Ozone gas reduces the growth of tropical forests, leaving an estimated 290 million tons of carbon uncaptured annually, new research shows.
The ozone layer in the stratosphere protects our planet from harmful ultraviolet radiation – and protecting it is one of the greatest successes of environmental action.
But ground-level ozone – formed by the combination of pollutants from human activities in the presence of sunlight – interferes with plants’ ability to absorb carbon dioxide. Ozone is also harmful to human health.
The new study, published in the journal Natural Geosciencescalculates that ground-level ozone reduces new annual growth in tropical forests by an average of 5.1%.
In some regions the effect is stronger: Asia’s tropical forests are losing 10.9% of new growth.
Tropical forests are vital ‘carbon sinks’: they capture and store carbon dioxide that would otherwise remain in the atmosphere and contribute to global warming.
“Tropical forests play a crucial role in reducing our carbon emissions,” said co-lead author Dr Alexander Cheesman from James Cook University and the University of Exeter.
“Our research shows that air pollution can jeopardize this crucial ecosystem service.
“We estimate that ozone has prevented the capture of 290 million tons of carbon per year since 2000. The resulting cumulative loss amounts to a 17% reduction in carbon removal by tropical forests so far this century.”
The researchers conducted experiments to measure the ozone sensitivity of various tropical tree species and then incorporated the results into a computer model of global vegetation.
Urbanization, industrialization, fossil fuel burning and fires have led to an increase in ‘precursor’ molecules – such as nitrogen oxides – that form ozone.
“Ozone concentrations in the tropics are expected to rise further due to increased precursor emissions and changed atmospheric chemistry in a warming world,” said co-lead author Dr Flossie Brown, a recent graduate from the University of Exeter.
“We found that areas where forests are currently and in the future being restored – areas critical to climate change mitigation – are disproportionately affected by this increased ozone layer.
“It is clear that air quality will continue to play an important but often overlooked role in how forests absorb and store carbon.”
Professor Stephen Sitch from the University of Exeter added: “Embracing a future with greater environmental protection would lead to a reduction in ground-level ozone, improving air quality and providing the added benefit of improved carbon capture in tropical forests. “
