Plant vegetation has kept the planet cooler by absorbing billions of tons of carbon, especially during the past 60 years.
Earth’s land-based carbon “sink”—or carbon-storage capacity—has kept 186 billion to 192 billion tons of carbon out of the atmosphere since the mid-20th century, researchers report in the Proceedings of the National Academy of Sciences.
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 agriculture that, while larger scale, is higher yield.
At the same time, industries and automobiles continued to steadily emit carbon dioxide that contributed to a botanical boom. Although a greenhouse gas and pollutant, carbon dioxide also is a plant nutrient.
Had Earth’s terrestrial ecosystems remained a carbon source they would have instead generated 65 billion to 82 billion tons of carbon in addition to the carbon that it would not have absorbed, the researchers found.
That means a total of 251 billion to 274 billion additional 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 (ppm), the researchers report—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 one-third of a degree Celsius (or a half-degree Fahrenheit), which would have been a sizeable jump, the researchers report.
The planet has warmed by only 0.74 degrees Celsius (1.3 degrees Fahrenheit) since the early 1900s, and the point at which scientists calculate the global temperature would be dangerously high is a mere 2 degrees Celsius (3.6 degrees Fahrenheit) more than pre-industrial levels.
The study is the most comprehensive look at the historical role of terrestrial ecosystems in controlling atmospheric carbon, says first author Elena Shevliakova, a senior climate modeler in Princeton University’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 says.
“People always say we know carbon sinks are important for the climate,” Shevliakova says. “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.
“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 notes.
“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.”
If carbon dioxide levels in the atmosphere continue rising, more vegetation would be needed to maintain the size of the carbon sink Shevliakova and her colleagues report.
“There is surely some limit to how long increasing carbon dioxide can continue to promote plant growth that absorbs carbon dioxide,” Saleska says. “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.”
The National Oceanic and Atmospheric Administration, the US Department of Agriculture, and PEI’s Carbon Mitigation Initiative funded the project.
Source: Princeton University