Claims that climate scientists have abandoned their most dire scenario have been widely misunderstood. While the highest emissions pathway is now considered unlikely, evidence suggests the climate system may still be tracking toward dangerously high levels of warming.

Occasionally, climate science is big news. On 26 May, the New York Times headlined: “Why scientists retired the dire climate scenario used for over a decade”. A good story!

David Spratt

The Australian, true to form, went with “Climate doomsday scenarios just got a major rewrite”, and in Jeff Bezos’s Washington Post it was  “The climate apocalypse? Don’t count on it”. There were a host of similar headlines.

Climate deniers and Donald Trump used an old playbook to claim scientific fraud (surprise!), but were called out, with ‘Trump twisted a climate debate beyond recognition’ and ‘Factcheck: Trump’s false claims about the IPCC and ‘RCP8.5’ climate scenario’.

So what’s the real story? Did scientists get it wrong, and is warming now likely to be less severe than previously thought?

As in engineering and business and government, scenarios are used by climate scientists to think about plausible alternative futures and their risks. The commonly-used climate scenarios are based on different possible trajectories for human greenhouse gas emissions and the social path humanity takes, and the consequences. And remember, scenarios in the end are simply a product of the minds that imagined them.

Fifteen years ago, four scenarios called representative concentration pathways (RCPs) were developed for the fifth IPCC assessment report in 2014, with RCP2.6 the lowest and RCP8.5 the highest. The numbers are radiative forcing (RF) values in 2100 for each scenario, where RF is the difference between the incoming radiation energy and the outgoing radiation energy in a given climate system, which is an indicator of total expected warming.

In conventional climate science terms, each one unit of RF (in watts per square metre) would in the long run be expected to result in around 0.75°C of warming. This relationship between change in radiative forcing and change in temperature is known as climate sensitivity.

RCP8.5 was sometimes called a ‘business as usual’ scenario, but this was a misnomer, and it was based on an assumption of little or no curbing of  greenhouse gases. Modellers estimated it would result in the end of warming of 5 to 6°C, with a range of 3.0 to 12.6°C.

The sixth IPCC report in 2022 focused on a modified system called Shared Socioeconomic Pathways (SSPs), where the scenarios more explicitly considered social, economic, and technological trends. The SSPs were again expressed as RF values. Figure 1 illustrates both the RCP and SSP scenarios as they relate to total emissions.

Now, in preparation for the modelling project for the next IPCC report due in 2029, known as ScenarioMIP,  scientists have suggested that the highest, ‘worse-case’ RCP8.5 scenario be dropped, because emissions were tracking more in line with one of the middle scenarios, RCP4.5. Hence all those headlines.

So, the ‘worse-case’ global warming case is no longer realistic. Big sighs of relief!

Not so quick. The big question in the end is not the amount of emissions but how hot it gets: the temperature. The focus on emissions in RCPs/SSPs is a bit to one side.

And on the future temperature, here’s the bomb. In a recent post, Ryan Katz-Rosene showed CERES data where the effective radiative forcing (ERF) at the moment is tracking above RCP8.5:

CERES is a NASA project that uses satellite and other data to measure the amount of sunlight absorbed by Earth and the amount of infrared energy emitted to space. As Katz says “current forcing observations from CERES really do appear to show a high current ERF value, which (at least at this point in time) does seem to be above the mean ERF expected in RCP8.5.”

[Technically, RF measures the immediate change in energy balance at the top of the atmosphere due to an external driver, while ERF accounts for adjustments in temperature and other factors after the initial change. ERF gives a more comprehensive understanding of the climate response to these changes.]

With the actual radiative forcing higher than the worst-case scenario, all those headlines about things getting better look like a lot of hot air.

So how can actual and future warming, indicated by RF, be tracking the worst case when the emissions trajectory is a middle-of-the-road scenario? The RCP/SSP scenarios were built around greenhouse gas emissions, not around the full suite of forcings and climate feedbacks that determine what the climate system actually does in terms of heating.

The assumptions about the relationship between emissions and temperatures have been too conservative. For example, what is not getting said is that the best estimate of the climate sensitivity has been rising, with perhaps the world’s most eminent climate scientist, Jim Hansen, taking it beyond the IPCC upper-range estimate. In fact, even the current range of modelling, known as CMIP6, produces a higher climate sensitivity than previously thought.

Other factors include reduced aerosol masking, ice-reflection loss, the release of permafrost carbon, and weakening ocean sinks that are not adequately captured by the IPCC or in model assumptions about future warming. Yet they’re showing up in the real-world numbers right now.

What is happening is way beyond IPCC projections. The rate of warming has accelerated by half over the last two decades, driven by reduced aerosols emissions and diminishing cloud cover. Warming has reached 1.5°C, and with an approaching strong El Nino, 2026-27 is likely to be around 1.7°C. Earth’s Energy Imbalance, an indicator of future warming, has doubled in the last 15 years and continues to increase, suggesting a warming trend of 2°C by 2040 is likely. Even global warming of 1°C, a threshold already passed, risks triggering some tipping points. At 1.5°C, six out of 10 studied climate subsystems already show large-scale abrupt shifts across multiple models.

Katz says: “We have such large uncertainty by end of century on climate sensitivity and carbon feedbacks, such that we can’t preclude mean warming of up to 4°C by 2100 even if we successfully pursue an emissions pathway resembling that in RCP4.5. So, again, if sensitivity or carbon feedbacks are not in our favour, there are plenty of scientific findings based on RCP8.5 which could turn out to be right on the mark in meteorological terms later this century, despite being way off on anthropogenic fossil emissions assumptions.”

Any reputable climate scientist over a drink at the bar will tell you that by far the majority of the human population would likely not survive 4°C. And that sounds like a worst case to me.

https://johnmenadue.com/post/2026/06/avoiding-worse-case-climate-warming-is-big-news-but-is-it-true/

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