Warmer globe won’t cut plants’ carbon emissions

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Rising temperatures are likely to mean plants emit more carbon dioxide, according to new research.

All plants release CO2 into the atmosphere when they respire; the process emits an estimated 60 gigatonnes each year. Until now, climate models have assumed that higher temperatures will dramatically increase these plant emissions.

But researchers suspected that the true picture could be more complex, because plants would adapt to their new environment by slowing the growth in the rate of respiration. That suspicion turns out not to be true.

Scientists grew 19 species of plant at a range of temperatures, monitoring respiration rates. They calculated how temperature related to two common measures of leaf performance: leaf mass per unit of leaf area, and leaf nitrogen content.

They then used the results to predict the effects of higher temperatures on the respiration of whole ecosystems. They found that the ability of plants on land to absorb and sequester carbon is indeed likely to weaken as temperatures rise.

This is the first time plant respiration’s response to temperature, or ‘acclimation’, has been measured to produce an accurate cross-species picture that can be used in wider climate models.

The default setting of the climate model used in the research, produced by the Hadley Centre for Climate Prediction and Research, does not include acclimation. It assumes that as the climate gets hotter, plants’ CO2 emissions grow exponentially.

‘A commonly-held expectation is that including acclimation in models should slow the acceleration of plant respiration as temperatures rise’ explains Dr Rosie Fisher, a climate modeller at the University of Sheffield. ‘Actually, we found that it had very little effect on the predictions.’

The research, published in Global Change Biology, suggests that in some areas, such as tropical rainforests, carbon emissions may fall as temperatures rise. But worldwide the effect will be negligible. This is partly because forests in cold climate - 40% of the world’s woodland - would become less effective as stores of carbon.

No softening of climate change blow

‘These findings strengthen the case of those warning about the dangers posed by climate change,’ says Dr Jon Pitchford, a biological mathematician at the University of York and another of the paper’s authors. ‘They challenge the idea that the planet’s plant life will adjust to increased temperatures in a uniform fashion that will, in turn, help ameliorate global warming’, he adds.

As well as respiring, plants take in carbon dioxide and emit oxygen when they photosynthesise. ‘The caveat of this experiment is that we only looked at one side of the plant carbon economics equation, respiration,’ Fisher says. ‘There is no clear agreement on how photosynthesis acclimates to temperature; this is a more complicated question and more experimental research is needed.’

She adds that colleagues from Leeds University are currently working on this subject, and that in due course researchers should be able to incorporate acclimation of photosynthesis into climate models as well.

Another area that needs to more investigation is what the effect will be of a reduction in size of the Amazon rainforest. Most climate models predict a decreased area of forest as a result of changing climate. This is also the likely result of unmananged development and increasing demand for timber, cash crops and pasture land.

This dramatic response masks the more subtle effects of plant acclimation in the model. ‘It doesn’t matter how the Amazon rainforest responds to rising temperatures if we don’t have an Amazon rainforest any more,’ Fisher points out.

The research was a collaboration between scientists at the Australian National University in Canberra, Umea University in Sweden, and the UK Universities of Hull, Sheffield and York.