We have already examined Milankovitch Cycles, solar cycles, ocean cycles, volcanism, and cosmic rays (TBA), so now it is time to examine something that is in my opinion, the ONLY human impact on climate change; urbanization and land use changes, which likely account for somewhere between 5% and 10% of the warming that took place between 1970 and 2016.
Most people understand that cities, especially large ones are warmer than surrounding rural areas. You may notice it by commuting to work from the countryside to the city, where temperatures are noticeably warmer, or you may see it on TV weather graphics. A good example of this is Northern Virginia versus Washington D.C. Washington is clearly warmer than surrounding areas most of the time (Figure 1).
The warmer temperatures in cities are most noticeable on nights with clear skies and light winds, because these conditions allow more radiational cooling in the surrounding areas, whereas cities have excess heat that gets built up during the day, causing higher nighttime temperatures. When skies are cloudy with stronger winds, the effects are less noticeable due to mixing and cooling of the urban area due to the strong winds. This urban heating is referred to as the urban heat island effect in meteorology.
Due to such an effect, we tend to see cooler temperatures in rural areas and urban parks, rather than downtown, commercial, and residential areas, where we see temperatures as much as ten degrees warmer (Figure 2).
Urbanization simply refers to the buildup and growth of cities over time; it is a result of land use changes. Areas that were once forest, are now farmland, and areas that were once farmland are now shopping centers, etc. etc. Human activities (not including CO2, because we have concluded CO2 has little or no effect) have been changing the landscape around us for centuries, although we haven’t really noticed the effects until the past few decades.
The first noticeable effects of a moderated climate was recorded in the late 1700s in colonial America. Thomas Jefferson and Noah Webster had a big argument over it. Thomas Jefferson was sure that the climate in the States had gotten warmer since he was a child; the winters were warmer, snows were less deep and not as long-lasting, and it seemed to have some human cause. Noah Webster’s side of the argument was different. He stated that Jefferson was seeing something similar of an urban heat island effect due to changes in land use. He stated that when Jefferson was a child, many areas that were then forest, has been flattened into farmland. Jefferson admitted he was wrong and they moved on with life.
Indeed, Webster was right. Areas that are forest tend to see cooler temperatures due to lack of sunlight and deeper snowfall, thus it stays on the ground longer. When the forest is cleared, temperatures of course rise.
The urban heat island effect has been documented in many studies about surface air temperatures over the past 50 years or so. One of them, Critchfield (1983) examined the annual mean surface air temperatures in Paris, France, and its suburbs, which clearly shows an urban heat island (Figure 3).
The same report also examined temperature differences in Vienna versus its surroundings, which once again show a strong urban heat island effect (Figure 4).
Satellite measurements have also shown this warming effect in urban areas. Figure 5 below from a remote sensing satellite shows how Atlanta, Georgia is a massive urban heat island. These measurements record radiant emissions, which is energy reflected from the ground, or surfaces, such as roofs, pavement, vegetation, grass, and water.
Furthermore, the urban warming is seen from cloud-free satellite images, where the darker grey areas represent warm areas (urban), while the lighter grey areas represent rural areas (Figure 6).
WHY ARE CITIES WARMER THAN RURAL AREAS?
There are many reasons why cities are warmer than rural areas. I will go into more detail, but to put it short and sweet, the main reason is that there is a difference between the energy gains and losses between cities and rural areas.
For one, during the daytime hours, the solar energy that get absorbed near the ground evaporates from the vegetation and soil. This means that there is an equal gain and loss of energy through solar radiative heating and evaporative cooling. In urban areas, less vegetation results in more solar radiation gain due to the buildings, streets, and sidewalks being good absorbents of the energy. During the nighttime hours, the lack of vegetation results in higher nighttime temperatures.
For two, another contributing factor would be that of waste heat that is generated from buildings, cars, and trains. The heat from these objects will likely make its way into the atmosphere, causing up to a third of contribution that solar input has alone.
The other major factor of the urban heat island effect is that of the thermal properties of buildings and structures in a city, which add heat energy to the air through conduction. Roads, sidewalks, and buildings are larger conductors of heat energy than grass and vegetation, because they have a higher specific heat than paved surfaces, meaning that it takes more energy to heat those surfaces than paved ones. In addition, tall buildings within cities are generally lined up side by side, creating a “canyon,” which amplifies the warming seen near the surface.
WHAT IMPACTS DOES URBANIZATION HAVE ON CLIMATE?
Urbanization itself doesn’t directly influence climate. Because of the building material, and the way roads and sidewalks are paved, cities clearly have warmer temperatures than surrounding areas. Much of the global and local temperature data that the government uses comes from cities with populations over 100,000, which is where we get a lot of the warming from in recent decades.
Air temperatures are measured at about 1.5 to 2 meters above the ground, which is where the “2m air temperature” phrase comes from. Overnight temperatures have risen about 50% faster than daytime highs in the United States, and globally because of the “asymmetric” warming between maximum and minimum temperatures (Vose et al., 2005).
Meteorologist Roger Pielke Sr. has shown that minimum temperatures are extremely sensitive to the height of the actual temperature measurement and wind speed. Furthermore, nighttime temperatures are even more sensitive to land surface type (Carlson, 1986, and McNider et al., 2005) and longwave radiation from greenhouse gas forcing, such as water vapor (most importantly), and clouds (Eastman et al., 2001, and Pielke and Matsui, 2005).
There has been debate as to whether or not station data in urban areas should be adjusted to account for the urban heat island effect, station location, and/or station moves. These adjustments are meant to account for temperature trends for the location had no farms or houses been built. Urban areas are considered to be towns or cities with a population of over 10,000 people. Locations that house less than 10,000 people are considered to be rural, thus no need for adjustment. However, research has found that populations of cities or towns with at least 10,000 people have an urban heating of about 2.2°C (4°F).
A simple mathematical formula has been developed to show how much an urban heating is for any specific town or city based upon population. UHI = 0.73log₁₀(pop), where “pop” means population.
For instance, a town with a population of 10 has a warm bias of 0.73°C (1°F) (probably not, but this is theoretical), a town with a population of 100 has a bias of 1.46°C (2.6°F), while a town with a population of 1,000 has a warm bias of 2.2°C (4°F), and a large city with over a million people has a heating effect of 4.4°C (8°F).
Despite solar forcing accounting for about 35% of the warming between 1970 and 2016, ocean cycles accounting for 40%, and lack of volcanism for about 5%, increased land use change and continuous urbanization has likely had about 20% effect on the warming over the past 40+ years, up 15% from 1900 to 1970, mainly because most of our data comes from cities, unless you are like me and use GHCN station data, which has all the urban stations, plus all the rural ones. Many urban stations are poorly sited and have seen increased bias over the years, as meteorologist Anthony Watts has examined. I will talk about that in another section.
It isn’t that our climate has warmed that much; it is weather station siting and land development that has had effect, which seems to amplify the natural warming that we have seen since 1977. In the next section, we examine that carbon dioxide has little or no effect on the climate system, which is why I have concluded that the ONLY human impact on climate change is through land use changes and urbanization.
If there needs to be any sort of climate policy, the Paris Accord is not one of them. I personally feel the only solution to temperature bias is to stop building a McDonald’s or Walmart on every street corner and to stop building so many houses. There are simple solutions to simple problems folks.
Ack. “Urban Heat Island.” WXWISE Urban Heat Islands, WXWISE, cimss.ssec.wisc.edu/wxwise/heatisl.html.
D’Aleo, Joseph. “URBAN HEAT ISLAND .” ICECAP, ICECAP, icecap.us/images/uploads/URBAN_HEAT_ISLAND.pdf.
“Urban Heat Island Effect.” Southwest Urban Hydrology, Southwest Urban Hydrology, http://www.southwesturbanhydrology.com/urbanization-concerns/urban-heat-island-effect/.
“Weather Maps.” WJLA, Sinclair Broadcast Group, wjla.com/weather/maps.