Climate scientists are no longer using the highest emission, highest warming scenario in their analysis for the upcoming Intergovernmental Panel on Climate Change report. That report is considered a consensus document that drives global discussion and policy around climate change.
Scientists say the scenario, known as RCP8.5, is no longer a plausible projection of future warming. That scenario projected about four to five degrees Celsius of warming by 2100.
“The divergence between the real world and the world in the extreme scenarios became so great that it’s not realistic or feasible to continue carrying on with these scenarios as if they’re a plausible representation of the future,” said Roger Pielke Jr., a senior fellow at the American Enterprise Institute think tank.
University of British Columbia climate scientist Simon Donner says the RCP8.5 scenario “ assumed an unrealistic amount of coal burning.” But he warns against viewing revised projections as evidence that climate risks have been exaggerated.
“You want to plan for the clean energy revolution, for the fact that the world is starting to do a good job at shifting away from fossil fuels,” Donner said. “But in terms of impacts, you still want to plan for the worst-case scenario.”
Why the scenario is fading
RCP8.5 was developed by climate researchers in the late 2000s for use in climate model simulations that informed the IPCC’s reports.
RCP8.5 was one of four climate scenarios, with it representing the highest emissions pathway. The “8.5” referred to a very high level of radiative forcing — effectively a proxy for the heat-trapping effect of greenhouse gas concentrations in the atmosphere by 2100.
The model later came to be called SSP5-8.5, but its core assumptions remained similar: extremely high fossil fuel consumption, especially coal use, combined with limited climate policy intervention.
The scenario was never intended to represent the single most likely future, says Zeke Hausfather, a senior fellow at the Breakthrough Institute, a California-based environmental research organization.
Rather, the scenario was used partly to test how climate systems — such as sea-level rises, flooding or wildfire conditions — would be affected by extreme emission conditions.
But Hausfather says there was a tendency in the 2010s to treat RCP8.5 as the likely climate outcome if countries’ activities continued “business as usual.”
“I think in the 2010s, [that scenario] became quite widespread,” he said.
Part of the issue, he says, stemmed from a “game of telephone” between the people who developed the emission scenarios and the users of climate models.
Also, when the scenario was first developed, rapidly rising Chinese coal consumption and expensive renewable technologies made extremely high projections of future fossil fuel use appear more plausible.
But since then, the economics of energy have shifted dramatically. According to the International Energy Agency, global solar photovoltaic costs fell by roughly 90 per cent between 2010 and 2023. Wind and battery storage costs also declined sharply over the same period.
“The single biggest thing is the declining cost of clean energy globally,” Hausfather said.
Hausfather argues that at least part of the decline in RCP8.5’s plausibility reflects genuine progress in clean energy deployment and emissions mitigation.
“About half of the difference between RCP8.5 and where we think we’re headed today … is associated with [RCP8.5] never being particularly realistic … and about half of the difference is actual progress we’ve made in bending down that curve of future emissions.”
Pielke, of the American Enterprise Institute, takes a different view.
He says the tendency by some to now acknowledge the scenario as implausible incorrectly attributes this shift to “the wild successes of climate policy.”
But “that is just wrong,” he said.
“If you take a look at the reasons why the scenarios are implausible … it’s because they were wrong from the start, not because they were right and then the world went a different direction.”
“If you look at Canada and the U.S. and the world, and you look at that curve for decarbonization since the 1960s, it’s linear. It’s a straight line. There is absolutely no evidence that any policies that have been put in place in Canada, the U.S., or around the world have accelerated the rate of that decarbonization.
“So climate policy has done many things, but accelerate decarbonization is not one of those things.”
Canada’s planning dilemma
The debate around RCP8.5 carries particular relevance in Canada, where governments and researchers have relied on high-end warming scenarios when studying wildfire risk, Arctic warming, flooding and infrastructure resilience.
As one example, the Trudeau government’s 2019 climate report card that assessed how climate change is affecting Canada refers to the RCP8.5 scenario over a dozen times.
In an emailed statement to Canadian Affairs, a spokesperson for Environment and Climate Change Canada said RCP8.5 and SSP5-8.5 “were always viewed by the climate science community as an upper bound” and should be understood as stress tests, not the most probable outcome.
The department acknowledged that evolving technology and policy trends mean the scenarios are now “unlikely to occur.”
At the same time, the department said adaptation planning still requires governments to prepare for a range of possible future outcomes.
The spokesperson added that future Canadian climate simulations will follow newer scenarios that assume “substantially lower greenhouse gas emissions.”
What comes next
The debate over climate scenarios comes at a time when Canada’s climate policies are in flux.
Since taking office last March, Prime Minister Mark Carney has sought to position Canada as an “energy superpower,” and has signaled a greater willingness to relax climate policies and support energy development than his predecessor.
In one of its first acts, the Carney government eliminated a consumer carbon tax. And it recently agreed with Alberta to lower its industrial carbon price to $130 per tonne by 2040, down from the previous benchmark of $170 per tonne by 2030.
Donner, of the UBC, resigned in December from the federal government’s Net-Zero Advisory Body over concerns that the advisory group’s work had become “performative.”
While acknowledging the geopolitical and economic pressures Canada now faces, Donner says he worries Canada is heading down “the wrong path in the long term.”
“ I worry that the government is a bit stuck in the past. Not just a past that was heavily reliant on fossil fuels, but a past way of thinking about climate policy.
“And by that, I mean they are very focused on things that can incrementally reduce emissions from our existing activities rather than what we’re going to do to replace those activities.”
