As scenario-building went, this was the worst. RCP8.5 laid down how a future climate would look. RCP is short for “representative concentration pathways”. RCP 8.5—its successor SSP5-8.5—comes to pass if we burn fossil fuels like there is no tomorrow and spew planet-heating emissions.

(SSP is short for Shared Socioeconomic Pathways. In order to fully reflect reality, the United Nations Intergovernmental Panel on Climate Change (IPCC) merged in 2021 RCPs into SSPs. The idea was to include socio-economic development in climate modelling.)

The “8.5” is the amount of extra heat, measured in watts, trapped per square metre—radiative forcing—by 2100.

As for any scenario-building, some assumptions went into it. One was that there would be unbridled use of fossil fuels and extreme emission load-up in the atmosphere. As a result, carbon dioxide levels would be 1,135 parts per million. (Currently, CO2 levels stand at 429.91 ppm.) Temperatures would be around 4.5C higher than pre-industrial levels. That was the future RCP 8.5 laid out, back in 2011.

As renewables came on the scene, solar exploded, populations didn’t balloon as expected, so there needed to be a revision. Scientists did just that. The consortium of scientists led by Detlef P. Van Vuuren published a paper titled The Scenario Model Intercomparison Project for CMIP7 (ScenarioMIP-CMIP7). This lays out the next version of emission scenarios.

They state: “On the high-end of the range, the high emission levels (quantified by SSP5-8.5) have become implausible, based on trends in the costs of renewables, the emergence of climate policy and recent emission trends. At the low end, many emission trajectories have become inconsistent with observed trends in the 2020–2030 period.”

This means that as the worst-case scenario has become “implausible”, the best-case scenario, limiting warming to 1.5C above pre-industrial levels, has become untenable.

Vishal Dixit, Assistant Professor, Centre for Climate Science, IIT Bombay, tells Hot Rock: “Understanding future global warming relies on three foundational concepts: climate forcing—the heat-trapping imbalance caused by rising greenhouse gas concentrations; climate sensitivity—how high temperatures go up in response to that forcing; climate scenarios—the hypothetical pathways mapping out future human emissions.

“When originally designed, scenarios RCP8.5 or SSP5-8.5, were built on the best data available at the time, which assumed a massive, five-fold global expansion of coal-based energy production by the end of the century. But fresh data points on clean energy market shifts, tumbling renewable costs, and evolving social behaviour show global fossil fuel expansion is changing, making the exponential curves of RCP8.5 a significant departure from our actual energy choices.”

Zeke Hausfather, Glen Peters and Piers Forster at Climate Brink have this to say on the death of RCP8.5: “When RCP8.5 was first published global emissions were skyrocketing. Clean energy sources were expensive, electric vehicles largely non-existent, and the idea that we would continue to increase our use of coal, oil, and gas through the end of the century was not as far-fetched.

“Even in that context, RCP8.5 was chosen to represent the high end of the range. It was never a likely outcome even in a world that did not address climate change; rather it was intended to represent a worst-case scenario that pushed fossil fuel expansion to the max.”

Crucially, retiring these extreme pathways does not imply any scientific deficit or error in the physics of the older models, nor does it suggest that the climate crisis is less grave, Dixit cautions.

“Rather, it is a pragmatic operational shift to channel the limited computational and intellectual resources of the global scientific community toward modelling more realistic, plausible projections,” says  Dixit.

Expanding on what possible range of futures are likely, he says, even updated “middle-of-the-road” global temperatures are projected to rise by a dangerous 2.5C to 3C by 2100. Furthermore, large uncertainties in climate sensitivity and potential natural feedback loops—like permafrost thawing—mean severe high-end warming remains a dangerous tail risk even under moderate emissions. Abandoning the extreme case doesn’t absolve us from a climate catastrophe. It’s still on the cards.

As scientists at Climate Brink put it: “When we try to estimate how much the world will warm this century and beyond, we run into three fundamental uncertainties: our future emissions, the sensitivity of climate to increasing forcings, and the carbon cycle feedbacks that determine the portion of our emissions that remain in the atmosphere.”

They say that “while we tend to give the central estimate of future warming in 2100 associated with a given emissions scenario (e.g. 2.8C), this single number hides a pretty wide range of actual possible climate responses.” The median is 2.8C, but the 5th to 95th percentiles span 2.1C to 3.7C, and there is even a small (~2%) chance of 4C or more warming.

“The brutal math of climate change is this: as long as CO2 emissions remain above zero, the world will continue to warm. The medium scenario ends up closer to 3.7C by 2150, while the high scenario ends up more or less matching the warming in the old RCP8.5 scenario despite an assumption of flattening or modestly declining emissions after 2100.”

So, here we’re back to, well, the extreme scenario, maybe a few years later down the line. At that point, it will not be a thought-experiment or gaming-scenario. It will be reality.