Abstract accepted for EGS April 2004

Probabilistic stabilisation scenarios, within an interactive model for global dialogue.

The "Java Climate Model" (JCM), whose core science builds on the methodology of IPCC-TAR projections, has been applied to a probabilistic analysis of some problems related to interpretation of Article 2 of the UNFCCC. The challenge is to explore how we may safely achieve a stabilisation level which avoids specific climate change impacts, considering the bounds of uncertainties as well as inertia, within both natural and human systems. This inverse approach contrasts with the 
rather fatalistic concept of climate "prediction", whilst applying a similar methodology.
fatalistic concept of trying to predict future climate, although applying a similar methodology.  

Model results were compared with historical data for both temperature and CO2, to quantify the relative probabilities of many sets of model parameters. Coherent combinations were sought in which, for example, high climate sensitivity may be offset by high aerosol or lower solar forcing, or high carbon fertilisation offset by high respiration feedback or slow ocean mixing. Such parameter sets were then combined with an iterative algorithm to stabilise one of several indicators, such as CO2 concentration, radiative forcing (including all gases and aerosols), or global temperature, at a specified level. It is evident that choosing a stabilisation indicator later in the cause-effect chain reduces the burden of uncertainty concerning impacts and adaptation, whilst increasing the uncertainty with regard to emissions mitigation pathways.  Thus, alongside the identification of key vulnerabilities, of acceptable levels of changes, and of certainties to avoid such changes, the type of indicator(s) used to define interim policy goals is also a key risk/value judgement. 

One example is the EU's long-term policy goal to limit the global temperature rise below 2C above preindustrial levels. A  probability distribution of emissions pathways consistent with this goal was compared with some recent statements from individual countries, revealing apparently varying attitudes to risk. Similar methodology may also be used to explore other approaches, including optimisation within an integrated assessment framework. 

However the ultimate "integrated assessment model" to balance the risk and value judgements remains the global network of human heads. To help provide a quantitative framework for the global dialogue, JCM (jcm.chooseclimate.org) is made accessible through an interactive graphical interface in a web browser, is open source, extensively documented, and partially translated in ten languages. Indeed the development of the interactive tool preceded the systematic probabalistic analysis, since for both a fast model is essential, and role-play experiments with students help to identify new scientific questions. Thus, a tool which serves both research and outreach can assist the iterative feedback between models and citizens, to find strategies for coping with uncertainty.