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

The main activity of IPCC is in 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 for 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 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 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 :



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.







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