America's urban trees absorb 25.6 million tonnes of CO2 annually. That carbon storage is worth $50.5 billion a year, but can cities profit?

Urban trees in the U.S. absorb 25.6 million tonnes of carbon dioxide annually and help lower energy costs by shading the asphalt and concrete jungle. The US Forest Service estimates all that carbon storage is worth $50.5 billion and growing by $2 billion a year, according to a new study from the agency.

That’s a lot of green. Just one problem—how do you profit from all that photosynthesis?

Figuring out how to monetize the urban forest could be key to keeping it—and the planet—healthy. As urbanization continues—cities are expected to account for 8.1 percent of US land area in 2050, up from 3.1 percent in 2000—the population of street trees is falling by 4 million trees a year, the report says. That’s the equivalent of losing 20,000 acres (8,094 hectares) of trees annually.

Why? Office towers and apartment buildings encroach on green space. Meanwhile, planting and maintaining urban trees is expensive. That’s especially true for cities facing shrinking municipal budgets and struggling to keep enough cops and garbage collectors on the streets.

On a global level, the United Nation’s REDD initiative (Reducing Emissions from Deforestation and forest Degradation in Developing countries) aims to preserve tree cover by paying countries not to cut down their forests. The US lacks a national carbon market to help such efforts along. But the state of California’s new cap-and-trade system does allow companies to meet their greenhouse gas emission targets partially by buying credits from approved urban tree projects. As a result, budget-constrained cities like southern California’s Long Beach are considering enrolling their urban street trees in the cap-and-trade program to make money.

Unfortunately, the program is limited to cities, universities and utilities, and California’s cap-and-trade regulations get in the way. Cities must establish monitoring and reporting systems to verify their street trees are indeed storing the carbon claimed in the credits. Also, to qualify for the program, cities must ensure that the carbon absorbed by their trees remain stored for 100 years. Trees, of course, die of old age, or get knocked down by storms and errant drivers, and cities must keep the carbon books balanced.

Forests are easier to manage, which explains why oil giant Shell has said it intends to purchase 500,000 carbon offsets from a Michigan project to meet its California cap-and-trade obligations. So where are the most tree-loving cities in the US? Texas. The state’s street trees are storing 49.8 million tons of carbon, according to the Forest Service survey. Surprisingly, tree-hugging California comes in seventh place, capturing 34,600 tons of carbon.

Top image: Jessica Gow/Reuters. This post originally appeared on Quartz, an Atlantic partner site.

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