Gross primary production (GPP) is a sink for carbon dioxide, and a key parameter scientists consider when predicting the effects of a changing climate. If a warmed climate is less favourable to plant growth, less carbon dioxide will be taken out of the atmosphere, and the climate will warm further. “Uncertainty in this feedback loop is a large part of the uncertainty in future climate projections,” said Ian Williams at Lawrence Berkeley National Laboratory (LBNL).

It is often assumed that a warmer climate would increase stress on GPP in regions that are already warm, and reduce stress on GPP in regions that are currently cool. That assumption comes from studies of plants when temperatures are higher or lower than seasonal averages.

But Williams and his colleagues at LBNL have uncovered a more complex outlook. They examined the monthly output of GPP, soil moisture, leaf area, precipitation and solar radiation given by historical and future climate projections in nine climate models. To simplify the results, the researchers ran the data through another model, which gave stress readings for water, sunlight and temperature. This latter model, they found, could define the optimal temperature for GPP as the temperature that minimized the combination of water and sunlight stresses.

Williams and colleagues learned that there is a temperature where soil moisture begins to become stressed, and that this optimal temperature increases under global warming because of a corresponding increase in rainfall. Temperature extremes are always relative, however, and since the extremes are predicted to become more frequent in a warmed climate, the researchers say, the absolute rise in optimal temperature will have little effect, and GPP may well become limited anyway.

“Stated another way, the sensitivity of GPP to heat extremes in a warmer climate is similar to the present-day sensitivity – but the net stress is [greater] in a warmer climate when accounting for the increase in frequency of heat extremes, and not just the impact per degree Celsius,” said Williams.

According to Williams, the study shows how important it is to account for extremes of temperature, together with light availability and soil moisture, when assessing the impacts of climate change. “These variables tend to vary together as a set and they all affect GPP as well as other ecosystem processes,” he added.

Williams reckons that Earth system models provide us with an understanding of the climate system that may not be immediately intuitive from present-day observations. “With this new understanding we now have a better idea of what to look for in observations and experiments, to test and improve the models,” he said.

The study is published in Environmental Research Letters (ERL).

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