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.

An unusual and 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 0.74°C, with Boston registering a 100-year warming of 3.6°C.  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 unusual and different type of study, Dow and DeWalle (2000) studied trends in annual evaporation and Bowen ratio measurements on 51 eastern United States watersheds that had experienced various degrees of urbanization between 1920 and 1990.  They found that as residential development progressively occurred on what originally were rural watersheds, watershed evaporation decreased and sensible heating of the atmosphere increased.  From relationships derived from the suite of watersheds investigated, it was 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 4°C increase in the mean near-surface air temperature of the globe, indicative of an order-of-magnitude climate sensitivity of 1°C 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 13°C in near-surface air temperature over the central portion of the watershed, which is consistent with 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 1.3°C, 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 is far from true.

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 series 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.  He determined that the soil temperatures measured in the totally rural setting revealed the existence of a temperature increase from the decade of 1901-1910 to that of 1941-1950 that amounted to 0.4°C.  This warming is 0.2°C less than the 0.6°C warming determined for the same time period from data of the U.S. Historical Climatology Network, which is supposedly corrected for urban heating effects.  It is also 0.2°C less than the 0.6°C 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 with populations of less than 6,000 people as of 1990.  And it is 0.17°C less than the 0.57°C warming derived from data obtained from 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 0.2°C 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 0.3°C from 1890 to 1950."  Clearly, the meticulous efforts of this world-renowned climate specialist call all surface-based global air temperature records 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, they are significantly artificially inflated.

In one final study from North America, DeGaetano and Allen (2002) 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.

In conclusion, we note that 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 was also found by the last three 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!

Given these undeniable facts, it is presumptuous in the extreme to believe that the global surface air temperature record of the last two decades of the 20th century -- when world population rose over 35% -- has been adequately adjusted for tiny-town and large-city heat island effects.  We can probably safely assume, however, that the true warming was significantly less than has been claimed by essentially all assessments of the phenomenon conducted to date.

References
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.

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

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.