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The Twelfth Session of Working Group I of the United Nations Intergovernmental Panel on Climate Change took place in Stockholm, Sweden, from September 23 to 26 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 findings 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 1,400 years in the Northern Hemisphere;
- The Arctic summer sea ice retreat in the last three decades was unprecedented in at least the last 1,450 years;
- It is virtually certain that the upper ocean (0 to 700 metres below the surface) warmed from 1971 to 2010; and
- 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 100 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. The 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 of 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 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 to 4.8 degrees Celsius and 0.45 to 0.82 metres, respectively, above the 1986-2005 average.
The SPM can be downloaded from www.climatechange2013.org. Highlights of the SPM can be found in the Appendix.
Ends/Thursday, October 3, 2013
Issued at HKT 18:15
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