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Temperature (Urbanization Effects - North America) -- Summary
How significant are anthropogenic-induced temperature increases that are not caused by greenhouse gas emissions? In what follows, we briefly review what has been learned about this important subject from pertinent studies conducted throughout North America.

In studying the non-greenhouse-gas-induced urban heat island (UHI) of Houston, Texas, Streutker (2003) analyzed 82 sets of nighttime radiation data obtained from the split-window infrared channels of the Advanced Very High Resolution Radiometer on board the NOAA-9 satellite during March 1985 through February 1987 and from 125 sets of similar data obtained from the NOAA-14 satellite during July 1999 through June 2001. Between these two periods, it was found that the mean nighttime surface temperature of Houston rose by 0.82 0.10 C. In addition, Streutker notes that the growth of the Houston UHI, both in magnitude and spatial extent, "scales roughly with the increase in population," and that the mean rural temperature measured during the second interval was "virtually identical to the earlier interval."

This informative study demonstrates that the UHI phenomenon can sometimes be very powerful, for in just twelve years the UHI of Houston grew by more than the IPCC contends the mean surface air temperature of the planet rose over the entire past century, during which period earth's population rose by approximately 280%, or nearly an order of magnitude more than the 30% population growth experienced by Houston over the twelve years of Streutker's study.

A very different type of study was conducted by Maul and Davis (2001), who analyzed air and seawater temperature data obtained over the past century at the sites of several primary tide gauges maintained by the U.S. Coast and Geodetic Survey. Noting that each of these sites "experienced significant population growth in the last 100 years," and that "with the increase in maritime traffic and discharge of wastewater one would expect water temperatures to rise" (due to a maritime analogue of the urban heat island effect), they calculated trends for the 14 longest records and derived a mean century-long seawater warming of 0.74C, with Boston registering a 100-year warming of 3.6C. In addition, they report that air temperature trends at the tide gauge sites, which represent the standard urban heat island effect, were "much larger" than the seawater temperature trends.

In another different type of study, Dow and DeWalle (2000) analyzed trends in annual evaporation and Bowen ratio measurements on 51 eastern U.S. watersheds that had experienced various degrees of urbanization between 1920 and 1990. In doing so, they determined that as residential development progressively occurred on what originally were rural watersheds, watershed evaporation decreased and sensible heating of the atmosphere increased. And from relationships derived from the suite of watersheds investigated, they calculated that complete transformation from 100% rural to 100% urban characteristics resulted in a 31% decrease in watershed evaporation and a 13 W/m2 increase in sensible heating of the atmosphere.

Now climate modeling exercises suggest that a doubling of the air's CO2 concentration will result in a nominal 4 W/m2 increase in the radiative forcing of earth's surface-troposphere system, which has often been predicted to produce an approximate 4C increase in the mean near-surface air temperature of the globe, indicative of an order-of-magnitude climate sensitivity of 1C per W/m2 change in radiative forcing. Thus, to a first approximation, the 13 W/m2 increase in the sensible heating of the near-surface atmosphere produced by the total urbanization of a pristine rural watershed in the eastern United States could be expected to produce an increase of about 13C in near-surface air temperature over the central portion of the watershed, which is consistent with maximum urban heat island effects observed in large and densely populated cities. Hence, a 10% rural-to-urban transformation could well produce a warming on the order of 1.3C, and a mere 2% transformation could increase the near-surface air temperature by as much as a quarter of a degree Centigrade.

This powerful anthropogenic but non-greenhouse-gas-induced effect of urbanization on the energy balance of watersheds and the temperature of the boundary-layer air above them begins to express itself with the very first hint of urbanization and, hence, may be readily overlooked in studies seeking to identify a greenhouse-gas-induced global warming signal. In fact, the fledgling urban heat island effect may already be present in many temperature records that have routinely been considered "rural enough" to be devoid of all human influence, when, in fact, such may be far from the truth.

A case in point is provided by the study of Changnon (1999), who used a series of measurements of soil temperatures obtained in a totally rural setting in central Illinois between 1889 and 1952 and a contemporary set of air temperature measurements made in an adjacent growing community (as well as similar data obtained from other nearby small towns), to evaluate the magnitude of unsuspected heat island effects that may be present in small towns and cities that are typically assumed to be free of urban-induced warming. This work revealed that soil temperature in the totally rural setting experienced an increase from the decade of 1901-1910 to the decade of 1941-1950 that amounted to 0.4C.

This warming is 0.2C less than the 0.6C warming determined for the same time period from the entire dataset of the U.S. Historical Climatology Network, which is supposedly corrected for urban heating effects. It is also 0.2C less than the 0.6C warming determined for this time period by eleven benchmark stations in Illinois with the highest quality long-term temperature data, all of which are located in communities that had populations of less than 6,000 people as of 1990. And it is 0.17C less than the 0.57C warming derived from data obtained at the three benchmark stations closest to the site of the soil temperature measurements and with populations of less than 2,000 people.

Changnon says his findings suggest that "both sets of surface air temperature data for Illinois believed to have the best data quality with little or no urban effects may contain urban influences causing increases of 0.2C from 1901 to 1950." He further notes -- in a grand understatement -- that "this could be significant because the IPCC (1995) indicated that the global mean temperature increased 0.3C from 1890 to 1950."

Clearly, the meticulous efforts of this world-renowned climate specialist call all near-surface global air temperature histories into question. Therefore, until the challenge of very-small-town urban heat island effects is resolved, the so-called "unprecedented" global warming of the past century, and especially the past quarter-century, cannot be accepted at face value. In all likelihood, the latter two warmings are artificially inflated.

Moving on, DeGaetano and Allen (2002b) used data from the U.S. Historical Climatology Network to calculate trends in the occurrence of maximum and minimum temperatures greater than the 90th, 95th, and 99th percentile across the United States over the period 1960-1996. In the case of daily warm minimum temperatures, the slope of the regression line fit to the data of a plot of the annual number of 95th percentile exceedences vs. year was found to be 0.09 exceedences per year for rural stations, 0.16 for suburban stations, and 0.26 for urban stations, making the rate of increase in extreme warm minimum temperatures at urban stations nearly three times greater than the rate of increase at rural stations less affected by growing urban heat islands. Likewise, the rate of increase in the annual number of daily maximum temperature 95th percentile exceedences per year over the same time period was found to be 50% greater at urban stations than it was at rural stations.

Working on the Arctic Coastal Plain near the Chuckchi Sea at Barrow, Alaska -- which is described by Hinkel et al. (2003) as "the northernmost settlement in the USA and the largest native community in the Arctic," the population of which "has grown from about 300 residents in 1900 to more than 4600 in 2000" -- the four researchers installed 54 temperature-recording instruments in mid-June of 2001, half of them within the urban area and the other half distributed across approximately 150 km2 of surrounding land, all of which provided air temperature data at hourly intervals approximately two meters above the surface of the ground. In this paper, they describe the results they obtained for the following winter. Based on urban-rural spatial averages for the entire winter period (December 2001-March 2002), they determined the urban area to be 2.2C warmer than the rural area. During this period, the mean daily urban-rural temperature difference increased with decreasing temperature, "reaching a peak value of around 6C in January-February." It was also determined that the daily urban-rural temperature difference increased with decreasing wind speed, such that under calm conditions (< 2 m s-1) the daily urban-rural temperature difference was 3.2C in the winter. Last of all, under simultaneous calm and cold conditions, the urban-rural temperature difference was observed to achieve hourly magnitudes exceeding 9C.

Four years later, Hinkel and Nelson (2007) reported that for the period 1 December to 31 March of four consecutive winters, the spatially-averaged temperature of the urban area of Barrow was about 2C warmer than that of the rural area, and that it was not uncommon for the daily magnitude of the urban heat island to exceed 4C. In fact, they say that on some days the magnitude of the urban heat island exceeded 6C, and that values in excess of 8C were sometimes recorded, while noting that the warmest individual site temperatures were "consistently observed in the urban core area."

These results indicate just how difficult it is to measure a background global temperature increase that is believed to have been less than 1C over the past century (representing a warming of less than 0.1C per decade), when the presence of a mere 4500 people can create a winter heat island that may be two orders of magnitude greater than the signal being sought. Clearly, there is no way that temperature measurements made within the range of influence of even a small village can be adjusted to the degree of accuracy that is required to reveal the true magnitude of the pristine rural temperature change.

Moving south, we find Ziska et al. (2004) working within and around Baltimore, Maryland, where they characterized the gradual changes that occur in a number of environmental variables as one moves from a rural location (a farm approximately 50 km from the city center) to a suburban location (a park approximately 10 km from the city center) to an urban location (the Baltimore Science Center approximately 0.5 km from the city center). At each of these locations, four 2 x 2 m plots were excavated to a depth of about 1.1 m, after which they were filled with identical soils, the top layers of which contained seeds of naturally-occurring plants of the area. These seeds sprouted in the spring of the year, and the plants they produced were allowed to grow until they senesced in the fall, after which all of them were cut at ground level, removed, dried and weighed.

Ziska et al. report that along the rural-to-suburban-to-urban transect, the only consistent differences in the environmental variables they measured were a rural-to-urban increase of 21% in average daytime atmospheric CO2 concentration and increases of 1.6 and 3.3C in maximum (daytime) and minimum (nighttime) daily temperatures, respectively, which changes, in their words, are "consistent with most short-term (~50 year) global change scenarios regarding CO2 concentration and air temperature." In addition, they determined that "productivity, determined as final above-ground biomass, and maximum plant height were positively affected by daytime and soil temperatures as well as enhanced CO2, increasing 60 and 115% for the suburban and urban sites, respectively, relative to the rural site."

The three researchers say their results suggest that "urban environments may act as a reasonable surrogate for investigating future climatic change in vegetative communities," and those results indicate that the twin evils of the radical environmentalist movement (rising air temperatures and CO2 concentrations) tend to produce dramatic increases in the productivity of the natural ecosystems typical of the greater Baltimore area and, by inference, probably those of many other areas as well.

Three years later, George et al. (2007) reported on five years of work at the same three transect locations, stating that "atmospheric CO2 was consistently and significantly increased on average by 66 ppm from the rural to the urban site over the five years of the study," and that "air temperature was also consistently and significantly higher at the urban site (14.8C) compared to the suburban (13.6C) and rural (12.7C) sites." And they again noted that the increases in atmospheric CO2 and air temperature they observed "are similar to changes predicted in the short term with global climate change, therefore providing an environment suitable for studying future effects of climate change on terrestrial ecosystems," specifically noting that "urban areas are currently experiencing elevated atmospheric CO2 and temperature levels that can significantly affect plant growth compared to rural areas." Consequently, for the 80% of the U.S. population that reside in urban areas, the future (environmentally speaking) is now, and it's good for earth's plants.

Working further south still, LaDochy et al. (2007) report that "when speculating on how global warming would impact the state [of California], climate change models and assessments often assume that the influence would be uniform (Hansen et al., 1998; Hayhoe et al., 2004; Leung et al., 2004)." Feeling a need to assess the validity of this assumption, they calculated temperature trends over the 50-year period 1950-2000 to explore the extent of warming in various sub-regions of the state, after which they attempted to evaluate the influence of human-induced changes to the landscape on the observed temperature trends, and determine their significance compared to those caused by changes in atmospheric composition, such as the air's CO2 concentration.

In pursuing this protocol, the three researchers found that "most regions showed a stronger increase in minimum temperatures than with mean and maximum temperatures," and that "areas of intensive urbanization showed the largest positive trends, while rural, non-agricultural regions showed the least warming." In fact, they report that the Northeast Interior Basins of the state actually experienced cooling. Large urban sites, on the other hand, exhibited rates of warming "over twice those for the state, for the mean maximum temperature, and over five times the state's mean rate for the minimum temperature." Consequently, they concluded that "if we assume that global warming affects all regions of the state, then the small increases seen in rural stations can be an estimate of this general warming pattern over land," which implies that "larger increases," such as those they observed in areas of intensive urbanization, "must then be due to local or regional surface changes."

Noting that "breezy cities on small tropical islands ... may not be exempt from the same local climate change effects and urban heat island effects seen in large continental cities," Gonzalez et al. (2005) describe the results of their research into this topic, which they conducted in and about San Juan, Puerto Rico. In this particular study, a NASA Learjet -- carrying the Airborne Thermal and Land Applications Sensor (ATLAS) that operates in visual and infrared wavebands -- flew several flight lines, both day and night, over the San Juan metropolitan area, the El Yunque National Forest east of San Juan, plus other nearby areas, obtaining surface temperatures, while strategically-placed ground instruments recorded local air temperatures. This work revealed that surface temperature differences between urbanized areas and limited vegetated areas were higher than 30C during daytime, creating an urban heat island with "the peak of the high temperature dome exactly over the commercial area of downtown," where noontime air temperatures were as much a 3C greater than those of surrounding rural areas. In addition, the eleven researchers report that "a recent climatological analysis of the surface [air] temperature of the city has revealed that the local temperature has been increasing over the neighboring vegetated areas at a rate of 0.06C per year for the past 30 years."

In discussing their findings, Gonzalez et al. state that "the urban heat island dominates the sea breeze effects in downtown areas," and they say that "trends similar to those reported in [their] article may be expected in the future as coastal cities become more populated." Indeed, it is probable that this phenomenon has long been operative in coastal cities around the world, helping to erroneously inflate the surface air temperature record of the planet and contributing to the infamous hockeystick representation of this parameter that has been so highly touted by the Intergovernmental Panel on Climate Change.

One year later, Velazquez-Lozada et al. (2006) evaluated the thermal impacts of historical land cover and land use (LCLU) changes in San Juan, Puerto Rico over the last four decades of the 20th century via an analysis of air temperatures measured at a height of approximately two meters above ground level within four different LCLU types (urban-coastal, rural-inland, rural-coastal and urban-inland), after which they estimated what the strength of the urban heat island might be in the year 2050, based on anticipated LCLU changes and a model predicated upon their data of the past 40 years. In doing so, their work revealed "the existence of an urban heat island in the tropical coastal city of San Juan, Puerto Rico that has been increasing at a rate of 0.06C per year for the last 40 years." In addition, they report that predicted LCLU changes between now and 2050 will lead to an urban heat island effect "as high as 8C for the year 2050."

Last of all, and noting that a mass population migration from rural Mexico into medium- and large-sized cities took place throughout the second half of the 20th century, Juregui (2005) examined the effect of this rapid urbanization on city air temperatures, analyzing the 1950-1990 minimum air temperature series of seven large cities with populations in excess of a million people and seven medium-sized cities with populations ranging from 125,000 to 700,000 people. This work indicated that temperature trends were positive at all locations, ranging from 0.02C per decade to 0.74C per decade. Grouped by population, the average trend for the seven large cities was 0.57C per decade, while the average trend for the seven mid-sized cities was 0.37C per decade.

In discussing these results, Juregui says they "suggest that the accelerated urbanization process in recent decades may have substantially contributed to the warming of the urban air observed in large cities in Mexico." Once again, therefore, we are reminded of the huge magnitude of the urban heat island effect compared to the global warming of the past century, as well as the urban heat island's dependence upon the nature of the urban landscape. And this fact further suggests to us that it is next to impossible to adequately adjust surface air temperature measurements made within an urban area to the degree of accuracy required to correctly quantify background or rural climate change, which may well be an order of magnitude or two smaller than the perturbing effect of the city.

In conclusion, the results of these several North American studies demonstrate that the impact of population growth on the urban heat island effect is very real and can be very large, vastly overshadowing the effects of natural temperature change. In addition, over three decades ago Oke (1973) demonstrated (as has also been demonstrated by several of the studies reviewed above) that towns with as few as a thousand inhabitants typically create a warming of the air within them that is more than twice as great as the increase in mean global air temperature believed to have occurred since the end of the Little Ice Age, while the urban heat islands of the great metropolises of the world create warmings that rival those that occur between full-fledged ice ages and interglacials!

So what have we learned about the urban heat island effect from data obtained in North America? We've learned that it is large and growing in large-and-growing cities, as well as in small towns. Given these undeniable facts, it is presumptuous in the extreme to believe that the global surface air temperature record of the last few decades has been adequately adjusted for small-town and large-city heat island effects; and we can thus be fairly confident that the true warming of the planet has likely been far less than what has been claimed by essentially all assessments of the phenomenon conducted to date.

Changnon, S.A. 1999. A rare long record of deep soil temperatures defines temporal temperature changes and an urban heat island. Climatic Change 42: 531-538.

DeGaetano, A.T. and Allen, R.J. 2002. Trends in twentieth-century temperature extremes across the United States. Journal of Climate 15: 3188-3205.

Dow, C.L. and DeWalle, D.R. 2000. Trends in evaporation and Bowen ratio on urbanizing watersheds in eastern United States. Water Resources Research 36: 1835-1843.

George, K., Ziska, L.H., Bunce, J.A. and Quebedeaux, B. 2007. Elevated atmospheric CO2 concentration and temperature across an urban-rural transect. Atmospheric Environment 41: 7654-7665.

Gonzalez, J.E., Luvall, J.C., Rickman, D., Comarazamy, D., Picon, A., Harmsen, E., Parsiani, H., Vasquez, R.E., Ramirez, N., Williams, R. and Waide, R.W. 2005. Urban heat islands developing in coastal tropical cities. EOS, Transactions, American Geophysical Union 86: 397,403.

Hansen, J., Sato, M., Glascoe, J. and Ruedy, R. 1998. A commonsense climatic index: Is climate change noticeable? Proceedings of the National Academy of Sciences USA 95: 4113-4120.

Hayhoe, K., Cayan, D., Field, C.B., Frumhoff, P.C. et al. 2004. Emissions, pathways, climate change, and impacts on California. Proceedings of the National Academy of Sciences USA 101: 12,422-12,427.

Hinkel, K.M. and Nelson, F.E. 2007. Anthropogenic heat island at Barrow, Alaska, during winter: 2001-2005. Journal of Geophysical Research 112: 10.1029/2006JD007837.

Hinkel, K.M., Nelson, F.E., Klene, A.E. and Bell, J.H. 2003. The urban heat island in winter at Barrow, Alaska. International Journal of Climatology 23: 1889-1905.

Intergovernmental Panel on Climate Change. 1995. Climate Change 1995, The Science of Climate Change. Cambridge University Press, Cambridge, U.K.

Juregui, E. 2005. Possible impact of urbanization on the thermal climate of some large cities in Mexico. Atmosfera 18: 249-252.

LaDochy, S., Medina, R. and Patzert, W. 2007. Recent California climate variability: spatial and temporal patterns in temperature trends. Climate Research 33: 159-169.

Leung, L.R., Qian, Y., Bian, X., Washington, W.M., Han, J. and Roads, J.O. 2004. Mid-century ensemble regional climate change scenarios for the western United States. Climatic Change 62: 75-113.

Maul, G.A. and Davis, A.M. 2001. Seawater temperature trends at USA tide gauge sites. Geophysical Research Letters 28: 3935-3937.

Oke, T.R. 1973. City size and the urban heat island. Atmospheric Environment 7: 769-779.

Streutker, D.R. 2003. Satellite-measured growth of the urban heat island of Houston, Texas. Remote Sensing of Environment 85: 282-289.

Velazquez-Lozada, A., Gonzalez, J.E. and Winter, A. 2006. Urban heat island effect analysis for San Juan, Puerto Rico. Atmospheric Environment 40: 1731-1741.

Ziska, L.H., Bunce, J.A. and Goins, E.W. 2004. Characterization of an urban-rural CO2/temperature gradient and associated changes in initial plant productivity during secondary succession. Oecologia 139: 454-458.

Last updated 21 January 2009