More Q&As will be added later, please stay tuned!

 

1. What Is the difference between weather and climate?

Although both "Weather" and "Climate" are used to describe the condition of the atmosphere, they are very different in terms of the time scale considered.  "Weather" describes the combined atmospheric situation in a place at the time or within a very short time (several hours to a few days), such as wind speed, temperature, cloud amount, rainfall, pressure, etc.  "Climate" refers to the average of the meteorological condition and pattern in a place over a longer period of time.  In other words, "Climate" can be described as the "Average Weather".  According to the definition of the World Meteorological Organization (WMO), the reference period for compiling the climate statistics should be at least 30 years.

More information on the climate of Hong Kong is available from the Observatory's "Climatological Information Services" webpage: http://www.weather.gov.hk/cis/climat_e.htm

 

Back to the top

2. What is climate change?

According to the Intergovernmental Panel on Climate Change (IPCC), climate change refers to any change in climate over time, whether due to natural variability or as a result of human activity. This usage differs from that in the United Nations Framework Convention on Climate Change (UNFCCC), where climate change refers to a change of climate that is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and that is in addition to natural climate variability observed over comparable time periods.

 

Back to the top

 

3. What is IPCC?

The Intergovernmental Panel on Climate Change (IPCC) is a scientific intergovernmental body tasked to evaluate the risk of climate change caused by anthropogenic activity. Climate change is a very complex issue; policymakers need an objective source of information about the causes of climate change, its potential environmental and socio-economic consequences and the adaptation and mitigation options to respond to it. That is why IPCC was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP).

The IPCC is the authority on climate change. The main activity of IPCC is providing its authoritative assessment reports at regular intervals. The findings of the first IPCC Assessment Report of 1990 and the second Assessment report of 1995 played a decisive role in leading to the United Nations Framework Convention on Climate Change (UNFCCC) and the Kyoto Protocol. The Third Assessment Report of 2001 as well as Special and Methodology Reports provided further information relevant for the development of the UNFCCC and the Kyoto Protocol. The Fourth Assessment Report of 2007 confirmed that warming of the climate system is unequivocal and most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the increase in anthropogenic greenhouse gas concentration.

Useful Links:

IPCC

UNFCCC

Kyoto Protocol

 

 

 

Back to the top

 
4. What is greenhouse effect?

The heat content at the surface of the earth is mainly derived from the sun. When solar radiation mainly in the form of visible light reached the earth, it heats up the earth. To balance the absorbed incoming energy, the earth will radiate the same amount of energy of infra-red radiation back to space. Greenhouse gases in the atmosphere, such as carbon dioxide, methane and nitrous oxide, absorb part of the infra-red radiation emitted from the earth and then return part of the re-emitted radiation to the earth. This is so-called the greenhouse effect. As such, greenhouse gases act like a blanket and prevent the earth from losing excessive heat. If the greenhouse gases concentration in the atmosphere increases, the earth surface will retain more heat than before, the air temperature of the earth surface will rise. But, if greenhouse gases are not present in the earth, its average surface temperature would be very low of around -18^oC rather than the about 14.5^oC found today.

 

 

Back to the top

How do human activities contribute to climate change?

Rapid development of economic and industrial activities since the 18th century has lead to excessive use of energy and resources. In particular, the burning of fossil fuels (such as coal and oil) emits large amounts of greenhouse gases into the atmosphere.  The increase in anthropogenic (human induced) greenhouse gas concentrations in the atmosphere enhances the greenhouse effect.  This can be visualized as the thickening of an invisible blanket covering the earth, resulting in global warming.  The human impact on climate during this era greatly exceeds that due to known changes in natural processes, such as solar changes and volcanic eruptions.

The main greenhouse gases added through human activities, including carbon dioxide, methane and nitrous oxide, will reside in the atmosphere for decades or even centuries.  The resulting global warming and its effect are thus long lasting.  As such, it is considered by scientists and policy makers to be one of the most serious problems that mankind has to face, not only now but for generations to come.

The Three Major Greenhouse Gases Produced by Human Activities

Ranking	Greenhouse Gas	Main human emission sources
1	Carbon dioxide (CO2)	Fossil fuel use and change in land use
2	Methane (CH4)	Agriculture and fossil fuel use
3	Nitrous oxide (N2O)	Agriculture

 

Atmospheric concentrations of key greenhouse gases (carbon dioxide, methane, and nitrous oxide) from 0 to 2005 (Source: IPCC, 2007) 
 

Back to the top

How are temperatures on Earth changing?

According to the Fourth Assessment Report of IPCC, the global mean temperature rose by 0.74 °C during the hundred year period between 1906 and 2005.  The rising rate was 0.13°C per decade in the 50 years from 1956 to 2005, nearly twice the rate in the past 100 years. Eleven (1998, 2005, 2003, 2002, 2004, 2006, 2007, 2001, 1997, 2008, 1999) of the last twelve years (from 1997 to 2008) rank among the 12 warmest years on record.

Observed changes in global average surface temperature. Changes are relative to corresponding averages for the period 1961-1990. Smoothed curve represents decadal averaged values while circles show yearly values. The shaded areas are the uncertainty intervals. (Source: IPCC, 2007)

Back to the top

What changes in precipitation can we expect?

Precipitation is the general term for rainfall, snowfall and other forms of frozen or liquid water falling from clouds.  Water on land and of the ocean becomes water vapour in the atmosphere through the processes of evaporation and evapotransporation. When water vapour rises, it cools down and when it reaches a certain height, it condenses as cloud and then falls back to ground as precipitation.  

Global warming (land and ocean) will affect the atmospheric moisture, precipitation and atmospheric circulation.  Increases in temperature lead to increases in the moisture-holding capacity of the atmosphere and enhance the hydrological cycle, altering the characteristics of precipitation amount, frequency, intensity, duration, type, etc.

Analysis of long term data shows that, unlike the global temperature rise, the regional variation in precipitation trends is large.  Some regions had a rising trend while some had a downward trend.

 

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: IPCC, 2007)

Back to the top

Is the amount of snow and ice on the Earth decreasing?

Yes. According to IPCC's Fourth Assessment Report, the observed decreases in snow and ice extent are consistent with the global warming. Mountain glaciers and snow cover on average have declined in both hemispheres. Since 1900, the extent of seasonal frozen ground in the Northern Hemisphere has decreased by about 7%, with a decrease in spring of up to 15%.

 

Average snow covered area in the Northern Hemisphere in March-April (Source : IPCC, 2007)

 

Satellite data since 1978 show that sea ice in the Arctic is shrinking in all seasons, especially in summer with a decreasing rate reaching 7.4% per decade.

 

 

The decrease of Arctic sea ice, minimum extent in 1982 and 2007

(Sources: Hugo Ahlenius, UNEP/GRID-Arendal ( http://maps.grida.no/go/graphic/the-decrease-of-arctic-sea-ice-minimum-extent-in-1982-and-2007-and-climate-projections) and National Snow and Ice Data Center)

 

Back to the top

Is sea level rising?

Yes. There is strong evidence that global sea level gradually rose in the 20th century and is currently rising at an increased rate. Global average sea level has risen since 1961 at an average rate of 1.8 mm per year and since 1993 at 3.1 mm per year. The two major causes of global sea level rise are thermal expansion of the oceans (water expands as it warms) and the melting of ice and snow over ground (glaciers, ice caps, and polar ice sheets). Since 1993 thermal expansion of the oceans has contributed about 57% of the sum of the estimated individual contributions to the sea level rise, with decreases in glaciers and ice caps contributing about 28% and losses from the polar ice sheets contributing the remainder.

 

 

Observed changes in global average sea level rise from tide gauge (blue) and satellite (red) data. Changes are relative to corresponding averages for the period 1961-1990. Smoothed curve represents decadal averaged values while circles show yearly values. The shaded areas are the uncertainty intervals. (Source: IPCC, 2007)

 

Back to the top

Are extreme weather events becoming more frequent ?

An extreme weather event (e.g. heavy rainfall, heat wave, cold spell, drought, etc.) is an infrequent event. Taking temperature as an example, the probability of occurrence of a temperature usually follows a normal distribution with a very low probability of occurrence (usually less than 5%) for extremely high or low temperature.  

In a changing climate, a relatively small shift in the mean of the distribution can result in substantial changes in the frequency of extreme events. According to the Fourth Assessment Report of IPCC, over the last 50 years, there were widespread changes in extreme temperatures. In many places, hot days, hot nights and heat wave have become more frequent, while cold days, cold nights and frost have become rarer. Moreover, the frequency of heavy rain events has also increased over most land areas. Since the 1970s, more intense and longer droughts have been observed over wider areas, particularly in the tropics and subtropics. In Hong Kong, it is observed that cold episodes have become rarer while very hot days and heavy rain events are becoming more frequent over the last 120 years or so.

 

 

Taking temperature as an example, an increase in the mean can result in substantial changes in the frequency of extreme events.

 

Back to the top

11. Can individual extreme events be explained by climate change?

It is not suitable to determine whether an individual extreme event is due to climate change alone because extreme weather events are usually caused by a combination of factors and a wide range of extreme events is a normal occurrence even in an unchanging climate. According to the Fourth Assessment Report of IPCC, observations over the past century suggested that the likelihood of some extreme events, such as heat waves and heavy precipitations, had increased due to climate change, and that the likelihood of others, such as cold spells and frost, had decreased.

 

Back to the top

12. Why snowstorms and extremely cold weather still occur in some regions under global warming?

The cold event in one place at one time (say, a week or a month) is just weather, and says nothing about climate. Global warming refers to a long term rising trend of globally averaged temperature attributed to human activity since the 20^th century, in addition to natural climate variability. Snowstorms and extremely cold weather are extreme climate events against a background of rising temperatures. Such events are part of natural climate variability and are not precluded by global warming. However, global warming has reduced the frequency of occurrence of extremely cold weather over past few decades. The frequency of extremely cold events is expected to decrease further if the global temperature keeps rising in the future.

 

Back to the top

Can the warming of the 20^th century be explained by natural factors?

The natural factors affecting climate include solar activity, volcanic activity, the Earth's orbital variation, etc.

Recent satellite observations confirmed solar irradiance has an 11-year cycle related to sunspots. However, there is no increasing trend in solar irradiance in the last few decades, while global temperatures have increased significantly. Since the Industrial Revolution, additional man-made greenhouse gases have far more impact on the climate change than the variation of the Sun's irradiance. Therefore, solar activity is not the main cause of the climate warming in the 20^th century.

Volcanic eruptions occasionally eject large amounts of dust and suspended particulates high into the atmosphere, temporarily shielding the Earth, reflecting sunlight back to space. This will decrease the solar energy received by the Earth's surface, causing short-term climate cooling.

The Earth's orbital variation brings itself closer or further away from the sun in predictable cycles, which could be related to the past ice-ages and very-long-term changes in the climate. Since the cycles have the periods of tens of thousands of years, they do not have much impact on the climate change observed over the past century.

Climate models cannot reproduce the warming observed in recent decades when only natural factors are considered. According to model simulation, we should have observed a decreasing trend in the global average temperature in the last few decades if only natural factors are considered, but we have observed a significant increasing trend in the global temperature. On the other hand, models can simulate the observed temperature changes in the 20^th century when human factors, such as greenhouse gas emissions, are included. Therefore, it is very unlikely that the 20^th century warming can be explained only by natural causes. Climate modeling results show that most of the global warming observed over the last 50 years is very likely due to human activities.

 

 

Modeling result of global temperature change by considering natural and anthropogenic factors. (relative to the corresponding average for 1901-1950) (Source: IPCC, 2007)

 

Back to the top