One of the most fundamental questions in climate research asks the following: What will the world look like when we reach a certain point of warming?
How will it change after 2 degrees? 4 degrees? Even warmer?
More than a decade ago, scientists designed a set of hypothetical scenarios to help them model the climate. Their goal was to answer these very questions. Each scenario assumes a different level of future greenhouse gas emissions and global carbon dioxide concentrations, translating to different levels of warming.
The mildest scenario assumes a bold effort globally to reduce emissions over the coming decades where global temperatures would rise only about 2 degrees Celsius above their preindustrial levels through the end of the century. Several intermediate scenarios assume slightly higher levels of warming. And the most severe trajectory, which presumes that emissions will grow throughout this century, could result in as much as 5 degrees C — that's about 9 degrees Fahrenheit — of additional global warming by the year 2100.
These scenarios have been used in thousands of climate studies since they were first developed.
Now there may be a problem with one of them. The most severe climate scenario of the bunch might be so extreme that it's no longer a likely outcome, experts say.
In one sense, that's good news for the world. It means that the most extreme visions of the future, outlined in climate studies over the years, probably won't come to pass.
But it also suggests that climate scientists may want to rethink the way they conduct modeling studies moving forward.
According to some experts, presenting the public with unreasonable climate scenarios could hurt global efforts to address climate change. It could suggest that global climate targets, like the goals of the Paris Agreement, are less achievable than they actually are.
That's one concern outlined in a comment published yesterday in Nature by University of California, Berkeley, climate scientist Zeke Hausfather and climate policy expert Glen Peters, research director at the Center for International Climate Research in Norway.
The scenario in question — dubbed Representative Concentration Pathway 8.5, or RCP8.5 — is often referred to in climate studies as a "business-as-usual" trajectory. In other words, it's framed as the outcome the world will experience if current conditions are allowed to continue into the future, with no future climate policy enacted.
But that's not actually the case, the authors argue.
"We still have a huge lift to go to get warming below 2 degrees," Hausfather said in an interview with E&E News. "But at the same time, we've started to exclude some of the truly catastrophic outcomes of high-end warming."
The worst-case scenario, RCP8.5, was originally intended "to explore an unlikely high-risk future," the authors say. The dangerous outcomes outlined in this scenario would require extreme emissions of future greenhouse gases, the sort that would probably have to be driven by massive expansions in global coal consumption over the coming decades.
When the pathways were developed, that grim future was still a potential concern.
"If we go back to when the scenario was developed in 2007, what was happening in the world of energy — we saw China building up coal combustion, building an unbelievable number of coal plants," said Justin Ritchie, a postdoctoral research fellow at the University of British Columbia, whose research has focused on the likelihood of high-end emissions scenarios.
"The question was, what if that continued?" Ritchie said. "If that continued for 90-plus years, then you get RCP8.5."
It was a possible, although extreme, scenario to begin with. But in the years since, Ritchie notes, a combination of global commitments to the Paris Agreement and changes in the global energy landscape have made this scenario much more unlikely.
Under these present-day conditions — what might be more accurately considered the new "business as usual" — most experts estimate the world is on track for global warming in the realm of about 3 degrees C. That's significantly less than the warming implied by RCP8.5.
This doesn't mean that the models used by climate scientists are wrong or can't be trusted. On the contrary, research suggests that climate models generally do a good job of predicting how the world will respond to given amounts of greenhouse gas emissions.
If global carbon emissions really reached the levels implied by RCP8.5, the models probably do a good job of projecting how the earth system would respond. The comment simply suggests that this particular emissions scenario — the assumptions fueling the most extreme model projections — are probably no longer plausible.
There's no doubt that RCP8.5 makes for an impressive, even apocalyptic, narrative in climate research. Under this trajectory, modeling studies suggest, the world would experience a stunning increase in deadly heat waves, devastating droughts, catastrophic flooding, widespread agricultural failures, extreme hurricanes and wildfires, and dizzying losses in polar ice.
But while global fossil fuel consumption no longer aligns with the RCP8.5 scenario, some experts still worry that other factors might drive future warming higher than current estimates suggest.
Thawing permafrost in the Arctic, for instance, is a major uncertainty when it comes to future climate change. As it warms up, permafrost can release large quantities of potent methane and carbon dioxide into the atmosphere. Some scientists worry that this process, and other similar natural feedbacks, could eventually accelerate the progress of climate change.
These are questions currently at the forefront of climate research. For the time being, however, there isn't enough evidence to suggest that the world is currently on track for the kind of scenario outlined in RCP8.5.
Some scientists are pointing out that RCP8.5 isn't a totally useless tool. The world may not necessarily reach 5 degrees of warming by the year 2100 — but if the world is still emitting carbon past the end of the century, global temperatures will continue to rise beyond that point, Hausfather noted.
That means, even under a milder emissions trajectory, the world could still reach the same level of warming further into the future. And modeling RCP8.5 does give climate scientists a view of what the world might look like with extreme levels of warming, even if the timeline is too fast.
Bob Kopp, a climate scientist at Rutgers University, has previously pointed out on Twitter that "from a climate science perspective, RCP 8.5 is very useful, since we would like to know how models simulate a 5C world," adding that climate models rarely run beyond the year 2100.
But from a policy standpoint, Peters and Hausfather argue, continuing to present RCP8.5 as a plausible business-as-usual scenario could be counterproductive to global climate efforts. When considering the effort required to keep global warming below 2 degrees over the next few decades, the costs may seem "exorbitant" to policymakers if RCP8.5 is their basis of comparison.
"The marginal investments required to move from 3C of warming to well below 2C ... will be much less than moving from 5C to well below 2C," the authors write. "A narrative of progress and opportunity can make the Paris targets seem feasible, rather than seemingly impossible."
Now may be a convenient time to start changing the narrative, they suggest.
Over the last few years, scientists and modeling experts have been building a new set of potential future pathways, known as the "shared socioeconomic pathways," or SSPs. They contain much more detailed built-in assumptions about global conditions than the RCPs, including future population growth, economic and technological change, and their associated greenhouse gas emissions.
Some of them do assume no future climate policy but don't necessarily include the other extreme assumptions that led to scenarios like RCP8.5.
Climate scientists are already beginning to incorporate the SSPs into new modeling studies, and their results will be coming in over the next few years. Switching over to the new pathways is an opportunity to begin focusing on more realistic scenarios, Peters and Hausfather suggest, or at least making sure that the most extreme trajectories aren't painted as likely outcomes.
"Going forward, we have a much wider toolbox available to look at no-policy baselines," Hausfather said. "We just need to make sure that we use it."