Representation Concentration Pathways (RCPs)

Climate scenarios are used to represent a range of plausible outcomes for climate change depending on how our climate system responds to high levels of greenhouse gasses and how quickly we can act to limit our emissions globally. The four Representation Concentration Pathways (2.6, 4.5, 6.0, 8.5) are used by the Intergovernmental Panel on Climate Change as trajectories for the future of greenhouse gas concentrations in our atmosphere. Each pathway represents a possible global outcome based on the cumulative volume of emissions released by 2100. The higher-numbered RCP scenarios assume the release of more carbon dioxide into the atmosphere by humanity. Some of the most common options include RCP4.5, and RCP8.5, which are both used in ClimateCheck analysis.

The RCP scenarios are not predictions, which means that none are chosen as having more of a likelihood of occurring than the others. However, each scenario may be better or worse as a tool for modeling short, medium, and long-term emissions horizons.

Why We Use RCP8.5 and 4.5

In ClimateCheck analysis for climate conditions through mid-century, we use RCP8.5 as our default scenario. We believe that it provides the best match for the levels of emissions that have already occurred as well as the best projections for short and medium term risks. RCP is the most aggressive scenario. It assumes that we will continue to emit globally at high rates and that coal use will continue to expand. It was therefore the best pathway to choose in order to avoid underestimating the physical risks, a potentially harmful miscalculation.

We also include information on RCP4.5. We think that providing multiple scenarios is an important component of understanding the full picture of climate change.While neither RCP8.5 or RCP4.5 will match reality exactly, they are both useful guides. ClimateCheck risk ratings reflect relative risk compared to the U.S. as a whole. This means the differences between our data between RCP4.5 and RCP8.5 do not significantly change the ratings, they slightly change the magnitude of the risk rating.

RCPs each represent a future that has the possibility of occurring, depending on the timing and intensity of global action against climate change. RCP4.5 represents a fairly optimistic future in our view; it assumes that a temperature stabilization will occur shortly after 2100. Many nations are falling behind on implementing emissions targets, which is why we feel that presenting RCP8.5 through mid-century as our default risk still best conveys the risks we face.

Many scientific authorities have weighed in on the issue of which RCP scenario most accurately reflects our future. According to Schwalm, Glendon, and Duffy (2020), historical cumulative CO2 emissions are most consistent with RCP8.5 (within 1 percent between 2005 and 2020). It will therefore be a good match for quantifying physical climate risk within the near through midterm time horizons.

Other Perspectives

RCP8.5 is often characterized as the “business as usual” scenario, which unfortunately encourages the misconception that 8.5 is inherently more likely or preferred. Hausfather and Peters argue that new information indicates that fossil CO2 emissions and land use emissions will decline faster than the RCP scenarios assume. Based on this information, they propose that the emphasis on RCP8.5 is misplaced, and that 4.5 presents a more accurate picture of our future. At ClimateCheck, for use in our risk models, we believe that total atmospheric carbon is the most important factor, rather than fossil CO2 emissions or land use emissions. For more information, see this rebuttal to Hausfather and Peters.

Some research indicates that other types of emissions scenarios may provide a more accurate picture than the RCPs. For example, the Shared Socioeconomic Pathways (SSPs) and the World Energy Outlook (WEO) each look at other types of factors, such as the influence of political and economic factors and technological innovation on future emissions levels. Some of our risk ratings also include information on SSPs for this reason.

Conclusion

RCPs, and emissions scenarios in general, help us understand what our future could look like. At ClimateCheck, we’ve used our data and expertise to choose RCP8.5 and 4.5 as the most probable futures. There have been many different types of emissions scenarios proposed in the last several decades due to our expanding understanding on the science of climate change and our global response to the issue.