Under the influence of global climate change, the average annual rainfall in Hong Kong will increase during the latter half of the 21st century. It is expected that, in the last 10 years of this century (2090-2099), the average annual rainfall recorded at the Hong Kong Observatory Headquarters will reach 2572 mm, 248 mm (11%) higher than the 1980-1999 average of 2324 mm.

Apart from the increase in the average annual rainfall in the latter half of the 21st century, the year-to-year variability in rainfall would also increase. The number of extremely wet years will increase significantly from 3 during the period 1885-2008 to 10 in the 21st century and the corresponding figure for extremely dry year is also expected to increase from 2 to 4. Here, the extremely wet and extremely dry year refer to a year with the annual rainfall at the Hong Kong Observatory Headquarters above 3187 mm and below 1282 mm respectively.

In the 21st century, the number of days with heavy rain is also likely to increase. During the last 30 years of this century, that is 2070-2099, the average number of days in a year with hourly rainfall at the Hong Kong Observatory Headquarters exceeding 30 mm would be about 6.5 days, about 1 day more than the 1980-1999 average of 5.8 days.

Further details on updated rainfall projections for Hong Kong can be found in the paper "Rainfall Projections for Hong Kong based on the IPCC Fourth Assessment Report".

Past and projected change in annual rainfall for Hong Kong
(based on IPCC AR4 annual mean projection data)

Note:

Climate projection uncertainties

It is important to note that climate projection is very different from weather or seasonal forecasts. Climate projection involves assumptions in future socio-economic and technological developments and greenhouse gas emission scenarios and aims at describing the plausible change in the future climate from a long term perspective, rather than depicting the "day to day" or "year to year" variations in weather.

Although a majority of the model projections suggests in general consistent trends for the changes in the climate of the 21st century, inter-model differences are still rather large with a divergence in the projections for the future climate. This, to a certain extent, reflects that climate projection is still subject to various uncertainties in the model simulation of the future climate, which depend very much on such factors as future greenhouse gas emission scenarios, the choice of models, model skills, the downscaling methodology, the stability of the statistical downscaling relationships in the future. The technique is expected to continue to improve with time as scientists know more about various climate processes and atmospheric processes that impact on the climate.