SINGAPORE Wetlands, forests and farmlands soak up large amounts of carbon dioxide but rising amounts of the gas in the atmosphere mean these carbon "sinks" could become less effective at fighting climate change.
Scientists say land ecosystems are an essential brake on the pace of climate change because plants soak up large amounts of carbon dioxide (CO2) as they grow. This also boosts the level of carbon in soils.
But in a study published in the British journal Nature on Thursday, scientists say rising levels of planet-warming CO2 will trigger an increased release of two other far more potent greenhouse gases from soils, rice paddies and wetlands.
"Our results suggest that the capacity of land ecosystems to slow climate warming has been overstated," the authors, led by Kees Jan van Groenigen of Northern Arizona University in the United States, conclude.
Scientists say many plants will grow faster as CO2 levels rise, leading to more CO2 mopped up from the atmosphere. But rising levels of nitrous oxide and methane offsets some of the benefit.
Van Groenigen and colleagues calculated that a surge in the release of greenhouse gases from soils would negate at least 16.6 percent of the previously estimated climate change fighting potential of increased carbon storage in the landscape.
This means the pace of global warming could in fact be faster than previously thought and that complex computer models that scientists use to project the impacts of climate change would need to be adjusted, van Groenigen told Reuters.
Nitrous oxide is about 300 times more powerful as a greenhouse gas than CO2 and methane about 21 times as powerful. Both gases are produced by microbes in the soil.
Van Groenigen said plants become more efficient at using water as CO2 levels rise, leaving more moisture in the soil that in turn leads to microbes producing more nitrous oxide. Bigger root systems lead to more carbon in the soil, fuelling yet more nitrous oxide to be released, he said.
"More organic matter in the soil means more microbes being active and one product of decomposition by microbes is nitrous oxide," he told Reuters from Flagstaff, Arizona. It was the same for methane.
"Under elevated CO2 levels, more carbon makes its way into the soil and this carbon is the source of the methane that is being produced," he added.
The results are based on an analysis of dozens of other studies and field observations on the impacts of rising CO2 levels.
"If we are correct, that would mean models over-estimate the effect (this) has on the climate because they don't take into account the extra amount of nitrous oxide and methane that are being produced," he said.
Carbon dioxide is essential for plant life and keeping the planet warm.
Scientists say CO2 emissions from burning fossil fuels and deforestation are overwhelming the natural cycle, driving global warming.
(Editing by Sugita Katyal)