Re: [HPAC] Comment on Recent Posting
John Nissen
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Dear Tom,
Overall, a very nice article and I agree completely on your conclusion that mitigation and CDR cannot be scaled up in time to keep the increase in the global average temperature below 2 C, much less pull the global warming back to well less than 1 C, which is where it would seem desirable to be. And restabilizing the ice sheets might well require going down to 0 C or below for few decades; while such a depression in temperature might adversely affect agriculture and some ecosystems, the equilibrium sea level sensitivity to the increase in global average temperature over the Pleistocene (last few million years) has been of order 15 meters (!) per degree of warming and there are presently 60 meters of so of sea level equivalent tied up in the Greenland and Antarctic ice sheets.
On the time needed to implement SRM, it is needed well before 2050, 2040, and even 2030, and I’d suggest there are ways that such a moderating influence could (with a dedicated effort) be initiated over less than a decade. What this would require, however, is changing the mindset we use in undertaking the effort. The scientific framing considered in your paper focuses on doing research until we have really high confidence in our projections, establishing a comprehensive global governance system, and full global agreement. I agree with your analysis that this would take at least two decades, which is why the decision-framing used by the scientific community in the face of the existential crisis that the world faces has to be changed.
Changing the decision framing is not without precedent. As the HIV/AIDS crisis spread and was killing tens of thousands each year, medical researchers were making progress on drug regimens that were giving preliminary indications of success. The traditional scientific framing was not to release drugs for use until proven safe, even though deaths were increasing (just as the COVID vaccine was not made available for children until it was proven safe even though children were dying). The HIV/AIDS community then raised their voices, noting that many would be dead before the drug regime was proven safe, so what could be a worse fate for them if the drugs were provided to them even if not proven safe to the level of traditional scientific confidence—and so emerged a compassionate exception decision-making standard in cases where there existed an existential threat, in that case of deaths of millions. And what happened was that the early release of approaches showing indications of potential benefit were made available and over time proved out and no longer was the diagnosis certain death. And as time went on, so did improvements—it was a get started, learn and adjust, and improve and proceed approach--and it worked, saving thousands of lives.
The same transformation is needed for dealing with climate change. As your paper indicated, SRM (in its multiple forms) is the last available approach that we have with the potential to avoid catastrophic consequences to life and agriculture as a result of extreme heat waves and wildfires, flood-inducing storms, the displacement and generation of environmental refugees from these events and greatly accelerated rates of sea level rise, and an array of tipping points that would amplify the consequences further. The question is at what point we question the decision-making framework we are using and switch to one with, we hope, the potential to moderate the consequences that we face.
For stratospheric aerosol injection (SAI) we have the not perfect, but useful, analog of volcanic eruptions that make clear that reflection of some of the incoming solar radiation can, at the very least, moderate warm season temperatures over land (and permafrost). Yes, there are uncertainties, but their consequences are far less than going forward without SAI, and, even with only the present level of understanding and model simulation, can be worked out over time by getting started and learning and adjusting as the effort gets going and improved as the effort proceeds. As a starting objective, seek to counterbalance further warming and as experience and understanding increase gradually increase the effort to start pulling warming back down to its mid-20th century level or a level found that might be more effective. Having the global average temperature go up to 2 C and beyond will have more and more catastrophic consequences—there are virtually no indications that either global conditions or conditions in most regions would be worse than they are now nor indications that any regions would be better off on decadal and longer scales without intervention.
Technologically, existing air freighters can fly above the tropopause in high latitudes and could quite readily be equipped with sprayers. In those latitudes, the sulfate shield is only needed for a few months per year, from the time in late spring after the winter snow cover has melted and the surface albedo has dropped until the time in early autumn when the sun angle has become low in the sky. Indeed, the injections would best be made at altitudes that would lead to the aerosols entering the troposphere and being rained out in the fall. There is no need to know precisely how much needs to be injected to get a specific amount of sulfate aerosol—theoretical estimates are possible and the injection-response relationship can be learned as efforts proceed and build up over time. Cooling the two Polar Regions, which are where warming has been greatest and the cryosphere most affected, would also pull warmth out of lower latitudes, moderating warming there even without aerosol injections. Complement polar SAI with Marine Cloud Brightening (MCB) and reflective tropospheric aerosols out over ocean areas (so not like SO2 now put out by coal-fired power plants that are located where people are) to reduce heat absorption by the oceans and one has an initial global way to moderate warming that could complement more aggressive mitigation, especially of short-lived species like methane and black carbon, and ecosystem restoration and possibly enhanced ocean to pull at least some CO2 out of the atmosphere. Such a triad of efforts is all possible within a decade.
It is true that global SAI would require global aerosol coverage to implement, and this would require higher flying aircraft, which would take time to develop—but why more than two decades if done on an aggressive basis. While there are likely limits to how much of a cooling offset could be accomplished, by the time that limit is reached, space-based reflective approaches might well be possible—planning in that area seems to be moving quite rapidly given the decreasing costs of lofting materials to Earth orbit or manufacturing capabilities are built on the Moon, proposed three decades ago as the likely least expensive approach. And space mirrors would not suffer the limiting influence of SAI.
With respect to the attribution issue, even with all of the proposed approaches, the dominant human influence on climate will still be the increased concentration of CO2 and other greenhouse gases—not climate intervention. The notion that the baseline for attributing the causes of regional extremes should be the increasing global average temperature due to ongoing CO2 emissions seems bizarre, if that was what was implied. I do realize that what matters will likely be perceptions rather than what a comprehensive scientific analysis shows, but it is the warming influences of greenhouse gases that are driving increased extremes—climate interventions may have larger or lesser moderating effects by location and season, but the problem is the higher GHG concentrations.
With respect to governance, the most vulnerable countries are generally those in low and high latitudes and those islands and countries with low coastal elevations. Those living in low latitudes generally lack air-conditioning and have the largest fractions of people with outdoor occupations growing food and involved in construction and transportation. The suggestion that all nations have to agree, especially those in developed nations with extensive air-conditioning raises, in my view, very serious environmental justice and ethical issues. Were a coalition of leaders of a few of the most vulnerable nations to find a way to get started with polar SAI in order to reduce their vulnerability, would the developed nations really take actions to stop them. Who would be suffering harm from such an effort—does any nation really want unconstrained warming? Would other vulnerable nations say stop? Many of the developed nations have serious vulnerabilities that are not as life-threatening, but potentially damaging—would they really take actions to stop an effort led by the most vulnerable nations? The idea that full global governance has to be completely worked out before doing anything is another example of letting those a particular decision framework control others who are actively seeking to protect their nations and peoples.
As the article indicates, climate intervention is the last option, much like a tourniquet that is done to stabilize emergency conditions when an existential injury has occurred. All this talk about the need for decades of process before getting started is like searching for the perfect sterile medically approved tourniquet while the patient bleeds to death rather than grabbing what is available and getting started and taking advantage of better options as they come along—waiting and waiting to get started is the one thing one does not do. In my view, our decision framework needs to be changed from the typical expert approach to a get started, learn, adjust and proceed approach or calamitous global disruption and destruction will be the outcome.
I do realize I am out a good bit ahead of the community on this—and have been for quite some time. I’d be interested in a credible alternative approach—not one based on idealistic possibilities but on realistic indications of what will really get done. For the 21st century, CO2 mitigation and CDR only limit how much more warming we will have; without SRM, there is really no way to go back to the climatic conditions and slow sea level rise that global societies need to build better lives for their peoples.
Michael MacCracken, climate change scientist
April 8, 2026
PS--I would have posted in comments on Substack, but it wants a shorter comment and a lot needs to be said.
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Ron Baiman
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