Pro: Scientists Account for UHI in Their Measurements
These images from the NASA/USGS satellite Landsat show the cooling effects of plants on New York City’s heat. On the left, areas of the map that are dark green have dense vegetation. Notice how these regions match up with the dark purple regions—those with the coolest temperatures—on the right. Credit: Maps by Robert Simmon, using data from the Landsat Program. Learn more at NASA’s Climate Kids.
While urban areas are warmer than surrounding rural areas, the urban heat island effect has had little to no effect on our warming world because scientists have accounted for it in their measurements.
Urban heat islands are not a newly-discovered phenomenon. Using simple mercury thermometers, weather-watchers have noticed for some two centuries that cities tend to be warmer than surrounding rural areas.
Likewise, researchers have long noticed that the magnitude of heat islands can vary significantly between cities. However, they are able to filter out those effects from the long-term trends. Overall, the urban heat island effect has not contributed very much to our warming world. Other human activities, primarily the burning of fossil fuels, are the main culprit.https://climate.nasa.gov/faq/44/can-you-explain-the-urban-heat-island-effect/
Scientists have been very careful to ensure that UHI is not influencing the temperature trends. To address this concern, they have compared the data from remote stations (sites that are nowhere near human activity) to more urban sites. Likewise, investigators have also looked at sites across rural and urban China, which has experienced rapid growth in urbanisation over the past 30 years and is therefore very likely to show UHI. The difference between ideal rural sites compared to urban sites in temperature trends has been very small:
Annual average temperature anomalies. Jones et al (dotted green and brown) is a dataset of 42 rural and 42 urban sites. Li et al (solid green and brown) is an adjusted dataset of 42 rural and 40 urban sites. Li (blue) is a non-adjusted set of 728 stations, urban and rural. CRUTEM3v (red) is a land-only data set (Brohan et al., 2006). This plot uses the 1954–83 base period.
Con: Urban Heat Islands Distort the Temperature Record
Urban heat islands, which grow along with the size of cities, create artificial warming at many long-term temperature stations. On average, urban heat islands increase the global surface temperature trend by almost 50 percent. Surveys have found that almost 90 percent of U.S. temperature stations have been compromised by urbanization effects. In fact, almost half of the reported U.S. warming disappears when reporting only uncorrupted stations.
The majority of U.S. temperature stations utilized for NOAA and NASA temperature records have been compromised by encroachment of artificial surfaces like concrete, asphalt, buildings, and air conditioner exhausts. This creates a substantial false warming trend that is responsible for almost half of reported U.S. warming. When only pristine temperature stations are used, warming trends are quite minimal. Figures 1 and 2, below, illustrates this.
Figure 3, below, shows that temperature stations that have not been corrupted by urban heat island impacts report much less warming than temperature stations corrupted by urban heat island impacts. Yet, corrupted temperature stations are a majority of the stations used to report official U.S. temperature data.
There is strong evidence of this same sort of siting problem around the world at many other official weather stations, suggesting that the same upward bias manifests itself in the global temperature record. Because the U.S. temperature record is thought to be “the best in the world,” it follows that the global database is likely similarly compromised and unreliable.
- New study of NOAA’s U.S. Climate Network shows a lower 30-year temperature trend when high quality temperature stations unperturbed by urbanization are considered, American Geophysical Union, December 16, 2015, https://fallmeeting.agu.org/2015/press-item/new-study-of-noaas-u-s-climate-network-shows-a-lower-30-year-temperature-trend-when-high-quality-temperature-stations-unperturbed-by-urbanization-are-considered/
- Is the US Surface Temperature Record Reliable?, https://www.heartland.org/_template-assets/documents/publications/SurfaceStations.pdf
- Ronald D. Leeper, John Kochendorfer, Timothy Henderson, and Michael A. Palecki, “Impacts of Small-Scale Urban Encroachment on Air Temperature Observations,” Journal of the American Meteorological Society, June 12, 2019, https://journals.ametsoc.org/doi/10.1175/JAMC-D-19-0002.1