Guest Post by Willis Eschenbach

I got to thinking about the phenomenon known as the “Urban Heat Island” effect, or UHI. Cities tend to trap heat due to the amount of black pavement and concrete sidewalks, the narrow canyons between buildings that slow down the wind, and the sides of the buildings reflecting sunlight downwards.

As a result, cities are often warmer than the surrounding countryside. In some cities, it’s hot enough that it affects the local weather. Here’s a simplified diagram:

What I was curious about, however, was another kind of urban heat effect. This is the heat from all of the energy used within the city—electricity, fuel for transport, fuel for heating buildings, all of it. Eventually, almost all energy ends up as heat. So I went and got the energy usage for 27 huge “megacities”, along with the area of the city itself. I then combined the two to give me a measure of citywide energy usage in watts per square metre (W/m2). As a measure for comparison with Figure 2 below, a doubling of CO2 is said to increase the “forcing”, the total radiant energy impinging on the surface, by 3.7 W/m2.

This was surprising to me. I hadn’t expected the effect to be so large.

Finally, I converted the forcing to an equivalent warming. The Intergovernmental Panel on Climate Change (IPCC) fifth assessment report, completed in 2014, gave a likely “Transient Climate Response” of about 1°C to 2.5°C for each additional 3.7 W/m2 of forcing. I’ve used 1.5°C because the heat generation persists over time. As a result, the cities have had time to equilibrate to the additional heating. Figure 3 shows that result.

Not much more to say about all of that. Note that this is a “first cut” analysis, I make no overarching claims about the accuracy of the results. I’ve used conservative assumptions and the best data I could find. It looks to be a significant additional heating source due to direct energy usage in the densest of the largest cities, one that is not generally included in the calculations of the Urban Heat Island.

Here, I’m in the Forest Cool Island, life is good. I spent my morning crawling around under my house successfully putting a new “generator” into my floor furnace so we now have a warm house again, a less than pleasant job that came complete with a veritable plethora of bad words. Then I got out the pumice stone and scrubbed a toilet bowl until it sparkled, and this has been my afternoon project … do I know how to have fun, or what?

My very best wishes to each of you,

w.

DATA: I’ve appended the data below, in comma-delimited form. PJ is petajoules, 10^15 joules.

City, PJ/yr, Km^2 New York, 2824, 8622 Tokyo, 2438, 13839 Moscow, 1984, 12197 Seoul, 1848, 10197 Los Angeles, 1848, 3884 Shanghai, 1644, 24256 Guangzhou, 1474, 14043 Osaka, 1258, 2691 Tehran, 1145, 8154 Mexico City, 1099, 9041 London, 1065, 8825 Beijing, 952, 21516 Manila, 918, 1780 Buenos Aires, 702, 3054 Paris, 657, 2229 Sao Paulo, 589, 12252 Jakarta, 589, 10075 Istanbul, 464, 14025 Rio De Janeiro, 384, 6718 Shenzhen, 350, 8378 Lagos, 350, 861 Dhaka, 350, 8906 Karachi, 339, 14910 Delhi, 316, 11034 Cairo, 282, 10230 Mumbai, 191, 12478 Kolkata, 78, 4486