In the hectic life of cities, urban parks are a source of tranquility. Trees are proven to provide great mental health benefits. But in addition to comforting us, they have a crucial function for our physical health: cleaning the air we breathe.
Particulate matter, which is especially harmful to the lungs, is retained on the surfaces of trees, whose leaves act as filters for polluting gases, according to the study The Effects of Trees and Forests on Air Quality and Human Health in the United States.
This report also cautions that while trees can mitigate the effect of air pollution, deposits of air pollutants on leaves can affect photosynthesis "and therefore affect removal of pollution by trees."
The cooling effect of trees
Trees can significantly lower temperatures in cities. In hot climates, tree canopies can help cut energy expenditure on air conditioners, while reducing the consumption of fossil fuels for power generation.
Experimental research in the United States has shown that shade from trees can reduce air conditioning costs for single-family homes by 20-30%.
"Trees could lower city temperatures by as much as 8 ° C, reducing air conditioning use and related emissions by as much as 40%," says SimoneBorelli, United Nations Food Organization Urban / Peri-Urban Forestry and Agroforestry Officer. and Agriculture (FAO).
Urban tree planting has to be done right. The planted species should be the most effective at trapping contamination, generally those with large leaves. Managers must also take into account other details, such as wind patterns and the spacing between trees. If water is scarce, they will want to consider drought tolerant species.
Conservation of urban trees and reforestation is increasingly important as urbanization is accelerating: the proportion of the world's population living in cities will be 60% in 2030 and 66% in 2050. Almost 90% of this increase will occur in Africa and Asia. To address the impacts of this rapid growth and related challenges, a large-scale effort is needed.
The Great Green Wall of Cities, under construction
At nearly 8,000 km long and 15 km wide, the Great Green Wall is a reforestation initiative of epic proportions that began in 2007 with the intention of greening North Africa from end to end, a semi-arid region that stretches from Senegal to Djibouti. After a decade and some 15% progress, the initiative is slowly bringing some of Africa's degraded landscapes back to life, while providing food security, jobs and a reason to stay for millions of people.
FAO and other partners are working on an initiative of this nature, dubbed the Great Green Wall of Cities, on the eve of the United Nations Climate Summit in New York in September 2019. The goal is to create up to 500,000 hectares of new urban forests and restore or maintain up to 300,000. Existing natural forests in and around 90 cities in the Sahel and Central Asia by 2030. Once established, this wall would capture 0.5 to 5 gigatons of carbon dioxide per year and store carbon for centuries.
On March 1, 2019, the United Nations General Assembly established the United Nations Decade for the Restoration of Ecosystems 2021-2030, which should further boost reforestation efforts.
"UN Environment promotes tree planting as a key way to mitigate climate change and boost land-based biodiversity, of which 80% is found in forests," explains Tim Christophersen, Director of the Freshwater, Land and Climate Branch. of UN Environment, and president of the Global Alliance for Forest Landscape Restoration.
“We are working with partners across the planet to drive tree planting and ecosystem restoration. There is scope to plant a billion more trees, in addition to the 3 billion that already exist on Earth. But this has to be done well; planting native trees, with the support of local communities, is a good way to start ”.
A moss-based solution
In forest ecosystems, trees are not alone in their role as air purifiers. An ambitious project by Greencity Solutions in Berlin, Germany, seeks to unite high-tech applications with another natural filter: moss.
"The ability of certain moss crops to filter pollutants like particulates 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 can barely survive due to their dependence on water and shade. This problem can be solved by connecting different mosses with a fully automated supply of water and nutrients, based on the unique technology of the Internet of Things, ”explains the company.
Another solution would be to plant more trees that provide the necessary cover and moisture for the moss to take hold and grow.
Air pollution is the theme of World Environment Day 2019, which is celebrated on June 5. The quality of the air we breathe depends on the lifestyle choices we make every day. Learn more about how pollution affects you and what is being done to clean the air we breathe at worldenvironmentday.global/en. Join the conversation on social media using the hashtag # SinContaminationDelAire.