Rising carbon dioxide levels in the atmosphere have increased plant growth across the planet over the past three decades, a new study has found.

Key points: A few areas on Earth have browned but the vast majority have greened

A few areas on Earth have browned but the vast majority have greened Plant coverage has grown by 18 million square kilometres in a few decades

Plant coverage has grown by 18 million square kilometres in a few decades The main cause is increase in atmospheric CO2

The main cause is increase in atmospheric CO2 Nitrogen from agricultural fertilisers has also contributed

The most comprehensive modelling of remote sensing data so far shows the area on Earth covered by plants in this time has increased by 18 million square kilometres — about 2.5 times the size of the Australian continent — largely due to the fertilising effect of carbon dioxide (CO2).

"[The greening] has the ability to fundamentally change the cycling of water and carbon in the climate system," said Dr Zaichun Zhu, from Peking University in China and lead author of the new study, which appears today in the journal Nature Climate Change.

"This is a snapshot of humans' global influence on the functioning of the entire global biosphere," said co-author CSIRO's Dr Pep Canadell of the Global Carbon Project.

The new findings come from analysis of data from the past 33 years taken by three different satellite missions that measured the green light reflected by photosynthesising leaves.

Dr Canadell said the remote sensing data was run through 10 global environmental change models to identify the cause of the greening.

A map of the Earth showing how much different areas have greened since 1982 ( Ranga Myneni )

About 85 per cent of Earth's ice-free land is covered by plants, and each year photosynthesis soaks up about one quarter of the nearly 10 billion tonnes of carbon emitted by humans.

Drs Zhu, Canadell and colleagues found that the 46 parts per million increase in atmospheric CO2 between 1982 and 2009 was responsible for 50 to 70 per cent of the observed greening.

"Carbon fertilisation is the dominant process for greening across the globe, particularly in the tropics because there's so much leaf area there," Dr Canadell said.

Scientists have long established humans as a key source of atmospheric CO2 through the use of chemical signatures that distinguishes carbon from different sources.

"The growth of CO2 in the atmosphere is almost exclusively due to fossil fuel burning and deforestation," said Dr Canadell.

The new study found other causes of the greening, including nitrogen from agricultural fertilisers.

'Surprising' finding given increase in droughts

Dr Canadell said the greening has surprised scientists who expected to see more browning, given the increase in droughts associated with global warming.

While the researchers found between 25 to 50 per cent of all vegetated areas of the land have become greener, only 4 per cent have become browner.

These included Mongolia, Argentina and areas of North America close to Alaska.

While south-eastern Australia also showed browning, overall the Australian continent was greening, said Dr Canadell.

While a greener Earth might seem like a positive from CO2-induced global warming, along with milder winters and longer growing seasons, he said there were many more negative impacts — including rising sea levels and severe weather.

"These will eventually outweigh by far any benefit from the greening," he said.