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