The tolerable windows approach: Theoretical and methodological foundations

The tolerable windows (TW) approach is presented as a novel scheme for inte grated assessment of climate change. The TW approach is based on the specif ication of a set of guardrails for climate evolution which refer to various climate-related attributes. These constraints, which define what we call t olerable windows, can be purely systemic in nature - like critical threshol ds for the North Atlantic Deep Water formation - or of a normative type - l ike minimum standards for per-capita food production worldwide. Starting fr om this catalogue of knock-out criteria and using appropriate modeling tech niques, those policy strategies which are compatible with all the constrain ts specified are sought to be identified. In addition to the discussion of the basic elements and the general theory of the TW approach, a modeling ex ercise is carried out, based on simple models and assumptions adopted from the German Advisory Council on Global Change (WBGU). The analysis shows tha t if the global mean temperature is restricted to 2 degrees C beyond the pr eindustrial level, the cumulative emissions of CO2 are asymptotically limit ed to about 1550 Gt C. Yet the temporal distribution of these emissions is also determined by the climate and socio-economic constraints: using, for e xample, a maximal tolerable rate of temperature change of 0.2 degrees C/ de c and a smoothly varying emissions profile, we obtain the maximal cumulativ e emissions, amounting to 370 Gt C in 2050 and 585 Gt C in 2100.