IPCC in action: Part II

The primary purpose of the Intergovernmental Panel for Climate Change (IPCC) is to assess the available scientific knowledge about climate change, not to initiate new research. The next IPCC report (Assessment Report 4, or AR4) is due in 2007, and in order to update of the state of knowledge it will only consider papers published in peer-review scientific journals between 2000 and papers submitted by May 1st 2005 (must be accepted before December 2005). It is essential that the papers be published in scientific quality journals in order to ensure the credibility of the results. Nevertheless the IPCC reports undergo several additional reviews and revisions involving a large number of independent referees. Thus, the IPCC reports undergo a more stringent review process than common papers in the scientific literature.

Independent people are assessing the performance of the models experiments intended for the AR4. Recently, a number of modelling groups around the world have put a great deal of effort into producing model results for a set of emission scenarios (pre-industrial control, 20th century, ‘comittment’, SRES B1, SRES A1b, SRES A2, 1% compound CO2 emission and doubled CO2 stabilisation). The modelling endevour is unprecedented in terms of number of participants and and the degree of complexity that must be coordinated. An archive of the model results has been established at the PCMDI.

Although there is still some disagreement in the preliminary results (eg the description of polar ice caps), a lot of things appear to be quite robust as the climate models for instance indicate consistent patterns of surface warming and rainfall trends: the models tend to agree on a stronger warming in the Arctic and stronger precipitation changes in the Topics (see crude examples for the SRES A1b scenarios given in Figures 1 & 2; Note, the degrees of freedom varies with latitude, so that the uncertainty of these estimates are greater near the poles). Some model differences in the stratospheric response to e.g. volcanos may be related to how the volcanic forcing is represented in the model experiments. Clouds are still one of the most difficult aspects of climate modelling, and a great deal of effort is going into resolving uncertainties concerning clouds.

Fig. 1: Zonal mean temperature change for AR4 multi-model ensemble

Fig. 2: Zonal mean precipitation change for AR4 multi-model ensemble

Climate models may over-estimate the so-called ‘water vapour feedback‘, they nevertheless seem to capture the right sign of the water vapour feedback. However, it appears increasingly unlikely that the net water vapour feedback could be negative, as proposed by Lindzen’s with his ‘Iris Effect’. Observations of the humidity in the upper troposphere and its relation with sea surface temperature in areas of deep convection point to an overall positive climate feedback by water vapour in the upper troposphere, which is inconsistent with the Iris effect. The Iris effect has been a controversial topic in the climate debate and has been used as an argument against a significant clobal warming, e.g. in Lomborg’s ‘the Skeptical Environmentalist’.

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