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EXPERT REACTION: Carbon dioxide drives massive plant growth

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Over the last 30 years, increased carbon dioxide (CO2) has upped plant growth so much it is as if a green continent more than twice the size of Australia (18 million km²) has been added to the world's land, say scientists including Australians. Plants use carbon dioxide as food to fuel their growth by turning it into sugar, and when there is more carbon dioxide plants produce more sugar, a process known as CO2 fertilisation. The researchers looked at what was driving the increase in plant growth between 1982 and 2009 and found that CO2 was the main culprit, and that up to half the world's land is becoming greener as a result. The study authors note that the greening has the ability to fundamentally change the cycling of water and carbon in the climate system.

Journal/conference: Nature Climate Change

Organisation/s: CSIRO

Media Release

From: Boston University

CO2 Fertilization Greening the Earth

International team reports CO2 fertilization prompted plants and trees to sprout extra green leaves equivalent in area to two times the continental USA, or nearly 4.4 billion General Shermans (Largest Giant Sequoia Tree)

BOSTON—An international team of 32 authors from 24 institutions in eight countries has just published a study titled “Greening of the Earth and its Drivers” in the journal Nature Climate Change showing significant greening of a quarter to one-­half of the Earth’s vegetated lands using data from the NASA-­MODIS and NOAA-­AVHRR satellite sensors of the past 33 years. The greening represents an increase in leaves on
plants and trees. Green leaves produce sugars using energy in the sunlight to mix carbon dioxide (CO2) drawn in from the air with water and nutrients pumped in from the ground. These sugars are the source of food, fiber and fuel for life on Earth. More sugars are produced when there is more CO2 in the air, and this is called CO2 fertilization.

“We were able to tie the greening largely to the fertilizing effect of rising atmospheric CO2 concentration by tasking several computer models to mimic plant growth observed in the satellite data” says co-­author Prof. Ranga Myneni of the Department of Earth and Environment at Boston University, USA. Burning oil, gas, coal and wood for energy releases CO2 in to the air. The amount of CO2 in the air has been increasing since the industrial age and currently stands at a level not seen in at least half-­a-­million years. It is the chief culprit of climate change.

About 85% of the Earth’s ice-­free lands is covered by vegetation. The area of all green leaves on Earth is equal to, on average, 32% of the Earth’s total surface area -­ oceans, lands and permanent ice sheets combined. “The greening over the past 33 years reported in this study is equivalent to adding a green  continent about two-­times the size of mainland USA (18 million km2), and has the ability to fundamentally change the cycling of water and carbon in the climate system,” says lead author Dr. Zaichun Zhu, a researcher from Peking University, China, who did the first-­half of this study as a Visiting Scholar in the Department of Earth and Environment at Boston University, USA, together with Prof. Myneni.

Every year, about one-­half of the 10 billion tons of carbon emitted in to the atmosphere from human activities remains temporarily stored, in about equal parts, in the 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,” says co-­author Prof. Shilong Piao of the Chinese Academy of Sciences Center for Excellence in
Tibetan Plateau Earth Science, Beijing, China.

The beneficial aspect of CO2 fertilization in promoting plant growth has been used by contrarians, notably Lord Ridley (hereditary peer in the UK House of Lords) and Mr. Rupert Murdoch (owner of several news outlets), to argue against cuts in carbon emissions to mitigate climate change, similar to those agreed at the 21st Conference of Parties (COP) meeting in Paris last year under the UN Framework on Climate Change (UNFCCC). “The fallacy of the contrarian argument is two-­fold. First, the many negative aspects of climate change, namely global warming, rising sea levels, melting glaciers and sea ice, more severe tropical storms, etc. are not acknowledged. Second, studies have shown that plants acclimatize, or adjust, to rising CO2 concentration and the fertilization effect diminishes over time,” says co-­‐‑author Dr. Philippe Ciais, Associate Director of the Laboratory of Climate and Environmental Sciences, Gif-­suv-Yvette, France and Contributing Lead Author of the Carbon Chapter for the recent IPCC Assessment Report 5.

CO2 fertilization is only one, albeit a predominant, reason why the Earth is greening. The study also identified climate change, nitrogen fertilization and land management as other important reasons. “While the detection of greening is based on measurements, the attribution to various drivers is based on models, and these models have known deficiencies. Future works will undoubtedly question and refine our results,” says co-­author Dr. Josep Canadell of the CSIRO Oceans and Atmosphere Division in Canberra, Australia and leader of the Global Carbon Project.


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Expert Reaction

These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.

Professor Rodney Keenan is from the School of Ecosystem and Forest Sciences at the University of Melbourne

This study shows the significant global impacts of increasing CO2 in the atmosphere and how it is changing the functioning of our ecosystems. The results demonstrate the positive role that plants and trees play in the carbon cycle and their value in offsetting emissions from burning fossil fuels and land clearing for agriculture. Increased CO2 and modest warming can have benefits for vegetation. However, there are clear doubts about whether these carbon storage benefits can persist under an increasingly warmer climate. As the climate warms further, extended droughts in some regions may result in rapid loss of vegetation cover and the release of stored carbon.

Increased disturbances predicted under future climates, such as fire, insect pests or diseases are also likely to impact on forests and other vegetation. The study shows the value of forest and vegetation management in expanding forest cover in some regions. Managing for the potential impacts of future climate will become increasingly important in sustaining forests in a rapidly changing world.

Last updated: 03 Nov 2016 7:35pm
Dr Anna Ukkola is a Research Associate at the ARC Centre of Excellence for Climate System Science, UNSW

Vegetation greening has potentially serious consequences for water resources. Denser, greener vegetation can consume more water, in turn reducing the amount of rainwater that would otherwise run off into rivers. Greening vegetation has been shown to significantly reduce river flows in drier parts of Australia in the recent decades, and can thus have important implications for regions with already limited water resources.

Last updated: 03 Nov 2016 7:31pm
Pep Canadell, CSIRO Research Scientist, and Executive Director of the Global Carbon Project

The global greening trend we report is perhaps the most compelling evidence yet showing that human activities are affecting every corner of the globe and that humans are now a major force in shaping how the Earth is functioning. Yes, we are indeed in the Anthropocene.

Last updated: 03 Nov 2016 7:17pm
Professor Peter Rayner is Leader of the Clean Air and Urban Landscapes NESP hub, at the University of Melbourne

The paper shows we've pumped up the volume on the global carbon cycle with more carbon going in and coming out. What matters is how much of this carbon stays out of the atmosphere and how long it's likely to stay there. That's not the job of this paper.

The results track well with the studies used in the IPCC that showed that vegetation activity and carbon uptake would continue to increase for a while as CO2 increased. In those models it's a few years before climate change really begins to bite and that carbon we're storing now gets returned to the atmosphere. The effect is certainly good for the planet, nature is doing its best to clean up after us but it can't keep up with fossil fuel emissions.

Whether it's good for humanity depends on the impact on crops and the best evidence here is that increasing yield is offset by decreasing nutritional value.

Last updated: 03 Nov 2016 4:44pm
Professor Derek Eamus is a Professor of Environmental Sciences at the University of Technology Sydney

"This new study by Zhu and colleagues used a combination of satellite-derived information about how many leaves there are sitting above each square meter of land and the outputs of several models of ecosystem function.  The satellite data are available back to 1982 and therefore allow Zhu and colleagues to address the question – have there been any consistent trends in changes in the amount of leaf area in canopies. This is an important question because the amount of leaf area present determines how much sunlight is absorbed by vegetation. The more leaf area present, the larger the amount of sunlight absorbed. This in turn, determines how much carbon is taken up by vegetation and hence how much growth (of trees, of crops, of pasture grasses) occurs each year.  

These authors were able to show that there was a persistent and widespread increase in the number of leaves present across 25 – 50 % of the world. They were also able to demonstrate using models, that most of this increase was the result of the increase in CO2 concentration in the atmosphere that has occurred in the past 35 years.  This result confirms the earlier studies undertaken by Australian workers, including that of Randall Donohue, Michael Roderick, Graham Farquhar and our own modelling work at UTS. These Australian teams have long established that rising atmospheric CO2 levels can stimulate plant growth and support increased leaf area, especially in arid and semi-arid regions that characterise most of Australia.

A key feature of the work of Zhu et al was their examination of the potential impacts of changes in nitrogen deposition to soils and the impacts of land-use change by running multiple models several times with changes to individual factors made one-at-a-time. Using this approach they showed that while changes in CO2 concentration accounted for 70 % of the increase in leaf area, changes in nitrogen deposition and climate changes accounted for a further 9 and 8 % respectively. This is an important contribution to the study of how vegetation responds to increased atmospheric CO2.

What might this mean for Australia landscapes?  One potential impact is a stimulation of so-called “woody-thickening” whereby the number of trees across Australian landscapes will increase. This increases the amount of carbon locked-up in biomass but potentially can reduce the growth of pasture grasses that support our beef and sheep industries. This woody thickening is being seen globally, as well as in Australia. The effect of increased CO2 in the atmosphere is likely to be larger in warm and drier locations (ie Australia) because increased atmospheric CO2 levels reduce the amount of water required to capture each kg of CO2. Consistent with this expectation is the observation in 2011, when sea-levels dropped because of massive rainfall across the southern hemisphere, arid-zone Australia accounted for about 60 % of the massive global increase in the annual uptake of CO2 that occurred in 2011.

Last updated: 03 Nov 2016 4:35pm

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