Science & Environment

Rise in atmospheric CO2 slowed by green vegetation

vegetation Image copyright SPL
Image caption The global pause in temperatures rises since 1998 has caused plants to release less CO2 into the atmosphere

The growth in the amount of CO2 in the Earth's atmosphere has been slowed by the increased ability of plants to soak up the gas.

A new study says that green vegetation has helped offset a large fraction of human related carbon emissions between 2002 and 2014.

Plants and trees have become more absorbent say the authors, because of so much extra CO2 in the atmosphere.

The slowdown, though, can't keep pace with the overall scale of emissions.

Over the past 50 years, the amount of CO2 absorbed by the Earth's oceans, plants and vegetation has doubled and these carbon sinks now account for about 45% of the gas emitted each year because of human activities.

Researchers now report that since the start of the 21st century there has been a significant change in the amount of carbon dioxide taken up by the plants and trees. The new analysis suggests that between 2002 and 2014 the amount of human caused CO2 remaining in the atmosphere declined by around 20%.

Reports earlier this year indicated that there has been an increase in the number of trees and plants growing on the Earth, the so-called greening of the planet. But the authors of this new study believe that this isn't the main cause of the slowdown in the rise of CO2.

Image copyright BERKELEY LAB
Image caption The black line is the observed growth rate and the beige line is the modelled rate. The blue line indicates no increasing trend between 2002 and 2014.

"There have been reports of the greening of the land surface but what we found was that was of secondary importance to the direct effect of CO2 fertilisation on the plants that are already there," lead author Dr Trevor Keenan told BBC News.

"We have a huge amount of vegetation on the Earth and that was being fertilised by CO2 and taking in more CO2 as a result."

Another important element in the story is the impact of a hiatus in global temperature increases on the behaviour of plants. Between 1998 and 2012 temperatures went up by less than in previous decades. This has impacted the respiration of vegetation.

"The soils and ecosystem are respiring so as temperatures increase they respire more, releasing more CO2 into the atmosphere," said Dr Keenan.

"In the past decade or so there hasn't been much of an increase in global temperatures, so that meant there wasn't much of an increase in respiration and carbon release so that was fundamentally different in the past decade or so compared to previous periods."

One consequence of a warming world that has been expected to increase was the number of droughts around the world. However, this new study suggests that, on a global scale, there has been little or no change in the prevalence of drought over recent decades.

Overall though the slowdown caused by vegetation hasn't stemmed the total rise of carbon which has now passed the symbolically important level of 400 parts per million (ppm) in the atmosphere.

Image copyright SPL
Image caption Green vegetation has limited the growth of CO2 in the atmosphere over the last decade

"This study highlights just how sensitive the natural environment is to a changing climate and how important it is to protect natural vegetation so it continues to absorb part of our carbon emissions," said Prof Corinne Le Quéré, director of the Tyndall Centre at the University of East Anglia, who wasn't involved in the study.

"Fundamentally, though, the carbon sinks help but their help is not enough to stop the planet getting warmer - far from that - carbon emissions have to drop to almost zero to stop global warming."

One of the big lessons from the new report is that land carbon sinks are not set in stone and do have the potential to change over time. If they could be managed properly, it might help some countries to cut their emissions and limit climate change.

The authors of the study say that the pause in the growth of atmospheric carbon will almost certainly be a temporary phenomenon. As temperatures rise, these green sinks could in fact become sources of CO2.

"Now we are seeing plants slow down the rate of climate change," said Dr Keenan.

"But if we are not careful and we don't do anything about climate change all that CO2 could be put back in the atmosphere later and that would really accelerate the rate of warming.

"It may be hitting the brakes right now but it can really punch the accelerator later."

The study has been published in the journal Nature Communications.

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