Thread by @hausfath

Nature just published a comment by @Peters_Glen and I pointing out that the high-emissions RCP8.5 emissions scenario – with its 500% increase in coal use by 2100 – is increasingly unlikely in a world of falling clean energy prices. nature.com/articles/d4158…

A thread: 1/11

RCP8.5 was never intended to be business-as-usual. It was originally selected to represent roughly the 90th percentile of no-policy (after 2005) outcomes in the literature. In the intervening 15 years it has become increasingly unlikely. carbonbrief.org/explainer-the-… 2/11

While some scientific papers correctly contextualized it as a very high end scenario, others referred to it as business-as-usual or indicated it was the current pathway that world was on. This traces back to poor communication between energy modelers and climate scientists. 3/11

While emissions used to generate RCP8.5 are increasingly unrealistic, we can't be as confident that warming in RCP8.5 is beyond the realm of possibility. Current policies seem to be leading us to a 3C (+/- 1C) world, but large uncertainties remain: thebreakthrough.org/issues/energy/… 4/11

For example, if climate sensitivity is on the high end its possible to get 4C+ warming even current policies. Carbon cycle feedbacks are poorly represented in current model projections (ScenarioMIP) and could lead to 8.5-level forcings under, say, a SSP3-7.0 scenario. 5/11

The world also doesn't end in 2100, even though most of our models do. As long as emissions remain above net-zero the world will continue to warm. At current emission levels we would still see RCP8.5-type outcomes in the 22nd century. 6/11

At the same time, the fact that the world is not on track for emissions consistent with 5C warming by 2100 is reason for hope. A world on track for ~3C is one where its easier to bend the curve down toward 2C or below than a world on track for emissions consistent with 5C. 7/11

So what are the broad takeaways? We suggest three steps for the climate science community moving forward:

First, the new SSPs have a wider range of intermediate baselines that can be used to explore more likely emissions scenarios: carbonbrief.org/explainer-how-… 8/x

Second, we need at least some assessment of relative risk of future emission scenarios. When specialists refuse to assign probabilities, users often do so themselves, and often poorly because they do not have a deep understanding of the underlying assumptions. 9/11

At a minimum this could include an incorporation of IEA WEO and UNEP assessments of likely emissions outcomes implied by current policies and commitments. We should not be afraid to say that RCP8.5 is less likely today than, say, RCP6.0, assuming current policies remain. 10/11

Finally, we aren't arguing RCP8.5 should be banished from the literature completely, just that it shouldn't be used as the sole no-policy baseline in studies. When RCP8.5 (and its SSP5-8.5 successor) are used it should be labeled as a worst case rather than BAU outcome. 11/11

A technical addendum: if we can't exclude a real possibility of RCP8.5 (or SSP5-8.5) levels of warming, doesn't that mean we should continue using RCP8.5?

There are two primary uncertainties at play: climate sensitivity and carbon cycle feedbacks. 1/A

For climate sensitivity, future warming scenarios are specifically set up so that uncertainties in emissions and uncertainties in sensitivity are independent. 2/A

If you want to explore high sensitivity outcomes it makes a lot more sense to look at high sensitivity models in a more realistic scenario (say SSP4-6.0 or SSP3-7.0) than use the average of all models in a scenario driven by with less realistic emissions (SSP5-8.5). 3/A

For example, high sensitivity models may show different regional patterns of changes that you'd miss if you were using all RCP8.5 models as a (poor) proxy of a lower-forcing higher-sensitivity outcome. 4/A

The more compelling argument in favor of RCP8.5 is that the IPCC scenario runs commonly featured exclude many important carbon cycle feedbacks. This is a real problem, and the reasons for it are a tad complex. 5/A

About half of climate models are Earth System Models (ESMs) that include biogeochemical processes governing carbon cycle feedbacks. These models include feedbacks from changing vegetive cover, melting permafrost, etc. 6/A

However, the other half of models are more simple climate models that do not include these feedbacks. In order to compare all the models, the IPCC uses scenarios where concentrations of greenhouse gases are fixed. Models are run on future concentrations rather than emissions. 7/A

So the commonly presented IPCC figures of future warming exclude many important carbon cycle feedbacks. There is a separate set of experiments – C4MIP – where ESMs are run with future emissions rather than concentrations, and where more carbon cycle feedbacks are present. 8/A

These experiments show a wide range of potential feedbacks, but on the high end they are enough to turn a SSP3-7.0 forcing scenario into a SSP5-8.5 one, for example. 9/A

I'd argue that the solution here isn't to make up for a lack of carbon cycle feedbacks with a purposefully unrealistic emissions scenario. Rather, C4MIP results should be more prominently featured in future projections shown in assessments. 10/A

Its also important to emphasize even including carbon cycle feedbacks doen't make RCP8.5/SSP5-8.5 likely. You'd need the combination of feedbacks on the high end of the model range and an emissions scenario (like SSP3-7.0) above current policy projections to get there. 11/A

For this reason while we think that the RCP8.5/SSP5-8.5 emissions scenario is increasingly unlikely, we don't think that level of forcing should be excluded from future assessments. Rather, it needs to be correctly contextualized as a very-high-end or worst-case outcome. 12/A

@Peters_Glen (Note that the link should be live shortly - the issue just went up on their website)