Emissions Scenarios

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Appendix I: SRES Terms of Reference New IPCC Emissions Scenarios

It is proposed that Working Group III coordinate the development of new emissions scenarios that assume no additional climate policy initiatives.

I.1. Background

In 1992 the Intergovernmental Panel on Climate Change (IPCC) released six emissions scenarios (Leggett et al., 1992) providing alternative emissions trajectories spanning the years 1990 through 2100 for greenhouse-related gases, Carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), nitrous oxide (N2O), nitrogen oxyde (NOx), and sulfur dioxide (SO2). These scenarios were intended for use by atmospheric and climate scientists in the preparation of scenarios of atmospheric composition and climate change. The work updated and extended earlier work prepared for the IPCC first assessment report. These six scenarios are referred to as the IS92 scenarios.

In many ways the IS92 scenarios were pathbreaking. They were the first global scenarios to provide estimates of the full suite of greenhouse gases. At the time, they were the only scenarios to provide emission trajectories for SO2. Alcamo et al. (1995) reviewed the scenarios and found that the fossil fuel carbon emissions trajectories spanned more than half of the open literature emissions scenarios reviewed. Other emissions trajectories had received less scrutiny in the open literature and, while the IS92 cases were not dissimilar to those in the open literature, the open literature was extremely sparse in many instances.

Much has changed in the period following the creation of the IS92 scenarios. Sulfur emissions have been recognized as a more important radiative forcing factor than other non-CO2 greenhouse-related gases, and some regional control policies have been adopted. Restructuring in the states of Eastern Europe and the Former Soviet Union has had far more powerful effects on economic activity and emissions than were foreseen in the IS92 scenarios. For some regions these scenarios are not representative of those found in the literature. The advent of integrated assessment (IA) models has made it possible to construct self-consistent emissions scenarios that jointly consider the interactions between energy, economy, and land-use changes.

Alcamo et al. (1995) found that for the purposes of driving atmospheric climate models, the CO2 emissions trajectories of the IS92 scenarios provided a reasonable reflection of variations found in the open literature. However, scenarios are also required for other purposes, and the IS92 scenarios are not suitable for purposes for which they were not developed. It was concluded that, if the scenarios were intended to have broader uses than simply a set of emissions trajectories to drive climate models, new scenarios should be developed. Further, a new approach should be adopted. The new approach should open the process to the broader research community.

I.2. Approach

It is proposed that new scenarios should be developed through a coordinated effort that draws upon the expertise of all researchers in the relevant community. A three-step process is envisaged. First, key input assumptions would be reviewed and provided to modelers. Second, modelers would be asked to construct emissions scenarios based on the input assumptions provided. Finally, the model results will be used to develop new emissions scenarios in the form of average results for participating models or results from a representative model.

A writing team would be established to consider key input assumptions (such as population projections and technologic change) and emissions from specific sources (such as SO2 emissions and CO2 emissions due to land-use change), possibly with the assistance of specialized task groups. The writing team will also stipulate a set of geographical reporting regions, reporting years, units of measure, etc., designed to provide climate modelers, impact assessment analysts, and other users with the detail they need for their work. Finally, the writing team would ensure that the range of results reflects the underlying uncertainty and, to the extent possible, that the assumptions for specific scenarios are internally consistent.

Scenario development will be an open process. There will be no "official" model. There will be no "expert teams." Any research group with the capability of preparing scenarios for any region can participate. This means that, while modeling teams which employ global coverage will be able to participate, so too will regional modelers. By opening the process in this way, developing and developed region researchers with local expertise can participate even if they do not have global coverage. Modeling teams will be provided with information on the input assumptions and other necessary information such as, for example, the world oil price, to regional modeling teams.

Once the modeling teams have completed their work, a set of scenarios will be chosen. This will likely be the inputs and outputs of a "representative" model, but it could also be the average of the participating models or some other representation of the model results.

I.3. Reporting and Distribution of Results

To maximize the usefulness of the new scenarios, two steps should be taken. First, arrangements should be made with an organization whose mission it is to disseminate information to provide a means by which users can access scenario results. All results from research institutions will be included in the database along with associated assumptions. In addition, for research teams willing to participate, the associated models will also be made available so that users can not only have access to scenario assumptions and outputs, but have the capability of independently creating derivative scenarios.

I.4. Timing and Coordination

It is proposed that the writing team begin work before the end of 1996. The team should establish the parameters - geographic reporting regions, reporting years, time horizon, units, etc. - by the end of the first quarter of 1997. Reports of the expert groups on the range of values for each of the input assumptions should be available by the end of the third quarter of 1997. The scenario results corresponding to these input assumptions should be available from participating modeling groups during the first quarter of 1998. Peer and government review should be complete by the end of 1998.


Alcamo, J., A. Bouwman, J. Edmonds, A. Gr´┐Żbler, T. Morita, and A. Sugandhy, 1995: An Evaluation of the IPCC IS92 Emission Scenarios. In: Climate Change 1994, Radiative Forcing of Climate Change and An Evaluation of the IPCC IS92 Emission Scenarios, Cambridge University Press, Cambridge, UK, pp. 233-304.

Leggett, J., W.J. Pepper and R.J. Swart, 1992: Emissions Scenarios for IPCC: An Update. In: Climate Change 1992. The Supplementary Report to the IPCC Scientific Assessment [Houghton, J.T., B.A. Callander and S.K. Varney (eds.)], Cambridge University Press, Cambridge, UK, pp. 69-95.


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