Dark City

Anyone who has spent much time in a city has at some point exited a building on a summer’s afternoon only to slam face first into an oven-like wall of heat, and perhaps thought Why is it so hot in this part of town?? And most urbanites will have noticed the relatively sudden onset of sweet cool relief upon entering a park, or passing beneath the shade of a leafy tree on a hot day. These oddities in urban climate, bane of the poor summer pedestrian, are linked to the Urban Heat Island effect.

The Urban Heat Island (UHI), described in London as far back as 1818, describes the general state of hotter temperatures in cities compared to the surrounding countryside, and is present in most urbanized places on earth today. The origin of the UHI has to do with the physical differences between urban land cover compared to undisturbed land: Urban areas are paved, set about with tall buildings, and have less trees and soil per square mile than the less-developed places on which they were built. These basic physical changes to the urban landscape alter the way that solar energy is absorbed by the land, transferred around, and finally dissipated, with one of the net effects being that the air near the ground in core urban areas can be considerably hotter than suburban or rural places. This phenomenon can make cities during heat waves not just unpleasant, but outright deadly. And when we start to look ahead to how cities will feel with the combined effects of climate change thrown on top, urban planners and municipal decision-makers start to get nervous about how to keep their cities livable into the future.

A thermal image shows the hot pavement and relatively cool trees (and shade) in Melbourne, Australia (Image: City of Melbourne, via The Guardian)

One of the ways of tamping down the UHI is to fit in more trees : Places like parks or areas with more tree cover tend to stay cooler because trees act like “swamp coolers”, trading water evaporation for heat and keeping the air temp down. But another means of keeping the town cool is to do like those charming Greek hillside villas do: Use bright surfaces to keep the amount of sunlight absorbed to a minimum.

The quality of a surface that describes how bright or dark it is – the ratio between how much light bounces off versus how much it is exposed to – is termed albedo. Low albedo things like charcoal (albedo near 0) absorb most of the light that hits them; high albedo things like snow (albedo near 1) reflect nearly all the light that hits them. It has long been thought that one of the reasons why cities stay hotter is because they are just simply darker than the un-built areas around them, a result of the materials they’re made of or the way that tall tightly-packed buildings trap sunlight. Urban scientists have been modeling how we might use brighter surfaces on rooftops or roads to hold solar energy absorption to a minimum and help keep the temperature down. Some places have even tried it.

Whitewashed walls have high albedo and don’t absorb much of the solar energy that strikes them (Image: Quora)

I started thinking about urban albedo when I went one day to try to look up what the albedo of a city is to help a colleague on another project – we were curious about how much climate change effect we could expect with future expansion of urban land cover. What I found was that not much has been done to simply measure exactly what the albedo of a city even is, or how it varies across the landscape. There were, and remain, many questions: What controls the albedo of a city? How much scope do we have for changing urban albedo to correct for UHI effect, and where should we start? Are we even putting the right numbers into our models?

I spent the past couple of years working on measuring the albedo of the city of Boston and its surrounding communities. Using some clever satellite data put together by our collaborators at U. Mass Boston, we measured albedo with unusual precision and looked at what other measures of urban land cover might be behind the variations we saw.

What we found confirmed our suspicions with concrete numbers. The urban land surface really is darker in the most densely developed parts of town, exactly where daytime temperatures are hottest. Urban land surface shows lower albedo (more sunlight absorbed) where it is most densely paved and most densely populated, and somewhat higher albedo (less sunlight absorbed) where there is less pavement, more tree cover, and fewer people. Overall, my part of town is both the most paved, highest density, least leafy, and hottest part of the greater Boston metropolitan area – and it is also the best at absorbing sunlight.

Albedo falls with greater paved cover (L) and rises with greater tree cover (R) in the Boston region; higher-density communities are both hotter and darker than less densely developed areas. (Trlica et al., 2017. Earth’s Future 5(11): 1084–1101.)

Our project gives us reason to believe that attacking the UHI from the direction of raising the albedo of our buildings and streets might work, focusing first on the hottest, darkest, most densely developed parts of town. It would take some additional computer modeling to estimate how much of an albedo change we could achieve, and how those change might precisely affect our summer temperatures. But there’s no reason physically why it shouldn’t help.

So the next time you’re huffing down the street on a hot summer afternoon blinking sweat from your eyes and you bump face first into a heat-addled city council member going the other way, you grab them by the lapels (gently, civilly – really don’t grab anybody by their lapels) and tell them the land surface around here is just too darn dark, and I’m not going to take it anymore!

If they don’t believe you, point them to our paper. Engaged citizens are cool citizens!



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