Avoiding 'worse-case' climate warming is big news. But is it true?
David Spratt
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!
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.
Jan Umsonst
HI all, that the RCP8.5 is dead is nonsense - all climate scientist who follow that view have no idea on what is observed at the ground.
It's kind of funny how they loose the oversight :D Therefore here an opinion from another climate scientist who does not have lost the oversight - it's all these recent assessments that natural systems will emit much more GHGs than projected and that the climate is much more sensitive than we thought if recent paleontological studies are taken into account:
Daniel Swain:
"I've got to say that I'm really disappointed in the recent discourse surrounding the RCP8.5 scenario. The lack of context and understanding is something I expected from the usual suspects, but it's frustrating to see such bad takes from people and outlets who should know better."
https://skywriter.blue/@weatherwest.bsky.social/3mmoxvqzt3s2j
We are now in a runway global warming which has just not realized itself. Just the new emission predictions on Arctic continental emissions would be enough - some 1-3Gt CO2e per year or more end of century even in a moderate warming scenario. And just the a study on the Tibetan plateau came to a similar result our of other reasons. At the same time the terrestrial carbon sink is nearing a reversal into a source while ocean carbon uptake plateaued the last years. All quite worrying signals...
In terms of shortwave feedbacks - highly important here marine heatwaves and SST patterns driven by marine heatwaves in the low cloud regions - we have now a OLR feedback going on - here the first paper on this recent weakening of the plank feedback - it's a vicious cycle between the atmosphere and upper oceans we trigger now:
It's a preprint but the signal is widely acknowledged in other papers that OLR did not increase as much as expected the last years - they see the Northeast Pacific as one main regions being responsible for this trend. But further studies will follow if this signal will persist. Also water vapor increases faster then modeled could play a role I would guess...
"Recent Weakening of the Global Radiative Feedback"; https://arxiv.org/pdf/2603.12515
Best
Jan
p.s. we so fucking hell in a worst case now that one only wants to vomit!
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Bruce Melton -- Austin, Texas
What is not being recognized in these discussions of how much further warming we will encounter, is tipping. It is active in about half of Earth systems (already degrading), and it does not reverse unless we remove the forcing that started it. There is no magic here. Once degradation begins from warming effects, only removing the warming effects can stabilize the degradation in almost all systems. Critically, as was cited previously in this thread, it appears that once degradation begins, systems do not simply begin to lose sequestration capacity, they move directly into emissions.
By mid-century, these already activated tipping elements complete their tipping irreversibly (too much of the systems will have been degraded for them to self-restore even if the degradation forcing is removed). The results will be the risk of re-emissions of ten times all the greenhouse gases humans have ever emitted that have been stored in these systems for millennia. This means that current warming is too much. The discussion about how much we will warm, climate sensitivity, IPCC scenarios - all are no longer relevant.
These discussions are acceptable science discourse of course, they can help guide knowledge advancement, but the results of "how much warming?" are now moot. Current warming is too much.
MeltOn
Image: The mesa tops of Mesa Verde National Park: Twenty years
after the epic drought, beetle kill, and fires that happened
around the turn of the century, the forested tops of Mesa Verde
National Park have seen zero regeneration. It is simply too warm
and dry. The former pinon-juniper woodland is no more. Only
grasses, pioneers, and scrub are returning.
Floyd et al., Effects of
Recent
Wildfires in Pinon-Juniper Woodlands of Mesa Verde, Bio One,
February 2021.
(Researchgate – Free subscription required) https://www.researchgate.net/publication/349013945_Effects_of_Recent_Wildfires_in_Pinon-Juniper_Woodlands_of_Mesa_Verde_National_Park_Colorado_USA
Director, Climate Change Now Initiative, 501c3
President, Melton Engineering Services Austin
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Tom Goreau
“Current warming is too much”
IMHO, TFM!
Sincerely yours,
Coral reefs (more ecosystems to follow), as dictated to the Coral Guru:
Thomas J. F. Goreau, PhD
President, Global Coral Reef Alliance
Chief Scientist, Biorock Technology Inc.
Technical Advisor, Blue Guardians Programme, SIDS DOCK
37 Pleasant Street, Cambridge, MA 02139
gor...@globalcoral.org
www.globalcoral.org
Phone: (1) 857-523-0807 (leave message)
Books:
Geotherapy: Innovative Methods of Soil Fertility Restoration, Carbon Sequestration, and Reversing CO2 Increase
Innovative Methods of Marine Ecosystem Restoration
On the Nature of Things: The Scientific Photography of Fritz Goro
Coral Reef Natural History From Beginning To End (in preparation)
Geotherapy: Regenerating ecosystem services to reverse climate change
No one can change the past, everybody can change the future
It’s much later than we think, especially if we don’t think
Those with their heads in the sand will see the light when global warming and sea level rise wash the beach away
“When you run to the rocks, the rocks will be melting, when you run to the sea, the sea will be boiling”, Peter Tosh, Jamaica’s greatest song writer
“The Earth is not dying, she is being killed” U. Utah Phillips
“It is the responsibility of intellectuals to speak the truth and expose lies” Noam Chomsky
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:
To view this discussion visit https://groups.google.com/d/msgid/healthy-planet-action-coalition/0ff54655-26ee-420b-beeb-1e74124696aa%40earthlink.net.
Jan Umsonst
Hi Tom and Bruce, I can only agree
Fun fact: I waited for years that rate dependent tipping and feedback strength became a topic in climate science. Since 1-3 years rate dependent tipping becomes the thing in the tipping cascade discussion.
Main principle is simple: with the rate of warming the subsystems of Earth are pushed further away from their climate optimum under which they could still exist while they still do "exist" as a kind of "zombie" system which then leads to regime shifts becoming more abrupt which increases the chances of a chain reaction as with the rate of warming accelerating more abrupt and intense regime shifts happen in closer temporal vicinity increasing the change of cascading collapse non-linear with further increases in the warming rate. Will be interesting thou to see where the thresholds lie as we seem to be eager to find out...
Sea ice is a perfect example, or sudden peaks in exceptional fire seasons, marine heatwave intensification because of non-linear increases in upper ocean stratification as the oceans take up way too fast heat from the top, sudden abrupt declines in Northern Hemisphere snow cover which lost its resilience towards sudden further shifts, permafrost soils starting to thaw over much larger regions at the same time, worldwide collapsing coral reefs due to accelerated warming etc. pp.
Hence, a super duper El Nino which I strongly reject because of its consequences would be a likely candidate to trigger a highly fascinating chain reaction in the Earth system under current boundary conditions of the climate system.
It's a fundamental difference between Arctic tundras burning gradually or if they go off in flames all at once - also from a black and dark brown carbon perspective being emitted at much higher quantities at the same time into the troposphere and straosphere. Should be non-lienear related to warming rates. Same counts for the Amazon or the Rainforests as such. Or Canada that lost some 7-10% of its forests during 2023 and 2025 alone - what a insane signal already at 1.2°C to 1.5°C happening.
Add another temperature jump by a possible super EL Nino, then let's see what striking signals we will get and how rate dependent the feedback's really are as we just had a large jump.
We live in truly exciting times and hopefully no extreme El Nino happens, as the system needs some time to digest the last jump...
Best
Jan
“Current warming is too much”
IMHO, TFM!
Sincerely yours,
Coral reefs (more ecosystems to follow), as dictated to the Coral Guru:
Thomas J. F. Goreau, PhD
President, Global Coral Reef AllianceChief Scientist, Biorock Technology Inc.
Technical Advisor, Blue Guardians Programme, SIDS DOCK
37 Pleasant Street, Cambridge, MA 02139
gor...@globalcoral.org
www.globalcoral.org
Phone: (1) 857-523-0807 (leave message)
Books:
eGeotherapy: Innovative Methods of Soil Fertility Restoration, Carbon Sequestration, and Reversing CO2 Increase
Tom Goreau
It’s important to note that while the Kuro Shio is pumping western Pacific heat up into the North Pacific, the Gulf Stream has reduced its heat transport in recent months, so the North Atlantic has cooled dramatically.
However please note as always that the Gulf Stream and AMOC are distinctly separate phenomena!
Noise fluctuation or long term trend? Too soon to know…..