Radiant Heat Effect on Global Warming

June 28, 2013 10:28 AM

Does the mass of human construction radiate enough heat to contribute to global warming?

Last weekend a group of our friends were over and we were talking about everything. When we were talking about global warming I thought of something I have not heard about before. Google didn't help in finding information about it.

Without people there are rocks, rivers, trees, grasses, and mother natural "things". We have added to that cars, busses, roads, parking lots, ships, bridges, houses, malls, warehouses, high rises, stadiums and other "things" that heat up during the day and then radiate heat, often for hours, after the sun sets.

Have we calculated how much of global warming is from heat radiated from what we have built?

If you stand in the middle of a hundred acres of trees next to a hundred acres of parking lot - which one will be hotter? (The answer is obvious.)

posted by Leenie to Science & Nature (11 answers total)

There are efforts afoot to paint roofs white for this reason.
posted by seemoreglass at 10:33 AM on June 28, 2013 [1 favorite]

Best answer: What you want to Google is Urban Heat Island, to wit:

Monthly rainfall is greater downwind of cities, partially due to the UHI. Increases in heat within urban centers increases the length of growing seasons, and decreases the occurrence of weak tornadoes.

The UHI decreases air quality by increasing the production of pollutants such as ozone, and decreases water quality as warmer waters flow into area streams, which stresses their ecosystems.

Not all cities have a distinct urban heat island. Mitigation of the urban heat island effect can be accomplished through the use of green roofs and the use of lighter-colored surfaces in urban areas, which reflect more sunlight and absorb less heat.

Despite concerns raised about its possible contribution to global warming, comparisons between urban and rural areas show that the urban heat island effects have little influence on global mean temperature trends.
posted by jquinby at 10:35 AM on June 28, 2013

Was just about the link the same Wiki article. Basically, you're right in a local sense, but not in a global sense. Lots of pavement will give you a hot time during summer in the city, but there's no effect on global warming. Only a very very tiny percentage of the earth is paved.
posted by echo target at 10:39 AM on June 28, 2013 [1 favorite]

I think an important thing to think about global warming and urban heat islands is not so much the effect of the heat islands on the global system but how it effects people in those areas. During periods of extreme heat the problem is so much worse in urban areas and effects lots of people.
posted by JayNolan at 10:47 AM on June 28, 2013 [1 favorite]

Urban heat islands don't influence climate change directly, except insofar as human settlements displace vegetation that previously provided natural carbon sinks. However, urbanization does exacerbate the effects of global warming through urban heat islands and lots of impermeable surfaces (pavements) that hinder ground-water absorption and increase the risk of flooding.
posted by voiceofreason at 10:59 AM on June 28, 2013

I would imagine that the power requirements and indirect effects, e.g. through the need for cooling technologies as a result of the the UHI, would be responsible for a degree of global warming.
posted by idb at 10:59 AM on June 28, 2013 [1 favorite]

This is pretty complicated actually. The urban heat island effect is as others have said. It doesn't have much effect, by itself, on global mean temperature. But land use changes are the other part of this. If you cut down a forest to put in a suburban housing development, you're going to do all kinds of things to the climate.
posted by natteringnabob at 11:13 AM on June 28, 2013 [1 favorite]

I used to live in Florida next to a tree-covered vacant lot. Then some developer bought the lot, cut down the trees, built a house, and put in a huge concrete driveway that just soaked up the heat and radiated it back to me. It definitely affected my microclimate, and led me to use more power staying cool.

You also have to look at the energy used to build all of those structures, to create the materials out of which they're built, to dispose of the trees or whatever used to be on that site, etc.

And then what about all of the trucks/trains/planes that bring necessary and luxury commodities to the city?
posted by mareli at 11:15 AM on June 28, 2013

You're sort of right but for wrong reasons. Construction stuff doesn't absorb more solar energy than mother natural stuff. Bright concrete pavement presumably absorbs less energy than dark pine needles, pine seems cooler simply because it more rapidly dissipates that heat into the atmosphere. That it takes longer for the pavement to cool doesn't necessarily mean more energy is involved, it means different ratios of surface area to thermal mass, heat conductivity, etc.

Urban heat comes from our intense use of energy - for example, water moving through a hydro dam miles away results in gigawatts of energy released in the city, which ends up as heat in the nearby atmosphere and water.

As the physicist said to the economist "at a 2.3% growth rate, we would reach boiling temperature in about 400 years", (where "we" is "the surface of the earth".)

But it's not really a factor in global warming - the heat energy we add to the atmosphere is considerably less than the cumulative heat energy we add via the solar-heat-trapping pollution we produced to make our energy. But either way, we need to decouple economic growth from growth in energy use.
posted by anonymisc at 11:27 AM on June 28, 2013 [2 favorites]

Some of these transformations have contributed to other environmental issues. Rivers are warmer where trees have been torn down and replaced by buildings or pavement, for example, and this is bad for the survival of endangered salmon here in the Pacific Northwest. This is not just an issue in very urban areas, but also in small towns and rural housing developments that probably don't have the heat bubble effect.
posted by croutonsupafreak at 11:57 AM on June 28, 2013

Another good search term you want is albedo, which is the reflection of light back off the surface of the earth.
posted by bilabial at 1:17 PM on June 28, 2013