Emissions and concentration scenarios

Introduction: Climate impacts depend on the magnitude of climate change, as well as planning practices and the level and effectiveness of adaptive responses. In turn, the magnitude of future climate change is uncertain, depending not only on uncertainty in the Earth system and natural variability but also on uncertainty in future emissions and their underlying human system drivers. As such, consideration of different potential emissions futures is essential for understanding and preparing for future climate change. 

This chapter briefly reviews the role of uncertainty in emissions projections. It then addresses the key drivers of emissions of greenhouse gases and other pollutants and how uncertainty in those drivers affects projections of future emissions. 

The Intergovernmental Panel on Climate Change (IPCC) has played a dominant role in developing emissions and concentration scenarios that are used by the climate community, and ultimately, these provide a range of climate scenarios of potential use to those assessing climate impacts. The IPCC approach has focused on scenario development, eschewing formal uncertainty analysis. We review the IPCC approach to scenario development and how it has changed through successive assessment reports (AR). We then describe approaches that use formal uncertainty analysis techniques and how this differs from scenario analysis, as well as methods that can connect probability distributions back to individual scenarios of interest. Finally, we summarize lessons learned from more than 40 years of socioeconomic effort to create scenarios to support climate change research.

Summary: With 40+ years of scenario development for climate analysis, there are some key takeaways: 

• Scenario uncertainty can be as important as uncertainty in the science of climate change, but its contribution to overall climate uncertainty doesn’t emerge as a main driver until the second half of the century.

• Various scenario approaches have been developed to capture a wide range of future emissions and concentration outcomes. 

• The IPCC scenario process has had a strong influence on the scenario development community. 

• The IPCC process has eschewed formal uncertainty techniques but has looked to the broader literature to develop target or marker scenarios. That literature also includes formal uncertainty analysis.

• Scenarios are often conditioned on specific assumptions, such as specific policy goals or broader storylines that define some elements of the scenario. 

• Expectations about future climate and what it means for climate impacts and adaptation require further judgment about the likelihoods of different scenarios/their various conditional assumptions, such as whether policy goals will be achieved. 

• Applying newer approaches like scenario discovery with probabilistic approaches can combine the benefits of distinct scenarios with likelihoods of outcomes of interest, providing a richer foundation for mitigation and adaptation studies.