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Large stretches of arid land have become greener since the 1980s due to rising concentrations of carbon dioxide, which fertilizes plant growth, a new study shows.
While this greening has long been noted in satellite imagery, its direct link to carbon dioxide (CO2) has been difficult to prove, explained study leader Randall Donohue, an environmental scientist at Australia's Commonwealth Scientific and Industrial Research Organization.
"There are so many processes occurring simultaneously that affect plant behavior, it is very difficult to determine which process is responsible for any given change," he told NBC News in an email. Teasing out a CO2 fertilization effect amongst the other processes "hasn't been done before," he added.
CO2 is also a major player in global climate change, which is making the planet warmer and, in places, wetter. Warmer temperatures in cold regions and increasing precipitation in dry areas are also expected to spur plant growth, he noted.
Increases in CO2 also fertilize plant growth by making more carbon available to plants and allowing plants to lose less water to the air during the process of photosynthesis. Plants need carbon and water for growth. More of both, means more growth, Donohue explained.
To detect the effect in nature, he and colleagues focused on satellite imagery of warm and dry environments around the world where rainfall — the biggest factor in plant growth — is limited. This makes it easier to see vegetation growth in satellite imagery and account for the effect of rainfall.
"If you go to a rainforest, it is much harder to detect this (CO2 fertilization effect) in the canopy cover because it is already covered," Ramakrishna Nemani, an Earth scientist at the NASA Ames Research Center in California, explained to NBC News.
Nemani, who was not part of the research, said the team's approach and finding of 11 percent more foliage due to the CO2 fertilization effect is "quite plausible and theoretically correct."
The praise in only tempered by the length of the dataset, which, he noted, is about a decade longer than what he used for his own "global greening" research published in 2003. Still, he said, "30 years is not really that long." Further monitoring should increase confidence that "this is happening."
The greening effect of increased CO2 is a global phenomenon. It is even seen in areas that are getting drier due to reduced rainfall and warmer temperatures as a result of global climate change, the researchers noted.
"If a brown place is getting drier, we can expect that the 'browning' won't be as severe as it would have been if CO2 levels were unchanged," Donohue explained. "Similarly, we can expect that the greening that would occur when a dry place gets wetter will be greater now because of higher CO2 levels."
The implications of the findings are potentially significant, he added. For example, it could change how much carbon is soaked up by plants and the amount of woody fuel available for forest fires.
"It needs to be considered as an important piece of the overall global-change puzzle that we are still trying to figure out."
The findings are reported in a paper accepted for publication in the journal Geophysical Research Letters.
John Roach is a contributing writer for NBC News. To learn more about him, visit his website.