...A recent “tree census” in New York City, conducted at the behest of Mr Bloomberg, values the city's nearly 600,000 trees at $122m. A rough breakdown: $11 million for filtering out air pollutants; $28m saved in energy consumption (less need for air conditioners); $36m for stemming storm-water run-off; and $53m in “aesthetic benefits”. The Forest Service values the urban canopy in all of America at $14.3 billion.
What is the use of all these (rather shady) numbers? Mr Bloomberg cannot sell off trees to patch a hole in his budget, after all. They are, literally, a fixed asset. But for politicians, numbers help. By claiming that every $1 put into New York's trees returns $5.60 in benefits, he may find it easier to galvanise New Yorkers to plant more and chop down fewer.
From The Economist