Should we throw out the RCP8.5 scenario?


In a recent letter from the Dutch Minister of Economic Affairs and Climate Policy to the Speaker of the Dutch House of Representatives about gas extraction in the Waddenzee, the Minister addresses the issue of sea level rise. The Minister states that the Representative Concentration Pathway 4.5 scenario (RCP4.5) would already take into account more sea level rise than the Paris agreement, as the Paris agreement is comparable to RCP2.6, which would be equivalent to a warming of less than 2 degrees Celsius by the year 2100. Then the minister says that the highest scenario, RCP8.5, has been critiqued in a recent Nature article because RCP8.5 assumes that no climate measures have been taken. What the article says is that RCP8.5 has been widely used as a “business as usual” scenario, while it was actually intended as a way “to explore a high-risk future”. But does this mean that we should throw out the scenario depicted by RCP8.5?

What is RCP8.5?

The RCP8.5 scenario is part of a set of four pathways (RPC 2.6, 4.5, 6 & 8.5) that were developed as a basis for climate modelling experiments. Before RCP, there were other scenarios such as the IS92 scenarios and the SRES scenarios that were used as common sets of scenarios in the scientific community to ease communication between researchers. But then new scenarios were needed. Scenarios that would use more detailed information and that would be able to explore the different impacts from climate policies while also still exploring the impacts of using no climate policies. And thus RCP was created.

The name Representative Concentration Pathways reflects two important characteristics of the scenarios. “Representative” because the RCPs should represent “the full range of emissions scenarios available in the current scientific literature, with and without climate policy“, and “concentration pathway” because the RCPs are not the final integrated scenarios. By using “concentration” instead of “emissions” they emphasise that “concentrations are used as the primary product of the RCPs, designed as input to climate models“.

The four RCP scenarios all represent a scenario with a different radiative forcing level by the year 2100, namely 8.5, 6, 4.5 and 2.6 W/m2. As said before, a radiative forcing level of 2.6 would be equivalent to a warming of less than 2 degrees Celsius. The highest scenario, with a radiative forcing of 8.5 W/m2, would be equivalent to a warming of 5 degrees Celsius by the end of the century. Now the real important thing to remember here is that these are 4 scenarios with a certain radiative forcing as outcome, I will come back on this later.

So what is the problem?

Lately, there has been criticism on the emissions scenario that was used to create RCP8.5 because of the assumptions it makes around high future emissions and because it assumes that there is a considerable increase in coal use. But they are cutting corners, as it is not as important where the CO2 emissions are actually coming from, but what they sum up to be.

As said before, RCP8.5 stands for a certain scenario that has as outcome a radiative forcing of 8.5 W/m2 by the year 2100. But the RCP scenario is not the only way to get to a radiative forcing of 8.5 W/m2, around 40 scenarios with a similar forcing level exist in the energy modelling literature. Thus, it should not matter that we do not use as much coal as was used in the RCP8.5 scenario, as there are other things that could generate the CO2 emissions necessary for a radiative forcing of 8.5 W/m2. And then there is also the fact that there are large uncertainties in carbon cycle feedbacks. Even low emissions could potentially lead to a higher forcing than was previously assumed. As models improve, such as the CMIP6 models that incorporate thawing permafrost, there may be an increase in the magnitude of the carbon cycle feedbacks.

I believe that RCP8.5 should not be left out when looking at possible climate change scenarios, as it is not a representation of a final scenario, but more of what would happen if a certain amount of greenhouse gasses is released into the atmosphere that creates a radiative forcing of 8.5 W/m2. Which just so happened to be represented by burning a lot of coal in the original RCP report, but could also be represented by previously not included factors such as thawing permafrost.

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