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Change in mean nighttime temperature from urban to rural areas in Hong Kong ... The warming at the Hong Kong Observatory Headquarters has become ...

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The Hong Kong Polytechnic University

Department of Applied Physics

GE Subject : Climate and Our Environment

Global and Local Climate Change

22 Sept 2008



  • Climate and Weather
  • Greenhouse Effect & Global Warming
  • Global Climate Change
  • Hong Kong Connection & Urbanization Effect
  • Future Projections
  • Potential Impacts of Climate Change
  • What can we do ?

What is Climate ? What is Weather ?

"Climate is what you expect; weather is what you get.“

by Robert A. Heinlein

  • Weather is the day-to-day state of the atmosphere
  • Climate describes the long-term character of all weather variations -- the ‘expected’ weather
  • Climate ≈ average of weather

Greenhouse Gases & Global Warming

Air composition



carbon dioxide 0.038%

Air consists of nitrogen (N2), oxygen (O2), carbon dioxide (CO2), noble gases (argon (Ar), helium (He) etc) and variable amount of water vapour (H2O)


Condition for no greenhouse gas in the atmosphere




Heating = Heat Dissipation

Surface temperature ≈ -18 oC



Greenhouse Effect




Average Temperature about 15℃

partly absorbed

re-emitted infra-red

Heat-trapping greenhouse gases act like a blanket and keep the surface and the lower atmosphere warmer than it would be without them.


greenhouse gases

Condition for greenhouse gases in the atmosphere; greenhouse gases include carbon dioxide (CO2), nitrous oxide (N2O), methane(CH4), chlorofluorocarbons (CFCs) , ozone(O3)and water vapour (H2O)

human activities produce greenhouse gases ghg
Human activities produce greenhouse gases (GHG)

Burning of fossil fuels and long term deforestation have been increasing the concentration of GHG in the atmosphere, thickening the greenhouse blanket

energy production, industry:carbon dioxide(CO2)

waste landfill: nitrous oxide(N2O)

husbandry: methane(CH4)

vehicle exhaust :ozone(O3)

freezer, aerosol spray:chlorofluorocarbons(CFCs)


carbon dioxide (ppm)

methane (ppb)

nitrous oxide (ppb)

time (before 2005)

time (before 2005)

time (before 2005)

Rising trends of greenhouse gases

(Source: Intergovernmental Panel on Climate Change)

Since 1750, the concentrations of global atmospheric carbon dioxide, methane and nitrous oxide have risen sharply due to human activities


Climate Change

  • Natural factors
  • Solar activity,
  • Volcanic gases
  • Dust in the atmosphere
  • Distribution of heat in the ocean
  • Anthropogenic (human-causes) factors
  • Greenhouse gas emission
  • Land use changes / Deforestation
  • Global Warming
  • Warming Of The Climate System Is Unequivocal
  • Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations
  • (IPCC)

The Consequence of Global Warming

Enhance greenhouse effect

Rising temperature

Thermal expansion of sea water & melting of snow on land

Sea level rise

Regional differences in precipitation

Enhance the water cycle

Change in atmospheric circulation and chemical composition

Increase in occurrence of extreme weather and climate events


The world has been warming !

Global mean temperature has been rising at a rate of 0.13 degree per decade in the past 50 years. This rising trend is double that of the trend in the past 100 years

Eleven (1998, 2005, 2003, 2002, 2004, 2006, 2001, 1997, 1995, 1999, 2000) of the last twelve years rank among the 12 warmest years on record. The warmest year being 1998.

(Source: Intergovernmental Panel on Climate Change)


Annual mean temperature trend in 1979-2005

Temperature has been rising in almost all regions, larger rises are observed in high-latitude than low-latitude areas, and in land areas than oceans


(white crosses represent trends are statistically significant at 5% level, areas in grey represent not having enough data for computation of reliable trends)

(Source: Intergovernmental Panel on Climate Change)


Sea level rise

Global mean sea level has been rising at 1.8 mm per year in 1961-2003. The rate of sea level rise is higher at 3.1 mm per year in 1993-2003.

blue:tidal gauge datared:satellite data

(Source: Intergovernmental Panel on Climate Change)


Water cycle

Global warming will enhance the water cycle, causing the mean global precipitation to increase. Precipitation here includes rain and snow.












Regional differences in land precipitation

The diagram shows the precipitation trends (1900 – 2005) at various regions. Precipitation curves with white background are having rising trends and those with yellow background falling trends.

(Source: Intergovernmental Panel on Climate Change)

weather and climate extremes
Weather and climate extremes

Global warming leads to increase in occurrence of heat wave, drought and flooding events, and possibly the increase in tropical cyclone intensity of the Atlantic.

Heat wave



Tropical cyclone


(Source: US National Oceanic Atmospheric Administration)


Climate of Hong Kong

Hong Kong's climate is sub-tropical, tending towards temperate for nearly half the year.

HOT & WET in Summer

COOL & DRY in Winter


Climate Change in Hong Kong : Observations

Climate change in HK = Global Warming + Local Urbanization Effect

  • Rise in mean air temperature (global warming and urban heat island)
  • Increase in rainfall
  • Rise in sea level
  • Decrease in wind speeds (urban sheltering)
  • Rise in the frequency of reduced visibility
  • Increase in cloud amount
  • Reduce the amount of solar radiation

Urbanization Effect on Local Climate

  • Due to urban development, change in land use, increase in population, human activities, etc.
  • A main characteristic is the significant difference in the temperature between urban and rural (the countryside) areas. Average temperature in the urban area is higher than that of the rural area.
  • Large difference in diurnal variation :
  • Daytime --- Urban temperature < Rural temperature
  • Nightime --- Urban temperature > Rural temperature
  • 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.


Change in mean nighttime temperature from urban to rural areas in Hong Kong

(Average of 0500H temperature in 2007)  


Mean hourly temperature difference between HKOHq and Ta Kwu Ling

(1989-2007 average)

HKOHq > TKL (red area)

HKOHq < TKL (blue area)


Annual mean temperature recorded at the Hong Kong Observatory Headquarters (1885-2007)

There was an average rise of 1.2°C per 100 years from 1885 to 2007.

the global average surface temperature rose by 0.6℃ (IPCC)


Annual mean temperature recorded at the Hong Kong Observatory Headquarters (1947-2007)

The warming at the Hong Kong Observatory Headquarters has become significantly faster in the period 1989 to 2007, at a rate of 0.34°C per decade.


Comparison of recent trends in annual mean temperature in Hong Kong


HKO Headquarters is a station in the urban area of Kowloon

Ta Kwu Ling is a rural station in the northern part of the New Territories

(Data period:

Global: 1989-2005, HKO Headquarters and Ta Kwu Ling : 1989-2007)


Annual Mean Number of Very Cold Days (Minimum Temperature =< 7oC) in a Decade

(No data in 1880-1884 ; 1937-1946 ; 2008-2009)


Annual rainfall at the Hong Kong Observatory Headquarters


The annual total rainfall at the Hong Kong Observatory Headquarters has been risen at a rate of 46mm/decade, though not statistically significant at 5% level.


Annual mean sea level at North Point/Quarry Bay (1954-2007)

The mean sea level in the Victoria Harbour has risen 0.13 m from 1954 to 2007, at an average rate of 2.4 mm per year


Annual average of 12-hourly 10-minute mean wind speed

of King’s Park and Waglan Island (1968-2007)

Growing of tall buildings increases the roughness of the surface underlying the atmosphere and exerts a drag on the low-level winds


Annual total number of hours with visibility at the Hong Kong Observatory Headquarters below 8 km from 1968 to 2007 (relative humidity below 95 % and not counting rain, mist or fog)

Caused by suspended particulates of one kind or another thrown up by human activities in the city


Annual mean cloud amount recorded at the

Hong Kong Observatory Headquarters (1961-2007)

Urbanization causes the increase in the concentration of condensation nuclei in the air (a factor favourable to the formation of cloud)


Annual mean daily total global solar radiation at King's Park (1964-2007)

the annual mean daily global solar radiation has decreased at a rate of 0.84 MJm-2 per decade from 1964-2007.


Long term trend in annual total evaporation, 1961-2005

Attributed to greatly decreased prevailing wind speed and reduced amount of solar radiation reaching the ground


Climate Change in Hong Kong :

Future Projections in the 21st century

Temperature : Downscaling based on IPCC’s Fourth Assessment Report (AR4)

Rainfall : Downscaling based on IPCC’s Third Assessment Report, being updated using AR4

Sea level : The sea-level at the South China Sea including Hong Kong is likely to be close to the global average in the long run. According to IPCC AR4, the global average sea-level will rise by 0.18 to 0.59 m at the end of 21st century relative to the period 1980 to 1999.


Past and projected annual mean temperature anomaly for Hong Kong


+6.8 oC

middle-of the-road

+4.8 oC

Annual mean temperature anomaly (oC)


+3.0 oC




(Projection of Global Mean is about +1.8 to 4 oC, IPCC)


Past and projected number of cold days in winter



Number of cold days in winter

1980-1999 average : 14 days

middle-of the-road





Temperature Projections for Hong Kong

  • Temperatures
  • The average temperature will continue to increase (Middle condition:4.8ºC, low-end:3.0ºC, high-end:6.8ºC)
  • More very hot days and hot nights in summer
  • Less cold days in winter

(相片來源:水務署Photo from Water Supplies Department)

(相片來源:渠務署 Photo from Drainage Services Department)


A Quick Summary of Climate Change in Hong Kong

  • In the Past
  • The average temperature has increased by 1.2 degrees in the past century
  • The mean sea level has risen by 0.12 metre in the past 50 years, at an average rate of 2.3 millimetres per year; and
  • The annual total rainfall has been risen at a rate of 46mm/decade, though not statistically significant at 5% level.
  • Projections for the 21st century
  • Temperatures will continue to increase, the mean temperature in the decade 2090-2099 is expected to rise by 4.8 ℃. “long summer, no winter“.
  • Annual rainfall will increase at a rate of about 1% per decade, with more heavy rain days and increase in the year-to-year variability in rainfall.

Possible Impacts :-

  • Fresh Water Resources
  • Ecosystems
  • Food and forest products
  • Coastal systems and low-lying areas
  • Industry, settlement and society
  • Health

Melting of ice caps and glaciers

Global warming leads to the melting of ice caps over polar land areas and the glaciers on high mountains. The melted ice-water flows into the sea and contributes to the sea level rise.

Muir Glacier, Alaska's Glacier Bay

August 13, 1941

August 31, 2004

(Image Credit: National Snow and Ice Data Center, W. O. Field, B. F. Molnia)

Between 1941 and 2004 the glacier retreated more than twelve kilometers and thinned by more than 800 meters.


Sea level rise causes flooding of coastal areas easier

waves caused by typhoon


rose in sea level


(Source: US National Oceanic Atmospheric Administration)

  • Flooding of the coastal areas becomes easier during typhoon approaches or heavy rain

Sea level rise leads to the increase flooding risk in coastal areas

( Source : US Geological Survey)


Extreme weather threatens life and property

(Source: Apple Daily)

  • (Source: Geotechnical Engineering Office)

An Overview of Weather & Health


Thermal Stress

- cardiovascular and respiratory

morbidity and mortality


Ecologically mediated

Vector-borne diseases

Marine-borne diseases

Food productivity


- malaria, dengue

- toxic algae, cholera

- malnutrition

  • deaths & injuries
  • damage to health infrastructure
  • increase risk of infectious diseases
  • civil disorder/conflicts

Weather disasters


With a death toll estimated to exceed 30 000, the heat wave of 2003 is one of the ten deadliest natural disasters in Europe for the last 100 years and the worst in the last 50 years. Elderly people were most affected.

Source :

- Kovats S, Wolf T, Menne B. Heatwave of August 2003 in Europe: provisional estimates of the impact on mortality. Eurosurveillance Weekly. 11 March 2004; 8(11).

- Environmental Alert Bulletin, United Nations Environment Programme


Public Health & Communicable Diseases

More mosquitoes, easier to transmit dengue fever and malaria

More ticks, easier to transmit some infectious diseases

Mosquitoe bites in a warm winter ?

Mosquitoes & ticks are expanding their territory ?


What can we do ?

The major cause of global warming is the excessive consumption of energy and resources by human beings. As we are all contributors to global warming, we should make effort to reduce global warming.

We could adopt a simple life style in our daily life to reduce global warming.


Save energy

  • Use compact fluorescent bulbs and energy-efficient electrical appliances.
  • Turn off electrical appliances and lighting when they are not in use.
  • Use less air-conditioning. Set the temperature at 25.5°C.
  • More use of renewable energy

(Source : Electrical and Mechanical Services Department)


On the road

  • Drive less and use public transport. Consider walking or cycling.
  • Drive smart, don’t rush into the traffic jam
  • Buy fuel-efficient vehicles.
  • Switch off idling vehicle engines.

Save Water

Don’t waste water. Take a shower instead of a bath.

Never brush your teeth under a running tap.

Only use your washing machine when you have a full load and cut down the rinse cycle if possible.

  • Use less paper and plant more trees
  • Disseminate information by electronic means where possible.
  • Print both sides of the paper and minimize photocopying.
  • Don't over wrap your gifts.
  • Plant more trees to absorb carbon dioxide in the atmosphere.

Reducing waste and recycling

  • Change the habit of excessive consumption and extravagant spending.
  • Before purchasing a commodity, think whether it is needed. Use recyclable products.
  • Enhance waste separation and recovery for recycling.

Large amount of energy is used in producing commercial productsand releasing carbon dioxide


Encourage others to conserve energy and resources

Promote public awareness and understanding of climate change

The educational package on climate change produced by the Observatory


Talks on Climate Change for Schools

In order to promote awareness and understanding of climate change to students in Hong Kong, a team of professional meteorologists of the Observatory has been delivering talks on climate change for primary and secondary school children.

website of climate change
Website of Climate Change