Super clarifying of all he sensitivity numbers! Kudos Dean. Am saving this for reference as my brain matter delays with age and news 😲. Wish I could do more than just restack your post to share.
For stream 4: It seems like the general consensus is moving towards higher cloud sensitivity estimates? Is that your read as well?
Lastly, Hansen makes all sorts of predictions for warming in a given year which he argues vindicates his predictions. If, for example, this El Niño year is very hot, does that have any bearing on ECS?
Thank you — these are exactly the right questions, and they all turn on the same distinction, which is the one the essay is built around.
Start with your instinct about acceleration, because it's a good one. You can see it in my own Figure 1: the 1975–2025 slope (2.93 K/doubling) is steeper than the 1850–2025 slope (2.37). That steepening is consistent with the acceleration recent work — and this preprint — are picking up. But here's the key: both are transient slopes, warming the system has expressed so far while the forcing is still being applied and the ocean is still taking up heat. Neither is ECS. Acceleration changes how fast we're traveling; ECS is where we arrive. A steeper approach doesn't move the destination.
That frame answers the rest. The preprint is a satellite lower-troposphere temperature study (not quite EEI, though related) — worth reading, but it's a preprint, the authors themselves say the drivers are uncertain, and it's a curve-fit projection. It speaks to the near-term rate, not the equilibrium. EEI has roughly doubled since 2000, which is real and important, but it's the numerator in the energy-budget calculation — it constrains the transient picture, not ECS directly. On clouds: the feedback is now robustly positive, firmer than a decade ago, but that isn't "ECS rising" — Cooper et al. (2026) actually nudge the central estimate down to ~2.8 via the pattern-effect correction, and the AR6 ~3 °C center has been remarkably stable. And a hot El Niño year doesn't bear on ECS at all — you can't read an equilibrium quantity off a single year. Hansen's year-ahead calls are a separate matter from his sensitivity argument.
The through-line: all four point at the transient — the rate, the near-term, the journey. None relocates the equilibrium. That's the whole reason it's worth separating TCR, ECS, and the slow-feedback timescales in the first place.
That all makes a lot of sense and I hope it is true that ECS is still ~3C. The thing is, Hansen himself does not agree with the statement that, “Hansen's year-ahead calls are a separate matter from his sensitivity argument.” He is very adamant that, “A steeper approach” actually does “move the destination.”
In the abstract, he makes it all clear, explaining, “The basis for our projection of record 2026 global temperature is high climate sensitivity, with its implication that aerosol cooling was still increasing during the period 1970-2005.”
The whole newsletter is worth reading in full as it does parallel yours in some interesting ways. He addresses similar ‘streams’ as you and concludes that ECS is actually very high. His treatment of the paleo data contrasts in an interesting way with specific citations. He also discusses aerosols in a rigorous way. Both conversations are sort of out of my league. I can’t assess the evolving aerosol or paleoclimate consensuses in any educated way. I am also unable to adequately interpret his argument about the reduction in the earth’s albedo.
I obviously hope you are right and he is wrong (and appreciate your explanation of where his disagreement lies). I will admit that the whole debate is giving me considerable anxiety about the future. If Hansen is right, his findings imply that human civilization is toast barring massive degrowth or geoengineering.
Thank you — and I want to take the anxiety seriously rather than wave it off.
You're right that Hansen disagrees with my framing, and it's worth being precise about how. His high-sensitivity case (4–5 °C) rests on four independent lines — mostly paleo and the satellite albedo record — and the 2026–27 prediction is a test of that framework, not the basis for it. So when he says a steeper approach moves the destination, he means the recent acceleration is evidence the underlying sensitivity is high. It's a real, falsifiable position, and I take it seriously.
What might ease the dread is how narrow the visible disagreement actually is. On 2027 — the year both single out — Zeke Hausfather's statistical model gives ~1.61 °C and Hansen ~1.7 °C. A tenth of a degree apart, both expecting a new record. And Mann's new PNAS study ran 40,000 simulations and found 2024's record was entirely consistent with standard models plus El Niño — no broken models, no hidden high sensitivity required. So two of these three independent reads sit near ECS ~3, and Hansen's higher number is a bet the next two years will test.
One honest caveat: Mann also says models may underestimate some impacts — ice-sheet loss, persistent heat and flood extremes. So it isn't "relax." The central temperature sensitivity looks stable, while the real tail risk is in the slow, long-term response — which is exactly where I try to keep the focus.
Because here's what steadies me: the long-term case doesn't depend on who wins the ECS argument. Whether it's 3 or 5, the committed multi-millennial departure is the same. The reason to act is the long game — and it holds at either number.
I like simple solutions, just as my mentor Cesare Emiliani, and it cost him plenty of criticism from 1956 to 1976. By the later date he was proven more or less correct. Thus, I value the actual record and stand by it. Temp rise of 5-C = 100ppm CO2, (@180-280ppm). Now, let's see, we are at + 150ppm CO2 (430ppm) beyond that and may (if linear) expect a temp of +7.5C. Simple. Cheers.
Peter — I'm with you on respecting the actual record, and Emiliani is good company to keep. But I think your number is the right calculation read on the wrong clock.
That ~5 °C per 100 ppm from the glacial–interglacial record is an equilibrium slope — CO₂ and temperature fully co-evolved over millennia, with the ice sheets, vegetation, and ocean all responded. So your +7.5 °C is roughly the warming 430 ppm eventually commits us to, held for thousands of years — not what we'll feel this century. The warming the system has actually expressed so far runs closer to 2.4–2.9 °C per doubling — the transient slope, the modern record in the figure above.
So the record isn't giving one number, it's giving two: the steep equilibrium slope you used, and the shallow transient slope we're riding now. The gap between them is the warming that only expresses over millennia. Your calculation didn't get it wrong — it found the destination and read it as the arrival time. Cheers.
HI Dean, sorry to bother you again but when we take into account the solar cycle and moving into and out of glacial periods as we have over the last million years, has there been an agreed natural background interglacial base for co2 that we measure from for human forcing? Many thanks for the above clarification.
From my understanding the average used is 280ppm and the mean temperature is 13.2-13.5 as an interglacial average so if we currently sit just shy of 1.5c and the mid point for co2 we have currently added is 1990 then the rest is man made biosphere breakdown, So if each of these measurements has a sensitivity derived from an intact biosphere should we not recalibrate these to the biosphere we currently have or do you believe that these are a clear representation of such?
Theodore — good question, and yes, there's an agreed baseline. It's 280 ppm: the preindustrial Holocene interglacial level. The ice cores show CO₂ sat right around 280 (give or take a few ppm) for the whole ~10,000 years before 1850 — a long, stable plateau at the top of the interglacial range. That stability is what makes it a clean reference point. The glacial–interglacial cycles you mention swing between roughly 180 ppm (deep glacial) and 280 ppm (warm interglacial), so 280 is the natural interglacial ceiling we'd have stayed near without industrialization. Everything above it — we're near 430 now — is the human addition, measured from that 280 line.
One clarification on the source: that ~150 ppm rise is fossil-fuel carbon. The fingerprints are unambiguous — the isotopic signature is fossil, atmospheric oxygen is dropping in step with combustion, and both the ocean and land are net absorbing carbon, so neither is the source. Cheers.
true but reduction in carbon sink is in essence a source , so if the longer term calibrations are historically grounded with a fully functioning biosphere would we need to recalibrate to the one we have. From my limited research, currently the biosphere stress response are what we would expect at a full degree more of warming so one would assume the time line response to co2 would change although the overall effect would not. Ie the curve would steepen and shorten. This would also change the time where the land and ocean would become net emitters. Thanks again
Super clarifying of all he sensitivity numbers! Kudos Dean. Am saving this for reference as my brain matter delays with age and news 😲. Wish I could do more than just restack your post to share.
This is spectacular, especially the explanation of the Hansen sensitivity explanation. This has been confusing me lately.
For stream 2: Isn't EEI much much higher now than models predicted? I have been looking at this pre-print (it has been circulating around the "doomer" side of the internet), which is incredibly alarming if accurate: https://assets-eu.researchsquare.com/files/rs-9283491/v1/31f020ea-96a4-430c-8c52-ec96f1a4bb4d.pdf?c=1775640338. Curious what you think about it and how it might implicate ECS estimates.
For stream 4: It seems like the general consensus is moving towards higher cloud sensitivity estimates? Is that your read as well?
Lastly, Hansen makes all sorts of predictions for warming in a given year which he argues vindicates his predictions. If, for example, this El Niño year is very hot, does that have any bearing on ECS?
Thank you — these are exactly the right questions, and they all turn on the same distinction, which is the one the essay is built around.
Start with your instinct about acceleration, because it's a good one. You can see it in my own Figure 1: the 1975–2025 slope (2.93 K/doubling) is steeper than the 1850–2025 slope (2.37). That steepening is consistent with the acceleration recent work — and this preprint — are picking up. But here's the key: both are transient slopes, warming the system has expressed so far while the forcing is still being applied and the ocean is still taking up heat. Neither is ECS. Acceleration changes how fast we're traveling; ECS is where we arrive. A steeper approach doesn't move the destination.
That frame answers the rest. The preprint is a satellite lower-troposphere temperature study (not quite EEI, though related) — worth reading, but it's a preprint, the authors themselves say the drivers are uncertain, and it's a curve-fit projection. It speaks to the near-term rate, not the equilibrium. EEI has roughly doubled since 2000, which is real and important, but it's the numerator in the energy-budget calculation — it constrains the transient picture, not ECS directly. On clouds: the feedback is now robustly positive, firmer than a decade ago, but that isn't "ECS rising" — Cooper et al. (2026) actually nudge the central estimate down to ~2.8 via the pattern-effect correction, and the AR6 ~3 °C center has been remarkably stable. And a hot El Niño year doesn't bear on ECS at all — you can't read an equilibrium quantity off a single year. Hansen's year-ahead calls are a separate matter from his sensitivity argument.
The through-line: all four point at the transient — the rate, the near-term, the journey. None relocates the equilibrium. That's the whole reason it's worth separating TCR, ECS, and the slow-feedback timescales in the first place.
That all makes a lot of sense and I hope it is true that ECS is still ~3C. The thing is, Hansen himself does not agree with the statement that, “Hansen's year-ahead calls are a separate matter from his sensitivity argument.” He is very adamant that, “A steeper approach” actually does “move the destination.”
See his recent newsletter: https://www.columbia.edu/~jeh1/mailings/2026/2026GlobalTemperature.2026.06.12.pdf
In the abstract, he makes it all clear, explaining, “The basis for our projection of record 2026 global temperature is high climate sensitivity, with its implication that aerosol cooling was still increasing during the period 1970-2005.”
The whole newsletter is worth reading in full as it does parallel yours in some interesting ways. He addresses similar ‘streams’ as you and concludes that ECS is actually very high. His treatment of the paleo data contrasts in an interesting way with specific citations. He also discusses aerosols in a rigorous way. Both conversations are sort of out of my league. I can’t assess the evolving aerosol or paleoclimate consensuses in any educated way. I am also unable to adequately interpret his argument about the reduction in the earth’s albedo.
I obviously hope you are right and he is wrong (and appreciate your explanation of where his disagreement lies). I will admit that the whole debate is giving me considerable anxiety about the future. If Hansen is right, his findings imply that human civilization is toast barring massive degrowth or geoengineering.
Thank you — and I want to take the anxiety seriously rather than wave it off.
You're right that Hansen disagrees with my framing, and it's worth being precise about how. His high-sensitivity case (4–5 °C) rests on four independent lines — mostly paleo and the satellite albedo record — and the 2026–27 prediction is a test of that framework, not the basis for it. So when he says a steeper approach moves the destination, he means the recent acceleration is evidence the underlying sensitivity is high. It's a real, falsifiable position, and I take it seriously.
What might ease the dread is how narrow the visible disagreement actually is. On 2027 — the year both single out — Zeke Hausfather's statistical model gives ~1.61 °C and Hansen ~1.7 °C. A tenth of a degree apart, both expecting a new record. And Mann's new PNAS study ran 40,000 simulations and found 2024's record was entirely consistent with standard models plus El Niño — no broken models, no hidden high sensitivity required. So two of these three independent reads sit near ECS ~3, and Hansen's higher number is a bet the next two years will test.
One honest caveat: Mann also says models may underestimate some impacts — ice-sheet loss, persistent heat and flood extremes. So it isn't "relax." The central temperature sensitivity looks stable, while the real tail risk is in the slow, long-term response — which is exactly where I try to keep the focus.
Because here's what steadies me: the long-term case doesn't depend on who wins the ECS argument. Whether it's 3 or 5, the committed multi-millennial departure is the same. The reason to act is the long game — and it holds at either number.
Thank you
You're welcome. Thanks for reading.
I like simple solutions, just as my mentor Cesare Emiliani, and it cost him plenty of criticism from 1956 to 1976. By the later date he was proven more or less correct. Thus, I value the actual record and stand by it. Temp rise of 5-C = 100ppm CO2, (@180-280ppm). Now, let's see, we are at + 150ppm CO2 (430ppm) beyond that and may (if linear) expect a temp of +7.5C. Simple. Cheers.
Peter — I'm with you on respecting the actual record, and Emiliani is good company to keep. But I think your number is the right calculation read on the wrong clock.
That ~5 °C per 100 ppm from the glacial–interglacial record is an equilibrium slope — CO₂ and temperature fully co-evolved over millennia, with the ice sheets, vegetation, and ocean all responded. So your +7.5 °C is roughly the warming 430 ppm eventually commits us to, held for thousands of years — not what we'll feel this century. The warming the system has actually expressed so far runs closer to 2.4–2.9 °C per doubling — the transient slope, the modern record in the figure above.
So the record isn't giving one number, it's giving two: the steep equilibrium slope you used, and the shallow transient slope we're riding now. The gap between them is the warming that only expresses over millennia. Your calculation didn't get it wrong — it found the destination and read it as the arrival time. Cheers.
HI Dean, sorry to bother you again but when we take into account the solar cycle and moving into and out of glacial periods as we have over the last million years, has there been an agreed natural background interglacial base for co2 that we measure from for human forcing? Many thanks for the above clarification.
From my understanding the average used is 280ppm and the mean temperature is 13.2-13.5 as an interglacial average so if we currently sit just shy of 1.5c and the mid point for co2 we have currently added is 1990 then the rest is man made biosphere breakdown, So if each of these measurements has a sensitivity derived from an intact biosphere should we not recalibrate these to the biosphere we currently have or do you believe that these are a clear representation of such?
Theodore — good question, and yes, there's an agreed baseline. It's 280 ppm: the preindustrial Holocene interglacial level. The ice cores show CO₂ sat right around 280 (give or take a few ppm) for the whole ~10,000 years before 1850 — a long, stable plateau at the top of the interglacial range. That stability is what makes it a clean reference point. The glacial–interglacial cycles you mention swing between roughly 180 ppm (deep glacial) and 280 ppm (warm interglacial), so 280 is the natural interglacial ceiling we'd have stayed near without industrialization. Everything above it — we're near 430 now — is the human addition, measured from that 280 line.
One clarification on the source: that ~150 ppm rise is fossil-fuel carbon. The fingerprints are unambiguous — the isotopic signature is fossil, atmospheric oxygen is dropping in step with combustion, and both the ocean and land are net absorbing carbon, so neither is the source. Cheers.
true but reduction in carbon sink is in essence a source , so if the longer term calibrations are historically grounded with a fully functioning biosphere would we need to recalibrate to the one we have. From my limited research, currently the biosphere stress response are what we would expect at a full degree more of warming so one would assume the time line response to co2 would change although the overall effect would not. Ie the curve would steepen and shorten. This would also change the time where the land and ocean would become net emitters. Thanks again