Urban Heat Islands and Land Use Changes
The IPCC theoretical predictions for surface warming range from 0.09 to 0.27 C/decade at the surface and greater in the mid-troposphere (about 0.11 to 0.32 C/decade).
In support of the theoretical claim, it is often pointed out that the surface is warming by about 0.18 C/decade (e.g., Jones et al., 2003), based upon using surface thermometers.
Given this claim of strong surface warming in the last 25 years, one would expect to see a very strong signal in the proxy measurements of temperature, but it is definitely not present in the tree ring data, the ice core data, or the Mg/Ca isotope data. Some of the proxy series show strong increases up to 1980 but then have strong reverses in the 1980s (e.g., some of the northern tree line series.) Tree ring densities have also declined consistent with cooling rather than warming. From looking at proxy data, one would never know that 1998 was "the warmest year of the millennium". It is likely temperatures have not risen as much as claimed, although one could argue that the proxies cannot reproduce climate, in which case the hockey stick reconstruction present by Mann et al. (1998) becomes invalid (see also McIntyre and McKitrick, 2003). These proxies provide no support for the assertion that the last 10 years are warmer than any years in the last millennium which is often claimed. Another proxy often used to assert temperatures are rising is the recession of glaciers, but glaciers respond to precipitation, evaporation, sunlight, cloud cover, aerosol deposition, and many other factors besides temperature so they are not a good proxy for temperature alone. For example, Kilimanjaro in Africa is often cited as evidence for rising temperatures, but actually it is receding due to local land use changes and lack of precipitation that have nothing to do with greenhouse gases (Molg et al., 2003).
The proxies do not support the strong warming seen in the surface thermometer network. The warming at the surface is not supported by balloon borne pressure transducers (Pielke et al., 1998), by balloon borne thermistors (Pielke et al., 1998; Christy et al., 2003), or by satellite observations (e. g., Christy et al., 2003). These three independent measurements of temperature in the mid-troposphere give a warming about 0.07 C/decade, or about one third the IPCC warming trend. The balloon borne pressure sensors allow the lower 75 millibars of the atmosphere to be sampled and these are consistent with very modest warming in the mid-troposphere ands strongly indicate there are problems with the surface thermometer measurements. Since the satellite observations are validated by the balloon observations, they are the best temperature measurements available and their results are inconsistent with the IPCC theoretical predictions.
There are many things wrong with the surface network such as poor locations of thermometers (Davey and Pielke, 2004), poor geographical coverage with less than 30% of the globe having sensors (Santer et al., 2000), urban heat island contamination(e.g., Torok et al., 2001; Oke, 1973), land use changes (Marland et al., 2003; Kalnay and Cai, 2003), soot on snow causing warming (Hansen et al., 2003), and so forth. These problems with the surface thermometers lead to measurements of greater warming than is actually occurring compared to a situation where we had perfect measurements. For example Kalnay estimates 40% of the surface warming is coming from land use changes (i., e., 0.07 C/decade). Marland et al. (2003) say more than half the warming may be coming from these land use changes (i.e., >0.09 C/decade). Hansen says 25% of the twentieth century warming may come from increased soot on snow which is a land use change (i.e., 0.015 C/decade). Urban heat islands and poor siting of the thermometers may account for yet more spurious warming. Subtracting these spurious signals from the claimed warming of 0.18 C/decade reduces it to less than 0.08 C/decade. Based on climate models, the surface warming from carbon dioxide must be less than the mid-tropospheric warming of 0.07 C/decade and correcting for these identified problems in the surface network brings the surface observations into near agreement with the satellite observations.
In summary, the warming of the globe is only about 0.07 C/decade and even if all of were attributed to greenhouse gases, it is far less than the warming predicted by the IPCC models. The models are not validated by the magnitude of the temperature trends.
Christy, J. R. And R. W. Spencer, 2004. 25 years of satellite data show Â€̃global warming' of only 0.34 . Press release, University of Alabama, Huntsville, AL (http://uahnews.uah.edu/scienceread.asp?newsID=196).
Christy, J. R., R. W. Spencer, W. B. Norris, W. D. Braswell and D. E. Parker, 2003. Error estimates of Version 5.0 of MSU/AMSU bulk atmospheric temperatures. Journal of Atmospheric and Oceanic Technology, 20, 613-629.
Davey, C. A. and R. A. Pielke Sr., 2004: Microclimate exposures of surface-based weather stations - implications for the assessment of long-term temperature trends. Bull. Amer. Meteor. Soc., submitted.
Hansen, J., Sato, M. and Ruedy, R. 1995. Long-term changes of the diurnal temperature cycle: Implications about mechanisms of global climate change. Atmospheric Research, 37, 175-209.
Hansen, J., and L. Nazarenko 2003. Soot climate forcing via snow and ice albedos. Proc. Natl. Acad. Sci., 101, 423-428.
Jones, P. D. and A. Moberg, 2003. Hemispheric and Large-Scale Air Temperature Variations: An Extensive Revision and Update to 2001. Journal of Climate, 16, 206-223.
Kalnay, E. and M. Cai, 2003. Impact of urbanization and land-use change on climate. Nature 423, 528 - 531
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.
Marland, G., R. A. Pielke, Sr., M. Apps, R. Avissar, R.A. Betts, K.J. Davis, P.C. Frumhoff, S.T. Jackson, L. Joyce, P. Kauppi, J. Katzenberger, K.G. MacDicken, R. Neilson, J.O. Niles, D. dutta S. Niyogi, R.J. Norby, N. Pena, N. Sampson, and Y. Xue, 2003. The climatic impacts of land surface change and carbon management, and the implications for climate-change mitigation policy. Climate Policy, accepted.
McIntyre, S. and McKitrick, R. 2003. Corrections to the Mann et al. (1998) proxy data base and Northern Hemispheric average temperature series. Energy and Environment,14, 751-771.
Molg, T., D. R. Hardy, and G. Kaser, 2003. Solar-radiation-maintained glacier recession on Kilimanjaro drawn from combined ice-radiation geometry modeling, J. Geophys. Res., 108, 4731.
Oke, T.R. 1973. City size and the urban heat island. Atmospheric Environment, 7 769-779.
Pielke, R. A., Sr., J. Eastman, T. N. Chase, J. Knaff, and T. G. F. Kittel, 1998. The 1973-1996 trends in depth-averaged tropospheric temperature, J. Geophys. Res., 103, 16,927-16,933.
Pielke, R. A., Sr., J. Eastman, T. N. Chase, J. Knaff, and T. G. F. Kittel, 1998. Correction to "The 1973-1996 trends in depth-averaged tropo-spheric temperature," J. Geophys. Res., 103, 28, 909-911.
Santer, B. D., et al., 2000. Interpreting differential temperature trends at the surface and in the lower troposphere. Science, 287, 1227-1232.
Torok, S. J., Morris, C. J. G., Skinner, C. and Plummer, N. 2001. Urban heat island features of southeast Australian towns. Australian Meteorological Magazine, 50, 1-13.
Some thoughts on UHI’s and rural stations
(adapted from an essay by P. M. Pollock)
What constitutes an urban site versus a rural site? Peterson and others who support the IPCC viewpoint consider a town with a population of less than 10,000 people to be rural and not to require any adjustment for urbanization. Nothing could be further from the truth. Oke (1973) and Torok et al (2001) show that even towns with populations of 1000 people have urban heating of about 2.2 C compared to the nearby rural countryside. Since the UHI increases as the logarithm of the population or as about 0.73 log (pop), a village with a population of 10 has an urban warming of 0.73 C, a village with 100 has a warming of 1.46 C, a town with a population of 1000 people already has an urban warming of 2.2 C, and a large city with a million people has a warming of 4.4 C (Oke, 1973).
Try this thought experiment: In 1900, world population is 1 billion and in 2000, it is 6 billion for an increase of a factor of six. If the surface measuring stations are randomly distributed and respond to this population increase, it would equal 2.2 log (6) or 1.7 C, a number already greater than the observed warming of 0.6 C. If however we note that UHIs occur only on land or 29% of the Earth’s surface, than the net global warming would be 0.29*1.7 or 0.49 C which is close the observed warming. It is not out of the realm of possibility that most of the twentieth century warming was urban heat islands.
Even “towns” with zero population can have an urban heat island.
1. Consider Nowheresville, population zero. In 1900 it consisted of a single store at the intersection of two dirt roads. In 2000, the store is abandoned and the roads are paved. The surface is now darker than in 1900 and the region around the store is warmer. If you are 500 feet from the road, your thermometer will show a 0.1 C warming and two of my correspondents have found such warming. The building itself will heat up during the day and release the heat at night, further adding to the warming. The net effect is warmer nights and slightly warmer days with a decrease in the diurnal temperature range (DTR). Decreases in DTR have been observed worldwide.
2. Next consider Podunk Corner, population 5. A road house at the crossing of two gravel wagon roads, Stevenson Screen in the open over by the corral, exposed to the prevailing wind. However the motorcar has arrived. The roads are paved, the hitching area and former corral likewise converted into parking. All that asphalt absorbing and then emitting energy. The saplings of 1910 growing into ever large windbreaks. Population still five. The net result is still an urban warming even with no change in population.
3. Or try a village of 30 people in five houses going to 150 people in 30 houses. All with their roofs, driveways and roads. An experimenter taking temperatures as he drives through such villages will find several degree higher readings within. But a village of 150 gets no UHI adjustment.
4. Then there is the maintenance problem: many of the remaining stations have intermittent signals, implying the likelihood of poor maintenance when they are up. A Stevenson screen that needs new white paint or that is dirty is a warmer screen. The response of the screen will change if the type of paint is changed and the new paint has a different thermal emissivity. A screen with its louvers occluded by spiders, birds nests, etc. is a warmer screen. Bushes and trees growing around the screens will block outgoing thermal radiation at night and their slow growth will be a warming signal, but not one related to AGHG. Earlier we called the last effect the “changing skyline hypothesis”. There is inadequate documentation of all these effects and no real interest in testing their validity, presumably because climate scientists are happy with the warming signal they see and are happy to attribute it to AGHGs.
Since satellite measurements began in 1979, the world’s population has approximately doubled leading to an UHI signal of 0.67 C over land and 0.19 C globally. The observed surface warming is about 0.36 C over the same time period, so a substantial portion may be just uncorrected UHI effects. Other effects include land use changes, increased darkness of vegetation, direct heat from fossil fuel burning, a brighter sun, changes in cosmic ray intensity, soot on snow, more soot in the atmosphere, and greenhouse gases (and this list is not exhaustive). There are many competing theories for the recent warming and some of them do a better job at explaining the observations than do greenhouse gases.
The land surface stations were designed to provide local climatology. They were not designed to detect climate change. Quality control of the surface network is inadequate.
Useful sites on UHI’s:
Hans Erren: http://hanserren.cwhoutwijk.nl/co2/homogen.htm
Warwick Hughes: http://www.warwickhughes.com
Torok summary: http://www.co2science.org/journal/2002/v5n20c3.htm
Bohm summary: http://www.co2science.org/journal/2000/v3n33c1.htm