Higher CO2 levels greening Earth: study

A quarter to half of Earth's vegetated lands has shown significant greening over the last 35 years largely due to rising levels of atmospheric carbon dioxide (CO2), a new study has found.

Green leaves use energy from sunlight through photosynthesis to chemically combine CO2 drawn in from the air with water and nutrients tapped from the ground to produce sugars, which are the main source of food, fibre and fuel for life on Earth, researchers said.

Studies have shown that increased concentrations of CO2 increase photosynthesis, spurring plant growth, they said.

CO2 fertilisation is not the only cause of increased plant growth - nitrogen, land cover change and climate change by way of global temperature, precipitation and sunlight changes all contribute to the greening effect.

To determine the extent of CO2's contribution, they ran the data for CO2 and each of the other variables in isolation through several computer models that mimic the plant growth observed in the satellite data.

"Results showed that CO2 fertilisation explains 70 per cent of the greening effect. The second most important driver is nitrogen, at 9 per cent. So we see what an outsized role CO2 plays in this process," said Ranga Myneni from Boston University in the US.

A team of 24 institutions from eight countries used satellite data from NASA's Moderate Resolution Imaging Spectrometre and the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiometre instruments to help determine the leaf area index, or amount of leaf cover, over the planet's vegetated regions.

About 85 per cent of Earth's ice-free lands is covered by vegetation. The area covered by all the green leaves on Earth is equal to, on average, 32 per cent of Earth's total surface area - oceans, lands and permanent ice sheets combined.

The extent of the greening over the past 35 years "has the ability to fundamentally change the cycling of water and carbon in the climate system," said Zaichun Zhu from Peking University in China.

While rising CO2 concentrations in the air can be beneficial for plants, it is also the chief culprit of climate change, researchers said.

The gas, which traps heat in Earth's atmosphere, has been increasing since the industrial age due to the burning of oil, gas, coal and wood for energy and is continuing to reach concentrations not seen in at least 500,000 years, they said.

The impacts of climate change include global warming, rising sea levels, melting glaciers and sea ice as well as more severe weather events, researchers said.

"The beneficial impacts of CO2 on plants may also be limited. Studies have shown that plants acclimatise, or adjust, to rising CO2 concentration and the fertilisation effect diminishes over time," said Philippe Ciais from Laboratory of Climate and Environmental Sciences in France.

The findings were published in the journal Nature Climate Change.