The video below is part of a CNN documentary. It finishes in the middle of an interesting discussion of the UNFCC’s “hockey stick” graph, which has been so persuasive to many in convincing them that Climate Change is really caused by anthropological activity. However, their alternative conclusion is lost when the video ends abruptly. Nevertheless this video is still well worth viewing.
March 2007 Update: This page below was written in 2004. This page provides a snapshot of the discussions and issues current at that time. Things have changed since this page was written. More than 90% of the world’s climate change scientists now believe that the scientific evidence shows that climate change is a proven fact and the current climate change report is the UNFCCC’s 3rd Report (published January 2007).
One of the arguments from scientists who do not accept global warming is happening, say that while surface records may show that temperatures on the earth’s surface are rising, satellite temperature records are unable to detect any warming occurring higher up in the troposphere. Climate change models have predicted that this level of the atmosphere should be experiencing warming, at a level similar to that occurring at the earth’s surface. Data from radiosonde balloons and using satellites to measure global temperature appeared to show cooling of the troposphere by approximately 0.05Oc per decade (Spencer, and Christy, 1990). This data became a crucial part of the argument against the occurrence of global warming.
However, Wentz and Schabel (1998) carried out a study re-examining the data, and came to the opposite conclusion, finding that the troposphere was indeed warming as predicted by the climate change models. They found that the troposphere was experiencing a warming trend of approximately 0.2Oc per decade. The raw data contained no pronounced warming trend, as observed by Spencer and Christy (1990), but Wentz and Schabel (1998) found that the data was being confused by signals from the stratosphere, which is a higher level of the atmosphere 10-50km above the earth’s surface. The stratosphere is known to cool quickly due to ozone depletion and increased greenhouse gases, making this area of the atmosphere cool fives times faster than the troposphere below, is warming. Wentz and Schabel (1998) went on to eliminate the error caused by the stratosphere, and remaining data showed that the troposphere was warming as predicted. From this research the lower atmosphere temperature trend can be seen to match surface temperature trends.
So we have seen the problems associated with using satellites to measure temperature, but there is also the problem of inaccuracy when measuring surface temperatures. It has been suggested that the apparent change in regional surface temperatures is due to the urban heat island effect, where cities act to trap heat, and create obstacles to wind, thus raising the temperature locally (Soon at al 1999). The IPCC (2001) reviews the urban heat island (UHI) effect in detail. It can be seen that there has been an observed increase in land air temperature over the last two decades, caused by a decrease in the diurnal temperature range, which is thought to be due to the urban heat island effect. Urban heat islands raise nightime temperatures more than they raise daytime temperatures. On average mean minimum temperature is increasing twice as fast as mean maximum temperature, causing the daily temperature range to decrease by approx 0.4OC. However, the urban heat island effect may not be the only factor reducing the daily temperature range. As this trend has been observed there has also been a subsequent increase in precipitation and cloud cover, and it has been suggested that these physical changes may also be a contributor (Easterling et al 1997).
When using satellite and weather balloon data the effect of the urban heat island becomes apparent. Over land areas experiencing the urban heat island effect, lower troposphere temperatures and surface temperatures show no significant difference. In contrast, when data is examined over an area where no urban heat effect is being experienced, surface temperatures exhibit substantially more warming than temperatures taken from the troposphere. The IPCC (2001) go on to state that the urban heat effect is a localised effect, and is not representative of climate change in larger areas. A method of removing the urban heat island effect is to take averages from the northern and southern hemispheres respectively. The southern hemisphere has a lower urban heat island effect compared to the northern hemisphere, so has warmed at a considerably slower rate. When warming is calculated a system is used which gives much higher weight to the southern, compared to northern hemisphere. Therefore urban heat island is accounted for and removed from the calculations (Hansen et al 1999).
Urban effects on 20th century globally and hemispherically averaged temperature readings do not exceed over approximately 0.05oC (IPCC 2001). Furthermore changes in borehole temperatures, the recession of glaciers, and changes in ocean temperature, are not subject to urban heat effect, but agree with surface warming estimates. However, greater urbanisation influences cannot be discounted in future research (Folland et al, 2001). Uncertainties like the urban heat island certainly need to be taken into account, and has been done by the IPCC review, which concludes, “the warming substantially exceeds the calculated uncertainties”. The urban heat island effect is not conclusive in proving the argument against anthropogenic global warming. It is not the exclusive factor causing surface temperatures to warm. Additional factors must be involved, and greenhouse gases are still certainly one of the most likely possibilities.
If you think that issues such as Urban Heat Islands have been given undue emphasis, and that the scientists have actually been under politically motivated attack in the US, amounting to undue pressure against heir views, you may want to visit the Petition Site Com, and support the work of the vast majority consensus of climate change scientists.