SCMP
| Apr 25, 2012 |
|
||||
|
I refer to the article by Wyss Yim (“Hot? Blame the urban heat island”, April 1). To enable your readers to better understand the issue on the urban heat island effect in Hong Kong and the roles of carbon dioxide and water vapour in global climate change, we would like to provide the following information. As the Observatory headquarters is situated in the heart of Kowloon, where significant development has taken place over the past half century, the long-term temperature increase over the last 125 years or so can be attributed to both global warming and local urbanisation. We recently further studied the headquarters’ temperatures utilising, as reference, the long-term data of Macau and air temperature data at higher altitudes above Hong Kong which are less affected by the urbanisation effect. The results suggest that the two factors mentioned above have roughly contributed equally to the observed warming in the past few decades. While the higher annual mean temperature at the headquarters, compared with Waglan Island, may be partly attributable to the urbanisation effect, the year-to-year fluctuation in the temperature difference between the two sites is more complicated, being affected by other factors including oceanic effects. Water vapour is a condensable greenhouse gas which cycles through the atmosphere quickly (the water cycle) and its concentration adjusts to global temperatures. Generally speaking, a warmer atmosphere can hold more water vapour. With warming on a macro scale induced by the increase in the concentration of non-condensing and long-lasting greenhouse gases (for example, carbon dioxide and methane), the atmosphere is able to absorb more water vapour which will further intensify the greenhouse effect, forming a positive feedback loop. Therefore, although the concentration of carbon dioxide in the atmosphere is lower than that of the water vapour, carbon dioxide (together with other non-condensing greenhouse gases) is a principal controlling factor affecting the earth’s climate. Thus, the rapid increase in the concentration of human-induced greenhouse gases in the atmosphere is considered by scientists and policymakers to be one of the most serious problems that mankind has to face, not only now but for generations to come. T. C. Lee, for director of the Hong Kong Observatory |
|||||
Before the temperature record of the Observatory’s headquarters station can be used for obtaining temperature trends for global warming adequate correction for the influence of the urban heat island effect is needed. A good way to resolve this is through a comparison of the 1968-2011 temperature record available from the headquarters station and the Waglan Island Station. The oceanic influence present at the site of the latter is what makes it good for removing the urban heat island effect (UHI). Based on higher mean annual temperature of 0.64 degree C found at the headquarters station no more than 0.14 degree C (or less than 25%) may be attributed to global warming after correction.
The collection of temperature data at higher altitudes above Hong Kong is not a solution for resolving the UHI but is more useful for future comparison. This is because of factors including the move of the Kai Tak Airport to Chek Lap Kok, the rapid increase in air traffic, land and sea transportation growth, population growth including the creation of new urban heat islands in the Tolo Harbour basin at Sha Tin and Tai Po, etc. have already taken place. Furthermore, in spite of the ‘short’ distance of the Waglan Island Station from the headquarters significant temperature differences can still be found. Additionally, records of temperature data at higher altitudes pre-dating the establishment of the urban heat island effect are unavailable.
I have already presented arguments why water vapour may be the most influential greenhouse gas in the April 1, 2012 SCMP article. After the IPCC Assessment Report 4 appeared in 2007, Susan Solomon the lead author has with her other colleagues published in 2010 findings of an analysis of observational records of stratospheric water vapour showing that its role in global warming was important and has been underestimated.
Carbon dioxide has been demonized by some governments and used as a means of increasing revenues through the introduction of a carbon tax. Because carbon dioxide is beneficial to plant life including crops, my concerns are for the problem created by human impact on the natural water cycle through deforestation, land use, dams and reservoir construction, irrigation schemes, etc. Afterall we are converted deserts into farmland through irrigation schemes.