From a quarter to half of Earth's vegetated lands have shown significant greening over the last 35 years largely due to rising levels of atmospheric carbon dioxide, according to a new international study which used satellite data to map changes in ‘leaf area’.

The results of the study, conducted by experts from 24 institutions around the globe and published in the journal Nature Climate Change this week, suggest there has been a huge increase in the leaves on plants and trees - equivalent to an area two times the size of the continental United States since the early 1980s.

Green leaves use energy from sunlight through photosynthesis to chemically combine carbon dioxide drawn in from the air with water and nutrients tapped from the ground to produce sugars and allow plants to grow. Research has suggested that, as emissions increase, raising levels of carbon dioxide in the atmosphere, so photosynthesis increases, spurring plant growth. However, with the predicted growth, negative impacts such as hotter surface temperatures, shifting rainfall patterns and increases in ozone are likely to take a toll on plant health - and on the wellbeing of people and animals - in the long term.

Even so, the NASA study shows carbon dioxide fertilisation isn't the only cause of increased plant growth: nitrogen, land cover change and shifts in temperature and rainfall associated with climate change all appear to be contributing to the greening effect. To determine the extent of carbon dioxide's contribution, researchers ran the data for carbon dioxide 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 carbon dioxide fertilisation explains 70 percent of the greening effect, according to co-author Ranga Myneni, a professor in the Department of Earth and Environment at Boston University. "The second most important driver is nitrogen, at 9 percent. So we see what an outsized role CO2 plays in this process,” Prof Myneni said.

About 85 percent of Earth's ice-free lands are covered by vegetation. The area covered by all the green leaves on Earth is equal to about 32 percent of Earth's total surface area. 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 lead author Zaichun Zhu, a researcher from Peking University, China, who did the first half of this study with Myneni as a visiting scholar at Boston University.

Every year, about half of the 10 billion tons of carbon emitted into the atmosphere from human activities remains temporarily stored, in about equal parts, in the Earth’s oceans and plants. "While our study did not address the connection between greening and carbon storage in plants, other studies have reported an increasing carbon sink on land since the 1980s, which is entirely consistent with the idea of a greening Earth," said co-author Shilong Piao of the College of Urban and Environmental Sciences at Peking University.

Although their study showed that climate changing emissions currently appear to be having a greening effect on the Earth’s plants, in the long term, climate change has the power to lead to massively unsettled weather patterns and extreme weather events like flooding and drought.

The scientists behind the NASA study are under no illusions about the serious threats posed by climate change. Dr Philippe Ciais, another of the study’s co-authors and associate director of the Laboratory of Climate and Environmental Sciences, Gif-suv-Yvette, France, said that on top of this, the beneficial impacts of carbon dioxide on plants may also be limited. Dr Ciais said, "Studies have shown that plants acclimatise, or adjust, to rising carbon dioxide concentration and the fertilisation effect diminishes over time."

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