The rate of growth in atmospheric carbon dioxide has slowed, despite an increase in CO2 emissions from human activity, due to an increased uptake of the greenhouse gas by the planet's plants, a new study has suggested.

Key points: Increasing CO2 has enhanced photosynthesis so plants absorb more of the greenhouse gas

Increasing CO2 has enhanced photosynthesis so plants absorb more of the greenhouse gas At same time, slowdown of global temperatures reduced amount of CO2 plants breathed out

At same time, slowdown of global temperatures reduced amount of CO2 plants breathed out These two factors mean more carbon was taken up by plants than released

Changes in the rates of photosynthesis and respiration in the Earth's ecosystems have created a larger-than-expected terrestrial carbon sink, an international team of scientists has reported today in Nature Communications.

Co-author and CSIRO research scientist Dr Pep Canadell said increases in atmospheric carbon dioxide levels between 2002 and 2014 had led to enhanced photosynthesis in plants, the process by which plants take up carbon dioxide.

During the same period, a slowdown in the rise of global temperatures over land — known as the hiatus period — led to a slowdown in respiration, the process by which plants 'breathe' out carbon dioxide.

These two factors combined meant the world's vegetation absorbed more carbon dioxide and slowed the growth rate in the amount of CO2 in the atmosphere by about 2.2 per cent a year between 2002 and 2014.

"The take-home message is that we have this incredible climate change discount — like half of the CO2 [humans emit] gets taken in by the natural carbon sinks," Dr Canadell, executive director of the Global Carbon Project, said.

"So we want to do whatever we can to maintain the sink and enhance it if we can through programs such as reforestation and even before that avoiding deforestation in the first place."

Oceans and land-based vegetation removed about 45 per cent of the CO2 emitted by humans each year, the researchers noted.

While absolute atmospheric CO2 levels had been increasing since the Industrial Revolution, there was significant year-to-year variability in the rate at which this increase occurred, largely driven by annual differences in plant growth.

Dr Canadell said satellite observations also showed the globe was "greening", with areas previously too cold or dry now sustaining more plant life and areas with vegetation "greener than they were before".

However, this increase in vegetation played a minor role in the increased uptake of carbon dioxide.

Slowdown may be temporary

Dr Canadell warned the slowdown in the growth rate of atmospheric CO2 could be temporary because the hiatus period, which was essential for decreased respiration, has now ended.

"The arrival of El Nino in late 2014 ended the dynamics [of decreased respiration] described in the paper," he said.

While it was uncertain how the terrestrial sinks would change under the latest conditions "the implication is … if we continue to warm the planet, this will always work towards making the land sinks grow more slowly [as respiration will increase]".

Without real cuts to the annual 40 billion tonnes of anthropogenic emissions, future climate change remained a reality.

The findings were based on climate change modelling using data from the Global Carbon Project, in combination with data from satellite remote sensing and a global network of observational stations including CSIRO's atmospheric tracking centre at Cape Grim in Tasmania.