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Working Group I of Intergovernmental Panel on Climate Change releases Summary for Policymakers of Fifth Assessment Report
(4 October 2013)

The Twelfth Session of Working Group I (WGI) of the United Nations Intergovernmental Panel on Climate Change (IPCC) took place in Stockholm, Sweden from September 23 to 26, 2013 to approve the Summary for Policymakers (SPM) of the Fifth Assessment Report (AR5) "Climate Change 2013: The Physical Science Basis". The Assistant Director of the Hong Kong Observatory, Mr Edwin Lai, joined the meeting as a member of the Chinese delegation.

Released on September 27, the SPM reaffirmed the finding of the Fourth Assessment Report (AR4) on the observed and unequivocal warming of the Earth's climate, highlighting a number of unusual or unprecedented changes on time scales of decades to millennia:

  • the last 30 years (1983-2012) was likely the warmest 30-year period of the last 1400 years in the Northern Hemisphere;
  • the Arctic summer sea ice retreat in the last three decades was unprecedented in at least the last 1450 years;
  • it is virtually certain that the upper ocean (0-700 m below surface) warmed from 1971 to 2010;
  • the rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia.

    The atmospheric concentration of carbon dioxide (CO2), the main driver of global climate change in the last hundred years or so, has increased by 40 per cent since pre-industrial times. The increase is primarily due to burning of fossil fuels and secondarily due to deforestation. Present-day concentration of CO2 is the highest in the last 800,000 years.

    Human influence has been detected in warming of the atmosphere and the ocean, changes in the global water cycle, reductions in snow and ice, global mean sea level rise, and changes in some climate extremes. This evidence for human influence has grown since AR4. It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.

    For future climate projections, a new set of four scenarios referred to as the Representative Concentration Pathways (RCP) was designed in AR5 to consider different atmospheric greenhouse gas concentration trajectories in the 21st century. In the high greenhouse gas concentration scenario, global mean surface temperature (Figure 1) and global mean sea level (Figure 2) for 2081-2100 will likely be in the range 2.6 - 4.8 oC and 0.45 - 0.82 m, respectively, above the 1986-2005 average.

    The SPM can be downloaded from Highlights of the SPM can be found in the Appendix.


    Photo 1
    (From left to right) The Minister for the Environment of Sweden, H E Lena Ek; the Co-Chair of Working Group I of IPCC, Professor Thomas Stocker and Dr Qin Dahe; the Chair of IPCC, Dr Rajendra K. Pachauri; and the Secretary of the IPCC, Dr Renate Christ declared the Twelfth Session of Working Group I of IPCC open.

    Photo 2
    The Assistant Director, Mr Edwin Lai (left), is photographed with the Administrator of the China Meteorological Administration and also the head of the Chinese delegation, Dr Zheng Guoguang (middle), and the Co-Chair of IPCC WGI, Dr Qin Dahe (right).

    Figure 1
    Figure 1: Multi-model simulated time series of change in global annual mean surface temperature relative to 1986-2005

    Uncertainty is represented by shading. The mean and uncertainty averaged over 2081-2100 for all Representative Concentration Pathways (RCPs) are given as vertical bars on the right hand side. The high greenhouse gas concentration scenario is labelled as RCP8.5. (source: SPM of IPCC WGI AR5)

    Figure 2
    Figure 2: Projections of global mean sea level rise over the 21st century relative to 1986-2005

    The assessed likely range is shown as a shaded band. The high greenhouse gas concentration scenario is labelled as RCP8.5. (source: SPM of IPCC WGI AR5)

    Last revision date: <20 Mar 2014>