Shading Our Cities Against Global Warming

(Credit: NASA’s Earth Observatory)

While we talk about “going green” to fight global warming, NASA says our cities should do so literally. By combining data from multiple satellites to model temperature variations, they determined that on average cities are 1.9oC (3.4oF) warmer than neighboring vegetated areas during the summer and 1.5oC (2.7oF) warmer during the winter (see graphic above). They evaluated cities across the continental United States through different times of the day and year, expanding upon previous studies that focused on individual cities or single snapshots in time. To cool things down, cities need to get back to nature.

The Environmental Protection Agency says an increase of just 1.0oC (1.8oF) can raise energy demands for air conditioning by 5 to 20 percent. Cities can save energy, and reduce carbon emissions in what becomes a virtuous cycle, by increasing their ratio of plants to concrete and steel. However, not all green spaces are created equal. Broadleaf plants and trees, because of the water evaporating from their leaves, provide more cooling than grasses or trees with needles.

Typically the conversation about land use changes’ role in climate change invokes images such as clearcutting the Amazon for farming, not the roads and skyscrapers of cities. NASA demonstrated that urban land use changes matter too. They estimated the carbon lost due to urbanization at 1.8% of total uptake over the continental U.S., even though cities occupy only 1.1% of the land. Thus, the hot concrete of a city sidewalk is not just making us miserable, it is having a disproportionate affect on carbon in the atmosphere.

Urbanization of the world’s population is a given. The bad news is cities that pave over more than 35% of their natural environment will experience degrees greater warming than the rest of the planet. The good news is that cities that expand their green spaces, particularly with broadleaf vegetation, can at least spare their residents from even higher temps and conserve energy in the process. Maybe cities will start showing Arbor Day some love.


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