RCPs vs. SRES scenarios

If you know anything about climate change, you’ve probably heard of the IPCC. And not the IPCC that investigates police failures laughably poorly, but the Intergovernmental Panel on Climate Change. The IPCC synthesises the most current peer-reviewed scientific literature from an array of disciplines to come to conclusions about the ‘state of knowledge’ on many different facets of climate change. Its nature, and focus on consensus, makes it inherently conservative, but also a pillar of scientific reliability and methodological solidity. It is famously described as neutral, and policy relevant, rather than policy prescriptive, which further exemplifies this.

The most recent IPCC assessment report (AR5) was released at the end of 2013/beginning of 2014, and included a methodological change that reflects the current trends in the climate literature. Since the last report in 2007, many scientists have drifted towards the consideration of cumulative emissions in climate targets rather than annual emissions. A study in 2011 by Bowerman et al. found that the relationship between cumulative emissions and temperature change was nearly linear, meaning that temperature and cumulative emissions increase proportionally to each other (Figure 1). What that means scientifically is that cumulative emissions are the most important factor in explaining temperature change, and that they are therefore the best way to conceptualise possible futures. This sort of sentiment has been echoed in important work by people like Meinshausen and colleagues (their paper in 2009 has been cited 409 times now, according to Reuters’ scientific journal search engine) and Oxford University’s Myles Allen & co. in the same year. You’d be hard pressed now to find reputable papers that don’t mention cumulative emissions, so great has been the sea change in the literature.

Figure 1 - the rate of warming (y axis, on the left) as a result of emissions (x axis, on the bottom).  Each graph, a-f, shows a different factor which can explain temperature change. The relationship is closest and approximately linear in a), suggesting that as one increases, the other increases. From Bowerman et al. (2011)

Figure 1 – the rate of warming (y axis, on the left) as a result of emissions (x axis, on the bottom). Each graph, a-f, shows a different factor which can explain temperature change. The relationship is closest and approximately linear in a), suggesting that as one increases, the other increases. From Bowerman et al. (2011)

This paradigm shift (if you can call it that) happened around the same time that the IPCC decided to introduce Representative Concentration Pathways (RCPs) to replace the previous generation of scenarios, SRES. Previously, the IPCC had used SRES emissions scenarios that explicitly considered the effects of prescribed levels of emissions into the atmosphere. However, there was (and still is) enormous uncertainty regarding contributing factors such as population growth, economic development and technological advances, hence the move towards RCPs. Whereas SRES scenarios describe ‘what if’ situations if a given amount of carbon dioxide equivalent was emitted, RCPs relate to concentrations of greenhouse gases in the atmosphere, and are essentially cumulative emissions budgets. As such, this means that they are end-points that can be reached via multiple pathways, and are less pre-determined than the original scenarios, which generally ordained when emissions would have to peak, at what levels, who by, etc. This made them complicated, and bitterly contested, especially by the actors who stood to lose out most under the scenarios, such as heavy industry and fossil fuel giants. However, the IPCC does not embroil itself in all the political wrangling that surrounds climate policy, and instead synthesises a ‘Summary for Policymakers’ and leaves it at that.

The primary reason for the IPCC’s switch to RCPs in AR5 was rather banal: functionality. Previously, all three working groups had relied on each other’s outputs to carry out research, meaning the final working group ended up waiting around for the first two to do their bit of the science (don’t even let me begin on the lack of integration and inter-disciplinarity in science). Rather than relying on a linear research process, where the scientists working on emissions scenarios (working group 2) generated the concentrations, converted into radiative forcings, to force the climate models (working group 1), the outputs of which were used by impact modellers (working group 3) to determine the effects of change, working groups 2 and 3 could work simultaneously. From the concentrations in the RCPs, they could figure out a) the likely pathways and situations to achieve the concentrations shown, and b) their likely effects. It also introduced some common units, which made the report more coherent between working groups – rather than the physical scientists talking about radiative forcing potential, the emissions modellers thinking about emissions in parts per million (ppm), and the impacts modellers discussing the effects of 4°C warming, all 3 could use a consistent metric that made their conclusions more comparable.

So although the principal cause for the IPCC was to improve the practicalities of the research process, the other reason why it’s good that they’ve moved towards RCPs of course is because emissions don’t directly cause climate change. Emissions cause concentrations of greenhouse gases to increase, and greenhouse gases cause climate change by changing the energy balance of the Earth. All the energy we get in should be balanced by what goes out, and gases like CO2 selfishly cling on to some of that escaping energy, thereby warming the atmosphere – you’re probably familiar with the greenhouse effect; and this is essentially it. Greenhouse gases have what’s called a radiative forcing effect (that’s the way we measure that perturbation to the energy – or radiation – balance), and each RCP is named, very imaginatively, after the level of radiative forcing that would result from the concentrations stipulated in that scenario (i.e. RCP 6.0, RCP 8.5 etc.). In that way, RCPs are more directly related to the issue at hand, and remind us that climate changes as a result of atmospheric concentrations of greenhouse gases.

However, while this is scientifically pleasing, and arguably more correct, this introduces problems for the general public, and for communicating climate science. Science communication is notoriously difficult anyway, particularly with the mainstream media seemingly conspiring to take things out of context and misreport at every opportunity. RCPs are another thing to be confused and manipulated, and act as another barrier to action by individuals and governments. Emissions scenarios, initially developed in 2000 in the seminal Special Report on Emissions Scenarios (SRES, for which mitigation scenarios in the third and fourth IPCC reports are named), are explicitly about human activity – it’s in the name. It’s hard to ignore the effect of driving your car, flying to Australia for a conference or leaving the heating on while you’re out if you frame scenarios in terms of emissions. Emissions scenarios relate to emissions, and although they are notoriously difficult to quantify (due to the economic and political incentives to misreport, as well as the sheer size of the feat required to collate all the data) and predict, they are more in-your-face about the effects of all that fossil fuel burning. RCPs step out of the debate about normative (‘to reach this target you should do x/y/z’) vs. descriptive (‘if x happened, y and z might result’) scenarios – they can be anything because they aren’t about emissions. The multiplicity of possible trajectories that could result in the concentrations demonstrated in each RCP means they essentially don’t need to think about how these concentrations arise.

In essence, that’s my point: from a scientific standpoint, and an educational one, RCPs represent serious progress in scientific thinking. But from a communication and policy perspective, I think they are regressive, and will distract people from what they need to do to prevent the worst effects of climate change. Allowing the Daily Mail to suggest that emissions are nothing to do with the concentrations observed in the atmosphere will result in a lot of “your cows fart more than ours, you should cut your emissions first” and not a lot of “oh shit, we’re screwing up the planet, let’s do something about this” from policy makers at big meetings like the upcoming Paris COP summit, heralded as the “last chance saloon” by many commentators. To get policy makers to do something, you need to shake them up a bit, and then present them with realistic methods to achieve what they need to achieve. Oh, and give them as little ammunition for geopolitical bitching as possible. The end.

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About shakkka

Londoner, climate scientist, extremist. All views are my own.

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