by Neil Auwarter
The good ship Earth
Imagine you are captain of an ocean liner that has struck an iceberg and is taking on water. A rescue SOS is sent. The crew and passengers work the pumps furiously to keep the ship afloat. The ship’s engineer estimates the vessel could sink in eight hours. But then, based on the progress made by those at the pumps, the engineer makes a revised estimate: the ship might remain afloat indefinitely! As captain, you should:
A) Urge passengers and crew to remain at the pumps and prepare to be rescued
B) Celebrate by abandoning the pumps and holding a luxury buffet in the ship’s grand ballroom
C) Denounce the ship’s engineer as an alarmist and an incompetent elite
If you are Captain Donald Trump, the answers are of course B and C. This allegory conveys the essence of Trump’s recent Truth Social post celebrating climate scientists’ retirement of one particularly dire climate scenario known as RCP 8.5.
What on planet Earth is “RCP 8.5”?
Climate scientists at institutions like NOAA (National Oceanographic & Atmospheric Administration), Caltech, MIT and their international counterparts develop forecasts of Earth’s future climate. These forecasts attempt to model different climate “pathways” based on varied future concentrations of atmospheric greenhouse gasses, all within a plausible range of future events. The most salient future events are the increase or decrease in fossil fuel use and the related rate of development and adoption of greener energy sources.
Climate models are, like any attempt to predict the future, inherently speculative. Future greenhouse gas concentrations depend on factors such as population growth, politics and human behavior – factors climate scientists do not control.
To accommodate these uncertainties, climate scientists develop multiple scenarios, each based on a different set of assumptions about future events. A prominent example is the “representative concentration pathway” (RCP), created by a consortium of scientists in 2014 and adopted by the IPCC (Intergovernmental Panel on Climate Change) in its Fifth Assessment Report (“AR5”). The numerical designations correspond to each scenario’s predicted level of “radiative forcing” (heat trapping) in Watts per square meter in calendar year 2100.
AR5 set out four alternative scenarios, with RCP 8.5 being the most dire. RCP 8.5 assumes large, rapid increases in future use of fossil fuels, particularly coal – the black sheep of an already dirty family. On the optimistic end of the spectrum is RCP 2.6, which assumes aggressive adoption of greener energy sources and prompt reductions in fossil fuel use. RCP 4.5 and RCP 6.0 are more moderate, positioned in the middle.
The following chart illustrates the expected range of future global temperature change expected under each RCP scenario through 2100.
Temperature change relative to 1986-2005 baseline. Shaded bands show likely ranges (5th-95th percentile). Data source: IPCC AR5 SPM Figure SPM.7. Chart generated by Claude AI Sonnet 4.6.
As the chart illustrates, under RCP 8.5 Earth’s temperature could increase 4.8°C (8.5°F) by 2100. This would almost certainly be catastrophic. Under the rosiest scenario, RCP 2.6, aggressive mitigation would result in a much more manageable increase of only about 1°C (1.8°F) by 2100.
The worst and best RCP scenarios are discarded
This spring, 12 years after AR5, a new consortium of scientists, including several authors of AR5, issued an updated set of climate scenarios in a report known as CMIP7. CMIP7 sets out seven climate scenarios. Gone are the numerical RCP designations, in favor of plain language labels, such as “high,” “medium,” “low” and “very low” emissions. Also gone is any scenario as dire as RCP 8.5, or as optimistic as RCP 2.6.
This effective retirement of RCP 8.5 is what Trump’s post alluded to. Trump may have a point because there are credible experts who have long argued that RCP 8.5 was never plausible — because it assumed an unrealistic explosion in coal use. And RCP 8.5 certainly was not a “business as usual” scenario, as many media accounts and academics unfortunately labeled it. But Trump’s post misses the crucial point that it is progress in green energy technology and increases in energy conservation and efficiency that made RCP 8.5 unrealistically dire. Even the foremost critic of RCP 8.5 argued the scenario was unrealistic because it overestimated coal use and failed to consider the existing trend toward affordable solar energy. In other words, RCP 8.5 was unrealistically gloomy because it failed to factor in a clear trend toward renewable energy.
So as in the allegory of the distressed ocean liner, it is the efforts of those at the pumps — those who work to advance energy efficiency and conservation and to develop and adopt green energy sources — that have made RCP 8.5 unrealistic and secured the planet a more promising future. Trump’s assertion that climate science is a “scam” and “alarmism” is utter nonsense, likely the result of inhaling the fumes of $100 million in fossil fuel campaign money in 2024.
Here is a chart illustrating the new scenarios set out in CMIP7:
Temperature above pre-industrial baseline (1850-1900). Bands show estimates from van Vuuren et al., 2026. Chart generated by Claude AI Sonnet 4.6.
How should we celebrate this more promising path?
Under the CMIP7 scenarios, the estimated range of temperature increase through 2100 is substantially lower than the 2014 RCP set. This is truly something to celebrate. And since the new scenarios’ baseline is the Earth’s pre-industrial temperatures, rather than the RCP set’s baseline of more recent global temperatures, the new scenarios are even more promising than the numbers on their face suggest. But while an upper end global increase of 3.8°C (7°F) under CMIP7 set would certainly be better than the upper end of 4.8°C (8.5°F) under the AR5 set, the upper temperatures under CMIP7 would still severely disrupt the Earth’s climate.
We are not out of the woods, but the more tolerable temperature scenarios are within our reach. The question remains, will we heed our captain’s call to regress in an orgy of fossil fuel consumption, or will we remain at the pumps to keep afloat the ship that will carry our grandchildren and theirs?