How Likely Is The Worst-Case Climate Scenario?

How Likely Is The Worst-Case Climate Scenario?

As a climate reporter, I’m not used to reporting any remotely good news on a global scale. But according to a new comment in Nature, things might be looking up a bit. This (potential) good news is super technical, but it’s also super important.

To describe the future of our planet under different circumstances, climate scientists use a set of scenarios called “representative concentration pathways” (RCP). Each scenario models the future out to 2100 under different levels of fossil fuel emissions and takes into account how much the planet has warmed, oceans have acidified, and crucially, how able or unable the planet is to respond to those changes and restore order.

The higher the RCP number, the scarier the situation for the planet (and by extension, us). And the most commonly referred to worst case scenario is RCP8.5, one often labelled “business as usual.” But in the comment in Nature published on Wednesday, two climate experts show that referring to it that way is misleading.

RCP8.5 describes the planet if world leaders don’t do anything to curb emissions between now and 2100. The scenario and all other RCPs start in 2000, and RCP8.5 assumes no new climate policy was enacted after 2005. That leads to runaway coal and fossil fuel use around the world and catastrophic climate change as a result. That’s not how business as usual has gone since nor was it necessarily how scientists envisioned it when they started working on the scenarios more than a decade ago according to the new comment.

“RCP8.5 was intended to explore an unlikely high-risk future,” wrote energy systems analyst and environmental economist Zeke Hausfauther, and Research Director at the Centre for International Climate Research Glen Peters. They go on to note in the comment that the high risk future is probably not where the world will end up. “Happily — and that’s a word we climatologists rarely get to use — the world imagined in RCP8.5 is one that, in our view, becomes increasingly implausible with every passing year,” they wrote.

Take coal use. RCP8.5 illustrates a world that increases coal burning fivefold by the end of the century. But there are signs that global coal use has already peaked and will flatline within the next few decades as it becomes economically unviable in the face of cheaper renewables (and for now, natural gas).

Then there’s climate policy. While there certainly haven’t been enough strides made to curb fossil fuel use and carbon emissions, there have been some unlike RCP8.5. Most notably, the Paris Agreement sets up a global framework to reduce carbon emissions. The Kigali Amendment, meantime, puts a cap on some of the worst greenhouse gases. And there are signs some nations are stepping up to the plate as well.

“We have had a lot of climate policy (and technological advancements) in the past 15 years, and RCP8.5 is much a much, much less likely outcome,” Hausfather told Gizmodo in an email.

That’s not say the world will be fine or that we’re on track for RCP2.6, one of the best case scenarios. But those policies and the economics of coal are serious signs we’re moving away from RCP8.5 from a societal standpoint. But there are still other parts of the scenario that have inspired (and will continue to inspire) heated debate. And for a wonky climate scenario, it really gets people in their feelings! Seriously, I invite you to search “RCP8.5” on Twitter. To wit:

I present to you a RCP8.5 meme as Exhibit B.

Much of the disagreement is about feedback effects, which amplify changes in the climate. The clearest example of this is thawing permafrost. The frozen soil has begun to melt due to global heating. But when permafrost melts, it actually releases more greenhouse gases into the atmosphere. Similarly, wildfires are more intense and frequent (hello, Australia) because of global heating and dryness. They also emit a ton of carbon that trees sucked out of the atmosphere before, uh, burning to the ground.

Some scientists believe the RCPs underestimate climate “forcing”—the term for the changes that push the climate system further into crisis—based on the effects of those feedback loops. Katherine Hayhoe, who directs the Climate Science Centre at Texas Tech University, told Gizmodo that though coal use is plateauing and renewables are on the rise, the world may still reach RCP8.5 because of those feedbacks.

“The emissions scenarios that were developed to go with RCP8.5 are thankfully looking like they are too high now,” she said. “But I think the forcing scenario itself is not yet off the table.”

Other scientists believe the RCP8.5 scenario is still possible because it underestimates the world’s rate of economic growth. “We cannot necessarily consider RCP8.5 to be a high emissions baseline, because of the likelihood that economic growth rates are understated in that scenario,” Peter Christensen, an assistant professor at the University of Illinois, told Gizmodo in an email. “The implication is that baseline (no policy) emissions growth could be quite a bit higher than projected, putting an even greater importance on regulatory and other policies to reduce the future the damages associated with climate change.”

Scientists aren’t in agreement about RCP8.5. But they are in agreement that no matter how likely we are to meet it, we desperately need to stop greenhouse gas emissions as soon as possible.

“I’m not sure it matters very much,” Kate Marvel, an associate research scientist at NASA Goddard Institute for Space Studies, told Gizmodo in an email while speaking about whether we’re on track for an RCP8.5 atmosphere or not. “[W]ithout targeted climate action, we’re almost certainly on track to blow past the Paris Agreement targets of limiting warming to 1.5 or 2 degrees C. I think the important thing to remember is that what controls the future climate isn’t physics, it’s people. We can make RCP8.5 even more implausible by cutting emissions. I think we should do that.”

Hausfather and Peters think that being clear about how likely the RCP8.5 is could help cut emissions. “Overstating the likelihood of extreme climate impacts can make mitigation seem harder than it actually is,” they wrote. “This could lead to defeatism, because the problem is perceived as being out of control and unsolvable.”

But the most important part? “This admission does not make climate less urgent,” they wrote.