Earth's Plant Life Keeping Planet from Getting Warmer

Earth is being kept cooler by its plant life, especially during the past six decades, in which the enhanced growth of leafy greens has slowed the planet's transition to warmer temperatures, according to a Princeton University-led study, which is the first to specify the extent to which plants have prevented climate change since pre-industrial times.

The researchers, writing in the Proceedings of the National Academy of Sciences, contend that Earth's ability to store carbon in its plants has kept between 186 billion and 192 billion tons of carbon out of the atmosphere since the mid-20th century. That carbon storage has, in turn, kept the Earth's temperature cooler than it would have been, the researchers report.

A shift in how humans typically used land also played a role in the transition, as substantial logging and deforestation campaigns between the 1860s and 1950s played a large role in carbon emissions into the atmosphere. After the 1950s, however, humans made a greater effort to restore forests.

"Changes in carbon dioxide emissions from land-use activities need to be carefully considered," said Elena Shevliakova, a senior climate modeler in Princeton's Department of Ecology and Evolutionary Biology. "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."

"People always say we know carbon sinks are important for the climate," Shevliakova said. "We actually for the first time have a number and we can say what that sink means for us now in terms of carbon savings."

If Earth's terrestrial ecosystems were to have remained a strong carbon source, they would have contributed to between 65 billion and 82 billion tons of carbon in addition to the carbon it would not have absorbed, the researchers found, which they say means as much as 192 billion tons of carbon that is not in the atmosphere today would be.

The researchers provide a compelling argument for continuing forest restoration projects, said Scott Saleska, an associate professor of ecology and evolutionary biology at the University of Arizona. Saleska, who studies the interaction between vegetation and climate, was not involved in the study.

"There is surely some limit to how long increasing carbon dioxide can continue to promote plant growth that absorbs carbon dioxide," Saleska said. "Carbon dioxide is food for plants, and putting more food out there stimulates them to 'eat' more. However, just like humans, eventually they get full and putting more food out doesn't stimulate more eating."