New Climate Science 2006-2009

Climate change is perhaps the most complex issue facing the international community. At the end of 2009, the countries of the world will gather in Copenhagen to attempt the establishment of a new climate agreement. Science forms the basis of climate policy. Reports from the UN Intergovernmental Panel on Climate Change IPCC, the latest being their fourth assessment report from 2007, have played a vital role in substantiating and clarifying the threats posed by global warming. The IPCC was then able to consider research carried out up to 2006. Since then a considerable amount of new research has been put forward. The question could therefore be asked whether more recent research provides any cause to change or adjust the conclusions drawn by IPCC in 2007.

Against this background, Professors Erland Källén and Markku Rummukainen were asked to carry out an update of the scientific basis concerning climate change. This report presents the more important recent research results. The report has been subject to a limited scientific review process. The authors are responsible for its content.

Conclusions

Climate research has, over the last 2-3 years further extended knowledge about the climate system. Below is a summary of the points that may be considered to indicate a modification or to add nuances to the knowledge of the climate system as compared to what was reported in AR4:

• Greenhouse gas concentrations in the atmosphere continue to increase. The pace of increase is faster than earlier.
• The global mean temperature in 2008 was about 0.1°C lower than in the immediately preceeding years. 2008 still belongs to the top ten warmest years since 1850 and the latest ten year period is warmer than the previous decade. The temperature trend is one of increase.
• Data on sea level rise has been further studied for the periods 1961-2003 and 1993-2003. Results indicate that the pace of rise has been higher during the later period. When also data from the last few years is considered , it appears that the pace of rise has again become a little slower, but it remains higher than during the 1900s.
• The large-scale warming trend in the Arctic is probably closely connected to global warming.
• New studies of land ice sensitivity to warming and consequently their melting rate indicates that the sea level may rise by more than what was stated in AR4. Estimates are made at around one metre of rise over the 21st Century.
• A significant change in precipitation during the period 1950-2000 has been established from observations. This is in line with what is anticipated as a result of greenhouse gas warming.
• Examples of possible tipping points and elements in the climate system have been published. The dramatic decrease in sea ice in the Arctic (see Point 4 above) may be an example of such a feature if it becomes lasting. In that case this will be the first example of an observed tipping point.
• New research indicates that it may be more difficult than previously assessed to limit global warming to a maximum of 2°C.