Sniff Test Regarding Urbanization Biases

In another post, I covered some of the many reasons why scientists have concluded that urbanization biases are not responsible for any significant fraction of global warming. In order to avoid too much duplication with that post, I'll only briefly summarize the reasons:

1. Homogenization Corrects Urbanization Bias. While cities are warmer than rural areas, they warm at about the same rate as rural areas. The bias is caused by urbanization. That is, as rural areas become more urban, they will warm at a faster rate than rural and urban areas. This bias is effectively removed by homogenization.
2. Very Rural Stations Warm at Least as Rapidly as All Stations. Wickham et al 2013 compared the most rural land stations globally and compared them to all land stations. The study found that "very rural" stations were warming at least as rapidly as all stations. If urbanization biases were making a significant contribution to global warming, then the most rural stations would warm more slowly than all stations. But instead, they observed, "the opposite of an urban heating effect over the period 1950 to 2010, with a slope of -0.10 ± 0.24°C/100yr (2σ error) in the Berkeley Earth global land temperature average."[1]
3. CONUS Temperatures Show No Urbanization Bias. In the United States, we can compare the all-rural USCRN network with the larger (urban and rural) homogenized nClimDiv for the years they overlap (2005-2024). For these 20 years, the two datasets are nearly indistinguishable with USCRN warming marginally more rapidly than nClimDiv. No urbanization bias can be detected in CONUS temperature data. This has been noted in the older USHCN dataset as well.[2]
4. Cities are a Small Fraction of Global Surface Area. Since urban areas make up only ~0.5% of global land surface area,[1] the urbanization bias would have to be extraordinarily large to account for a significant fraction of the warming affecting global land areas, well beyond observed increases in urban temperatures.

I'd like to focus more on this last point, since it occurs to me that we can calculate how rapidly cities must be warming if urbanization biases account for a significant fraction of global warming. To make the math simpler and to make sure uncertainty favors the claim that urbanization significantly affects global warming trends, let's double the urban land fraction to 1%. GMST has warmed at 0.203 ± 0.018°C/decade (2σ) in HadCRUT5 between 1970 and 2024. Land has warmed more rapidly than oceans, so let's also use 0.2°C/decade to be conservative. So the question is, how rapidly must the 1% of the global surface that constitutes urban areas be warming to account for a 0.2°C/decade warming rate? This is easy to calculate. Let's call the urban warming rate Tu and the rural warming rate Tr and the global land warming rate Tg. From this we can say Tg = (1Tu + 99Tr)/100 = 0.2°C/decade (1.1°C global warming since 1970). We can solve for Tu as

Tu = 100Tg - 99Tr   

So let's run a few scenarios to see what the urban warming rate must be, given the fact that rural areas cover 99% of the globe, for various rural warming rates. Let's begin by assuming rural areas are not warming at all and then progressively allow for more rural warming. I'll calculate both the needed urban warming rate (in °C/decade) and the amount of warming since 1970 (5.5 decades) that cities would have warmed if Tu occurred.

1. If Tr = 0.0°C/decade (0.0°C rural warming), then Tu = 20°C/decade (110°C urban warming).
2. If Tr = 0.1°C/decade (0.55°C rural warming), then Tu = 10.1°C/decade (55.5°C urban warming).
3. If Tr = 0.15°C/decade (0.83°C rural warming), then Tu = 5.15°C/decade (28.3°C urban warming).
4. If Tr = 0.18°C/decade (1.0°C rural warming), then Tu = 2.18°C/decade (12°C urban warming).

As you can clearly see, if we assume that Tu explains all of global warming (Tr = 0.0°C/decade), you'd have to imagine that Tavg in cities have warmed by 110°C. This is absurd. But even if we allow rural areas to warm almost as rapidly as Tg (Tr = 0.18°C/decade) we still have to imagine that cities have warmed by 12°C since 1970 while rural areas warmed by just 1°C. But this just isn't happening.

Above I show two maps. The top map is of CONUS population density, where darker areas show the most densely populated areas - that is, cities. The lower map shows the annual CONUS temperature anomalies - deviation of 2015-2024 from 1941-1970. If cities only warmed by 12°C (21.6°F) since 1970 and rural areas warmed by 1°C (1.8°F) since 1970, this map should look pretty much identical to the population density map, since urban areas would have warmed by 12x more than rural areas. But instead, you can't even pick out the cities in the temperature map. Likewise, below I show a plot of GMST anomalies from the ERA5 reanalysis. I showed the deviation of 2015-2024 from the 1941-1970 mean. You can't pick out the cities in this map either, but if cities were warming 12x faster than rural areas (as would be required if rural areas were warming at 0.18°C/decade since 1970), you absolutely would be able to find the cities just by looking at this map. Instead, what is warming the fastest is the Arctic, where there are no major cities, consistent with theoretical expectations of climate science.

Given the extremely small land area that is urban globally, these results are not surprising. This is a point made by Wickham's study as well. They note that the fact that urbanization bias has no impact on GMST trends "is not surprising given that urban areas are only 0.5% of the land area (according to the MOD500 map)."[1] There is simply no evidence that urbanization biases are contributing a significant fraction to observed global warming.

Reference:

[1] Wickham C, Rohde R, Muller RA, Wurtele J, Curry J, et al. (2013) Influence of Urban Heating on the Global Temperature Land Average using Rural Sites Identified from MODIS Classifications. Geoinfor Geostat: An Overview 1:2. doi:10.4172/2327-4581.1000104
https://www.scitechnol.com/2327-4581/2327-4581-1-104.pdf

[2] Hausfather, Z., M. J. Menne, C. N. Williams, T. Masters, R. Broberg, and D. Jones (2013), Quantifying the effect of urbanization on U.S. Historical Climatology Network temperature records, J. Geophys. Res. Atmos., 118, 481–494, doi:10.1029/2012JD018509.