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Friday, 14th May 2010

How hot would it get ?

In answering a media question on 23 March 2010, the World Meteorological Day, I mentioned that the Observatory was updating the extreme temperature projections for Hong Kong based on the latest data. I would like to further elaborate on this.

In 2007, the Observatory conducted a study to assess the temperature trend for Hong Kong up to the end of this century. The study utilized the monthly temperature figures simulated by the global climate models participating in the Fourth Assessment Report (AR4) of the United Nations Intergovernmental Panel on Climate Change (IPCC). The study also made an attempt to estimate how frequent extremely hot or cold weather would become.

Since the 2007 study, more refined AR4 model simulation data, in the form of daily temperature figures, have become available to the Observatory. Thus the Observatory recently undertook further evaluation using such data. The much higher temporal resolution of the data, i.e. daily instead of monthly, allowed us to carry out further analyses to refine our projections.

The latest evaluation results suggest that, similar to the results of the 2007 study, there will be more very hot (maximum temperature at or higher than 33oC) while cold weather events will decrease significantly. However, they also suggest that cold days (minimum temperature at or less than 12oC) in Hong Kong may practically disappear towards the end of the century, i.e. less than 1 cold day per year. Such disappearance of cold days, which many people perceive as 'winter', is expected to occur later than that estimated in the 2007 study.

I would like to point out that both the old and new results have consistently depicted a significantly warmer climate in the 21st century with a range of temperature rises depending on the future greenhouse gas emission scenarios. The recent evaluation does not obscure that fact that warming in Hong Kong has been accelerating in recent decades.

Figure 1 below shows the trend of the number of cold days since records began in the 19th century in Hong Kong. Assuming a straight line for the change in the number of cold days with time, simple extension of the line into the future tells us that cold days would completely disappear by early next century. Such a crude estimate is not far off from the results of the recent evaluation discussed above.

Figure 1.    Annual number of cold days in Hong Kong (1885 - 2009).  A cold day is one where the temperature fell to 12 C or below.  The linear fit is statistically significant at 5% level.
Figure 1.      Annual number of cold days in Hong Kong (1885 - 2009). A cold day is one where the temperature
fell to 12oC or below. The linear fit is statistically significant at 5% level.


IPCC has unambiguously pointed out in AR4 that global warming is unequivocal and very likely human induced. Recently, the World Meteorological Organization (WMO) reported that, globally, the year 2009 is the fifth warmest on record since 1850. On the decadal scale, the decade of 2000-2009 was warmer than the decade of 1990-1999, which in turn was warmer than the pervious decade of 1980-1989 and earlier ones. Locally in Hong Kong, 2009 is also the ninth warmest year since records began in the late 19th century. While many Hong Kong people might feel that the past cool season was very like a 'typical' winter decades ago, the fact remains that the average temperature of the 3-month period from December 2009 to February 2010 was still 0.6oC higher than normal.

Figure 2.    Decadal global average temperature (C) (Source : WMO Statement on the Status of the Global Climate 2009 http://www.wmo.int/pages/publications/showcase/index_en.html)
Figure 2.      Decadal global average temperature (oC)
(Source : WMO Statement on the Status of the Global Climate 2009
http://www.wmo.int/pages/publications/showcase/index_en.html )


Scientific study on climate change is an on-going process that is constantly improving. We will provide further updates from time to time in the light of new data.


B Y Lee



Last revision date: <17 Jan 2013>