Plant a tree for your tomorrow remains good advice.
A new study looks at the benefits of trees in 10 of the world's mega-cities, including New York, London and Mexico City.
Researchers found that the trees in those cities save around $500 million a year in energy costs and reduce pollution.
Planting 20 percent more trees would double those benefits, the study says.
Environmental groups have created an online tool call i-Tree to help people find out where trees could be planted in cities for the greatest benefit.
The study was published in the journal Ecological Modelling.
Photo: Getty Images