Out with RCP and SSP, the modelling community has published the new scenario set for the next IPCC reporting round. They throw CDR & SRM requirements into sharp focus for meeting the Paris goals.
Tom, this is the clearest summary I've read of what CMIP7 actually implies. Three concerns worth adding, all in the direction of "the picture is worse than the picture shows":
First, the mitigation scenarios normalize CDR as a smooth curve on a chart. The line bends through net-zero into negative territory like turning a steering wheel. It understates the enormity of scaling an unproven technology to capture more carbon than humanity has emitted in all of industrial history, powered by energy systems not yet built, paid for by economies that have never funded anything comparable. Your "ambitious???" is doing a lot of work that the figures themselves don't carry.
Second, the figures' horizon hides what comes after. In the baseline scenarios, temperatures are still rising at the chart edge and remain elevated for centuries. In the mitigation scenarios, the apparent stabilization is illusory — the system continues to respond to today's CO₂ on timescales of millennia. Either way, what the chart shows is not where the system settles.
Third, I don't consider SRM a viable option. Stratospheric aerosol residence is ~1-2 years, so suppression must be continuous. Stop, and termination shock returns the masked warming in years. That means SRM only buys time against the success of CDR or emissions reduction — it inherits the implausibility of whichever it's bridging to. Indefinite maintenance across geopolitical regime changes for hundreds of years is not a strategy.
The gap between current trajectory and any livable outcome is now larger than the figures suggest.
Hi Dean, thanks. I agree with you completely, the predicament is indeed catastrophic. A year ago I would have agreed with you that SRM was only an option once the emissions curve had been bent. Now I think it will be tried just to buy time. It will need to be continuous and will last for as long as civilisation can afford to do so, and agree to collaborate on it. I’m glad I’m not in my 20s!
Termination shock is generally attributed to ceasing successful sai or mcb deployment action. Srm can be brightening all tropical urban zones(quickly needed) and afforestation/reforestation on land and sea ( algae ) with ccn production in mind and no termination shock
We gotta reflect to fo cdr well because reflection saves the carbon sinks.
I notice many say srm=sai. Not true. Sai is a subset of srm.
Jeff, fair correction on the taxonomy — SRM is broader than SAI, and the termination shock argument applies specifically to the short-residence methods (SAI and MCB). I was using SRM loosely where I should have said SAI.
The harder question is whether the persistent-albedo methods can do comparable work. Urban brightening has a real local heat-island literature behind it, but the global signal is bounded by area. Urban surfaces are well under 1% of Earth's surface, and modeling studies put maximum global cooling from full urban albedo enhancement at roughly 0.01–0.07°C. SAI can deliver whole degrees. That's two orders of magnitude difference.
Afforestation albedo effects are regionally complex — boreal forests are darker than the snow-covered land they replace, so the carbon and albedo effects can pull in opposite directions. Useful for sinks, ambiguous for forcing.
Your point about reflection protecting carbon sinks is worth taking seriously. Stabilizing temperature does plausibly preserve sink integrity that warming would otherwise compromise. But that protective work requires W/m²-scale forcing, which among SRM methods only SAI and MCB can deliver. So we're back to the methods with the termination shock problem.
Urban brightening is genuinely useful at the city scale; I just don't see it scaling to a global SRM role.
I think I posted my reply to you just a minute ago mistakingly to Tom. One thing that is so important that gets overlooked in my climate modelling/pathway/solution discussions is water. Specifically 💧 availability to sustain everything good on this planet.
Tom, thanks once again for your work. This is, by far, the best explanation that I have read of the models (model scenarios really), their changes, how it all hangs together and the resulting implications. The sheer complexity of understanding the climate change problem, the physics, feedback loops and interactions on economics and, ultimately, civilisation is truly mind-boggling. Thank you again for your expositions which help shed light on a lot of the elements of this.
Hi Tom I was running figures through the new forcing coefficients Ali was describing in "1802-the-climate-model-error-nobody checks" in relation to the spreader levee land management and rehydration system and the results were quite powerful. The accumulative energy rerouting and cloud albedo are high fractions of the co2 sequestration potential. The key is to be able to build the cloud deck under the Hadley cell suppression for warm rain activation and even this is not a binary constraint due to the summer winter tilt and due to the lack of higher altitude cloud. I look forward to his next article in the series calculating the top of the Atmosphere conversion. We may see the same order of magnitude over the oceans where nutrient loss is driving cloud changes. I may try and summarize this in an article but it is not my area of expertise. thanks again for the easily readable breakdowns of a highly complex problem.
Tom - you mentioned that the modeling community is moving towards adding methane and natural emissions as part of the models, but when you were researching this, did you get a sense for how far into the proverbial weeds they were willing to go to get better in tune to the naturally occuring emissions and feedbacks in the new models?
No, I have not looked into what the modelling will entail as yet. The models are getting more and more complex, so I’d expect that a lot more will be included in CMIP7. For example Greenland melt is not included in CMIP6 ocean current models, but should be in 7. Something to look into…
Tom, another high 5 to you! The model and pathway explanations are well deciphered by you. I reflect on what you write and that makes me want to reflect in other ways.
Really appreciate your objectivity and candor. I too question how carbon sequestration can be scaled to have any beneficial impact. Obviously is would be much less expensive not to create the emit the carbon in the first place. Cleaning up a mess is always more expensive than the money made or saved by making it in the first place. great edification ... ThankU!
You are absolutely right that sai and possibly mcb are projected to be more powerful,economical,and quicker in reducing W/m2. I like using nature as the tool for srm and there is no "tcb" term ( terrain cloud brightening ) and mcb refers to spraying salt air which I think should include plankton induced( fertilized by iron) DMS for ocean clouds. Point well taken on strategies are site dependent.
Urban brightening I include as a must have now not on total Earth brightening but how many lives we can save now. We all project pathways into the future and we gotta look at who hurts now or the immediate future too.
I think it is pretty clear the heat is turning carbon sinks to sources besides hindering them. When I look at it all we gotta give the sinks a chance to give cdr a chance and we shouldn't keep throwing carbon in the air.
Tom, this is the clearest summary I've read of what CMIP7 actually implies. Three concerns worth adding, all in the direction of "the picture is worse than the picture shows":
First, the mitigation scenarios normalize CDR as a smooth curve on a chart. The line bends through net-zero into negative territory like turning a steering wheel. It understates the enormity of scaling an unproven technology to capture more carbon than humanity has emitted in all of industrial history, powered by energy systems not yet built, paid for by economies that have never funded anything comparable. Your "ambitious???" is doing a lot of work that the figures themselves don't carry.
Second, the figures' horizon hides what comes after. In the baseline scenarios, temperatures are still rising at the chart edge and remain elevated for centuries. In the mitigation scenarios, the apparent stabilization is illusory — the system continues to respond to today's CO₂ on timescales of millennia. Either way, what the chart shows is not where the system settles.
Third, I don't consider SRM a viable option. Stratospheric aerosol residence is ~1-2 years, so suppression must be continuous. Stop, and termination shock returns the masked warming in years. That means SRM only buys time against the success of CDR or emissions reduction — it inherits the implausibility of whichever it's bridging to. Indefinite maintenance across geopolitical regime changes for hundreds of years is not a strategy.
The gap between current trajectory and any livable outcome is now larger than the figures suggest.
Hi Dean, thanks. I agree with you completely, the predicament is indeed catastrophic. A year ago I would have agreed with you that SRM was only an option once the emissions curve had been bent. Now I think it will be tried just to buy time. It will need to be continuous and will last for as long as civilisation can afford to do so, and agree to collaborate on it. I’m glad I’m not in my 20s!
Dean,
Termination shock is generally attributed to ceasing successful sai or mcb deployment action. Srm can be brightening all tropical urban zones(quickly needed) and afforestation/reforestation on land and sea ( algae ) with ccn production in mind and no termination shock
We gotta reflect to fo cdr well because reflection saves the carbon sinks.
I notice many say srm=sai. Not true. Sai is a subset of srm.
Jeff, fair correction on the taxonomy — SRM is broader than SAI, and the termination shock argument applies specifically to the short-residence methods (SAI and MCB). I was using SRM loosely where I should have said SAI.
The harder question is whether the persistent-albedo methods can do comparable work. Urban brightening has a real local heat-island literature behind it, but the global signal is bounded by area. Urban surfaces are well under 1% of Earth's surface, and modeling studies put maximum global cooling from full urban albedo enhancement at roughly 0.01–0.07°C. SAI can deliver whole degrees. That's two orders of magnitude difference.
Afforestation albedo effects are regionally complex — boreal forests are darker than the snow-covered land they replace, so the carbon and albedo effects can pull in opposite directions. Useful for sinks, ambiguous for forcing.
Your point about reflection protecting carbon sinks is worth taking seriously. Stabilizing temperature does plausibly preserve sink integrity that warming would otherwise compromise. But that protective work requires W/m²-scale forcing, which among SRM methods only SAI and MCB can deliver. So we're back to the methods with the termination shock problem.
Urban brightening is genuinely useful at the city scale; I just don't see it scaling to a global SRM role.
I think I posted my reply to you just a minute ago mistakingly to Tom. One thing that is so important that gets overlooked in my climate modelling/pathway/solution discussions is water. Specifically 💧 availability to sustain everything good on this planet.
Tom, thanks once again for your work. This is, by far, the best explanation that I have read of the models (model scenarios really), their changes, how it all hangs together and the resulting implications. The sheer complexity of understanding the climate change problem, the physics, feedback loops and interactions on economics and, ultimately, civilisation is truly mind-boggling. Thank you again for your expositions which help shed light on a lot of the elements of this.
Thank you Nick, I really appreciate that.
Hi Tom I was running figures through the new forcing coefficients Ali was describing in "1802-the-climate-model-error-nobody checks" in relation to the spreader levee land management and rehydration system and the results were quite powerful. The accumulative energy rerouting and cloud albedo are high fractions of the co2 sequestration potential. The key is to be able to build the cloud deck under the Hadley cell suppression for warm rain activation and even this is not a binary constraint due to the summer winter tilt and due to the lack of higher altitude cloud. I look forward to his next article in the series calculating the top of the Atmosphere conversion. We may see the same order of magnitude over the oceans where nutrient loss is driving cloud changes. I may try and summarize this in an article but it is not my area of expertise. thanks again for the easily readable breakdowns of a highly complex problem.
Tom - you mentioned that the modeling community is moving towards adding methane and natural emissions as part of the models, but when you were researching this, did you get a sense for how far into the proverbial weeds they were willing to go to get better in tune to the naturally occuring emissions and feedbacks in the new models?
No, I have not looked into what the modelling will entail as yet. The models are getting more and more complex, so I’d expect that a lot more will be included in CMIP7. For example Greenland melt is not included in CMIP6 ocean current models, but should be in 7. Something to look into…
Tom, another high 5 to you! The model and pathway explanations are well deciphered by you. I reflect on what you write and that makes me want to reflect in other ways.
Really appreciate your objectivity and candor. I too question how carbon sequestration can be scaled to have any beneficial impact. Obviously is would be much less expensive not to create the emit the carbon in the first place. Cleaning up a mess is always more expensive than the money made or saved by making it in the first place. great edification ... ThankU!
Thank you Robert, much appreciated.
Dean,
You are absolutely right that sai and possibly mcb are projected to be more powerful,economical,and quicker in reducing W/m2. I like using nature as the tool for srm and there is no "tcb" term ( terrain cloud brightening ) and mcb refers to spraying salt air which I think should include plankton induced( fertilized by iron) DMS for ocean clouds. Point well taken on strategies are site dependent.
Urban brightening I include as a must have now not on total Earth brightening but how many lives we can save now. We all project pathways into the future and we gotta look at who hurts now or the immediate future too.
I think it is pretty clear the heat is turning carbon sinks to sources besides hindering them. When I look at it all we gotta give the sinks a chance to give cdr a chance and we shouldn't keep throwing carbon in the air.
If we learn how to create clouds by reforesting with ccn production in mind would that be considered srm as well as cdr?