Third International Symposium on Non-CO2 Greenhouse Gases (NCGG-3) Maastricht, Netherlands, 21-23 January 2002 1 Evaluating indicators for the relative responsibility for climate change - alternatives to the Brazilian proposal and global warming potentials N. Höhne & J. Harnisch ECOFYS energy & environment, Cologne, Germany Keywords: Historical responsibility for climate change, Brazilian Proposal, Global Warming Po- tentials, GWP ABSTRACT: In this paper, several indicators to describe the responsibility for climate change are discussed and evaluated. During the negotiations of the Kyoto Protocol, the delegation of Brazil proposed to use the impact of historical emissions on the current temperature. The Kyoto Protocol uses current emissions weighted by 100-year global warming potentials (GWPs) as the basis. As a powerful indicator historical responsibility proved the radiative forcing due to current concentra- tions integrated from today over a finite period into the future. Such indicator could be interpreted as a GWP for concentrations. The indicator is ‘backward looking’ (takes into account historical emissions), it is ‘backward discounting’ (early emissions weigh less depending on the decay in the atmosphere) and ‘forward looking’ (future effects of the emissions are considered) and it is compa- rable for all gases. The choice of the start date, from when historical emissions are accounted, is a political one. 1 INTRODUCTION The ultimate goal of the United Nations Framework Convention on Climate Change (UNFCCC) is to stabilize greenhouse gas concentrations at a level that would prevent dangerous anthropogenic interference with the climate system. To reach this objective in the long-term, the burden to reduce emissions has to be shared among countries. This could be based on the relative responsibility of countries for climate change. To calculate the relative responsibility for climate change (Filho & Miguez 2000, Den Elzen 2000 and Enting 1998) one needs to consider the cause-effect chain which leads from emissions of greenhouse gases to changes in climate: emissions of greenhouse gases, precursors and aerosols change the concentration of these and other gases in the atmosphere. Changed concentrations influ- ence radiative forcing. Changed radiative forcing influences the global-average surface tempera- ture. The absolute change in temperature, as well as the rate of its change, influences the sea level and other parameters such as precipitation and related damages. During the negotiations of the Kyoto Protocol, the delegation of Brazil proposed to use the im- pact of historical emissions on the current temperature as an indicator for historical responsibility for climate change (UNFCCC 1997). In this paper we discuss and evaluate various indicators which could be used to describe the relative responsibility for climate change. For such consideration, the timing is important due to the delays in the respective effects. Many greenhouse gases, once emitted, are only slowly removed from the atmosphere. The resulting ra- diative forcing causes changes in the global-average surface temperature again with a certain time delay. The radiative effect of different greenhouse gases also depends on the composition of the atmosphere, which is changing over time. The effects of emissions of different greenhouse gases are generally compared using Global Warming Potentials (GWPs). For the calculation of the absolute GWPs, the radiative forcing from