Some examples of Urban Heat Island effect:
http://home.wanadoo.nl/bijkerk/tokyo.JPG is probable one of the most extreme, average tempreture compared here with the three closest rural weather stations.
But also moderate cities like http://home.wanadoo.nl/bijkerk/Louisville.jpg display clear UHI effect compared with it's closest rural weather station. Note that the temperature trend is a cooling here, just like central China, North Siberia, Argentine, Chile and Antarctica.
Anyway, this is how IPCC dictates the http://www.grida.no/climate/ipcc_tar/wg1/052.htm:
Clearly, the urban heat island effect is a real climate change in urban areas, but is not representative of larger areas. Extensive tests have shown that the urban heat island effects are no more than about 0.05°C up to 1990 in the global temperature records used in this chapter to depict climate change. Thus we have assumed an uncertainty of zero in global land-surface air temperature in 1900 due to urbanisation, linearly increasing to 0.06°C (two standard deviations 0.12°C) in 2000.
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As again http://www.globalwarming.org/article.php?uid=186 is that many weatherstations have closed in the 1980-1990 timeframe worldwide, most of them were rural stations obviously (nobody was interested in UHI effect at that time). Consequently part of the temperature rise may be due to bias caused by a change in ratio between urban and rural weather stations. In the USA however most stations remained in commission and curiously enough there is also little or no warming in the US. Many areas are actually cooling.
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The principles of greenhouse forcing are very well explained http://geosci.uchicago.edu/~archer/PS134/LabManual/lab.modtran.html all you like and compare effects of any concentration of CO2 and CH4 greenhouse gasses on absorption and emission. For instance: run the model with all defaults except CO2:
0 ppm: INTEGRATED RADIATION ENERGY FLUX= 249.096W/m2
10 ppm: INTEGRATED RADIATION ENERGY FLUX= 239.582W/m2
Hence difference 9,5 W/m2
1000 ppm: INTEGRATED RADIATION ENERGY FLUX= 223.16W/m2
2000 ppm: INTEGRATED RADIATION ENERGY FLUX= 220.428W/m2
Hence difference 2,7 W/m2
See how quickly the greenhouse gas effect saturates? Now, I have no idea at all how all of this has lead to scaring hype of global warming.
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Effects of clouds is still under heavy debate. High thin clouds are believed to nett reduce re-radiation and hence have a warming effect. Low dense clouds reflict more visible light and cause cooling. The recent albedo changes suggest changes in average cloud cover.
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Soot may be the single most important anthropogenic part of the current warming. Other reasons are:
bias due to ratio change in urban and rural weather stations,
the very strong el nino of 1997-1998
simulaneous change in several atmospheric and oceanic oscillations.