‘Rare, Happy News’ from Climatologists: Worst-Case Warming Now ‘Increasingly Implausible’
It doesn’t make the climate crisis any less urgent, but there’s a growing view among scientists that humanity’s decarbonization efforts so far have dodged the worst climate outcomes projected in the last global assessment report in 2014.
From scientists publishing in the journal Nature, to one of the world’s self-described climate “alarmists” writing for New York Magazine, multiple sources are celebrating the strong likelihood that RCP8.5, the extreme scenario from the Intergovernmental Panel on Climate Change (IPCC) that projects 5.0°C average global warming by 2100, won’t likely come to pass.
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“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,” write Zeke Hausfather, director of climate and energy at the Breakthrough Institute in Oakland, and Glen P. Peters, research director at Oslo’s CICERO Center for International Climate Research, for Nature.
“Assessment of current policies suggests that the world is on course for around 3°C of warming above pre-industrial levels by the end of the century—still a catastrophic outcome, but a long way from 5°C,” they add. “We cannot settle for 3°C; nor should we dismiss progress.”
“Over the last few weeks a new narrative about the climate future has emerged, on balance encouraging, at least to an alarmist like me,” added columnist and literary editor David Wallace-Wells in mid-December. “As best as we can understand and project the medium- and long-term trajectories of energy use and emissions, the window of possible climate futures is probably narrowing, with both the most optimistic scenarios and the most pessimistic ones seeming, now, less likely.”
Hausfather and Peters trace the shift in perspective to a choice that climate scientists and energy modellers made nearly a decade ago, in the lead-up to the IPCC’s Fifth Assessment Report (AR5) in 2014, producing consequences that are now “hotly debated” as the IPCC’s next big report moves into its final stages this year.
In the lead-up to AR5, “researchers developed four scenarios for what might happen to greenhouse gas emissions and climate warming by 2100,” the two authors explain. “They gave these scenarios a catchy title: Representative Concentration Pathways (RCPs). One describes a world in which global warming is kept well below 2°C relative to pre-industrial temperatures (as nations later pledged to do under the Paris climate agreement in 2015); it is called RCP2.6. Another paints a dystopian future that is fossil fuel-intensive and excludes any climate mitigation policies, leading to nearly 5°C of warming by the end of the century. That one is named RCP8.5.”
That scenario was only ever intended to explore “an unlikely high-risk future,” they add. “But it has been widely used by some experts, policy-makers, and the media as something else entirely: as a likely ‘business as usual’ outcome. A sizeable portion of the literature on climate impacts refers to RCP8.5 as business as usual, implying that it is probable in the absence of stringent climate mitigation. The media then often amplifies this message, sometimes without communicating the nuances. This results in further confusion regarding probable emissions outcomes, because many climate researchers are not familiar with the details of these scenarios in the energy modelling literature.”
Particularly when coverage and scientific reporting are built around RCP8.5 and RCP2.6, the “focus becomes the extremes,” they add, “rather than the multitude of more likely pathways in between.”
Contrary to that communications cascade, Hausfather and Peters look at today’s emissions trends, and they like some of what they see. “Emission pathways to get to RCP8.5 generally require an unprecedented, five-fold increase in coal use by the end of the century, an amount larger than some estimates of recoverable coal reserves,” they write. “It is thought that global coal use peaked in 2013, and although increases are still possible, many energy forecasts expect it to flatline over the next few decades.” And meanwhile, “the falling cost of clean energy sources is a trend that is unlikely to reverse, even in the absence of new climate policies.”
Hausfather’s and Peters’ conclusions aren’t unanimous within the climate science community: they note that some scientists see “feedback” effects, like greenhouse gases released from thawing permafrost, that are understated in the most recent generation of IPCC climate models and could pull the Earth back to an RCP8.5 pathway.
But “in our view, reports of emissions over the past decade suggest that they are actually closer to those in the median scenarios,” they write. “We contend that these critics are looking at the extremes and assuming that all the dice are loaded with the worst outcomes.”
“Asking ‘what’s the worst that could happen?’ is a helpful exercise. It flags potential risks that emerge only at the extremes,” they add. But “we must all—from physical scientists and climate impact modellers to communicators and policy-makers—stop presenting the worst-case scenario as the most likely one.” That’s because “overstating the likelihood of extreme climate impacts can make mitigation seem harder than it actually is,” and “this could lead to defeatism, because the problem is perceived as being out of control and unsolvable. Pressingly, it might result in poor planning, whereas a more realistic range of baseline scenarios will strengthen the assessment of climate risk.”
With the latest generation of climate models just recently published, the two authors suggest three steps the climate community can take in the next year before the IPCC publishes its Sixth Assessment Report, AR6:
• Explore the “space between the high-end and low-end scenarios” more deeply, and communicate the range of likely climate impacts more clearly;
• Recognize that different users need different modelling tools, and use the final AR6 synthesis to integrate those various perspectives on climate risk;
• Use “more plausible outcomes” in the multitude of climate impact studies that will flow from AR6. “When RCP8.5 or its successor SSP5-8.5 are deployed,” Hausfather and Peters write, “they should be clearly labelled as unlikely worst cases rather than as business as usual.”