A recently study showed that among the overall sources of carbon dioxide in urban environments, a fraction is from decaying trees, lawns, and other urban vegetation. This was more than researchers anticipated.
Urban greenery adds CO2 to the atmosphere when vegetation dies and decomposes, increasing total emissions.
Burning fossil fuels in densely populated regions greatly increases the level of the greenhouse gas, carbon dioxide.
The largest carbon dioxide sources are cars, trucks, ports, power generation, and industry, including manufacturing.
The team made this discovery by tracing carbon dioxide sources with carbon-14, a rare form of carbon that occurs naturally in Earth’s atmosphere and is absorbed by living things as they grow.
Carbon-14’s presence in organic materials is the basis of radiocarbon dating and serves as a powerful tool to distinguish the carbon dioxide produced by fossil fuel combustion from that produced by decomposing vegetation and other organic matter.
Researchers found that, over the course of a year, urban greenery accounts for about one-fifth of the excess carbon dioxide observed in the air.
Understanding these relationships can help planners design and manage green spaces in ways that pull as much carbon out of the atmosphere as possible and store it permanently.
To effectively reduce fossil-fuel emissions, researchers need to quantify the background sources of carbon dioxide in global cities to understand how it varies with climate, latitude, degree of industrialization.
Carbon-14 is mainly created by gamma rays from the Sun in Earth’s upper atmosphere, where it becomes chemically incorporated into carbon dioxide. It is transported by winds downward toward Earth’s surface and then around the planet.
Living organisms absorb carbon dioxide that contains both regular carbon (carbon-12) and carbon-14. Once an organism dies, the carbon-14, which is radioactive, decays away over time.
Scientists have long used carbon-14 as a natural “clock” for estimating the age of dead plants and animals as well as the age of materials made from them.