Unadjusted D.C. summer temperatures (average highs and lows - smoothed using a summer long moving average) since 1872. From June 1872 through June 1945, observations were taken either at 17th and G NW or 24th and M St. NW at an elevation of 100 feet. Since July 1945, observations have been taken at Reagan National Airport, at an elevation of about 60 feet. (Data courtesy National Weather Service and Ian Livingston)

In my August 1 column on July’s record-breaking heat in Washington, D.C., after describing the onslaught of heat records the last two summers, I wrote: “The two primary suspects for the warming trend are urbanization and the increasing concentration of greenhouse gases in the atmosphere from burning fossil fuels.”

Due to space constraints, a wonky, but important sentence was cut by editors: “Unpacking the relative contribution of urbanization and greenhouse gases to the warming is difficult locally, compounded by the fact the location of D.C.’s observing station changed from 24th and M St. for most years prior to 1945 to Reagan National airport thereafter.”

Since the column ran, I’ve tried to “unpack” the contribution and, it would appear, a large fraction is due to urbanization with some smaller , more nebulous contribution from greenhouse gases.

The easiest way to assess the cause of the warming is to analyze temperature trends from a NASA dataset which not only adjusts for station moves (such as the switch from 24th and M to Reagan National), but also corrects for urbanization. It uses night light brightness from satellite imagery to classify the location of weather stations as urban, near-urban or rural. As more warming generally occurs at urban station, NASA uses neighboring rural stations to adjust the long-term temperatures at urban stations.

Summer average temperature in Washington, D.C. since 1880 adjusted for urbanization (NASA)

But there’s a wrinkle.

Along with me, former Virginia state climatologist Pat Michaels (who taught me applied climatology at the University of Virginia), has been trying to sort out these complicated D.C. temperature trends and their cause. Whereas he found the Northern Hemisphere’s summer temperature has warmed at a steep rate of four degrees per century since the late 1970s, he discovered Washington D.C.’s summer temperature - adjusted for urbanization - hasn’t really warmed during this most recent period. (I also found no warming in the adjusted record since 1977; the unadjusted warming rate since 1977 is about 1.4 degrees per century or about the same as the rate since 1880)

In Michaels’ view, the lack of correlation between D.C. and the rest of the hemisphere rejects the hypothesis that greenhouse warming is affecting D.C.’s summer temperature. Or, as he said in a blog post at Forbes.com,: “That’s not a pretty picture for those claiming our [D.C.’s] recent suffering is because of our economic sins.”

I’m not convinced, however, the lack of correlation means much or that we should even expect correlation.

Consider that at the local level, a variety of factors affect the climate - factors which may or may not play prominently at larger scales. Some small scale cause (type unknown) could well have offset the greenhouse gas pressure locally.

I’d ask Michaels to consider the following: what would D.C.’s temperature history look like without the increase in greenhouse gases over the last few decades?

In any event, Michaels and I agree that if a strong greenhouse signal hasn’t prominently emerged to date, it will get ugly if it does so in the future.

Or as Michaels concluded his post:

Yes, the hell created by the culture of Gomorrah-on-the-Potomac will actually become hotter than hell.