Air Pollution Reduces Carbon Sequestration in Tropical Ecosystems

A new study published in Nature Geoscience estimates that 290 million tonnes of carbon are lost each year due to ozone pollution inhibiting the establishment of tropical forests.

One of the major achievements of environmental action has been the preservation of the ozone layer, which shields the Earth from harmful UV radiation. However, at ground level, the combination of pollutants from human activity and sunlight produces ozone, which hinders plants' ability to absorb carbon dioxide and negatively impacts human health.

A recent study found that tropical forests experience an average 5.1 % reduction in new annual growth due to ground-level ozone. In some regions, such as Asia, the effect is even more severe, with tropical forests losing 10.9 % of their fresh growth.

Tropical forests are crucial "carbon sinks," capturing and storing carbon dioxide that would otherwise contribute to global warming if left in the atmosphere.

Tropical forests play a crucial role in mopping up our carbon dioxide emissions. Our study shows that air pollution can jeopardize this critical ecosystem service. We estimate that ozone has prevented the capture of 290 million tonnes of carbon per year since 2000. The resulting cumulative loss equates to a 17 % reduction in carbon removal by tropical forests so far this century.

Dr. Alexander Cheesman, Study Co-Lead Author, University of Exeter

The researchers conducted studies to evaluate the ozone vulnerability of various tropical tree species and then integrated their findings into a global vegetation computer model.

Urbanization, industrialization, fossil fuel use, and fires have all increased the number of "precursor" molecules, such as nitrogen oxides, which contribute to the formation of ozone.

Ozone concentrations across the tropics are projected to rise further due to increased precursor emissions and altered atmospheric chemistry in a warming world. We found that areas of current and future forest restoration–areas critical for the mitigation of climate change–are disproportionately affected by this elevated ozone. It is clear that air quality will continue to play an important but often overlooked part in the way forests absorb and store carbon.

Dr. Flossie Brown, Study Co-Lead Author, University of Exeter

Professor Stephen Sitch, from the University of Exeter, added: “Embracing a future with greater environmental protection would lead to reduced ground-level ozone, thus improved air quality and the additional benefit of enhanced carbon uptake in tropical forests.”

Journal Reference:

Cheesman, A. W., et al. (2024) Reduced productivity and carbon drawdown of tropical forests from ground-level ozone exposure. Nature Geoscience. doi.org/10.1038/s41561-024-01530-1