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Estimates of 21st Century global-mean surface temperature increase have generally been based on scenarios that do not include climate policies. Newly developed multigas mitigation scenarios, based on a wide range of modeling approaches and socioeconomic assumptions, now allow the assessment of possible impacts of climate policies on projected warming ranges. This article assesses the atmospheric CO(2) concentrations, radiative forcing, and temperature increase for these new scenarios using two reduced-complexity climate models. These scenarios result in temperature increase of 0.5-4.4 degrees C over 1990 levels or 0.3-3.4 degrees C less than the no-policy cases. The range results from differences in the assumed stringency of climate policy and uncertainty in our understanding of the climate system. Notably, an average minimum warming of approximately 1.4 degrees C (with a full range of 0.5-2.8 degrees C) remains for even the most stringent stabilization scenarios analyzed here. This value is substantially above previously estimated committed warming based on climate system inertia alone. The results show that, although ambitious mitigation efforts can significantly reduce global warming, adaptation measures will be needed in addition to mitigation to reduce the impact of the residual warming.

How can dangerous interference with the climate system be avoided? Science can help decision-makers answer this political question. Earlier publications have focused on the probability of keeping global mean temperature change below certain thresholds by stabilizing greenhouse gas concentrations at particular levels. We compare the results of such "stabilization profiles" with a set of "peaking profiles" that reduce emissions further after stabilization and thus result in a concentration peak. Given the inertia in the climate system, stabilization profiles lead to ongoing warming beyond 2100 until the temperature reaches equilibrium. This warming partly can be prevented for peaking profiles. In this way, these profiles can increase the likelihood of achieving temperature thresholds by 10-20% compared with the likelihood for the associated stabilization profiles. Because the additional mitigation efforts and thus costs for peaking profiles lie mainly beyond 2100, peaking profiles achieving temperature thresholds with the same likelihood as the original stabilization profile, but at considerably lower cost (up to 40%), can be identified. The magnitude of the cost reductions depends on the assumptions on discounting. Peaking profiles and overshoot profiles with a limited overshoot may, in particular, play an important role in making more ambitious climate targets feasible.

The necessity for policy makers to follow the processes in relation to sustainability demands new tools for management, a major aspect of which is integrated attention to economic and sociocultural influences together with environmental issues. North and South have specific positions and responsibilities with regard to sustainable development, and management tools--such as indicators--should reflect them. Because the North-South dialogue is changing, different approaches to deal with the relationships are needed (e.g., introduction of reciprocity, equity, and participation), while at the same time doing justice to the individual social, economic, and cultural characteristics of a country. Ecosystem health can be defined by requirements such as resilience, vigor, organization, and relations, which may be used for totally different systems as well. Because integration is a crucial aspect of the new tools, it is argued that integrating conceptual tools such as the environment-economy-society triangle, the PSIR-causality chain, and the Daly triangle in addition to other described integration tools is fundamental for decision makers. Crucial issues when dealing with indicators for sustainable development are integration and balance, attention to users, use of system characteristics, and understanding southern perspectives in this context.