In this paper Moss et al provide an overview of how scenarios of the future are used in climate change research to aid understanding of how changes in technology, lifestyle, and policies can address the risks of climate change, and outline a new process for developing these scenarios.

Moss, Richard H., Edmonds, Jae A., Hibbard, Kathy A., Manning, Martin R., Rose, Steven K., van Vuuren, Detlef P., Carter, Timothy R., Emori, Seita; Kainuma, Mikiko; Kram, Tom; Meehl, Gerald A., Mitchell, John F. B., Nakicenovic, Nebojsa; Riahi, Keywan; Smith, Steven J., Stouffer, Ronald J., Thomson, Allison M., Weyant, John P., Wilbanks, Thomas J., 2010. The next generation of scenarios for climate change research and assessment. Nature, 463 pp. 747-756. Available at: http://dx.doi.org/10.1038/nature08823

The nature of climate change means that information needs to be incorporated from a range of disciplines including environmental science, sociology, and technology, and until now this process has been a slow one as each discipline took turns to incorporate their data. The new process aims to integrate all the information more efficiently with the different teams working in parallel rather than sequentially.

Evolution of climate-related scenario analysis

The earliest scenarios date back to the late nineteenth century when the first estimates of carbon dioxide-induced climate warming were made. They focussed on emissions of greenhouse gases through human activity, and since the 1960s these emissions-based scenarios have grown in complexity to include data on land use and assumptions about population growth and new technology. They typically ignore short term economic cycles and energy price fluctuations, instead looking at long term trends in energy use to explore the impact of new policies and technologies on future emissions. They also attempt to assign a probability to the likelihood of each scenario occurring, and a key criticism of this approach is whether such probabilities are anything more than a wild guess.

Variations on the traditional emissions scenario include the climate scenario, which focuses on factors such as temperature and rainfall to define the future, and environmental scenarios, which take a broader approach describing the potential future world in terms of water availability, sea levels, and land use, as well as air quality and climatic factors. Vulnerability scenarios also take into account social and economic factors to understand how humans will be affected by climate change-induced environmental changes.

Why new scenarios are needed

Historically, model based scenarios were developed along a linear pathway starting with identifying socioeconomic factors, next developing a narrative explaining how these factors will influence greenhouse gas emissions in the future, then considering the resulting atmospheric and climate process, and finally evaluating the impacts on humans and the environment. This was time consuming and made it difficult for the development of models to keep pace with the rapid growth in new information on environmental, technological and socio-economic factors.

With greater understanding of climate science has come a demand for more sophisticated climate models incorporating more physical processes and more detailed scenarios. The expected outcome of scenario analysis has also moved away from a basic ‘policy or no policy’ decision, towards a need to consider different (and much longer) time frames, account for potential extreme events, and evaluate the costs and benefits of a wide range of possible actions encompassing both mitigation and adaptation to climate change impacts.

The parallel approach to climate scenario analysis

The new faster approach to scenario development begins with defining four potential future outcomes for the world’s atmosphere, called “radiative forcings”. A radiative forcing is the change in balance between radiation coming in and out of the atmosphere caused by a change in the components of the atmosphere, such as the level of carbon dioxide. Each scenario also has a specific emissions scenario associated with it that could bring about that radiative forcing. The emissions scenario is just one of potentially many plausible scenarios that could lead to that radiative forcing, hence it is termed a “representative pathway”. These pathways could come about through a variety of combinations of socioeconomic, technological or political activity, so each scientific discipline must assesses the characteristics that would be required in their area to achieve each of the four pathways.

The intention of this approach is to focus research efforts on a few defined emissions pathways that are expected to lead to particular atmospheric concentration and magnitude of climate change, saving time on initial scenario development and providing some consistency of research across different disciplines which are able to work simultaneously rather than sequentially. The choice of the representative concentrations pathways (or RCP’s) is not intended to select the most likely outcomes, but to ensure a wide range of possible outcomes is covered.

Future research

Two sets of climate change projections will be developed using the new parallel approach – one focusing on the short term to 2035, and the other on the long term to 2100. Some may be extended even further out to 2300, and a narrative will be developed around the different emissions scenarios to help translate the predictions to the actual conditions at local level. Equally important is increasing cooperation between climate researchers, bringing in more expertise from developing countries, and continuing to expand knowledge of the physical climate system, as well as improving communication to the public. The ultimate aim is to better understand the interaction of human activity with natural climate processes and the potential costs and benefits of the choices and policies we make.

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