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Endless wildfires

According to the analysis of the World Meteorological Organization, 2019 was the second warmest year on record globally, with the past five years, 2015-2019, being the five warmest on record. Under global warming, extreme hot weather events ravaged different parts of the world. For example, Australia registered the hottest summer on record between the end of 2018 and early 2019. In December 2019, a high temperature near 50°C was recorded in southern Australia. Europe could not be spared and was hit by heatwaves twice within a month between late June and late July 2019. In the first heatwave, France registered a record breaking temperature of 46°C. The second heatwave was more pervasive, breaking high temperature records in Germany, the Netherlands, Belgium, Luxembourg and the United Kingdom. During the two heatwaves, more than 1,400 excess deaths as compared with the average were observed in affected regions.

Very often, hot weather is accompanied by wildfires (also known as bushfires and forest fires), especially in dry environment. In June 2019, more than a hundred intense and long-lived wildfires occurred in the Arctic Circle. In the same year, the Amazon forest experienced the most active fire season since 2010. Yet, the most significant wildfire in 2019 was the Australian bushfires which occurred in the second half of the year. The bushfires began in Queensland in September 2019, spreading south to New South Wales and Victoria (Figure 1), and gradually became under control in March 2020. The bushfires have killed many people, destroyed hundreds of homes and burned large areas of land, causing massive devastation to ecosystems and the environment. Analyses showed that the bushfires had burned 21 per cent of Australia’s forests, far exceeding similar records in other continents over the past 20 years. In addition, scientists estimated that 800 million animals were killed by the fires in New South Wales alone, and the total number of animals killed exceeded 1 billion for the whole nation.

Is this extreme bushfire event a random event, or its chance of occurrence has been enhanced by climate change? We can get some clues from the climate report published by the Bureau of Meteorology, Australia. The temperature of Australia has risen by 1°C since 1910, leading to an increase in the frequency of extreme heat events. In southeastern Australia, there has been a decline of around 11% of April-October rainfall since the late 1990s, and there is a long-term increasing trend in the Forest Fire Danger Index (Figure 2). 2019 was the warmest and driest year on record for Australia, setting the scene for the bushfires. In fact, many studies have pointed out the link between climate change and Australian bushfires. For instance, climate change made the 2018 Queensland bushfires four times more likely; and climate change will make firestorms more likely in southeastern Australia.

In light of the Australian bushfires, scientists have reviewed recent research results and drawn the conclusion that human-induced climate warming has already led to a global increase in the frequency and severity of fire weather, increasing the risks of wildfire. Wildfires not only pose direct threat to lives and properties but also release pollutants detrimental to human health and ecosystems (Figure 3). In addition, carbon dioxide released by wildfires exacerbates the greenhouse effect. Natural factors that trigger wildfires (e.g. lightning) are beyond human control. However, reducing greenhouse gas emissions to mitigate climate change impact could be under human’s control. The threats of climate change on the human daily lives and the ecosystems are imminent. We must make every effort to combat climate change proactively.

Figure 1  A river of smoke from bushfires in Victoria and New South Wales, Australia, on 2 January, 2020 (Source: National Aeronautics and Space Administration, US)

Figure 2  Trends from 1978 to 2017 in the annual sum of the daily Forest Fire Danger Index. Positive trends are indicative of an increasing length and intensity of the fire weather season. (Source: Bureau of Meteorology, Australia)

Figure 3  Wildfires devastating ecosystems

Why Carbon Dioxide is a Greenhouse Gas?

Solar radiation reaches the Earth in the form of shortwave radiation and heats up the Earth's surface. The Earth’s surface then emits infrared radiation (longwave radiation) in order to cool down. However, greenhouse gases in the atmosphere will absorb part of the infrared radiation emitted by the Earth, and then re-emit infrared radiation in all directions. Part of the infrared radiation will escape to space but part of it will go back to the Earth, heating up the Earth's surface and causing the greenhouse effect.

The Earth's atmosphere consists of nitrogen (~78%), oxygen (~ 21%), argon (~0.9%) and trace gases such as carbon dioxide, water vapour, methane, etc. Nitrogen and oxygen together account for more than 90% of the atmosphere but they are not greenhouse gases. Carbon dioxide accounts for just about 0.04% but it is the single most important greenhouse gas in the atmosphere and also the principle control knob of Earth's temperature. Why carbon dioxide is a greenhouse gas? To answer this question, we first need to understand that infrared radiation is an electromagnetic wave. Laws of physics require that molecular vibration must lead to a change in the relative distribution of charge for the gas to absorb electromagnetic radiation.

Let us take nitrogen as an example for illustration. A nitrogen molecule consists of two nitrogen atoms (blue balls, left side of Figure 1). The positive charges carried by the two nitrogen atoms are identical and symmetrical in distribution. The vibration of nitrogen atoms along the chemical bond does not change the relative distribution of charges (left side of Figure 1). Therefore, nitrogen cannot absorb infrared radiation. Then how about carbon dioxide? A carbon dioxide molecule consists of two oxygen atoms (red balls, right side of Figure 1) and one carbon atom (grey balls, right side of Figure 1). Three different modes of vibration are possible with such configuration (right of Figure 1). When the carbon atom moves along the chemical bond towards either one of the oxygen atoms, or moves up and down relative to the oxygen atoms, the relative distribution of the charges will be altered and hence carbon dioxide can absorb infrared radiation.

Carbon dioxide is the single most important greenhouse gas in the atmosphere. The annual mean concentration set a record high again in 2017, reaching 405.5 ppm which is about 46% above pre-industrial levels. A recent study showed that the present-day atmospheric concentration of carbon dioxide is likely the highest in the last three million years, which is a worrisome situation indeed.

Figure 1 Modes of vibration of nitrogen and carbon dioxide molecules

Corals facing the double whammy brought about by climate change

Corals are of high value to ecosystems and human. Coral reefs provide food and habitat for marine creatures, supporting the associated ecosystems. Coral reef structure along the coastal waters buffers against the threat of high waves whipped up by winds and storms, protecting the lives and property of coastal communities. However, climate change caused by human activities has brought about a double whammy to corals.

Human activities have released a large amount of greenhouse gases into the atmosphere, enhancing the greenhouse effect and trapping extra heat on the Earth. About 90 percent of the extra heat has been absorbed by the oceans, leading to ocean warming which, in turn, causing coral bleaching. The colours of corals originate from the symbiotic algae living in the coral structure. As sea water warms, corals tend to reject the symbiotic algae and turn themselves white. Since the major food source for corals comes from the photosynthesis products generated by the symbiotic algae, bleaching corals will become fragile and more vulnerable to diseases, and may even ultimately lead to their demise.

Besides, about 25 percent of the carbon dioxide emitted by human activities has been taken up by the oceans, leading to ocean acidification. The carbonate ions in more acidic sea water will be reduced, seriously affecting the calcification process and skeletal growth of corals.

The World Meteorological Organization announced that the atmospheric concentration of carbon dioxide had soared to 403.3 ppm in 2016, the highest level in the last 800,000 years. With an unabated increase in greenhouse gases, global warming continues. The average frequency of coral bleaching events has increased by four times in the past 40 years. According to the assessment of the United Nations Educational, Scientific and Cultural Organization, all World Heritage coral reefs are likely to disappear by 2100 unless there is a drastic reduction in carbon dioxide emission.

A turtle swimming over the bleached coral of Great Barrier Reef in Australia
(Photo source:The Ocean Agency / XL Catlin Seaview Survey / Richard Vevers)

Milankovitch Cycles

In the early 20th century, Milutin Milankovitch, a Serbian astronomer, proposed that the coming and going of ice ages on Earth were closely related to three orbital geometric parameters of Earth's revolution around the Sun. The first parameter is the shape of Earth's orbit around the Sun. The orbit changes from nearly circular to elliptical in a periodical manner and the whole cycle takes about 100,000 years. The orbital changes will affect the amount of solar energy reaching the Earth in different seasons.

Figure 1

Figure 1Left: circular orbit, right: elliptical orbit (source: US National Aeronautics and Space Administration, or NASA)

The second parameter is the tilt of Earth's rotational axis. The axial tilt varies between 22.1 and 24.5 degrees in a cycle of around 40,000 years. Changes in this parameter will not alter the total amount of solar energy reaching the Earth but will affect the latitudinal distribution of insolation.

Figure 2

Figure 2The variation in Earth's axial tilt (source: NASA)

The third parameter is the precession of Earth's rotational axis, i.e. the wobbling of Earth's axis. A full cycle of the wobbling takes about 26,000 years. Changes in this parameter will also affect the latitudinal distribution of insolation.

Figure 3

Figure 3The precession of Earth's rotational axis (source: NASA)

According to Milankovitch, the impact of these parameters on the amount of insolation reaching the high latitudes in the Northern Hemisphere, where most of the ice and snow on Earth are found, is particularly important. Variation in ice and snow cover can lead to a positive feedback mechanism. For example, when the amount of insolation reaching the northern high latitudes decreases, summer heat is not sufficient to melt all the ice and snow precipitated in the preceding winter, leading to an overall increase of ice and snow in the course of the year. Increasing ice and snow will reflect more sunlight back into space, thereby reducing the amount of heat absorbed on Earth. This will set up a viscous cycle that supports further growth of ice and snow. Persistent increase in ice and snow year after year will eventually push the Earth into an ice age.

Milankovitch's theory was finally accepted in the late 20th century after thorough examinations by scientists.

Is the melting of glaciers in West Antarctica unstoppable?

In May 2014, scientists claimed that the glacier loss in the Amundsen Sea sector of West Antarctica had passed the point of no return (see blog Point of no return), implying that the glaciers would eventually melt away.

The glaciers of concern sit on bedrock lying below sea level and flow towards the ocean across the grounding line, at which point they become floating ice shelf over the sea water. Using satellite data over the past two decades, researchers found that the grounding lines of the glaciers were all retreating inland rapidly, probably caused by warmer ocean currents under the floating ice. With the glacier beds below sea level and sloping downward in the inland direction, the more the grounding line retreats, the more vulnerable the ice becomes to the erosive effect caused by the incursion of warm ocean currents. Normally, ice outflow to the ocean increases with ice thickness at the grounding line. Here, the ice thickness at the grounding line increases as the latter continues to retreat, implying more ice outflow to the ocean. The only way to stop the vicious cycle of retreating grounding line and melting glaciers is to have some high obstructions rising from the bedrock at some point, which is exactly the natural blockage currently lacking in the landscape of West Antarctica.

Land-based ice sheet flowing to the ocean (source: IPCC)

How do scientists reconstruct the temperature tens of thousands of years ago?

Snowfall in the polar regions accumulates from year to year. As snow at the surface gets buried with time, it is compressed to form solid ice. Lower sections of the ice sheet are therefore older than the upper ones. Ice cores drilled from the Greenland and Antarctica ice sheets containing proxy data of air temperature can then be used to estimate temperature in the past.

Natural oxygen (O) comes in two major varieties (or isotopes in chemical terminology): O18 and O16. O16 contains 8 protons and 8 neutrons and O18 contains 8 protons and 10 neutrons. O16 is lighter than O18 and is more common. Both isotopes can combine with two hydrogen atoms to form water molecules (H2O). The ratio of O18/O16 in the ice core is higher in a warmer climate as more energy is available to evaporate the heavy water molecules which contain O18 from the ocean and to transport them to the polar regions. In a cooler climate, fewer heavy water molecules can be evaporated into the atmosphere and even fewer of them can reach the polar regions before they condense out elsewhere. Hence, by determining the ratio of O18/O16 in different sections of the ice core, scientists can estimate the temperature in the past.

A variety of methods are used to date an ice core. The most direct way is to count the annual cycle of the O18/O16 ratio (the ratio is higher in summer and lower in winter). Another useful technique is to identify events (e.g. volcanic eruptions) which are seen in other types of climate records, such as tree ring and sedimentary records. These events can be used to synchronize the age scales and a time series of temperature over the past tens of thousands of years could then be reconstructed.


Petit, J.R., D. Raynaud, C. Lorius, J. Jouzel, G. Delaygue, N.I. Barkov, and V.M. Kotlyakov. 2000: Historical isotopic temperature record from the Vostok ice core. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A. doi: 10.3334/CDIAC/cli.006

Australian Antarctic Division, Ice Cores

A scientist measuring an ice core. (source: NASA A global change master directory portal)

Temperature time series reconstructed from the ice core from Vostok, Antarctica. Temperature changes are relative to modern surface temperature value of -55.5oC.
(source: Petit et al., 2000)

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)


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.

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:

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.

What is IPCC?

The Intergovernmental Panel on Climate Change (IPCC), established under the auspices of the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), is a scientific body tasked to evaluate the risk of climate change caused by human activities.

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 its impacts. This is the key motivation behind the establishment of IPCC in 1988 as the authority on climate change.

The main activity of IPCC is the compilation of assessment reports on a regular basis. The First Assessment Report in 1990 played a decisive role in the establishment of the United Nations Framework Convention on Climate Change (UNFCCC). The Second Assessment Report in 1995 provided key input to the negotiations of the Kyoto Protocol. The Third Assessment Report in 2001 and a number of special reports provided relevant information for the development of the UNFCCC and the Kyoto Protocol. The Fourth Assessment Report in 2007 confirmed that warming of the climate system was unequivocal. The Fifth Assessment Report in 2013 reaffirmed this finding and concluded that it was extremely likely that human influence had been the dominant cause behind the observed warming since the mid-20th century.

Useful Links:



Kyoto Protocol

United Nations and Climate Change

How do human activities contribute to climate change?

Rapid development of economic and industrial activities since the 18th century has led to excessive use of energy and resources. In particular, the burning of fossil fuels emits large amounts of greenhouse gases into the atmosphere. The human-induced increase in atmospheric greenhouse gas concentrations enhances the greenhouse effect, resulting in global warming. The human impact on climate has exceeded the effects of other natural factors such as solar and volcanic activities.

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 effects 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


Greenhouse Gas

Main human emission sources


Carbon dioxide (CO2)

Fossil fuel use and change in land use


Methane (CH4)

Agriculture and fossil fuel use


Nitrous oxide (N2O)


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

Are extreme weather events becoming more frequent?

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 20th 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.

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

Please refer to the following blogs:

Can a quiet sun halt warming on Earth?
An ever-extending hockey stick
Volcanoes, weather and climate

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. Most of the extra heat goes to the oceans. The warming is not restricted to the sea surface. The sign of warming is observed down to 2000 m below the surface of the oceans. A direct consequence of ocean warming is sea level rise. According to the Fifth Assessment Report of IPCC, thermal expansion of sea water contributed to a global average sea level rise of about 1.1 mm per year during 1993-2010.

Global sea surface temperature anomalies (relative to 1961-1990) from various datasets (Source: IPCC, 2013)

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 .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 :

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)

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.


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 : 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.

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).


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.

What is carbon cycle?

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