CO2's leafy effects assessed
Over the last 30 years, increased carbon dioxide (CO2) has boosted plant growth by an amount equal to twice the size of Australia (18 million km²).
Plants use carbon dioxide as food to fuel their growth by turning it into sugar. When there is more carbon dioxide plants produce more sugar in a process known as CO2 fertilisation.
An international team of researchers has looked at what drove the increase in plant growth between 1982 and 2009, and found that CO2 was the main culprit.
They estimate that up to half the world's land is becoming greener as a result.
The authors also note that the greening could fundamentally change the cycling of water and carbon in the climate system.
“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,” says Pep Canadell, CSIRO Research Scientist, and Executive Director of the Global Carbon Project.
The study -“Greening of the Earth and its Drivers” - has been published in the journal Nature Climate Change.
It shows significant greening of up to half of the Earth’s vegetated lands, based on data from the NASA-MODIS and NOAA-AVHRR satellite sensors of the past 33 years.
“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 from Boston University.
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 driver of increased climate variability.
Professor Derek Eamus, a Professor of Environmental Sciences at the University of Technology Sydney, says it is a very significant development.
Dr Eamus said; “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... [and] 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.”
Dr Eamus said a key feature of the work was the “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 per cent of the increase in leaf area, changes in nitrogen deposition and climate changes accounted for a further 9 and 8 per cent 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.”