Sacred Headwaters #20: What are the RCPs?
Climate impacts are generally contextualized with references to different emissions scenarios. What are these scenarios, and what does "business as usual" mean in this context?
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Issue #20: What are the RCPs?
I wanted to write this issue on extreme heat: its already horrifying (and unjustly distributed) impacts, predictions about how it will affect different parts of the world going forward. But I realized that any discussion of the potential future impacts of ongoing climate change is grounded in the modelling of that change, and that most literature speaks to a range of impacts stemming from emissions scenarios. So, to better understand that context and the range of climate effects that we may see, I’m writing this issue about emissions scenarios, the RCPs (Representative Concentration Pathways), and climate sensitivity.
I’ve said it before and I’ll say it again: we don’t need to all be climate scientists or experts in any of these topics. But we do need to know enough to be able to actually understand when climate science is reported on in traditional media, and more importantly, to understand when it’s reported on in misleading ways.
In this issue, we’re going to read about what emissions scenarios (and specifically, the RCPs) are, what they aren’t, and how they continue to be useful despite departures from the policy scenarios that were used in their development. We’re also going to read about developments in the science of climate sensitivity: RCPs are scenarios based on policy, land use change, and more, that attempt to model greenhouse gas emissions. The product they output is atmospheric greenhouse gas concentration. That concentration can then be mapped to actual warming and the associated impacts of climate change, but that mapping (what’s called “climate sensitivity”) is another area of uncertainty that has seen new developments in the lead-up to the IPCC’s upcoming Sixth Assessment Report (AR6).
I think it’s important to have a working understanding of emissions modelling and the meaning of commonly referenced scenarios like the RCPs because they are so often used to classify impacts as attached to “high emissions” or “low emissions” scenarios without any real context about what that means in terms of where we are today. While you may not be directly familiar with the RCPs, you’ve almost certainly read articles that reference a “high emissions scenario” or “business as usual” usually with some degree of dismissal. That comes from a misunderstanding and mischaracterization of the RCPs.
A quick definition: “radiative forcing”
Radiative forcing is the change in the net, downward minus upward, radiative flux (expressed in Watts per square metre; W m-2) at the tropopause or top of atmosphere due to a change in an external driver of climate change, such as, for example, a change in the concentration of carbon dioxide (CO2) or the output of the Sun. WGIII
IPCC Data Distribution Centre.
This term comes up a lot in these readings so I wanted to include the IPCC definition and a short explanation. Essentially, radiative forcing is the change in the amount of heat energy trapped within Earth’s atmosphere caused by some exogenous factor (i.e. GHG emissions).
IPCC Fifth Assessment Report (AR5) Topic 2: Future Climate Changes, Risk and Impacts (10 minutes)
The link above is directly to the section in question. The full report is available at the IPCC website.
This short section of the AR5 (2014) explains the basic approach that the IPCC takes for forecasting the impacts both of climate change itself and of climate mitigation policies. There are a few interesting notes in Box 2.1, some of which may sound familiar from past newsletters: the IPCC indicates that while modelling has improved significantly since their AR4 report, there are still large areas of uncertainty in specific Earth processes, including the dynamics of Greenland and Antarctic ice. Box 2.2 introduces the RCPs, a set of emissions scenarios constructed from various mitigation, land use, and emissions assumptions to match a range of radiative forcing outcomes (the number attached to “RCP”). It’s important to note that the RCPs are meant to model emissions based on policy scenarios, but the translation of the RCPs to actual biophysical effects is not tied to those policies, it’s tied only to the forcing that the scenarios predict. In other words, it’s tied to the GHG concentrations in the atmosphere and the amount of warming those concentrations are expected to cause, not to the assumptions used to generate the scenario.
The representative concentration pathways: an overview (20 minutes)
Read the whole paper if you want, but if you’re in a hurry, just read the Introduction, Discussion, and Conclusion. It’s also worth glancing at Table 2.
This paper (2011) is an overview of the development of and recommended uses for the RCP scenarios. It provides some important context for how and why they were developed and how they were intended to provide a starting point for assessing the interplay of policies, emissions, and land-use with atmospheric GHG concentration and warming.
The words “concentration pathway” are meant to emphasize that these RCPs are not the final new, fully integrated scenarios (i.e. they are not a complete package of socio-economic, emission and climate projections), but instead are internally consistent sets of projections of the components of radiative forcing that are used in subsequent phases. The use of the word “concentration” instead of “emissions” also emphasizes that concentrations are used as the primary product of the RCPs, designed as input to climate models.
These two sentences from the introduction are important context for how the RCPs have been used in media coverage and popular understanding over the last decade. First: while the RCPs were developed with policy, technology, and land-use pathways, they are not specific to those pathways; other situations could lead to the same atmospheric concentrations and forcing. Second, Table 2 presents each RCP’s CO2 concentration next to its defining watts per square meter radiative forcing measurement. This relationship is a source of uncertainty that’s seen new developments over the last few years, meaning that the pathways to a specific radiative forcing rate could be quite different from the originally developed scenarios.
RCP8.5 tracks cumulative CO2 emissions (15 minutes)
If you haven’t seen much media coverage referencing the RCPs, you may not understand why I’m making the points I’m making. Basically, RCP8.5 has long been inaccurately characterized as “business as usual,” or low-to-no mitigation. That mischaracterization has opened a window for climate deniers like Michael Shellenberger to argue that RCP8.5 “assumed” a continuing increase of coal-burning, concluding that since coal emissions have fallen, RCP8.5 is not a meaningful scenario and is used by climate alarmists to propagate fear. I hope the readings above have made clear how deliberately wrong this interpretation is.
The authors of this paper argue that RCP8.5 remains an important reference point for a number of reasons. First, from 2005-2020, our emissions have tracked within 1% of those laid out by the RCP8.5 scenario. Second, predictions based on both announced policy intentions and current policies through 2050 yield cumulative emissions amounts landing between RCP4.5 and RCP8.5. The authors also note that natural feedback loops are not included by these 2050 emissions forecasts. In other words, Earth system feedback cycles that are accelerated by global warming — permafrost melt, soil dynamics, increased forest fire — produce additional emissions that are not included in those forecasts and would likely push cumulative emissions further towards the RCP8.5 scenario by 2050. This is not to say that RCP8.5 directly models the path we’re on — it doesn’t, and was never meant to. But it’s a useful tool, particularly since so much work has been done on modelling the possible impacts of the concentrations of CO2 it represents.
Just how sensitive is the climate to increased carbon dioxide? (10 minutes)
This may come as a surprise, but one of the most fundamental questions involved in predicting global warming is still subject to a great deal of uncertainty: given an atmospheric CO2 concentration, just how much will the Earth actually warm? The working assumption from more or less 1979 till 2020, was that a doubling of atmospheric CO2 would lead to warming of between +1.5C and +4.5C. An extensive multi-year study published this year has narrowed that range substantially, concluding that +2.3C-+4.5C is the “likely range.” Unfortunately, the authors suggest that the possibility of more than +4.5C is as high as 18%. It’s still a broad range of uncertainty, but eliminating the lowest end of the spectrum is an important development. One thing to think about when trying to understand what climate sensitivity is that’s obscured by the way it’s presented: these are not predictions or forecasts of global warming. They are a measure of how much a given change in CO2 concentration would warm the world, if changed immediately and then held constant. In the real world, CO2 concentrations are very unlikely to remain constant. If we act fast, we may be able to reach a point where atmospheric CO2 is diminishing. If we don’t, natural feedback cycles could work in concert with ongoing human emissions to continue raising atmospheric CO2 concentrations.
I don’t have a book recommendation this week, sorry! I’m open to recommendations re: climate modelling, emissions scenarios, or climate science more generally.
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