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A eucalyptus forest on Kangaroo Island, Australia

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Rising carbon dioxide levels have been boosting plant growth, but this “fertilisation effect” has been declining faster than predicted by computer models, according to an analysis of satellite records. This means plants will soak up less CO2 than forecast and we will need to make bigger cuts in carbon dioxide emissions than we thought to limit global warming.

Living organisms are made of chains of carbon, and plants get this carbon from the CO2 in the air. When plants have enough water and other nutrients, such as nitrogen and phosphorus, the level of CO2 in the atmosphere can be the factor that limits their growth.

The rising levels of CO2 since the start of the industrial age have boosted plant growth and led to a global greening effect. This fertilisation effect is why the land and seas have continued to soak up half of all the CO2 we emit even though we are emitting more than ever.

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Studies involving raising CO2 levels at small test sites suggest that the fertilisation effect fades rapidly as other limits kick in. For instance, in eucalyptus forests in Australia low phosphorus levels limit the effect. The models that inform projections of future warming predict a slow decline in the fertilisation effect.

Now Yongguang Zhang at Nanjing University in China and his colleagues around the world have analysed three different satellite records of global plant growth. They conclude that the fertilisation effect has been declining faster than models predict since at least 1982.

“This study shows a declining fertilisation effect from multiple lines of evidence in the real world due to nutrient limitations,” says Ranga Myneni at Boston University.

This means larger reductions in emissions will be needed to meet climate targets, but the team hasn’t quantified this.

However, Richard Betts at the UK Met Office has looked at how uncertainties about carbon-cycle feedbacks, which include CO2 fertilisation, could affect future warming.

His team’s analysis shows that when these uncertainties are taken into account, the range of warming is considerably wider than commonly reported. “Emissions scenarios consistent with current worldwide policies could still give global warming well above 4°C by the end of this century,” says Betts.

Journal reference: Science, DOI: 10.1126/science.abb7772

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