米俐的部落格

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

1. What Is the difference between weather and climate?

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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.

 

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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

 

 

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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.

 

 

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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) 
 

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How are temperatures on Earth changing?

According to the Fourth Assessment Report of IPCC, the global mean temperature rose by 0.74 during the hundred year period between 1906 and 2005.  The rising rate was 0.13 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)

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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)

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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)

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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)

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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.

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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.

 

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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.

 

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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)

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Are the oceans warming up too?

Human activities have increased the concentration of greenhouse gases in the atmosphere and enhanced the greenhouse effect, resulting in the accumulation of extra heat on Earth. The 'extra heat' has not only heated up the atmosphere and melted the ice/snow; it has also warmed up the oceans. The warming is not restricted to the sea surface. The sign of warming is observed down to 700m below the surface of the oceans. A direct consequence of ocean warming is the associated rise in the sea level. According to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), thermal expansion of sea water accounts for a rise of about 18 mm from 1993 to 2003 (i.e. about 1.6 mm per year).

Annual anomalies (relative to the 1961 to 1990 mean) of global sea surface temperature from 1850 to 2005 (Source : Fourth Assessment Report, IPCC)

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What is the "2°C Target"?

The 2°C target was first put forward by European Union (EU) in 1996 based on the impact studies of the 2nd Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) published in 1995. During the 1939th Council Meeting held in Luxembourg in 1996, EU indicated that the global mean surface temperature increase should not exceed 2°C above pre-industrial levels to avoid the risk of severe climate change impacts on human and ecological systems. The 2°C increase above pre-industrial levels corresponds to a 1.4°C increase above 1990-2000 levels. The estimated temperature rise in 1990-2000 relative to the pre-industrial levels (1750s or 1850s) is about 0.6°C.

This threshold was later adopted by some other countries and widely cited by many researchers, green groups and organizations as the target ceiling of global warming.

More information about EU's 2°C Target is available at :
http://ec.europa.eu/environment/climat/pdf/brochure_2c.pdf

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Are tropical cyclone activities changing?

Tropical cyclone is one of the most destructive weather systems on Earth. The possible change in tropical cyclone activity in a changing climate is a matter of great concern to the public and decision makers.

According to a study conducted by an expert team of the World Meteorological Organization (Knuston et al., 2010), it remains uncertain whether the changes in tropical cyclone activities based on the records of different basins in the last century or so, have exceeded the natural variability. This is because the trend detection is complicated by the large fluctuations in the frequency and intensity of tropical cyclones and the limitations in the availability and quality of global historical records. Looking into the future, theory and climate model simulations suggested that, if 21st century warming occurs as projected, the global frequency of tropical cyclones is expected to either decrease or remain unchanged. There will be some increase in the mean maximum wind speed of the tropical cyclones, although increases may not occur in all regions. The rainfall rates associated with tropical cyclones are likely to increase too.

In western North Pacific and the South China Sea (0-45°N, 100-180°E), analysis of the observational data shows that the annual number of tropical cyclones decreases from about 35 in the 1960s to about 27 after 2000. Locally, the annual number of tropical cyclones landing within 300 km of Hong Kong has decreased from about 3 in the 1960s to about 2.5 in the 2000s, but the trend is not statistically significant (Ginn et al., 2010).

Annual number of tropical cyclone landing within 300 km of Hong Kong (1961-2009)

Reference:

Knutson, T.R., J.L. McBride, J. Chan, K. Emanuel, G. Holland, C. Landsea, I. Held, J.P. Kossin, A.K. Srivastava and M. Sugi, 2010: Tropical Cyclones and Climate Change. Nature Geoscience 3, 157 - 163.

Ginn, W.L., T.C. Lee & K.Y. Chan, 2010: Past and Future Changes in the Climate of Hong Kong. ACTA Meteorologica Sinica, 24(2), 163-175, HKO Reprint No. 902.

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What is thermohaline circulation?

The thermohaline circulation is a large-scale density-driven circulation in the ocean, caused by differences in temperature (thermo) and salinity (haline). It is also driven by mechanical forces such as winds and tides. In the North Atlantic the thermohaline circulation consists of warm surface water flowing northward and cold deep water flowing southward (see the figure), resulting in a net poleward transport of heat, thereby moderating the tropics and warming the high latitudes of Europe.

There are concerns that greater rainfall and melting of land ice and snow associated with climate change may change the salinity of the oceans and slow down or even halt the thermohaline circulation. Up to the end of 20th century, parts of the thermohaline circulation exhibit considerable inter-decadal variability, but data do not support a coherent trend. According to the Fourth Assessment Report of IPCC, it is very likely that the Atlantic thermohaline circulation will slow down over the course of the 21st century, but very unlikely that it will undergo a large abrupt transition.

Simpified illustration of the Great ocean conveyor belt (Source: Climate change 2001 - Synthesis report, IPCC)

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What are global warming potential and carbon dioxide equivalent?

The ability of different greenhouse gases in trapping heat differs due to their different physical properties, lifetimes and their concentrations in the atmosphere. For a given greenhouse gas, its warming effect (greenhouse effect) over a time period (say 100 years) relative to that of carbon dioxide (CO₂) is represented by the Global Warming Potential (GWP). For example, the GWP (over a 100-year time frame) for methane is about 25 (25 times of that for CO₂). This means, in terms of contribution to global warming, the emission of 1 tonne of methane is equivalent to that of 25 tonnes of carbon dioxide.

Based on the above concept, carbon dioxide equivalent (CO₂-eq) is commonly used in carbon audit to gauge the combined greenhouse effect from a mixture of greenhouse gas emissions. CO₂-eq refers to the amount of CO₂, by weight, emitted into the atmosphere that would produce the same global warming potential as that of a given weight of other greenhouse gases being emitted. It is obtained by multiplying the emission of a greenhouse gas by its GWP for the given time horizon. As such, emission of 1 tonne of methane (GWP = 25) is equal to 25 tonnes of CO₂-eq.

Global Warming Potential of 3 major human-induced greenhouse gases

Greenhouse Gas	Global Warming Potential*
Carbon Dioxide (CO2)	1
Methane (CH4)	25
Nitrous oxide (N2O)	298

*Global warming potential of important greenhouse gases over the common 100-year time frame. (IPCC AR 4, WG1, Chapter 2, page 212-213, 2007)

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Climate change and extreme precipitation : Is there a connection ?

Although an individual extreme precipitation event cannot be solely attributed to climate change, as pointed out in scientific studies, climate change will likely affect the frequency of occurrence of such events in the long term. It is because the tropospheric warming due to increased anthropogenic (human induced) greenhouse gases can lead to an increase in the water-holding capacity of the atmosphere. The warming may also enhance the hydrological cycle and atmospheric instability. A less stable atmosphere with more water vapour in the air will provide a more favourable condition for intense precipitation events.

Moreover, some studies suggest that urbanization effect may also partly contribute to heavier rain in urban areas. This may be attributed to the urban heat island effect that enhances the convective activities, the increased roughness over a city that slows down the storm movement and the increase in the concentration of suspended particulates from urban activities that helps the formation and development of rain-bearing clouds.

Locally in Hong Kong, a study on the past occurrences of extreme rainfall indicates that heavy rain events in Hong Kong have become more frequent in the last 120 years or so.

Reference:

Weather extremes in a changing climate: Hindsight on Foresight, World Meteorological Organization, WMO-No. 1075, 2011

Min, S.K., X. Zhang, F.W. Zwiers and G.C. Hegerl, Human contribution to more-intense precipitation extremes, Nature 470, 378-381, 2011

Allan, R. P. and B. J. Soden, Atmospheric warming and the amplification of precipitation extremes, Science 321, 1481-1484, 2008

IPCC AR 4, WG1, Chapter 3, Section 3.4.2.1 : Surface and Lower-Tropospheric Water Vapour, page 272-273, 2007

Shepherd, J. M., H. Pierce, A. J. Negri, Rainfall modification by major urban areas: observations from spaceborne rain radar on the TRMM satellite, J. Appl. Meteor. 41, 689-701, 2002

Cao, K., Z. Ge, M. Xue and Y. Song, Analysis of Urban Rain Island Effect in Shanghai and Its Changing Trend, Water Resources and Power 27 (5), page 31-33, 54, 2009

Wong, M.C., H.Y. Mok and T.C. Lee, Observed Changes in Extreme Weather Indices in Hong Kong, Int. J. Climatol., October 2010, DOI: 10.1002/joc.2238, HKO Reprint No. 941

Note : Precipitation is the general term for rainfall, snowfall and other forms of frozen or liquid water falling from clouds.

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Do volcanoes emit more carbon dioxide than human activities?

Carbon dioxide emitted by volcanoes to the atmosphere is one of the natural factors contributing to variations in the ancient climate. However, various studies have shown that, in the last century, the annual amount of carbon dioxide released by human activities far exceeded that released by terrestrial and submarine volcanoes. The estimated amount of anthropogenic (i.e. human-induced) carbon dioxide emission in 2010 is about 35 gigatons, which is more than 100 times the estimated global volcanic carbon dioxide emission (about 0.26 gigaton per year).

Reference:

U.S. Geological Survey, Volcanic Gases and Climate Change Overview.

Gerlach, T., 2011 : Volcanic versus anthropogenic carbon dioxide. EOS, Transactions, American Geophysical Union, 92(24), 201-208.

Fredlingstein, P., R.A. Houghton, G. Marland, T. Hackler, T.A. Boden, T.J. Conway, J.G. Canadell, M.R. Raupach, P. Ciais, and C. Le Quere, 2010 : Update on CO2 emissions, Nat. Geosci., 3(12), 811-812.

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What is medieval warm period?

Medieval warm period (MWP) generally refers to the warm period roughly around 900-1300 AD in some regions of the Northern Hemisphere (e.g. North Atlantic, Southern Greenland, the Eurasian Arctic, and parts of North America). Due to the scarcity of data prior to 1600, the precise duration and areal extent of the medieval warmth and whether it was a global phenomenon are still areas of active research. As pointed out by the United Nations Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) and some recent studies, there is no sufficient evidence to support that MWP was as warm as the 20th century as a whole. According to IPCC AR4, the warming in the late 20th century is widespread over the globe and the average Northern Hemisphere temperature in the late 20th century is likely the highest in the past 1300 years.

The causes of warming during MWP and the late 20th century are also different. The MWP was mainly due to natural factors, such as solar and volcanic activity. However, as indicated in IPCC AR4, most of the warming since the middle of the 20th century very likely results from the human-induced increase in atmospheric greenhouse gas concentration.

Records of Northern Hemisphere temperature variation during the last 1300 years with 12 reconstructions using multiple climate proxy records (tree rings, boreholes, ice core/ice boreholes, etc.) shown in colour and instrumental records shown in black. (Source IPCC AR4 WG1 Fig. 6.10)

Reference:

IPCC AR 4, Working Group I, 2007: Chapter 6: Palaeoclimate.

Mann, M. E., Z. Zhang, S. Rutherford, R.S. Bradley, M.K. Hughes, D. Shindell, C. Ammann, G. Faluvegi and F. Ni, 2009 : Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly. Science, 326, 1256-1260.

P. A. Stott, S. F. B. Tett, G. S. Jones, M. R. Allen, J. F. B. Mitchell, and G. J. Jenkins, 2000 : External Control of 20th century temperature by natural and anthropogenic forcings. Science, 290, 2133-2137.

National Research Council (U.S.), Committee on Surface Temperature Reconstructions for the Last 2,000 Years, 2006 : Surface temperature reconstructions for the last 2,000 years, National Academies Press, ISBN 9780309102254.

Koch J. and J. J. Clague, 2011: Extensive glaciers in northwest North America during Medieval time. Climatic Change, 107, 593-613.

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將會有更多的氣候變化小百科推出，萬勿錯過!

1. 「天氣」與「氣候」有什麼不同？

雖然「天氣」與「氣候」同樣是形容大氣狀態的名詞，但它們所描述的時間尺度有很大分別。 「天氣」是指一個地區瞬時或較短時間內（幾小時到幾天）的大氣綜合狀態(包括風速、雲量、溫度、降雨、氣壓等氣象要素)。 「氣候」是指一個地區在一段較長時期裏的平均氣象狀況及變化特徵。簡單來說，「氣候」可以解作“平均的天氣”。根據世界氣象組織（WMO）的定義，用作氣候統計的參考年期為不少於30年。

有關香港的氣候特徵及資料，可瀏覽以下天文台的「氣候資料服務」網頁:

http://www.weather.gov.hk/cis/climat_c.htm

 

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2. 什麼是氣候變化？

根據政府間氣候變化專門委員會(IPCC)，氣候變化是指氣候隨時間的任何變化，無論其原因是自然變率，還是人類活動的結果。這有別於聯合國氣候變化框架公約(UNFCCC)中的用法。在公約中，氣候變化是指直接或間接歸因於改變全球大氣成分的人類活動，所引起的氣候變化，這種變化是疊加在同期觀測到的氣候自然變率之上的。

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3. IPCC是一個什麼的組織？

IPCC是政府間氣候變化專門委員會（Intergovernmental Panel on Climate Change）的英文簡稱。它是一個政府間的組織。認識到氣候變化是一個非常複雜的問題，政策決策者需要一個客觀的資訊來源來評估與理解人為引起的氣候變化，它的潛在影響，以及適應和減緩方案的科學基礎。於是世界氣象組織（WMO）和聯合國環境規劃署（UNEP）于1988年建立了IPCC，專責研究由人類活動所造成的氣候變化

。

IPCC是氣候變化的權威機構。其主要工作是定期對氣候變化的認知現狀進行評估。1990年發表的第一份評估報告及1995年發表第二份評估報告分別促使了《聯合國氣候變化框架公約》及《京都議定書》的確立。及後2001年發表的第三份評估報告和一些“特別報告”給予《聯合國氣候變化框架公約》及《京都議定書》發展的重要資訊。在2007年發表的第四份評估報告中確認全球氣候暖化，20世紀中期以來全球平均氣溫的上升，極有可能是由於人為的溫室氣體濃度上升所引致。

相關連結:

政府間氣候變化專門委員會

《聯合國氣候變化框架公約》

《京都議定書》

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4. 什麼是溫室效應？

地球表面的熱量主要來自太陽，太陽輻射主要以可見光形式抵達地球後，令地球受熱。為了平衡所吸收的入射能量，地球本身亦會向太空輻射出等量的紅外線。而大氣中的溫室氣體例如二氧化碳，甲烷及一氧化二氮會吸收部份地球釋放的紅外線，然後將部份重新釋放的紅外線輻射回地球，形成所謂溫室效應。因此，溫室氣體就像大棉被一樣，減少地球流失熱量。如果大氣中的溫室氣體濃度增加，地面吸收的熱量會比以前多，地球的氣溫就會上升。但是，如果大氣層沒有溫室氣體，地球表面的平均溫度不會是現在的約14.5度，而是十分低的零下18度左右。

 

 

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人類活動如何引起氣候變化？

自18世紀開始，經濟及工業活動急速發展，人類在這段期間大量耗用地球上的能源和資源，尤其是燃燒會釋放大量溫室氣體的化石燃料(例如煤和石油)。人類活動令大氣中溫室氣體濃度上升，溫室氣體就像無形的大棉被一樣覆蓋地球，引致溫室效應增強，並帶來全球變暖。人類在這個時期裡對氣候的影響遠超過了太陽活動、火山爆發等自然過程帶來的影響。

人類活動所產生的主要溫室氣體包括二氧化碳、甲烷和一氧化二氮。它們會在大氣中逗留一段非常長的時間，其暖化效果會持續數十年甚至數個世紀，對全球變暖有長遠影響。正因如此，很多科學家及政策決策者都認為全球變暖是人類現在及未來都要面對的最嚴重問題。

三大人為溫室氣體

排名	溫室氣體	主要人為排放源
1	二氧化碳 (CO2)	使用化石燃料及土地利用變化
2	甲烷 (CH4)	農業活動及使用化石燃料
3	氧化亞氮 (N2O)	農業活動

0-2005年主要人為溫室氣體(包括二氧化碳、甲烷和一氧化二氮) 在大氣中的濃度  (來源：IPCC, 2007)

 

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地球的溫度正在發生怎樣的變化？

根據政府間氣候變化專門委員會的第四份評估報告，全球平均氣溫在1906到2005年的一百年間升了0.74度。而在1956到2005年的50年，每10年上升0.13 度，升幅是過去100年的兩倍。過去12年中(1997-2008)，有11年(1998, 2005, 2003, 2002, 2004, 2006, 2007, 2001, 1997, 2008, 1999)名列最暖的12年內。

全球平均表面氣溫的變化。變化是相對於1961-1990年的平均值。平滑曲線代表十年際平均數值，圓點代表每年數值。陰影部份為不確定間距。(來源：IPCC, 2007)

 

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降水會有什麼變化？

降水是降雨、降雪或其他從雲中降落的凍結水或液體水的總稱。地面及海洋的水透過蒸發和蒸騰變成水汽，當水汽上升到某個高度便會凝結形成雲，然後降回地面，形成降水。

全球暖化（陸地和海洋）會影響大氣中的水汽含量、降水和大氣環流。溫度上升將會增加大氣中可容納的水汽及加劇水循環，改變降水量、頻率、強度、時間和種類等特徵。

跟全球氣溫上升不同，長期數據分析顯示不同地區的降水趨勢差異很大，有些地區有所增加，有些則減少。

 

水循環

 

圖中顯示各地區1900 至 2005年的降水趨勢 。白色背景降水曲線圖表示降水增加，色背景圖表示降水減少。(來源 : IPCC, 2007)

 

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地球上的冰雪是否正在減少？

是的。根據政府間氣候變化專門委員會的第四份評估報告，觀測到的冰雪面積減少趨勢與全球變暖趨勢一致。在南北半球，冰川和積雪平均面積已呈退縮趨勢。自1900年以來，北半球季節性凍土最大面積減少了大約7%，春季凍土面積的減幅高達15%。

 

北半球三月至四月平均積雪面積 (來源：IPCC, 2007)

  

從1978年以來的衛星資料顯示，北冰洋海冰在所有季節裡都在縮小，在夏季尤甚，退縮率達每十年7.4%。

  

在1982年及2007年北冰洋海冰最小覆蓋範圍的情況，顯示北冰洋海冰正在減少。

 (來源：Hugo Ahlenius, UNEP/GRID-Arendalz (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)

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海平面是否正在上升？

是的。有強力證據顯示全球海平面在20世紀逐步升高，而上升的速度正在加快。自1961年以來，全球平均海平面上升的平均速率為每年1.8毫米，而從1993年以來平均速率為每年3.1毫米。海平面上升的兩個主因是海水的熱膨脹（溫度上升時水體積膨脹）和陸地冰雪融化(冰川、冰帽和極地冰蓋)。自1993年以來，海洋熱膨脹對海平面上升的預估貢獻率占所預計的各貢獻率之和的57%，而冰川和冰帽的貢獻率則大約為28%，其餘的貢獻率則歸因於極地冰蓋。

 

由測潮器(藍色)及衛星(紅色)數據顯示的全球平均海平面上升變化。變化是相對於1961-1990年的平均值。平滑曲線代表十年際平均數值，圓點代表每年數值。陰影部份為不確定間距。(來源：IPCC, 2007)

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極端天氣事件是否越來越多?

極端天氣事件(例如:大雨、熱浪、寒潮、乾旱等)是不常發生的。以氣溫為例，出現某氣溫的機率多遵照常態分佈，只有很少的機率(通常少于5%)出現極端高溫或極端低溫情況。

氣候變化改變了氣候要素分佈的平均值，而這變化即使很少，都能使極端天氣事件出現的機率大大改變。根據政府間氣候變化專門委員會的第四份評估報告，在過去50年間，世界各地的極端氣溫出現情況已發生變化。在很多地方，熱晝、熱夜及熱浪的出現頻率越來越多，但冷晝、冷夜及霜凍的出現頻率則越來越少。此外，在大多數陸地上觀測到的大雨發生頻率有所增加。而自1970年代以來，在更大範圍地區，尤其是在熱帶和副熱帶地區，觀測到比從前更嚴重和更持久的乾旱。在香港，過去120多年寒冷天氣發生次數日漸減少，但出現酷熱天氣及大雨情況則越來越多。

  

 

以氣溫為例，氣候平均值升高能使極端天氣事件出現的機率大大改變。

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11. 個別極端天氣事件是否能用氣候變化解釋?

我們不宜將個別極端天氣事件單單歸因於氣候變化，這是因為極端天氣事件通常是由多個因素共同作用引發的，而且極端天氣事件也是正常氣候的一部份，即使在氣候沒有任何變化的情況下，極端天氣事件仍然會出現。不過，根據政府間氣候變化專門委員會的第四份評估報告，從過去一個世紀的觀察數據顯示，部分極端天氣事件(例如:熱浪、暴雨等)發生的可能性已因氣候變化而增加，但亦有部分極端天氣事件(例如:寒潮、霜凍等)出現的機會因此而減少。

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12. 在全球氣候變暖的背景下，為什麼在某些地區間中仍出現雪災和嚴寒天氣?

在某地某時（例如：一星期或一個月）出現的寒冷天氣事件只是天氣，並不是氣候。全球氣候變暖是指在自然的氣候變率下，自20世紀因人類活動造成的長期全球平均氣溫上升情況。雪災和嚴寒天氣是在全球升溫背景下的極端氣候事件。這些事件是自然氣候變率的一部分，並不會因全球氣候變暖而不再發生。不過，全球氣候變暖在過去數十年間減低了嚴寒天氣出現的頻率。如果全球升溫持續，嚴寒天氣出現的頻率會繼續減少。

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能用自然因素解釋20世紀的變暖嗎？

影響氣候的自然因素包括太陽活動、火山活動、地球軌道變化等。

最新的衛星觀測證實太陽輻射量具有與太陽黑子相關的11年週期變化，而太陽輻射量在過去幾十年間沒有增加的趨勢，但全球氣溫卻顯著上升。自工業革命以來，人為溫室氣體的增加對氣候變化的影響，遠大於因太陽輻射量變化造成的影響，所以太陽活動不是造成20世紀氣候變暖的主因。

火山爆發有時會噴發出大量灰塵和懸浮微粒到大氣層中，這些灰塵和懸浮微粒會暫時將部分到達地球的太陽光反射回太空，減少地球表面接收到的太陽能量，造成短期氣候冷卻。

而地球軌道的週期變化令地球週期性地接近或遠離太陽，這變化相信與過去冰河時期和非常長期的氣候變化有一定的關係。但由於地球軌道的變化週期是數以萬年計的，所以對於在過去一個世紀出現的氣候變化不會有很大的影響。

氣候模式在僅僅考慮自然因素時，無法模擬出最近數十年間觀測到的升溫現象。從這些氣候模式結果顯示，在自然因素的影響下，近數十年的全球平均溫度應該呈下降趨勢，但我們觀察到的卻是全球氣溫顯著上升。但當模式加入溫室氣體排放等人為因素時，就能模擬出在20世紀觀測到的氣溫上升趨勢。所以20世紀的變暖不太可能只用自然因素來解釋。氣候模式模擬結果表明，在過去 50年大部分觀測到的全球氣溫上升很有可能是由人類活動所致。

  

 

考慮自然及人為因素的全球氣溫模擬結果 (與1901-1950年的平均值相比) (來源：IPCC, 2007)

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海洋也在變暖嗎？

人類活動令大氣中溫室氣體濃度上升，引致溫室效應增強，令整個地球累積額外熱能。這些「額外熱能」不單使全球氣溫上升和冰雪溶化，還會令全球海洋暖化。而且暖化並不局限於海洋表面，觀測數據顯示在深至700米的海洋也出現暖化的跡象。海洋暖化的一個直接後果是海平面高度上升。根據政府間氣候變化專門委員會的第四份評估報告，在1993至2003年間，海水的熱膨脹使全球平均海平面上升約18毫米(即大約每年1.6毫米)。

  

 

自1850年至2005年全球海面温度的年距平(相對1961至1990年平均) (來源：政府間氣候變化專門委員會第四次評估報告書)

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什麼是「兩度目標」？

歐盟(European Union)於1996年根據政府間氣候變化專門委員會第二次評估報告書(1995)中的影響評估報告，首次提出「兩度目標」這個概念。歐盟在1996年於盧森堡舉行的第1939次會議中指出，假如能將全球表面平均氣溫維持在不超過工業革命前2℃或以下的水平，便能避免人類及自然生態系統因氣候變化而遭到嚴重衝擊。在1990-2000年時，全球平均氣溫已較工業革命前(1750年代或1850年代)高約0.6℃，亦即是說離開「兩度目標」只剩下1.4℃的空間。

「兩度目標」後來被一些國家所採用，而且這目標也被很多研究學者、綠色組織及機構視作為全球暖化的可接受上限。

關於歐盟「兩度目標」更多的資料可到以下網站瀏覽:
http://ec.europa.eu/environment/climat/pdf/brochure_2c.pdf (只有英文版本)

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熱帶氣旋活動有什麼變化？

熱帶氣旋是地球上其中一種最具破壞力的天氣系統，因此普羅大眾與決策者都非常關心氣候變化會否影響熱帶氣旋的活動。

根據世界氣象組織的專家小組所發表的研究報告(Knuston et al., 2010)，基於不同區域過去一世紀左右的熱帶氣旋記錄，現時仍不能確定熱帶氣旋活動的變化趨勢幅度，是否已超過了自然變化的波幅。這是由於熱帶氣旋的出現頻率和強度有很大的差異，而且過去全球熱帶氣旋記錄的完整度及質素有限，令熱帶氣旋活動趨勢變化的分析非常困難。至於未來的情況，理論與氣候模式結果都顯示，假如全球在21世紀如推算般繼續變暖，全球熱帶氣旋的出現頻率將會下降或維持不變。在部分地區熱帶氣旋的平均最高風速會上升，同時與熱帶氣旋相關的降雨也可能會增加。

在西北太平洋及南海 (赤道至北緯45度，東經100至180度)，從過去觀察資料顯示平均每年的熱帶氣旋數目由1960年代的35個，減少至2000年後的27個。而在本地方面，在香港300公里內登陸的熱帶氣旋數目，由1960年代的每年平均3個下降至2000年代的約2.5個，但從統計學來說，這變化趨勢並不顯著(Ginn et al., 2010)。

  

每年在香港300公里內登陸的熱帶氣旋數目(1961-2009)

參考:

Knutson, T.R., J.L. McBride, J. Chan, K. Emanuel, G. Holland, C. Landsea, I. Held, J.P. Kossin, A.K. Srivastava and M. Sugi, 2010: Tropical Cyclones and Climate Change. Nature Geoscience 3, 157 - 163.

Ginn, W.L., T.C. Lee & K.Y. Chan, 2010: Past and Future Changes in the Climate of Hong Kong. ACTA Meteorologica Sinica, 24(2), 163-175, HKO Reprint No. 902.

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什麼是溫鹽環流？

溫鹽環流是大尺度的海洋環流，由溫度和含鹽度的差異所致，也受風力和潮水所推動。在北大西洋，溫鹽環流的表面暖水向北流動，而深海冷水向南流動 (見附圖)，造成淨熱量從熱帶向北輸送，為歐洲高緯地區送暖。

有人擔心與氣候變化相關的降雨增加和陸上冰雪融化可能會令到海洋的含鹽度改變，使溫鹽環流減慢，甚至停止。直至20世紀末，部分大西洋溫鹽環流顯示出很大的年代際變化，但是從觀測資料上看不出它的強度有一致的變化趨勢。根據政府間氣候變化專門委員會的第四份評估報告，預料在21世紀，大西洋的溫鹽環流很有可能減慢，但不大可能出現突變。

  

 

輸送洋流的簡化示意圖(源自：氣候變化2001-綜合報告, IPCC)

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什麼是全球增溫潛勢和二氧化碳當量？

各種溫室氣體由於其物理特性不同，而且在大氣中的壽命和濃度各異，所以不同溫室氣體吸收熱量的能力會各有差異。某一溫室氣體在一段時間內（例如100年）的暖化效應(溫室效應) ，相對於二氧化碳（CO₂）的暖化效應便是全球增溫潛勢（GWP）。例如:甲烷的全球增溫潛勢（在100年的時段）約為 25（二氧化碳的25倍），這意味著，排放1噸甲烷與排放25噸的二氧化碳所造成的暖化效應是相同的。

基於上述概念，二氧化碳當量（CO₂-eq）通常用於碳排放審計，以評估各種溫室氣體的混合排放所造成溫室效應的總效果。對於某一重量的其他溫室氣體排放, 二氧化碳當量是指產生與它相同變暖潛力所需的二氧化碳重量。二氧化碳當量可從溫室氣體的排放重量乘以該溫室氣體在特定時段內的全球增溫潛勢（GWP）而得出。因此，排放1噸甲烷（GWP = 25）相等於25噸二氧化碳當量。

三大人為溫室氣體的全球增溫潛勢

溫室氣體	全球增溫潛勢*
二氧化碳 (CO2)	1
甲烷 (CH4)	25
一氧化二氮 (N2O)	298

*主要人為溫室氣體在100年時段內的全球增溫潛勢。 (IPCC AR4, WG1, Chapter 2, page 212-213, 2007)

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氣候變化與極端降雨事件：兩者是否有關連？

雖然個別極端降雨事件不能單單歸因於氣候變化，但有科學研究指出，長期來看，氣候變化有可能影響極端降雨事件的發生頻率。這是由於人為的溫室氣體使對流層暖化，令大氣的持水量增加，並可能加速水循環和使大氣的不穩定性上升。一個較不穩定和持水量較高的大氣會為強降雨事件發生提供有利的條件。

除此以外，一些研究指出城巿化效應也可能是令巿區出現較大雨的部分原因。這是因為城市熱島效應加強了城市地區的對流活動，同時城市化令城巿地面粗糙度增加，使風暴的移動速度減慢。而且各種城市活動增加了空氣懸浮粒子濃度，令雨雲更容易形成並發展。

本港方面，一項極端降雨事件的研究顯示，過去120多年以來在香港發生的大雨事件頻率有所增加。

參考資料:

Weather extremes in a changing climate: Hindsight on Foresight, World Meteorological Organization, WMO-No. 1075, 2011

Min, S.K., X. Zhang, F.W. Zwiers and G.C. Hegerl, Human contribution to more-intense precipitation extremes, Nature 470, 378–381, 2011

Allan, R. P. and B. J. Soden, Atmospheric warming and the amplification of precipitation extremes, Science 321, 1481–1484, 2008

IPCC AR 4, WG1, Chapter 3, Section 3.4.2.1 : Surface and Lower-Tropospheric Water Vapour, page 272-273, 2007

Shepherd, J. M., H. Pierce, A. J. Negri, Rainfall modification by major urban areas: observations from spaceborne rain radar on the TRMM satellite, J. Appl. Meteor. 41, 689-701, 2002

Cao, K., Z. Ge, M. Xue and Y. Song, Analysis of Urban Rain Island Effect in Shanghai and Its Changing Trend, Water Resources and Power 27 (5), page 31-33, 54, 2009

Wong, M.C., H.Y. Mok and T.C. Lee, Observed Changes in Extreme Weather Indices in Hong Kong, Int. J. Climatol., October 2010, DOI: 10.1002/joc.2238, HKO Reprint No. 941

附註 : 降水泛指降雨、降雪及其它由雲降下的固態或液態水。

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火山比人類活動釋放更多二氧化碳嗎？

由火山釋放到大氣的二氧化碳，是造成古氣候變化的自然因素之一。然而，各項研究顯示在過去一個世紀每年由人類活動排放的二氧化碳重量，遠遠超過由陸地和海底火山所釋放的總和。在2010年由人類活動引起的二氧化碳排放量估計約為 350億噸，是全球火山二氧化碳估計排放量（約每年2.6億噸）的100倍以上。

參考:

U.S. Geological Survey, Volcanic Gases and Climate Change Overview.

Gerlach, T., 2011 : Volcanic versus anthropogenic carbon dioxide. EOS, Transactions, American Geophysical Union, 92(24), 201-208.

Fredlingstein, P., R.A. Houghton, G. Marland, T. Hackler, T.A. Boden, T.J. Conway, J.G. Canadell, M.R. Raupach, P. Ciais, and C. Le Quere, 2010 : Update on CO2 emissions, Nat. Geosci., 3(12), 811-812.

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什麼是中世紀暖期？

中世紀暖期(Medieval Warm Period)一般是指大約在公元900至1300年在北半球一些地區(例如北大西洋、格陵蘭南部、歐亞大陸的北極地區和北美部分地區)的溫暖時期。由於缺乏公元1600年以前的數據，中世紀暖期的確實持續時間、溫暖地區的分佈範圍，以及它是否一個全球現象仍在研究當中。聯合國政府間氣候變化專門委員會（IPCC）的第四份評估報告和最近一些研究指出，現時仍沒有充分的證據支持中世紀暖期的整體溫度能與20世紀相比。根據 IPCC第四份評估報告，在20世紀後期的變暖遍及全球，而20世紀後期的北半球平均溫度很可能是在過去1300年中最高的。

中世紀暖期和20世紀後期的變暖原因也截然不同。中世紀暖期主要由自然因素引起，例如太陽活動和火山活動。然而，正如IPCC第四份評估報告指出，20世紀的變暖很有可能是由人類活動造成大氣中溫室氣體濃度增加所引致的。

  

各顏色線分別代表12種氣候代用記錄（樹木年輪、鑽孔、冰芯/冰鑽孔等）所重建過去1300年的北半球溫度變化記錄，而黑色線則是利用儀器記錄的溫度變化資料。 （來源： IPCC第四份評估報告，第一工作組，圖6.10)

  

參考:

IPCC AR 4, Working Group I, 2007: Chapter 6: Palaeoclimate.

Mann, M. E., Z. Zhang, S. Rutherford, R.S. Bradley, M.K. Hughes, D. Shindell, C. Ammann, G. Faluvegi and F. Ni, 2009 : Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly. Science, 326, 1256-1260.

P. A. Stott, S. F. B. Tett, G. S. Jones, M. R. Allen, J. F. B. Mitchell, and G. J. Jenkins, 2000 : External Control of 20th century temperature by natural and anthropogenic forcings. Science, 290, 2133-2137.

National Research Council (U.S.), Committee on Surface Temperature Reconstructions for the Last 2,000 Years, 2006 : Surface temperature reconstructions for the last 2,000 years, National Academies Press, ISBN 9780309102254.

Koch J. and J. J. Clague, 2011: Extensive glaciers in northwest North America during Medieval time. Climatic Change, 107, 593-613.

  

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