A recent flurry of activity on social media last week centered around a new study published in the Journal of Applied Meteorology and Climatology. According to the abstract, it found that "temperature observations (in an experiment with four observation sites) were warmest for the site closest to the built environment." This finding is not "News." We have known about the Urban Heat Island (UHI) effect for many decades, and I have previously described it in Forbes.
I suppose the novel caution in this new paper is that urban "creep" is possible at some temperature stations used for overall climate change assessment like the U.S. Climate Reference Network. My colleague Dr. Roger Pielke Sr. has a deep body of work on land changes and climate impacts so I suspect he is very pleased to see studies like this as am I. However, this essay is written as an antidote to wild claims spouting the "cliche" or "zombie theory" that climate warming is caused by urban heat bias. Not only is that flawed, there is a greater danger that many have overlooked.
One of my favorite climate scientists to follow on social media is Zeke Hausfather. A 2013 study published in Geophysical Research Letters by Hausfather and colleagues examined the urban bias on the U.S. Historical Climate Reference Network (USHCN). According to the NOAA website,
The U.S. Historical Climatology Network (USHCN) data are used to quantify national- and regional-scale temperature changes in the contiguous United States (CONUS). The USHCN is a designated subset of the NOAA Cooperative Observer Program (COOP) Network with sites selected according to their spatial coverage, record length, data completeness, and historical stability.
Hausfather and colleagues found that "urbanization accounts for 14% to 21% of the rise in unadjusted minimum temperatures since 1895 and 6% to 9% since 1960." However, the paper goes on to say that correction procedures have effectively removed the urban heat signal such that it has not caused a bias in temperature assessment over the past 50-80 years. They also noted that stations around the USHCN sites were "sufficiently" rural to limit urban biases when the data are blended together. However, the new study certainly raises a caution flag as we move forward.
Neil Debbage, a professor at the University of Texas - San Antonio, and I recently published a study in Computers, Environment, and Urban Systems on how to calculate the urban heat island. The calculation depends on how the reference "rural" sites are included and how the urban land cover is situated. In the study, we found that both dense and sprawling urban regions can enhanced the so-called urban heat island (UHI) effect.
Climate scientists are quite smart so it is baffling when someone claims the broader climate warming signal is "urban" related. In my head, I am thinking, "my Gosh, don't you think we thought of that too." The Hausfather study and numerous documentation on the NOAA NCEI website clearly indicates that we have. If you need a more common sense dismissal of the urban bias "zombie theory," simply look at the regions of the planet experiencing the most rapid growth: regions of the Arctic (see graphic below). The last time I checked, there are not many large cities to bias results there. Like the USHCN, there is also a Global Historical Climate Network (GHCN), which according to NOAA's website:
an integrated database of climate summaries from land surface stations across the globe that have been subjected to a common suite of quality assurance reviews. The data are obtained from more than 20 sources. Some data are more than 175 years old while others are less than an hour old.
Satellites and other ocean-based instrumentation is also used to complement land-based thermometers.
Brian Stone, a professor at the Georgia Institute for Technology, and colleagues sounded a warning bell that I often feel gets missed. Cities are made with heat-absorbing materials, have less vegetation to provide evapotranspirational cooling, and consist of anthropogenic heat from transportation and heating systems. Many buildings also contribute to the UHI by emitting its heat into urban corridors. Stone and colleagues have argued that scientists have been so careful to make sure we are not biasing the global temperature record (rightfully so) with the "urban signal" that we have overlooked that urban areas are warming with greater intensity. Why? Urban areas have the "double-whammy" of the background anthropogenic warming associated with greenhouse gas emissions plus the warming signal from the urban heat island. The world is an urbanized population. More than 50% of people live in urban spaces, and this is expected to increase rapidly in coming decades. If urban spaces are getting a "two for the price of one" warming rate, this will be increasingly challenging for planners, health officials, and energy managers in cities.
I end with this promise. Climate scientists really do understand the role that urbanization plays in temperature warming, and the new study also shows that they will be cognizant of urban encroachment on the observation. However, none of this changes the narrative about climate change that the planet currently faces.
