Scientific aspects of UNFCCC Article 2, Stabilisation scenarios, and Uncertainty. Ben Matthews Jean-Pascal van Ypersele Institut d’astronomie et de géophysique G. Lemaître, Université catholique de Louvain, Louvain-la-Neuve, Belgium Web: (UCL-ASTR) jcm.chooseclimate.org (interactive model) JCM also developed with: DEA-CCAT Copenhagen, UNEP-GRID Arendal, KUP Bern For European Strategy on Climate Change, Firenze 11 th Sept 2003 In preparation for WCCC2003 Moscow

Jean-Pascal van Ypersele Framework Convention on Climate Change (Rio, June 1992)  Ultimate objective (Art 2): 'stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved within a time frame sufficient - to allow ecosystems to adapt naturally to climate change, - to ensure that food production is not threatened and - to enable economic development to proceed in a sustainable manner.'

Article 2 in IPCC AR4 IPCC Plenary decided «the scientific, technical, and socio- economic issues associated with Article 2 » would be a CCT in AR4. Scoping paper suggests a structure: WEHAB framework to assess Key Vulnerabilities =>Critical Limits (thresholds) of climate parameters =>Critical Levels (e.g. Concentrations) =>Possible Emissions Pathways including Risk/Uncertainty framework

Jean-Pascal van Ypersele Temperature and « reasons for concern » Source: IPCC WG2 (2001)

EU 2C Target  In practice, global debate will revolve around simple indicators, e.g. Global Annual Average Temperature  EU Council Of Ministers 1996  "the Council believes that global average temperatures should not exceed 2 degrees Celcius above pre-industrial level and that therefore concentration levels lower than 550 ppm CO2 should guide global limitation and reduction efforts.  This means that the concentrations of all GHGs should also be stabilised. This is likely to require a reduction of emissions of GHGs other than CO2, in particular CH4 and N2O"

Jean-Pascal van Ypersele Stabilisation scenarios: Uncertainty depends on Indicator  Sets of scenarios stabilising Concentration, Forcing, Temperature, (Sea-level), all reaching equivalent 2C level on average  Core science very similar to IPCC-TAR models, Over 3000 variants of model parameters per set - probabilistic approach.

 Carbon cycle:  LUCF emissions,  CO2 Fertilisation,  T-Respiration feedback,  Ocean mixing  Other gas emissions:  Scaled so all gases (inc Sulphate and Ozone precursors) reduced by same proportion with respect to 6 SRES baselines  Forcing/Climate Sensitivity:  Vary Solar and Sulphate forcing  7 GCM parameterisations as IPCC TAR (apx9.1)  Constrain combinations by consistency with historical temperature record ( ¾ variants rejected )

Jean-Pascal van Ypersele

Caveats  The distributions are skewed (e.g. for stabilising Temperature=2C, mean CO 2 concentration is 475ppm, the lower end of plotted range. This is due to nonlinearities).  Climate Sensitivity may be higher than TAR model range (and increasing at higher temperatures). Not well constrained by historical temperatures.  Mitigation of other gases in proportion to CO 2 (reduction wrt SRES) is a simplistic assumption. Beter to have socioeconomic/landuse model.  Uncertainty in several factors not yet considered: Atmospheric chemistry, Ice-melting etc.  Shape of land-use emissions curve influences terrestrial carbon-cycle balancing

Jean-Pascal van Ypersele Inertia in the climate system Stabilising CO2 alone doesn't stabilise temperature (as below from TARSYR Q6) However stable CO2 may correspond to stable Temperature if other gases with shorter lifetimes are also mitigated to a similar extent.

Shifting the Burdern of Uncertainty  Fixing a temperature rather than a concentration target reduces the burden of uncertainty for impacts/adaptation  (assuming we commit to adjust emissions to reach it as the science evolves)  however it increases the uncertainty regarding emissions pathways.  Which is better?

To solve Article 2, we need Article 6  IPCC cannot decide interpretation of Article 2. More people must be involved in the global dialogue, to assess risk and value judgements as well as scientific uncertainties. The Ultimate Integrated Assessment Model remains the global network of Human Heads.  Java Climate Model  Interactive - explore cause-effect with draggable controls Available on web jcm.chooseclimate.org Transparent (open source code, 50,000 words documentation) Multi-lingual for global dialogue Applications for both research and teaching