The discussion of climate sensitivity is very valuable, especially as you point out that the next decade of action and emissions have huge consequences for the future.
The idea that ECS increases with warmer states makes intuitive sense since there are less slow feedbacks that will allow warming to happen faster. Ice is the best example you mention. Not only is albedo dependent on initial ice cover, but today, twice as much energy is being used to melt the world’s ice than the air. If you start from a warmer, ice free world, that energy can go into the oceans and land instead and accelerate the air warming.
Added to this is that its possible that some warming effects are rate dependent. Ocean stratification which drives marine heatwaves transferring heat around the planet and between the water and air is indeed rate dependent. Slow warming would see greater deep ocean heat storage. This could be another input into the PETM being so high.
In terms of net-zero, permafrost melt will have an impact on future warming depending on the net-zero temperature point. If we reach 3ºC, northern permafrost emissions will continue for centuries at a level equal to the US today. This could be another driver for your higher level band, if indeed emissions were lower than the other sinks. If not warming and CO₂ levels will continue to increase.
It would be interesting to also see more research on cloud formation and behaviour at elevated temperatures. A great deal of the energy imbalance is due to cloud reduction and dimming, some of which may be due to aerosol reduction, but certainly not all of it. This is a fast feedback, but we don’t know how cloud behaviour changes with warm house conditions.
ECS may not just be about CO₂ - I know that’s the definition but humans are doing a lot more than just GHG injection. A recent study showed that through deforestation we have lowered the Amazon’s tipping point from 4-5ºC to below 2ºC. Micro-plastics are shown to be warming both the air and the upper ocean layers. Deforestation is lowering cloud cover etc. There may even be such a thing as ACS (Anthropomorphic climate sensitivity) where our other actions and pollutions have a multiplying factor on the base ECS.
A couple of other observations.
The PETM took about 200,000 years to reach equilibrium following about 20,000 years of CO₂ growth. Some of the delay was due to vegetation changes, not just latitudinal migration but physical evolutionary changes required to cope with the warming. A recent study suggested that after 4.5ºC of warming, vegetation collapsed and took 70,000-100,000 years to recover before it could draw down CO₂ from the atmosphere. https://doi.org/10.1038/s41467-025-66390-8
James Hansen’s conclusion that ECS is in the order of 4.8ºC is not just based on the LGM, its also based on model work and recent observations. These he show arrive independently at the value he states.
Sorry for the long comment, but as you can see, the article really got me thinking.
Tom — thank you for such a thoughtful and substantive engagement. This is exactly the kind of exchange that makes the work worthwhile.
Your point about Hansen drawing on multiple lines of evidence is worth clarifying precisely, because it gets at something important about how the field defines ECS.
Hansen's 4.8°C is labeled Charney (fast-feedback) ECS — which by definition holds ice sheets fixed as a forcing rather than treating them as a feedback. Yet his primary derivation uses glacial-to-interglacial temperature change, which inherently includes ice sheet responses. He then uses the full Cenozoic temperature record as a supporting line of evidence — which is essentially the same empirical relationship my equilibrium band captures. So his multiple lines of evidence are largely paleoclimate-based, with the Cenozoic analysis as corroboration rather than an independent constraint.
This points to something the field genuinely needs to grapple with: the spread in ECS estimates — from Cooper's 2.8°C to Hansen's 4.8°C — partly reflects genuine scientific disagreement, but partly reflects definitional inconsistency. Different researchers are measuring subtly different things and calling them by the same name. A more consistent framework would help enormously.
Your ice energy budget intuition is a good physical argument for why ECS rises in warmer worlds — but I'd frame it as evidence for state-dependent sensitivity rather than a challenge to the canonical ECS definition. The definition isn't wrong, it's just incomplete for the timescales my loop is navigating.
you are considering that we will return to net zero. but net zero wont be achieved as it it was not achieved during the 10k yrs of the otimum climate where the human mild activities still had an impact on climate. to achieve net zero naturally we need a resilient planet but such resiliency is being destroyed by our activity. giving the immense size of our atmosphere dac will not dent. so i guess that 10k is really optimistical and not for the co2 that is limited by amount of fossil fuels that we can burn (2x of what we have already burned?) but by the reduced capabilities of our endangered biosphere. of course if tipping points do not trigger cascade events
You reaffirm ECS is bigger than mainstream "rationalizations".
My question is what do you suggest or propose as a doable adaptation/mitigation for helping save the low latitude people, water, and ag starting now? In "doable" I always hope people take into account ghg mitigation promises are total pipedreams because the powers always block them.
What would you do within the 20NS or 30NS lat bandwidth? I concentrate on rapid solutions in that area because of highest solar irradiance and people. The Arctic/Antartica only folks forget the climate is a heat engine and by far most of the incidence is around the equator.. and that heat if reflected there can save the poles faster too.
Jeff — thanks for engaging and for the important work you're doing on low-latitude adaptation. One clarification on the ECS framing: the post doesn't argue ECS is larger than mainstream estimates. Cooper et al. (2024) and AR6 anchor the fast-feedback response at 2.8–3.0°C, which the table presents as well-constrained. What Tierney's work raises is whether ECS is state-dependent — potentially higher in warmer worlds — which is a genuinely open question, but a different one from saying the mainstream is wrong.
My role here is science communicator. My goal is to make the underlying physics more accessible and contextual — to help people understand both the near-term and long-term consequences of where we are headed. My hope is that a clearer picture of the science makes it easier for policy makers and advocates to convey the seriousness of what we face. The work you're describing — near-term, actionable responses for the communities most immediately at risk — is exactly the kind of work this science should be in service of.
Sorry I said the ECS comment wrongly. I think Tierney's hypothesis is something that mainstream should adapt more openly as we accelerate in warming and we are in uncharted territory that in paleoclimate has not scene the "sequence of events". There are no ice cores showing the anthropocene except the top meters.
I still beg the question on what you would consider 💯 doable adaptation solutions that can also be done in a quick time frame
What is to you the biggest bang for the buck in save the most lives, ecosystems, heat gain, water, etc in the 20NS lat bandwidth now? I ask you because you know the numbers.
Jeff — your point about uncharted territory is well taken. The paleoclimate record has no analog for the rate and sequence of what's happening now.
On the 20N-S bandwidth — you're identifying exactly the right human stakes. The populations most exposed to heat stress, sea level rise, and agricultural disruption are precisely the ones least responsible for the problem. The loop I'm describing is the multi-generational reality those communities will live inside.
But specific adaptation strategy is genuinely outside my expertise. For the quantified "biggest bang for the buck" question, Project Drawdown is the most rigorous public resource I know of. For regional impacts, IPCC Working Group II's AR6 report covers the tropics and subtropics in detail. For coastal and sea level exposure specifically, Climate Central has good accessible tools.
Thank you Dean for your work and kind sharing of information. Have to look at the AR6 report info. Very familiar with J Foley's work and CC
I subscribe to too much.
BTW, am looking into "mitadaptation" ( my take of using reflection and evapotrans/greening to cool, hydrate, save carbon sinks and increase drawdown). I have my own name for my version of drawdown.. DrawdAwn. New better dawns.
My silver bucket list ( hopefully buckshot 2b):
Biogenic aerosols mcb or even just seaspray and paint it bright stuff desal water pay the indigent to heal their lands etc.
I call this project Agape Cool. Am looking for ngos govs religions private to unite on a common 2020 lat vision.
An excellent and thought provoking article.
The discussion of climate sensitivity is very valuable, especially as you point out that the next decade of action and emissions have huge consequences for the future.
The idea that ECS increases with warmer states makes intuitive sense since there are less slow feedbacks that will allow warming to happen faster. Ice is the best example you mention. Not only is albedo dependent on initial ice cover, but today, twice as much energy is being used to melt the world’s ice than the air. If you start from a warmer, ice free world, that energy can go into the oceans and land instead and accelerate the air warming.
Added to this is that its possible that some warming effects are rate dependent. Ocean stratification which drives marine heatwaves transferring heat around the planet and between the water and air is indeed rate dependent. Slow warming would see greater deep ocean heat storage. This could be another input into the PETM being so high.
In terms of net-zero, permafrost melt will have an impact on future warming depending on the net-zero temperature point. If we reach 3ºC, northern permafrost emissions will continue for centuries at a level equal to the US today. This could be another driver for your higher level band, if indeed emissions were lower than the other sinks. If not warming and CO₂ levels will continue to increase.
It would be interesting to also see more research on cloud formation and behaviour at elevated temperatures. A great deal of the energy imbalance is due to cloud reduction and dimming, some of which may be due to aerosol reduction, but certainly not all of it. This is a fast feedback, but we don’t know how cloud behaviour changes with warm house conditions.
ECS may not just be about CO₂ - I know that’s the definition but humans are doing a lot more than just GHG injection. A recent study showed that through deforestation we have lowered the Amazon’s tipping point from 4-5ºC to below 2ºC. Micro-plastics are shown to be warming both the air and the upper ocean layers. Deforestation is lowering cloud cover etc. There may even be such a thing as ACS (Anthropomorphic climate sensitivity) where our other actions and pollutions have a multiplying factor on the base ECS.
A couple of other observations.
The PETM took about 200,000 years to reach equilibrium following about 20,000 years of CO₂ growth. Some of the delay was due to vegetation changes, not just latitudinal migration but physical evolutionary changes required to cope with the warming. A recent study suggested that after 4.5ºC of warming, vegetation collapsed and took 70,000-100,000 years to recover before it could draw down CO₂ from the atmosphere. https://doi.org/10.1038/s41467-025-66390-8
James Hansen’s conclusion that ECS is in the order of 4.8ºC is not just based on the LGM, its also based on model work and recent observations. These he show arrive independently at the value he states.
Sorry for the long comment, but as you can see, the article really got me thinking.
Tom — thank you for such a thoughtful and substantive engagement. This is exactly the kind of exchange that makes the work worthwhile.
Your point about Hansen drawing on multiple lines of evidence is worth clarifying precisely, because it gets at something important about how the field defines ECS.
Hansen's 4.8°C is labeled Charney (fast-feedback) ECS — which by definition holds ice sheets fixed as a forcing rather than treating them as a feedback. Yet his primary derivation uses glacial-to-interglacial temperature change, which inherently includes ice sheet responses. He then uses the full Cenozoic temperature record as a supporting line of evidence — which is essentially the same empirical relationship my equilibrium band captures. So his multiple lines of evidence are largely paleoclimate-based, with the Cenozoic analysis as corroboration rather than an independent constraint.
This points to something the field genuinely needs to grapple with: the spread in ECS estimates — from Cooper's 2.8°C to Hansen's 4.8°C — partly reflects genuine scientific disagreement, but partly reflects definitional inconsistency. Different researchers are measuring subtly different things and calling them by the same name. A more consistent framework would help enormously.
Your ice energy budget intuition is a good physical argument for why ECS rises in warmer worlds — but I'd frame it as evidence for state-dependent sensitivity rather than a challenge to the canonical ECS definition. The definition isn't wrong, it's just incomplete for the timescales my loop is navigating.
Agreed. Maybe something will be formalised within AR7 that the community can then refer to.
Looks akin to an hysteresis loop in engineering? Things rarely go back to zero.
Thanks for your thoughts/work.
you are considering that we will return to net zero. but net zero wont be achieved as it it was not achieved during the 10k yrs of the otimum climate where the human mild activities still had an impact on climate. to achieve net zero naturally we need a resilient planet but such resiliency is being destroyed by our activity. giving the immense size of our atmosphere dac will not dent. so i guess that 10k is really optimistical and not for the co2 that is limited by amount of fossil fuels that we can burn (2x of what we have already burned?) but by the reduced capabilities of our endangered biosphere. of course if tipping points do not trigger cascade events
Dean,
You reaffirm ECS is bigger than mainstream "rationalizations".
My question is what do you suggest or propose as a doable adaptation/mitigation for helping save the low latitude people, water, and ag starting now? In "doable" I always hope people take into account ghg mitigation promises are total pipedreams because the powers always block them.
What would you do within the 20NS or 30NS lat bandwidth? I concentrate on rapid solutions in that area because of highest solar irradiance and people. The Arctic/Antartica only folks forget the climate is a heat engine and by far most of the incidence is around the equator.. and that heat if reflected there can save the poles faster too.
Jeff — thanks for engaging and for the important work you're doing on low-latitude adaptation. One clarification on the ECS framing: the post doesn't argue ECS is larger than mainstream estimates. Cooper et al. (2024) and AR6 anchor the fast-feedback response at 2.8–3.0°C, which the table presents as well-constrained. What Tierney's work raises is whether ECS is state-dependent — potentially higher in warmer worlds — which is a genuinely open question, but a different one from saying the mainstream is wrong.
My role here is science communicator. My goal is to make the underlying physics more accessible and contextual — to help people understand both the near-term and long-term consequences of where we are headed. My hope is that a clearer picture of the science makes it easier for policy makers and advocates to convey the seriousness of what we face. The work you're describing — near-term, actionable responses for the communities most immediately at risk — is exactly the kind of work this science should be in service of.
Dean,
Sorry I said the ECS comment wrongly. I think Tierney's hypothesis is something that mainstream should adapt more openly as we accelerate in warming and we are in uncharted territory that in paleoclimate has not scene the "sequence of events". There are no ice cores showing the anthropocene except the top meters.
I still beg the question on what you would consider 💯 doable adaptation solutions that can also be done in a quick time frame
What is to you the biggest bang for the buck in save the most lives, ecosystems, heat gain, water, etc in the 20NS lat bandwidth now? I ask you because you know the numbers.
Thanks!
Jeff
Jeff — your point about uncharted territory is well taken. The paleoclimate record has no analog for the rate and sequence of what's happening now.
On the 20N-S bandwidth — you're identifying exactly the right human stakes. The populations most exposed to heat stress, sea level rise, and agricultural disruption are precisely the ones least responsible for the problem. The loop I'm describing is the multi-generational reality those communities will live inside.
But specific adaptation strategy is genuinely outside my expertise. For the quantified "biggest bang for the buck" question, Project Drawdown is the most rigorous public resource I know of. For regional impacts, IPCC Working Group II's AR6 report covers the tropics and subtropics in detail. For coastal and sea level exposure specifically, Climate Central has good accessible tools.
Thank you Dean for your work and kind sharing of information. Have to look at the AR6 report info. Very familiar with J Foley's work and CC
I subscribe to too much.
BTW, am looking into "mitadaptation" ( my take of using reflection and evapotrans/greening to cool, hydrate, save carbon sinks and increase drawdown). I have my own name for my version of drawdown.. DrawdAwn. New better dawns.
My silver bucket list ( hopefully buckshot 2b):
Biogenic aerosols mcb or even just seaspray and paint it bright stuff desal water pay the indigent to heal their lands etc.
I call this project Agape Cool. Am looking for ngos govs religions private to unite on a common 2020 lat vision.