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Temperature (Urbanization Effects - Asia) -- Summary
The warming of near-surface air over non-urban areas of the planet during the past one to two centuries is believed to have been less than 1C.  Warming in many growing cities, on the other hand, may have been a full order of magnitude greater.  Thus, since nearly all near-surface air temperature records of this period have been obtained from sensors located in population centers that have experienced significant growth, it is absolutely essential that urbanization-induced warming be removed from all original temperature records when attempting to accurately assess what has truly happened in the natural non-urban environment.  In this Summary we briefly review some of the papers that lead to this conclusion based on studies carried out in different parts of Asia.

The first study we cite in this regard only provides a hint of an urbanization effect.  In it, Hasanean (2001) investigated surface air temperature trends with data obtained from meteorological stations located in eight Eastern Mediterranean cities: Malta, Athens, Tripoli, Alexandria, Amman, Beirut, Jerusalem and Latakia.  The period of analysis varied from station to station according to available data, with Malta having the longest temperature record (1853-1991) and Latakia the shortest (1952-1991).  Four of the cities exhibited overall warming trends and four of them cooling trends.  In addition, there was an important warming around 1910 that began nearly simultaneously at all of the longer-record stations, as well as a second warming in the 1970s; but Hasanean reports that the latter warming was "not uniform, continuous or of the same order" as the warming that began about 1910, nor was it evident at all of the stations.  One interpretation of this non-uniformity of temperature behavior in the 1970s is that it may have been the result of temporal differences in city urbanization histories that were accentuated about that time, which could have resulted in significantly different urban heat island trajectories at the several sites over the latter portions of their records.

In a more direct study of the urban heat island effect that was conducted in South Korea, Choi et al. (2003) compared the mean station temperatures of three groupings of cities (one comprised of four large urban stations with a mean 1995 population of 4,830,000, one comprised of six smaller urban stations with a mean 1995 population of 548,000, and one comprised of six "rural" stations with a mean 1995 population of 214,000) over the period 1968-1999.  This analysis revealed, in their words, that the "temperatures of large urban stations exhibit higher urban bias than those of smaller urban stations and that the magnitude of urban bias has increased since the late 1980s."  Specifically, they note that "estimates of the annual mean magnitude of urban bias range from 0.35C for smaller urban stations to 0.50C for large urban stations."  In addition, they indicate that "none of the rural stations used for this study can represent a true non-urbanized environment."  Hence, they correctly conclude that their results are underestimates of the true urban effect, and that "urban growth biases are very serious in South Korea and must be taken into account when assessing the reliability of temperature trends."

In a second study conducted in South Korea, Chung et al. (2004a) report there was an "overlapping of the rapid urbanization-industrialization period with the global warming era," and that the background climatic trends from urbanized areas might therefore be contaminated by a growing urban heat island effect.  To investigate this possibility, they say that "monthly averages of daily minimum, maximum, and mean temperature at 14 synoptic stations were prepared for 1951-1980 (past normal) and 1971-2000 (current normal) periods," after which "regression equations were used to determine potential effects of urbanization and to extract the net contribution of regional climate change to the apparent temperature change."  Twelve of these stations were growing urban sites of various size, while two (where populations actually decreased) were rural, one being located inland and one on a remote island.

In terms of change over the 20 years that separated the two normal periods, Chung et al. report that in Seoul, where population increase was greatest, annual mean daily minimum temperature increased by 0.7C, while a mere 0.1C increase was detected at one of the two rural sites and a 0.1C decrease was detected at the other, for no net change in their aggregate mean value.  In the case of annual mean daily maximum temperature, on the other hand, a 0.4C increase was observed at Seoul and a 0.3C increase was observed at the two rural sites.  Hence, the change in the annual mean daily mean temperature was an increase of 0.15C at the two rural sites (indicative of regional background warming of 0.075C per decade), while the change of annual mean daily mean temperature at Seoul was an increase of 0.55C, or 0.275C per decade (indicative of an urban-induced warming of 0.2C per decade in addition to the regional background warming of 0.075C per decade).  Also, corresponding results for urban areas of intermediate size defined a linear relationship that connected these two extreme results when plotted against the logarithm of population increase over the two-decade period.

In light of the significantly intensifying urban heat island effect detected in their study, Chung et al. say it is "necessary to subtract the computed urbanization effect from the observed data at urban stations in order to prepare an intended nationwide climatic atlas," noting that "rural climatological normals should be used instead of the conventional normals to simulate ecosystem responses to climatic change, because the urban area is still much smaller than natural and agricultural ecosystems in Korea."

Yet a third study of South Korea was conducted by Chung et al. (2004b), who evaluated temperature changes at ten urban and rural Korean stations over the period 1974-2002.  As a result of this exercise, they found that "the annual temperature increase in large urban areas was higher than that observed at rural and marine stations."  Specifically, they note that "during the last 29 years, the increase in annual mean temperature was 1.5C for Seoul and 0.6C for the rural and seashore stations," while increases in mean January temperatures ranged from 0.8 to 2.4C for the ten stations.  In addition, they state that "rapid industrialization of the Korean Peninsula occurred during the late 1970s and late 1980s," and that when plotted on a map, "the remarkable industrialization and expansion ... correlate with the distribution of increases in temperature."  Consequently, as in the study of Chung et al. (2004a), Chung et al. (2004b) found that over the past several decades, much (and in many cases most) of the warming experienced in the urban areas of Korea was the result of local urban influences that were not indicative of regional background warming.

Shifting attention to China, Weng (2001) evaluated the effect of land cover changes on surface temperatures of the Zhujiang Delta (an area of slightly more than 17,000 km2) via a series of analyses of remotely-sensed Landsat Thematic Mapper data.  They found that between 1989 and 1997, the area of land devoted to agriculture declined by nearly 50%, while urban land area increased by close to the same percentage.  Then, upon normalizing the surface radiant temperature for the years 1989 and 1997, they used image differencing to produce a radiant temperature change image that they overlaid with images of urban expansion.  The results indicated, in Weng's words, that "urban development between 1989 and 1997 has given rise to an average increase of 13.01C in surface radiant temperature."

In Shanghai, Chen et al. (2003) evaluated several characteristics of that city's urban heat island, including its likely cause, based on analyses of monthly meteorological data from 1961 to 1997 at 16 stations in and around this hub of economic activity that is one of the most flourishing urban areas in all of China.  Defining the urban heat island of Shanghai as the mean annual air temperature difference between urban Longhua and suburban Songjiang, Chen et al. found that its strength increased in essentially linear fashion from 1977 to 1997 by 1C.

Commenting on this finding, Chen et al. say "the main factor causing the intensity of the heat island in Shanghai is associated with the increasing energy consumption due to economic development," noting that in 1995 the Environment Research Center of Peking University determined that the annual heating intensity due to energy consumption by human activities was approximately 25 Wm-2 in the urban area of Shanghai but only 0.5 Wm-2 in its suburbs.  In addition, they point out that the 0.5C/decade intensification of Shanghai's urban heat island is an order of magnitude greater than the 0.05C/decade global warming of the earth over the past century, which is indicative of the fact that ongoing intensification of even strong urban heat islands cannot be discounted.

Simultaneously, Kalnay and Cai (2003) used differences between trends in directly observed surface air temperature and trends determined from the NCEP-NCAR 50-year Reanalysis (NNR) project (based on atmospheric vertical soundings derived from satellites and balloons) to estimate the impact of land-use changes on surface warming.  Over undisturbed rural areas of the United States, they found that the surface- and reanalysis-derived air temperature data yielded essentially identical trends, implying that differences between the two approaches over urban areas would represent urban heat island effects.  Consequently, Zhou et al. (2004) applied the same technique over southeast China, using an improved version of reanalysis that includes newer physics, observed soil moisture forcing, and a more accurate characterization of clouds.

For the period January 1979 to December 1998, the eight scientists involved in the work derived an "estimated warming of mean surface [air] temperature of 0.05C per decade attributable to urbanization," which they say "is much larger than previous estimates for other periods and locations, including the estimate of 0.027C for the continental U.S. (Kalnay and Cai, 2003)."  They note, however, that because their analysis "is from the winter season over a period of rapid urbanization and for a country with a much higher population density, we expect our results to give higher values than those estimated in other locations and over longer periods."

In a similar study, Frauenfeld et al. (2005) used daily surface air temperature measurements from 161 stations located throughout the Tibetan Plateau (TP) to calculate the region's mean annual temperature for each year from 1958 through 2000, while in the second approach they used 2-meter temperatures from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-40), which temperatures, in their words, "are derived from rawinsonde profiles, satellite retrievals, aircraft reports, and other sources including some surface observations."  This approach, according to them, results in "more temporally homogeneous fields" that provide "a better assessment of large-scale temperature variability across the plateau."

So what did they find?

Frauenfeld et al. report that over the period 1958-2000, "time series based on aggregating all station data on the TP show a statistically significant positive trend of 0.16C per decade," as has also been reported by Liu and Chen (2000).  However, they say that "no trends are evident in the ERA-40 data for the plateau as a whole."

In discussing this discrepancy, the three scientists suggest that "a potential explanation for the difference between reanalysis and station trends is the extensive local and regional land use change that has occurred across the TP over the last 50 years."  They note, for example, that "over the last 30 years, livestock numbers across the TP have increased more than 200% due to inappropriate land management practices and are now at levels that far exceed the carrying capacity of the region (Du et al., 2004)."  The resultant overgrazing, in their words, "has caused land degradation and desertification at an alarming rate (Zhu and Li, 2000; Zeng et al., 2003)," and they note that "in other parts of the world, land degradation due to overgrazing has been shown to cause significant local temperature increases (e.g., Balling et al., 1998)."

Another point they raise is that "urbanization, which can result in 8-11C higher temperatures than in surrounding rural areas (e.g., Brandsma et al., 2003), has also occurred extensively on the TP," noting that "construction of a gas pipeline in the 1970s and highway expansion projects in the early 1980s have resulted in a dramatic population influx from other parts of China, contributing to both urbanization and a changed landscape."  In this regard, they say that "the original Tibetan section of Lhasa (i.e., the pre-1950 Lhasa) now only comprises 4% of the city, suggesting a 2400% increase in size over the last 50 years."  And they add that "similar population increases have occurred at other locations across the TP," and that "even villages and small towns can exhibit a strong urban heat island effect."

In concluding their analysis of the situation, Frauenfeld et al. contend that "these local changes are reflected in station temperature records."  We agree; and we note that when the surface-generated anomalies are removed, as in the case of the ERA-40 reanalysis results they present, it is clear there has been no warming of the Tibetan Plateau since at least 1958.  Likewise, we submit that the other results reported in this Summary imply much the same about other parts of China and greater Asia.  Hence, we feel certain that the dramatic surface-generated late-20th-century warming of the world that is claimed by the IPCC, Mann et al. (1998, 1999) and Mann and Jones (2003) to represent mean global background conditions is significantly biased towards warming over the last 30 years and is therefore not a true representation of earth's recent thermal history.

Balling Jr., R.C., Klopatek, J.M., Hildebrandt, M.L., Moritz, C.K. and Watts, C.J.  1998.  Impacts of land degradation on historical temperature records from the Sonoran desert.  Climatic Change 40: 669-681.

Brandsma, T., Konnen, G.P. and Wessels, H.R.A.  2003.  Empirical estimation of the effect of urban heat advection on the temperature series of DeBilt (the Netherlands).  International Journal of Climatology 23: 829-845.

Chen, L., Zhu, W., Zhou, X. and Zhou, Z.  2003.  Characteristics of the heat island effect in Shanghai and its possible mechanism.  Advances in Atmospheric Sciences 20: 991-1001.

Choi, Y., Jung, H.-S., Nam, K.-Y. and Kwon, W.-T. 2003. Adjusting urban bias in the regional mean surface temperature series of South Korea, 1968-99. International Journal of Climatology 23: 577-591.

Chung, U., Choi, J. and Yun, J.I.  2004a.  Urbanization effect on the observed change in mean monthly temperatures between 1951-1980 and 1971-2000.  Climatic Change 66: 127-136.

Chung, Y.-S., Yoon, M.-B. and Kim, H.-S.  2004b.  On climate variations and changes observed in South Korea.  Climatic Change 66: 151-161.

Du, M., Kawashima, S., Yonemura, S., Zhang, X. and Chen, S.  2004.  Mutual influence between human activities and climate change in the Tibetan Plateau during recent years.  Global and Planetary Change 41: 241-249.

Frauenfeld, O.W., Zhang, T. and Serreze, M.C.  2005.  Climate change and variability using European Centre for Medium-Range Weather Forecasts reanalysis (ERA-40) temperatures on the Tibetan Plateau.  Journal of Geophysical Research 110: 10.1029/2004JD005230.

Hasanean, H.M.  2001.  Fluctuations of surface air temperature in the Eastern Mediterranean.  Theoretical and Applied Climatology 68: 75-87.

Kalnay, E. and Cai, M.  2003.  Impact of urbanization and land-use change on climate.  Nature 423: 528-531.

Liu, X. and Chen, B.  2000.  Climatic warming in the Tibetan Plateau during recent decades.  International Journal of Climatology 20: 1729-1742.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1998.  Global-scale temperature patterns and climate forcing over the past six centuries.  Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D.  2003.  Global surface temperatures over the past two millennia.  Geophysical Research Letters 30: 10.1029/2003GL017814.

Weng, Q.  2001.  A remote sensing-GIS evaluation of urban expansion and its impact on surface temperature in the Zhujiang Delta, China.  International Journal of Remote Sensing 22: 1999-2014.

Zeng, Y., Feng, Z. and Cao, G.  2003.  Land cover change and its environmental impact in the upper reaches of the Yellow River, northeast Qinghai-Tibetan Plateau.  Mountain Research and Development 23: 353-361.

Zhou, L., Dickinson, R.E., Tian, Y., Fang, J., Li, Q., Kaufmann, R.K., Tucker, C.J. and Myneni, R.B.  2004.  Evidence for a significant urbanization effect on climate in China.  Proceedings of the National Academy of Sciences USA 101: 9540-9544.

Zhu, L. and Li, B.  2000.   Natural hazards and environmental issues. In: Zheng, D., Zhang, Q. and Wu, S. (Eds.) Mountain Genecology and Sustainable Development of the Tibetan Plateau, Springer, New York, New York, USA, pp. 203-222.

Last updated 13 April 2005