Fifteen Eighty Four

If you’d like to visit the front lines of climate change, you need not travel as far as the North Pole, the nearest melting glacier, or the most recently inundated island.  Easier to access by road or rail, and far more revealing of the nature in which climate change is altering our everyday lives, is Louisville, Kentucky. One of several mid-sized cities situated along the Ohio River, Louisville distinguishes itself as the home of the Kentucky Derby – the world’s most widely watched horse race – the Louisville Slugger bat, and as an excellent place to consume bourbon.  But in recent years this city has distinguished itself from other Midwestern river towns in at least one additional respect: Louisville may now be the climate change capital of the urban world.

What sets apart Louisville from other large cities is not simply the extremity of its summer temperatures – lower latitude cities such as Phoenix, Dallas, and Miami tend to be hotter – but the rate at which Louisville is warming relative to its surrounding region.  Known technically as the “urban heat island effect,” the extent to which urban centers exhibit higher temperatures than nearby rural areas is a key measure of climate change at the urban scale, and a warming phenomenon that is largely independent of the global greenhouse effect.  As explored in my recently published book, The City and the Coming Climate, the urban heat island effect is amplifying the rate at which large cities around the planet are warming.  In fact, most large cities are not only warming faster than the planet as a whole, they are heating up at double the rate of global warming.  With rapidly rising temperatures in cities comes a growing threat of heat waves to human health.  In the period since the 1950s, the number of heat wave days in large U.S. cities has more than doubled, with extreme heat now accounting for more weather-related deaths per year than hurricanes, tornadoes, earthquakes, and flooding events combined.

So what can Louisville and other large cities do to effectively slow the greatly accelerating pace of warming at the urban scale?  Limiting greenhouse gas emissions is absolutely essential for reversing the process of global warming, but conventional mitigation holds little promise for measurably cooling down urban temperatures during the course of our lifetime.  Because it is a growing urban heat island effect that is most responsible for rising temperatures in cities today, urban governments must take steps not only to limit greenhouse gas emissions but to reduce as well the direct emission of heat energy into the local environment.

For most cities, a re-vegetation of the built environment is the single most effective strategy to slow climate change in the near term. Climate modeling studies find that efforts to plant up to a million new trees in cities such as New York, Los Angeles, and Denver will not only render these urban environments more beautiful, such campaigns are greatly enhancing the resilience of these cities to climate change.  In fact, a re-greening of cities through extensive tree planting, green roof installations, and parkland expansion, in combination with a resurfacing of building roofs and streets with more highly reflective materials, would have a more substantial cooling effect at the urban scale than a complete cessation of greenhouse gas emissions – and at a far lower cost.  So while residents of Louisville and other large cities may be surprised to find themselves on the front lines of climate change, they need not await the next international agreement to move forward in confronting the climate problem – the most effective tools to slow the pace of warming are to be found in their own backyards.  Let the planting begin.