November 18, 2016, by Tim Radford
Trees and other vegetation play a vital role in absorbing carbon dioxide emissions.
Image: marcovdz via Flickr
The greenhouse effect has prompted plants to consume more carbon dioxide from the atmosphere − but not enough to prevent global warming.
LONDON, 18 November, 2016 − The green economy has begun to respond to a greenhouse world, with new research suggesting that the vegetable kingdom has stepped up its appetite for carbon dioxide as emissions continue to grow.
The outcome is that although carbon dioxide proportions in the atmosphere have soared in less than two centuries, from 280 parts per million to 400 ppm now everywhere on the planet, the rate of increase has appeared to slow.
Unfortunately, the bad news from the researchers is that this increased appetite for carbon is nowhere near enough to halt human-induced climate change.
According to a study in Nature Communications, the rate at which CO2 increased in the atmosphere between 2002 and 2014 was contained at 1.9ppm per year. And researchers put this down to growing stimulus in the photosynthesis industry.
In what is sometimes called the fertilisation effect, trees, grasses, shrubs, reeds and even crops have responded to the extra CO2 – the raw material for all plant growth – by taking in more carbon, sparking even more growth and thus more carbon consumption.
The same research confirmed another factor at work. Plant respiration − the process by which plants take in oxygen and release CO2 − did not increase at the same rate as photosynthesis.
This is because respiration responds to temperature, which was affected by the much-contended rate of slowdown in global warming in the early years of this century.
Overall, during the latter half of the 20th century, the growth rate of atmospheric CO2 climbed steadily. In 1959, it was 0.75ppm per year, and by 2002 this had risen to 1.86ppm. But since 2002, the growth has remained flat.
“This study highlights just how important it is
to protect natural vegetation so it continues
to absorb part of our carbon emissions”
Human numbers and economic expansion have continued to swell, and with it the overall emissions of CO2 as a consequence of the combustion of fossil fuels. So some of the expected growth was being consumed naturally.
“We believed one of the planet’s main carbon sinks had unexpectedly strengthened − but the question was, which one?” says Trevor Keenan, a climate and ecosystem scientist at the Lawrence Berkeley National Laboratory in California.
The oceans had been identified as one possible agent, but not the dominant cause. That left variations in carbon uptake and plant respiration as the other possible answer.
So the researchers got to work with a set of computer-based models, and these suggested that the increase in CO2 levels had prompted terrestrial plants to double their carbon uptake from between one and two billion tonnes a year in the 1950s to between two and four billion tonnes a year now.
Humans now release between 9 and 10 billion tonnes of the greenhouse gas annually. So the plants have slowed the rate of increase by stepping up their own activity.
“Unfortunately, this increase is nowhere near enough to stop climate change,” Dr Keenan says.
“We’ve shown that the increase in terrestrial carbon uptake is happening, and with a plausible explanation why. But we don’t exactly know where the carbon sink is increasing most, how long this increase will last, and what it means for the future of the Earth’s climate.”
Corinne le Quéré, director of the Tyndall Centre for Climate Change Research at the University of East Anglia in the UK, adds: “This study highlights just how sensitive the natural environment is to a changing climate and how important it is to protect natural vegetation so that it continues to absorb part of our carbon emissions.
“Fundamentally, though, the carbon sinks help, but their help is not enough to stop the planet warming. Carbon emissions have to drop to almost zero to stop global warming.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.