If You Like a Greener Earth, Thank CO2

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Credit: doi:10.1038/nclimate3004 Credit: doi:10.1038/nclimate3004

If you remember photosynthesis from basic plant science, green leaves produce sugars using energy in 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. When there is more CO2 in the air, more sugars are produced -- a process known as CO2 fertilization -- and such 'greening' of Earth has happened more in a quarter to one-half of the Earth's vegetated lands, according to an analysis of data from the NASA-MODIS and NOAA-AVHRR satellite sensors of the past 33 years.

We have more plants and more trees.

Obviously, most of Earth is covered in water and there has been concern about land usage due to larger populations. It may seem like urban living has led to less green space, but it is actually the opposite. The additional greening over the past 33 years is enough to cover the entire continental USA -- twice over.

"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.

The amount of CO2 in the air has been increasing since the industrial age and about half of the 10 billion tons of carbon emitted in to the atmosphere from human activities remains temporarily stored in the oceans and plants. More green means an increasing carbon sink on land, which could be why forecasts of planetary harm have not come to pass. But the researchers say that is no reason to get complacent about emissions.

CO2 fertilization may be the key reason why the Earth is greening, but it is not the only one.

"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 coauthor Dr. Josep Canadell of the CSIRO Oceans and Atmosphere Division in Canberra, Australia and leader of the Global Carbon Project.