ENHANCED growth of the Earth's leafy greens during the 20th century has significantly slowed the heating of the planet, according to the first study to specify the extent to which plants have prevented climate change since pre-industrial times.
Researchers based at Princeton University found that land ecosystems have kept the planet cooler by absorbing billions of tons of carbon, especially during the past 60 years.
The planet's land-based carbon "sink" — or carbon storage capacity — has kept 186-192 billion tons of carbon out of the atmosphere since the mid-20th century, according to a report in the Oct. 15 Proceedings of the National Academy of Sciences.
According to Princeton, from the 1860s to the 1950s, land use by humans was a substantial source of the carbon entering the atmosphere because of deforestation and logging. After the 1950s, however, humans began to use land differently, such as by restoring forests and adopting agricultural practices that, while larger in scale, result in higher yield.
At the same time, industries and automobiles continued to steadily emit carbon dioxide that contributed to a botanical boom, the announcement said. Although a greenhouse gas and pollutant, carbon dioxide also is a plant nutrient.
Had the terrestrial ecosystems remained a carbon source, they would have instead generated 65-82 billion tons of carbon in addition to the carbon the Earth would not have absorbed, the researchers found. That means an additional 251-274 billion tons of carbon would currently be in the atmosphere.
That much carbon would have pushed the atmosphere's current carbon dioxide concentration to 485 parts per million, the researchers reported — well past the scientifically accepted threshold of 450 ppm at which the Earth's climate could drastically and irreversibly change. The current concentration is 400 ppm.
Those "carbon savings" amount to a current average global temperature that is cooler by 0.33 degrees C (or 0.5 degrees F), which would have been a sizeable jump, the researchers reported. The planet has warmed by only 0.74 degrees C (1.3 degrees F) since the early 1900s, and the point at which scientists say the global temperature would be dangerously high is a mere 2 degrees C (3.6 degrees F) more than pre-industrial levels.
The Princeton study is the most comprehensive look at the historical role of terrestrial ecosystems in controlling atmospheric carbon, explained first author Elena Shevliakova, a senior climate modeler in Princeton's department of ecology and evolutionary biology.
Previous research has focused on how plants might offset carbon in the future but overlooked the importance of increased vegetation uptake in the past, she said.
"Changes in carbon dioxide emissions from land use activities need to be carefully considered. Until recently, most studies would just take fossil fuel emissions and land use emissions from simple models, plug them in and not consider how managed lands such as recovering forests take up carbon," she said. "It's not just climate; it's people. On land, people are major drivers of changes in land carbon. They're not just taking carbon out of the land; they're actually changing the land's capacity to take up carbon.
"After the 1940s and 1950s, if you look at the land use change trajectory, it's been slowed down in the expansion of agriculture and pastures," Shevliakova said. "When you go from extensive agriculture to intensive agriculture, you industrialize the production of food, so people now use fertilizers instead of chopping down more forests. A decrease in global deforestation combined with enhanced vegetation growth caused by the rapid increase in carbon dioxide changed the land from a carbon source into a carbon sink."
Scott Saleska, an associate professor of ecology and evolutionary biology at the University of Arizona who studies interactions between vegetation and climate, said the researchers provide a potentially compelling argument for continued forest restoration and preservation by specifying the "climate impact" of vegetation. (Saleska is familiar with the research but had no role in it.)
"I think this does have implications for policies that try to value the carbon saved when you restore or preserve a forest," Saleska said. "This modeling approach could be used to state the complete 'climate impact' of preserving large forested areas, whereas most current approaches just account for the 'carbon impact.' Work like this could help forest preservation programs more accurately consider the climate impacts of policy measures related to forest preservation."
Although the researchers saw a strong historical influence of carbon fertilization in carbon absorption, that exchange does have its limits, Saleska said. If carbon dioxide levels in the atmosphere continue rising, more vegetation will be needed to maintain the size of the carbon sink, Shevliakova and her colleagues reported.
"Unless you really understand what the land use processes are, it's very hard to say what the system will do as a whole," Shevliakova concluded.