jatin Kala of Murdoch University in Perth, Western Australia, and colleagues factored in the responses of the plants to rising temperatures.

They looked at data from 314 species of plant from 54 research field sites. In particular, they investigated stomatas, tiny pores on the leaves through which plants absorb carbon dioxide and shed water to the atmosphere.

Response crucial

What matters is how vegetation responds to extremes of heat. Researchers have already established that plants respond, not always helpfully: extremes can alter the atmospheric chemistry unfavourably for plants, and certainly reduce crop yields

But other scientists have confirmed the so-called carbon dioxide fertilisation effect: as more carbon becomes available, plants use water more economically and so even though drylands may get drier the landscape can also get greener, and growth tends to begin ever earlier as winters get warmer, and spring arrives earlier.

Dr Kala and his fellow researchers used their field observation data to model the response of species, and types of plants, to higher temperatures, and to make some estimates of the balance of carbon taken up by the stomata, and the water released.

There has been an assumption that plants respond to temperature in roughly the same way. But there can be considerable variation.

The scientists found, overall, that the response of the plants became increasingly important: over Eurasia – they decided not to model the pattern in North America because cloudiness introduced extra uncertainties – needleleaf foreststundraand farmland would actually release lower levels of water into the atmosphere.

And since water in the atmosphere helps lower the daytime temperatures, this means that the temperatures would rise even higher than the models suggest.

“We often underestimate the role of vegetation in extreme temperature events as it has not been included in enough detail in climate models up to this point,” Dr Kala said.

“These more detailed results are confronting, but they help explain why many climate models have consistently underestimated the increase in intensity of heat waves and the rise in maximum temperatures when compared to observations.”

Unexpected results invite challenge. The test will be in the replication of the findings by other groups of scientists. And the Australian scientists intend to pursue the questions too. But they see their work as one of the rewards of the interdisciplinary approach: the marriage of ecology and climate simulation.

Surprise findings

Belinda Medlyn, a theoretical biologist at Western Sydney University, and a co-author, said: “Our study of stomata was originally intended just to learn more about how plants work. We were not really expecting to find these important implications for heat waves.”

The part played by the vegetation would not affect either the frequency or the duration of the heat waves. But the traffic between stomata and atmosphere could certainly affect their intensity.

The bottom line is, the scientists conclude, that there could be increases of 5°C by 2040 to 2059, and these increases would be “additive to those likely caused by increasing greenhouse gases over the same period.”

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