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How is Hong Kong affected? |
| Global Warming and Local Urbanization |
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The climate change in Hong Kong can be attributed to both global warming as a result of enhanced greenhouse effect, and localized urbanization. Details of the changes in essential meteorological parameters and potential impacts can be found below.
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| Temperature |
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At the Hong Kong Observatory Headquarters, temperature readings are available for the over 120 years period from 1885 to 2008, apart from a break during the World War II from 1940 to 1946. Analysis of the annual mean temperature data showed that there was an average rise of 0.12°C per decade in that period.
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Annual mean temperature recorded at the Hong Kong Observatory Headquarters (1885-2008). Data are not available from 1940 to 1946
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In post-war years from 1947 to 2008, the average rise amounted to 0.16°C per decade. The warming at the Hong Kong Observatory Headquarters has become faster in the period 1989 to 2008, at a rate of 0.23°C per decade.
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Annual mean temperature recorded at the Hong Kong Observatory Headquarters (1947-2008)
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Increase in annual mean temperature can be attributed to global warming as well as local effects such as urbanization. In Hong Kong, the rates of temperature rise at Ta Kwu Ling over the same period was smaller than that at the Hong Kong Observatory Headquarters. The Hong Kong Observatory Headquarters is situated at the urban areas of Kowloon whereas Ta Kwu Ling is in the rural areas of the New Territories.
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Comparison of recent trends in annual mean temperature in Hong Kong (1989-2008)
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The temperature trend at the Hong Kong Observatory Headquarters is typically the effects of urbanization where temperatures are higher than their rural surroundings. Temperature differences that develop between an urbanized area and the rural landscape are in essence a result of dissimilarity in radiative fluxes. Buildings and other concrete surfaces in the urban areas retain the heat produced by incoming solar radiation during the day and release the heat in the form of long-wave radiation during the night. High-rise buildings also inhibit the transfer of long-wave radiation to the atmosphere. This results in a slower fall of temperatures at night and a higher minimum temperature than when buildings were absent.
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| Rainfall |
In the 62-year period after the World War II, the annual total rainfall at the Hong Kong Observatory Headquarters has been risen at a rate of 56mm/decade, though not statistically significant at 5% level. The annual rainfall trend is small when compared to the year-to-year fluctuations. The strength of El-Nino Southern Oscillation (ENSO) and the winter monsoon in the preceding winter are important factors affecting the interannual variability of the rainfall.
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Annual rainfall at the Hong Kong Observatory Headquarters (1947-2008)
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The rainfall over different regions of Hong Kong was generally on a rising trend. The rate was higher over the urban areas than the New Territories, offshore islands and high grounds. Urbanization is one of the possible causes for the rainfall's increasing trend and the regional variation in Hong Kong. The higher temperature in urban areas enhances convective activities and the increase in concentration of suspended particulates from urban activities also favours the formation and development of rain-bearing clouds.
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| Visibility |
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One visible aspect of climate change is the turbidity in the sky which more and more local people are concerned about. It is caused by suspended particulates of one kind or another thrown up by human activities in the city. It may be purely dust and natural (e.g. loess from northern China). It could be particulates formed from combustion products (e.g. vehicle exhaust, kitchens and power generation) through photochemical processes.
The number of hours of reduced visibility below 8 km observed from the Hong Kong Observatory Headquarters each year was counted, with cases of visibility impairment that was concurrent with reports of fog, mist and rain or associated with high relative humidity (95% or more) were excluded. The visibility observed at the Hong Kong Observatory Headquarters from 1968 to 2008 has a deteriorating trend. Up to the late 1980s there was no significant trend. But from then onwards, there has been a dramatic rise in the frequency of reduced visibility. The occurrence of reduced visibility was increased at a rate of about 554 hours per decade in the period between 1988 and 2008 as compared with that of 53 hours per decade during 1968 to 1987.
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Annual total number of hours with visibility at the Hong Kong
Observatory Headquarters below 8 km from 1968 to 2008
(relative humidity below 95 % and not counting rain, mist or fog)
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| Sea Level |
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In Hong Kong, the tide gauge stations at North Point (1954-1985) and Quarry Bay (since 1986) have recorded the sea level of the Victoria Harbour for more than 50 years. An analysis of the records shows that the mean sea level in the Victoria Harbour has risen at an average rate of 2.4 mm per year during the period 1954 to 2008. The trend was similar to that over the South China Sea as measured from satellites using remote sensing techniques from 1993 to 2005.
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Annual mean sea level at North Point/Quarry Bay (1954-2008)
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A major impact of the mean sea-level rise in Hong Kong is an increase in sea flooding from storm surges caused by tropical cyclones. For a rise of 0.18 m or 0.59 m in the mean sea level and assuming no change in storm characteristics, the extreme sea-levels in the Victoria Harbour for various return periods would become higher as illustrated below:
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Return Period (years) |
Extreme sea-level above Principal Datum (m) |
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Current
mean sea level |
A rise of 0.18 m in
mean sea level |
A rise of 0.59 m in
mean sea level |
| 2 |
2.8 |
2.9 |
3.3 |
| 5 |
3.0 |
3.1 |
3.5 |
| 10 |
3.1 |
3.3 |
3.7 |
| 20 |
3.2 |
3.4 |
3.8 |
| 50 |
3.4 |
3.5 |
4.0 |
| 100 |
3.5 |
3.7 |
4.1 |
| 200 |
3.6 |
3.8 |
4.2 |
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| Cloud Amount |
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An increase in cloud amount can cause a decrease in diurnal temperature range by reducing the solar incoming radiation during day time and trapping of long-wave radiation at night. The diurnal temperature range has decreased since the 1950s worldwide and the coincidental increases in total cloud cover are often cited as a likely cause for the observed decrease in diurnal temperature range.
In Hong Kong, cloud amount is reported in oktas hourly by trained observers at the Hong Kong Observatory Headquarters. The daily mean cloud amount is taken as the average value of the observed hourly cloud amount in a day. Based on these daily mean cloud amounts, annual mean values were calculated.
The annual mean cloud amount has been increasing at a rate of 1.2% per decade in the period 1961 to 2008. One potential cause for the increase in cloud amount over Hong Kong could be the increase in the concentration of condensation nuclei in the air (that favoured the formation of clouds), which is known to be associated with urbanization.
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Annual mean cloud amount recorded at the Hong Kong Observatory Headquarters (1961-2008)
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| Solar Radiation |
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Both the increase in the concentration of suspended particulates and the increase in cloud amount would reduce the amount of solar radiation reaching the surface. At King's Park, the amount of solar radiation reaching Hong Kong is recorded continuously by thermo-electric pyranometers. Between 1964 and 2008, there has been a clear, board falling trend. For the whole period, the annual mean daily global solar radiation decreased at a linear rate of 0.74 MJm-2 per decade.
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Annual mean daily total global solar radiation at King's Park (1964-2008)
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| Tropical Cyclone |
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In the South China Sea, the annual number of tropical cyclones landing over the south China coast within 300 km of Hong Kong in the past 40 years or so from 1961 to 2008 has decreased from about 3 tropical cyclones in the 1960s to about 2.5 in the 1990s, but the rate of change is not statistically significant at 5 % level.
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Annual number of tropical cyclone landing over the south
China coast within 300 km of Hong Kong (1961-2008)
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| Potential Impacts |
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Information on potential impacts can be found at:
 Climate - Hong Kong (in Chinese only)
Characterizing the Climate Change Impact in Hong Kong
Climate Change and Its Impacts
Issues Related to Global Warming - Myths, Realities and Warnings
On Climate Changes Brought About by Urban Living
Long-term Trend Analyses of Weather Stress Indices for Human (in Chinese only)
Oasis-Climate-Hong Kong (in Chinese only)
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