Exhaust fumes and burning reduce the ability of rainforests to capture carbon by 0.29 billion tonnes each year. A new study by researchers from the University of Gothenburg shows how ozone at the ground-level formed by human combustion harms plant photosynthesis.
Rainforests are the most effective counterforce to the greenhouse effect. Tropical forests absorb 10-15 percent of the carbon dioxide released into the atmosphere by humans. Unfortunately, the emission of nitrogenous compounds and volatile hydrocarbons from burning creates large amounts of ground-level ozone, which is toxic to living organisms, both humans and trees.
"Combustion does double damage to our climate. It increases the amount of carbon dioxide in the atmosphere, and it increases the levels of ground-level ozone by 2 to 3 times. The ozone makes the plants' photosynthesis work less well and it inhibits the absorption of carbon dioxide from the atmosphere," says Johan Uddling, professor of botanical ecophysiology at the University of Gothenburg.
30 percent is taken up by plants
The impact of ozone on photosynthesis has been known for some time. The new study, published in Nature Geoscience, is the first to measure the damage to tropical forests, which is particularly important since these forests account for about half of the total carbon uptake by plants on Earth.
"Our study shows that the carbon drawdown to rainforests has decreased by 17 percent so far this century, if we assume that the effects on plant growth mirror those on photosynthesis. This means that the equivalent of 0.29 gigatonnes (billion tonnes) of carbon per year has not been captured from the atmosphere due to the damaging effects of ozone at ground-level," says Johan Uddling.
By comparison, total emissions from fossil fuel combustion and cement production are estimated to be equivalent to around 10 gigatonnes of carbon globally each year. Of all the carbon dioxide emitted worldwide, it is estimated that 30 percent is stored as carbon in plants, where the actual capture takes place through photosynthesis. 25 percent ends up in the ocean and the rest stays in the atmosphere, where it contributes to the greenhouse effect.
The ozone problem can be solved
Ozone is a secondary air pollutant formed at elevated levels during the combustion of nitrogen compounds and volatile hydrocarbons, such as fossil fuels. Ozone is then broken down relatively quickly, in a matter of hours to a few weeks at most.
In Europe, levels of ground-level ozone have decreased since the introduction of better exhaust gas treatment with catalytic converters. Instead, the problem has grown in countries with rapid industrialisation and growth, sometimes located in parts of the world where tropical forests grow.
"It is a question of transferring technologies to these areas. More resources need to be invested in combating harmful emissions from combustion in Asia, Africa and South America to reduce ground-level ozone there too. Forests that are not exposed to high levels of ground-level ozone are healthier and generate more ecosystem services for the local population as well as a greater uptake of carbon from the atmosphere," says Johan Uddling.
The study in Nature Geoscience: Reduced productivity and carbon drawdown of tropical forests from ground-level ozone exposure
