But the study also warns that while trees can mitigate the effect of air pollution, deposits of air pollutants on leaves can also affect photosynthesis “and therefore potentially affect pollution removal by trees”. As with everything, balance is key.
The cooling effect of trees
Trees can also significantly cool temperatures in cities. In hot climates, tree cover can reduce energy expenditure on air conditioning, while driving down the consumption of air polluting fossil fuels that power these cooling systems. Experimental investigations and modelling studies in the United States have shown that shade from trees can reduce the air conditioning costs of detached houses by 20–30 per cent.
“Trees could reduce temperatures in cities up to 8°C, lowering use of air conditioning and related emissions by up to 40 per cent,” says Simone Borelli, an Agroforestry and Urban/Periurban Forestry Officer with the Food and Agriculture Organization of the United Nations.
“When part of a wider landscape mosaic, large green patches within and around cities would also reduce emissions through avoided sprawl and excess mobility requirements,” he adds.
Urban tree-planting has to be done right. Species planted should be ones that are most effective at trapping pollution, typically those with large leaves. Officials also need to account for things like wind patterns and tree spacing. If water is scarce, they’ll want to consider drought-tolerant varieties, and avoid trees that increase pollen and allergies.
Action is all the more important given that urbanization is accelerating—the proportion of people living in cities will be 60 per cent in 2030 and 66 per cent in 2050. Nearly 90 per cent of this increase will occur in Africa and Asia. To address the impacts of this rapid growth and the related challenges, a large-scale effort is needed.
Building the Great Green Wall of Cities
Nearly 8,000 km long and 15 km wide, the Great Green Wall is an African-led movement of epic proportions initiated in 2007 to green the entire width of northern Africa, a semi-arid region extending from Senegal to Djibouti. A decade in and roughly 15 per cent under way, the initiative is slowly bringing life back to some of Africa’s degraded landscapes, providing food security, jobs and a reason to stay for the millions who live along its path.
An initiative of this nature in urban areas is being developed by the Food and Agriculture Organization and other partners in preparation for the UN Climate Summit in September 2019. It aims to create up to 500,000 hectares of new urban forests and restore or maintain up to 300,000 ha of existing natural forests in and around 90 cities of the Sahel and Central Asia by 2030. Once established, this “Great Green Wall of Cities” would capture 0.5–5 gigatonnes of carbon dioxide per year and stock carbon for centuries.
“UN Environment promotes the planting of trees as a key way to mitigate climate change and boost land-based biodiversity, 80 per cent of which is in forests,” says Tim Christophersen, head of UN Environment’s Freshwater, Land and Climate Branch, and Chair of the Global Partnership on Forest and Landscape Restoration. “We are working with partners across the planet to boost tree planting for ecosystem restoration. There is scope for planting one trillion more trees, in addition to the 3 trillion that already exist on Earth. But it has to be done right; planting indigenous trees, supported by local communities, is a good way to go.”
Let the stones gather some moss
In those forest ecosystems, trees are not alone in cleaning the air. An ambitious project by Greencity Solutions in Berlin, Germany, seeks to marry high-tech applications with another natural air purifier: moss.
“The ability of certain moss cultures to filter pollutants such as particulate matter and nitrogen oxides from the air makes them ideal natural air purifiers,” says Greencity Solutions.
“But in cities, where air purification is a great challenge, mosses are barely able to survive due to their need for water and shade. This problem can be solved by connecting different mosses with fully automated water and nutrient provision based on unique Internet of things technology,” it explains.
Or by planting more trees that will provide the cover and humidity, that will help moss take hold and grow.
Air pollution is the theme for World Environment Day on 5 June 2019. The quality of the air we breathe depends on the lifestyle choices we make every day. Learn more about how air pollution affects you, and what is being done to clean the air. What are you doing to reduce your emissions footprint and #BeatAirPollution?
The 2019 World Environment Day is hosted by China.