A new study on how air pollution impacts tropical forests is causing concern among scientists, highlighting how deteriorating air quality over the course of the 20th century has impacted the global carbon cycle.
Alexander Cheesman is a Senior Research Fellow at James Cook University and the University of Exeter in the UK and one of the lead authors of a new paper in the journal Nature Geoscience examining the phenomenon.
He said elevated levels of ozone at ground-level are a result of human activity.
"Urbanisation, industrialisation and fossil fuel usage all increase the emission of chemical precursors which react to form ozone. This ozone has a detrimental impact on human health and the ability of plants to collect CO2 from the atmosphere," said Dr Cheesman.
"However, the influence of ozone on tropical forests in particular, one of the largest absorbers of CO2 from the atmosphere, was unclear. We wanted to find how increased ozone exposure since the year 1900 has affected tropical-forest productivity and the global carbon cycle," he said.
The scientists measured the ozone susceptibility of various tropical tree species at a purpose-built experimental facility in Cairns, Queensland, and then incorporated these data into a dynamic global vegetation model.
"We found that current ozone caused by human activity resulted in a substantial decline in annual net primary productivity (NPP) – the net uptake of CO2 from the atmosphere - across all tropical forests, with some areas being particularly impacted," said Dr Cheesman.
He said between 2005 and 2014, ozone produced by human activity has probably reduced tropical-forest NPP by an average of 5.1%, varying from a decrease of just 1.5% across Central Africa to 10.9% in Asian tropical forests.
"We calculate that this productivity decline has resulted in a cumulative loss in carbon drawdown to tropical forests – the removal of carbon from the atmosphere by this critical ecosystem - of about 17% in the 21st century.
"Ozone concentrations across the tropics are projected to rise further still due to increased precursor emissions and altered atmospheric chemistry in a warming world," said Dr Flossie Brown, a recent graduate of the University of Exeter, UK and co-lead author on the paper.
"We also 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 component of determining tropical forest responses to a changing world," said Dr Brown.
