Open Letter in Support of Applied-Science Testing and Piloting of Near-term Global Climate Cooling Approaches

(Short Version)

(Endorsement link at bottom of Short Version endorser list)

August 25, 2026

While emissions reduction and long-term carbon dioxide removal are essential, these approaches are insufficient to prevent increasing suffering and dangerous and potentially catastrophic prolonged overshoot. Global society is on course to global warming of 2.0°C by 2040; with substantial risk of warming of 3.0°C after mid-century, and 4.0°C by 2100, that could make securing a stable climate impossible. Such risks would not be tolerated for any other critical infrastructure or international security threat and impose an unacceptable level of risk on global human societies. The world's nations are simply not moving fast enough to reduce emissions and avoid considerable further warming. This means that now, every moment of delay in application of cooling influences at local, regional and global scales, will increase the suffering and deaths of humans and other species. Stratospheric aerosol injection (SAI) is presently the most studied and highly scalable option for exerting a near-term, counter-acting cooling influence. As important as computational models are, systematic evaluation of these urgently needed technologies requires experiments in real world conditions. Several other proposed approaches also have the potential to be both effective and scalable to exert substantial global cooling influence. These approaches are worthy of intensified investigation as supplements to and perhaps replacements for SAI. We, the undersigned, therefore, call for developing a credible response to accelerating climate change by augmenting current emission-reduction and carbon removal efforts with an immediate initiation of transparent, applied-science testing and piloting of promising approaches for exerting a near-term global cooling influence.

Endorsers: 

As of Sep. 2, 2026.                        

(Organizational affiliations provided for the purpose of identification only.)

• Ron Baiman: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States and Benedictine University, Lisle, IL, 60532, United States

• Michael MacCracken: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States and Climate Institute: Washington, DC, 20001, United States

• Alan Gadian (Corresponding Author): University of Leeds, Leeds, England LS2 9JT, United Kingdom

• Anton Alferness: Paradigm Climate, Seattle, WA, 98133, United States

• Clif Alferness: Paradigm Climate, Seattle, WA, 98133, United States

• Roger Arnold: HPAC (Healthy Planet Action Coalition) Los Osos, CA, 93402, United States and Atlantis Project, Sunnyvale, California, 94087, United States

• William Barclay: CPEG (Chicago Political Economy Group), Chicago, Illinois, 60657, United States

• Gabriel Bernasconi, Retired from IAEA Seibersdorf Laboratory, Seibersdorf Laboratory, 2444, Austria

• Rebecca Bishop: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Adam Burke: Queensland University of Technology, Brisbane, QLD, 4000, Australia

• Jonathan Cole: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Robin Collins: Canadian Pugwash Group, Canada

• Clive Elsworth: (NOAC) Nature-based Ocean and Atmospheric Cooling, London, EC1V 2NX, United Kingdom

• Jim Elsworth: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States 

• Tim Foresman: International Center for Remote Sensing Education,Elkridge, MD, 21075, United States 

• Gene Fry: Citizens Climate Lobby, Coronado, CA,92118, United States and HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Dennis Garrity: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States and Global Evergreening Alliance, Burwood East, Victoria 3151, Australia 

• Paul Garver: Citizen, Acton, MA, 01720, United States

• Eduardo D. Greaves: Universidad Simón Bolívar, Caracas, DC, 1080A, Venezuela

• Brian von Herzen: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Sidney Hollander: CPEG (Chicago Political Economy Group), Chicago, Illinois, 60657, United States

• Luke Iseman: Make Sunsets, Elder, SD, 57719, United States

• Arnd Jurgensen. Science for Peace

• Veli Albert Kallio: Sea Research Society, Irmo, SC, 29063, United States

• Peter Lindenmayer: PRAG (Planetary Restoration Action Group) and HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States 

• Kevin Lister: Foundation for Climate Restoration, South Pasadena, CA, 91030, United States

• John MacDonald: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Aria Mckenna: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States 

• Bruce Melton: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States and Climate Change Now Initiative, Austin, TX, 78748, United States

• John Nissen: AMEG (Arctic Methane Emergency Group), London, England, W5, United Kingdom and PRAG (Planetary Restoration Action Group) London, England, W5, United Kingdom 

• Bru Pearce: Envisionation Ltd, Salcombe, Devon, TQ8 8NT, United Kingdom 

• Jesper Pedersen: JP ClimaTec, Holstebro, RM, 7500, Denmark

• Joseph Persky: CPEG (Chicago Political Economy Group), Chicago, Illinois, 60657, United States and University of Illinois at Chicago, Chicago, Illinois, 60607, United States

• Oswald Petersen: Atmospheric Methane Removal AG, Kreuzlingen, TG, 8280, Switzerland

• David T. Price: Retired from Natural Resources Canada, Sorrento, British Columbia, V0E 2W0, Canada 

• Dermott Reilly: NanoLandGlobal Ltd, St. Albans, England, AL3 4RY, United Kingdom

• Benjamin Redmond Roche: University College, London, WC1E 6BT,  United Kingdom

• Michael Routh: Climate Restoration Technologies, Loveland, CO 80538, United States and  HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Jose Ruiz-Camacho: Universidad Simón Bolívar, Caracas, DC, 1080A, Venezuela

• Andrew Song: Make Sunsets, Elder, SD, 57719, United States

• Graeme Taylor: BEST Futures, Brisbane, QLD 4067, Australia and HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Robert Tulip: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Michael Williamson: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States and Science Council for Global Initiatives, Boonville, CA, 95415, United States

Endorse Short Letter here: https://docs.google.com/forms/d/e/1FAIpQLScLswoaMrMgJ0-qSOk4YuQh6uxml8mrRiIG-9FFVoFrwsUwkg/viewform?usp=header

Open Letter in Support of Applied-Science Testing and Piloting of Near-term Global Climate Cooling Approaches

(Long Version)

(Endorsement link at bottom of Long Version endorser list and above Technical Appendix and References)

August 25, 2026

To: International, National, and Local Government Leaders; Indigenous Leaders; Climate Change and Impact Scientists; and Concerned Global Citizens

The climate crisis is accelerating with the failure to get on a path toward net-zero greenhouse gas (GHG) emissions as agreed to in the Paris Accord negotiated in 2015. Record heat waves, melting ice caps, more frequent extreme weather, and over-stressed ecosystems are no longer potential threats — they are our reality. Based on current trends and commitments, projections indicate that the increase in global average temperature is likely to reach 2.0°C by 2040 or earlier and may, without an unprecedented commitment to mitigation, cross 3.0°C in the second half of the 21st century. This trajectory risks exceeding irreversible tipping points and severe climate harms involving at least: permafrost thaw and diminished carbon sinks leading to a rising CO2 concentration even as direct human emissions are phased down; an increasing incidence of heat waves with dangerously high wet bulb temperatures; ice sheet destabilization with consequent acceleration of sea level rise; and a lengthened period of overshoot of 2°C global warming due to increased accumulation of trapped heat in the global ocean.

Continuing to treat emissions reduction and long-term carbon dioxide removal as our only approaches for limiting global warming will lead to global catastrophe. While essential, these approaches are insufficient to prevent increasing suffering and dangerous and prolonged overshoot while we work to get our atmospheric carbon budgets back into balance.  Increased research on and policy consideration of climate cooling interventions must begin now so that their potential to shave off peak warming and limit duration of climate-transforming change becomes an option for dealing with climate change.  The National Members of the UNFCCC Conference of the Parties (COP) have an obligation to ensure the international community is in the best possible position to moderate the intensifying impacts of climate change in coming years as the difficult task of ending fossil fuel emissions goes on.

In the face of accelerating global warming, a recent article proposed a ban on geoengineering research  (Siegert et al. 2025). We strenuously reject this proposal.  While Siegert et al. accurately describe many of the current and possible future calamitous impacts of climate change, they note that official estimates suggest that global warming can be limited to 2.6°C by 2100 if all Nationally Determined Contributions (NDCs) are fulfilled (UNEP 2024). Aside from 2.6°C of global warming, which is on average significantly higher over land where people live, very likely exceeding several key tipping point thresholds and causing exceedingly serious weather disruptions, fulfillment of NDCs is a very optimistic presumption given the lack of success in nations meeting past commitments. With cooling approaches now the only remaining means for avoiding what would widely be considered “dangerous anthropogenic interference with the climate system” (UNFCCC 1992), Siegert et al. assert that: “… further research into these [geoengineering] techniques would not be an effective use of limited time and resources.” [brackets ours].  We find this an untenable conclusion that is incompatible with the risk framing that society has long used in evaluating existential threats.

An analysis of data from multiple official sources suggests that, if current emission and mitigation trends continue, 2.0°C will be reached by 2040 or earlier (Foster and Rahmstorf 2025); and UN Environment Programme projections estimate that even if all current unconditional NDCs are met, global warming will peak, with 66% confidence at 1.9 - 3.3°C by 2050, and, with 90% confidence at 2.3 - 4.0°C by 2100 (UNEP 2025, Figure ES.6). It seems unacceptable that, based on conservative UNEP estimates, global society is on course to global warming of 2.0°C or more by 2040, and has a roughly 1 in 6 chance of calamitous overheating by more than 3.0°C by 2050, and 1 in 20 chance of catastrophic warming of 4.0°C by 2100. Such risks would not be tolerated for any other critical infrastructure or international security threat, and the notion of simply foregoing a potential moderating approach, in our view, would impose an unacceptable level of risk on global human societies near and far from the historic emissions themselves.

This call for a credible approach for addressing climate change has convinced us that continued opposition to deploying approaches tending to cool the Earth with sunlight reflection and other methods as soon as possible can no longer be scientifically or morally justified (Baiman et al. 2024a). In view of the need for action to reduce the likelihood and severity of global catastrophe, our evidence-based reasoning, which is discussed further in the Technical Appendix, has led us to the following conclusions:

1. The world’s nations are simply not moving fast enough to reduce emissions and avoid considerable further warming. This means that now every moment of delay in application of cooling influences at local, regional and global scale will increase the suffering and deaths of humans and other species. The limited results from COP-30 provide no indication that there will be a significant and imminent increase in the pace of emission reductions (Baiman et al. 2024a; UNEP 2025). Despite cost reductions for renewable energy technologies and significant improvements in energy efficiency, meeting the growing energy demands of the increasing global population and the need to raise the global standard of living has kept the share of global energy being derived from fossil fuels at roughly 80%. The world is not yet even on a path of declining emissions. There are just too many fossil-fuel using cars and trucks, buses and trains, ships and planes, farms and industries, warehouses and homes, and more that need to shift to non-climate changing technologies. Continued warming amplifies human suffering, species loss and feedback loops (e.g., carbon dioxide and methane release from thawing permafrost) and extends the projected duration of a dangerous overshoot of what will be, for most of the world’s people, virtually unendurable weather and environmental impacts. Failure to immediately begin to take advantage of viable global cooling influences would allow the global average temperature and weather disruption to continue to increase and make pulling back with global cooling approaches more difficult. The alternative to exerting cooling influences is unacceptable planetary heating.

Allowing warming to continue without exerting a cooling influence would extend the duration of the prospective global temperature “overshoot,” increase the rate of sea level-rise, reduce ocean and terrestrial uptake of carbon, and increase release of carbon dioxide and methane from permafrost, soils, and ocean sediments (Stokes et al. 2025; Baiman et al. 2024a). Delay would also increase the amount of heat stored in the ocean, prolonging the global warming overshoot and making efforts to cool the surface of the planet more difficult and slow (Boselius et al. 2025).

Finally, continued warming will lead to irreversibly exceeding harmful climate tipping points, a number of which are already being crossed (McKay et al. 2022), including coral reef die-off, ice sheet and glacier loss, and even possibly reductions in cloud cover  (Goreau and Hayes 2024; Hugonnet et al. 2021; Tselioudis et al. 2025). The likelihood of even more catastrophic climate disruption and crossing of other irreversible tipping points is also increasing, including the collapse of the Atlantic Meridional Overturning Circulation (AMOC), which would exert severe regional disruption of the weather and of vital ecosystems for critical life-supporting areas of the planet (Aðalgeirsdóttir et al. 2024).

2. Stratospheric aerosol injection (SAI) is presently the most studied and highly scalable option for exerting a near-term, counter-acting cooling influence. Applied-science research, engineering, testing and piloting of SAI within appropriate legal and governance frameworks (Brent et al. 2025), needs to begin now in order for there to be at least one viable and tested approach for moderating global warming (“peak shaving”) to be available within five years, which seems likely to be when the increasing intensity of adverse impacts may convince government leaders that more than emission reduction is urgently needed. The increasing pace of global warming and sea level rise; the increasing intensification of heat waves, tropical cyclones and extreme weather; and the increasing likelihood of racing past irreversible tipping points all portend disastrous impacts. Near-term deployment of climate intervention is the only means for offsetting the inevitable further climate disruption from the slow pace of decarbonizing the global energy system. To be ready when needed, the readiness of cooling systems must be rapidly upgraded because, once tipping points are crossed, climate intervention will likely be too little and too late to prevent greatly amplified impacts.  With SAI being the most studied and scientifically considered approach with near-term potential, investment in science, prototype testing, and the development of governance mechanisms need to be rapidly scaled up so that steps to make potential initiation a viable option can be accomplished as soon as over the next few years.

Focused and phased up research, engineering, and governance efforts would also offer the opportunity for enabling responsible and closely monitored SAI deployment to be started gradually in the high latitudes, adjusting and refining efforts over ensuing years as consideration continues regarding the need to develop and scale up capabilities for more directly addressing global climate change (Baiman et al. 2024a4, p. 17-19). Beginning slowly and gradually intensifying the effort will only be possible if there is a rapid phase up of applied scientific research and testing of approaches for stratospheric injection of aerosol precursors (Baiman et al. 2025).

3.  In addition to SAI, there are a number of additional approaches being proposed that  have the potential to be both effective and scalable for exerting substantial global cooling influence, making these approaches worthy of intensified investigation as supplements to and perhaps replacement for SAI. To ensure that ongoing intervention can be implemented with sufficient and optimal approaches, we therefore support research, evaluation and applied-science testing of potentially complementary and alternative global cooling approaches, including the following, which are further discussed in the Technical Appendix:

1.      Steps to enable more rapid reduction in the atmospheric methane concentration by accelerating cutbacks in emissions and catalytic removal of short-lived greenhouse gases such as methane (Stoerk et al. 2025);

2.      Steps that would moderate the reduction in global albedo that has resulted, at least in part, from the recent de-sulphurization of bunker fuels used in shipping. Doing so by relaxing shipping fuel sulfate regulations in the high seas would seem compatible with reducing the beneficial effect of emissions reductions on human health (Baiman et al. 2024b);

3.      Steps to accelerate the reduction of black carbon emissions (Harmsen, et al. 2019), especially emissions leading to deposition on snow and ice;

4.      Steps to brighten marine cloud layers through further research and pilot testing of Marine Cloud Brightening (MCB). This approach, which attempts to use fine sea salt aerosols to increase the upward reflection of incoming solar radiation by marine stratocumulus clouds, is already being piloted over the Great Barrier Reef (Hernandez-Jaramillo 2025); and

5.      Steps that could be taken to developing and deploying other methods for increasing solar reflection in the Earth’s cloudy and cloud-free regions (Baiman et al. 2024a, p. 11-17; Baiman et al. 2024b).

Figure 1: For “Peak Shaving” in 2035 Testing and Piloting Must Begin Now

 (Click "next" at bottom of form to view)

A common argument against global climate intervention is the so-called “moral hazard” concern that it would slow emission reduction efforts. There is, however, scant evidence that this is the case (Antoan et al. 2025; Mclaren  2016). The costs of globally impactful climate intervention to keep near-term warming below 1.5 C or lower are estimated to be orders of magnitude lower than those of essential short and long-term emissions mitigation, so that the economic tradeoff between these efforts would be insignificant (Alberti 2024; Smith 2020; Royal Society 2009). In addition, with renewable technologies for generating electricity becoming the lowest cost option, there is little risk and no economic basis for going back to the use of fossil fuels.

Another often-raised concern is that sustained climate intervention could lead to what is called “termination shock” or rapid warming if climate intervention were for some reason suddenly ceased. It is, however, not clear why the risk of an abrupt and prolonged cessation of a global climate cooling program, that included redundancy and protective measures, would be nearly as likely as continuing the relatively inexpensive and beneficial cooling efforts and slowly phasing these down gradually and responsibly as emissions are reduced and GHG drawdown is accelerated. Why would the saving of millions of lives and more not be undertaken to address this speculative and irrational risk? 

Most importantly, the opposition to global climate intervention fails to acknowledge the need to credibly address the certainty of increasingly calamitous harm to humans and other living species in the absence of intervention (Baiman et al. 2024a). Based on model simulations and analyses to date, it has generally been found that the impact risks of testing, piloting and responsible deployment of SAI or other near-term global climate cooling methods would be well less than the risks of continuing to allow global warming to increase without intervention, even if emissions reductions get the world on a path to net zero emissions during the second half of the 21st century. Indeed, there are numerous studies suggesting that the potential societal benefits of a responsible and transparent near-term global cooling program would greatly outweigh the risks (Irvine et al. 2019; Harding et al. 2024; Kebrich et al. 2025; Crook et al. 2015; Ricke et al. 2013; Irvine et al. 2010; Brody et al. 2025).

While it may be claimed that until recently fossil fuel impacts on the climate were inadvertent, for some decades that has no longer been the case. Especially now, with COP-30 failing to address emissions reductions, choosing not to act with sufficient urgency to limit global emissions is already a deliberate choice. 

Concluding Thoughts

The world’s decision makers are facing a challenge that is unique in human history—how to respond to the unprecedented rate and complexity of climate change,  and how to ensure the world’s peoples can sustain the cultural, physical, environmental, and economic capital that our predecessors have amassed. An inadequate response will lead to conditions leading to a very diminished and disrupted heritage.  We believe that our generation’s decision makers must take action to ensure a vital path forward for future generations.  We believe that providing hope for the future requires preparing to exert a cooling influence and this effort must start with our generation.

We, the undersigned, therefore, call for developing a credible response to accelerating climate change by augmenting current emission-reduction efforts with an immediate initiation of transparent, applied-science testing and piloting of promising approaches for exerting a near-term global cooling influence. 

Endorsers:                         

(Organizational affiliations provided for the purpose of identification only.

(Click “next” at bottom of form to add your name to endorsers)

• Ron Baiman: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States and Benedictine University, Lisle, IL, 60532, United States

• Michael MacCracken: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States and Climate Institute: Washington, DC, 20001, United States

• Alan Gadian (Corresponding Author): University of Leeds, Leeds, England LS2 9JT, United Kingdom

• Anton Alferness: Paradigm Climate, Seattle, WA, 98133, United States

• Clif Alferness: Paradigm Climate, Seattle, WA, 98133, United States

• Roger Arnold: HPAC (Healthy Planet Action Coalition) Los Osos, CA, 93402, United States and Atlantis Project, Sunnyvale, California, 94087, United States

• William Barclay: CPEG (Chicago Political Economy Group), Chicago, Illinois, 60657, United States

• Gabriel Bernasconi, Retired from IAEA Seibersdorf Laboratory, Seibersdorf Laboratory, 2444, Austria

• Rebecca Bishop: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Adam Burke: Queensland University of Technology, Brisbane, QLD, 4000, Australia

• Jonathan Cole: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• John Dixon: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States and University of Queensland, Brisbane, QLD, 4072, Australia

• Clive Elsworth: (NOAC) Nature-based Ocean and Atmospheric Cooling, London, EC1V 2NX, United Kingdom

• Jim Elsworth: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States 

• Tim Foresman: International Center for Remote Sensing Education,Elkridge, MD, 21075, United States 

• Gene Fry: Citizens Climate Lobby, Coronado, CA,92118, United States and HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Dennis Garrity: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States and Global Evergreening Alliance, Burwood East, Victoria 3151, Australia 

• Paul Garver: Citizen, Acton, MA, 01720, United States

• Eduardo D. Greaves: Universidad Simón Bolívar, Caracas, DC, 1080A, Venezuela

• Brian von Herzen: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Sidney Hollander: CPEG (Chicago Political Economy Group), Chicago, Illinois, 60657, United States

• Luke Iseman: Make Sunsets, Elder, SD, 57719, United States

• Veli Albert Kallio: Sea Research Society, Irmo, SC, 29063, United States

• Peter Lindenmayer: PRAG (Planetary Restoration Action Group) and HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States 

• Kevin Lister: Foundation for Climate Restoration, South Pasadena, CA, 91030, United States

• John MacDonald: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Aria Mckenna: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States 

• Bruce Melton: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States and Climate Change Now Initiative, Austin, TX, 78748, United States

• John Nissen: AMEG (Arctic Methane Emergency Group), London, England, W5, United Kingdom and PRAG (Planetary Restoration Action Group) London, England, W5, United Kingdom 

• Bru Pearce: Envisionation Ltd, Salcombe, Devon, TQ8 8NT, United Kingdom 

• Jesper Pedersen: JP ClimaTec, Holstebro, RM, 7500, Denmark

• Joseph Persky: CPEG (Chicago Political Economy Group), Chicago, Illinois, 60657, United States and University of Illinois at Chicago, Chicago, Illinois, 60607, United States

• Oswald Petersen: Atmospheric Methane Removal AG, Kreuzlingen, TG, 8280, Switzerland

• David T. Price: Retired from Natural Resources Canada, Sorrento, British Columbia, V0E 2W0, Canada 

• Dermott Reilly: NanoLandGlobal Ltd, St. Albans, England, AL3 4RY, United Kingdom

• Benjamin Redmond Roche: University College, London, WC1E 6BT,  United Kingdom

• Michael Routh: Climate Restoration Technologies, Loveland, CO 80538, United States and  HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Jose Ruiz-Camacho: Universidad Simón Bolívar, Caracas, DC, 1080A, Venezuela

• Andrew Song: Make Sunsets, Elder, SD, 57719, United States

• Graeme Taylor: BEST Futures, Brisbane, QLD 4067, Australia and HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Robert Tulip: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States

• Michael Williamson: HPAC (Healthy Planet Action Coalition), Los Osos, CA, 93402, United States and Science Council for Global Initiatives, Boonville, CA, 95415, United States

Endorse Long Letter here: https://docs.google.com/forms/d/e/1FAIpQLScFU5kqThA5DIZfW6iGGc_xItOtsVFwymwZNcsgvG9USkJl3Q/viewform?usp=header

Technical Appendix

In preparing the following analysis, we have attempted to compare all potentially globally impactful near-term climate cooling methods of which we are aware, including reducing emissions of short-term climate drivers or Short Lived Climate Pollutants (SLCPs) (Baiman et al. 2024a; IGSD 2025, C2ES 2021). Table 1 below displays what we have concluded are presently the ten most likely to be feasible and effective.

Note that as there is an unavoidable degree of subjectivity in the analysis that follows, it is intended only as a starting point for more detailed assessments serving to guide the allocation of resources to the various alternatives.

Table 1: Rank Ordered Near-Term Global-Hemispheric Cooling Influence Chart

(Click “next” at bottom of form to view)

Note: Column heading links go to descriptions of column values and row heading links go to references for methods as described below. 

We categorized these methods by the following criteria, as expressed in the column headings:

1. Method and specific objective for moderating global-scale warming and climate change: a list of possible hemispheric cooling methods. 

2. Type of Approach (adding to or pulling out materials from the atmosphere):   “Intervention to exert cooling influence by increasing reflection of solar shortwave radiation from out of the atmosphere” - “Solar Radiation Modification” (SRM) approaches; or “Intervention to reduce warming due to trapping of long wave, or solar shortwave, radiation  in the atmosphere” - Thermal Radiation Modification” (TRM) approaches, see (Baiman et al. 2024a, table 1 and p. 11-17).

3. Sustaining cooling requires a: continuing, or once and done, effort: “Continuing” - repeated effort; or “Once and done” - one-time effort, needed to sustain cooling benefit. 

4. Potential for plausible 2050 deployment in combination with other methods to contribute to reduction of risk of crossing tipping points: The potential of the method to reduce tipping point risk due to its potential for global cooling. Tipping point risk reduction is estimated to be: “Substantial” (dark green) - for methods with the potential to provide up to 1.5°C cooling, relative to estimated global warming without cooling, by 2050 (assessed in column G); “Possible” (light green) - for methods with the potential to provide up to 0.2°C cooling relative to estimated global warming without cooling, by 2050 (assessed in column G), “Unlikely” (very light green) - for methods with potential to provide at most 0.1°C cooling relative to estimated global warming without cooling by 2050 (assessed in column G); and “Very Unlikely” (light red) - for methods that will probably take more than 20 years to deploy at scale (assessed in column E). 

5. Realistic time to significant at-scale deployment: Assessment of how long it is likely to take to scale the method up to achieve the global cooling magnitude estimated in column G, divided into groups of: “<=5 years”, “<=10 years”, “<= 15 years” (dark green); “<= 20 years” (light green); and “> 20 years” (light red), all assuming global effort(s) are made to reach full-scale deployment of the method on or before 2050. 

6. Risk of adverse consequences compared to non-use: “Negligible” (dark green) - No known significant adverse impacts from deployment relative to the consequences of not deploying; “Small” (light green) - Possibly significant but likely small potential adverse impacts that could likely be mitigated, relative to the consequences of not deploying; “Moderate” (very light green) - Possible significant consequences from concentrated localized deployment that could likely be managed with responsible globally targeted or balanced deployment, relative the consequences of not deploying.

7. Potential magnitude of near-term global/hemispheric scale climate cooling from plausible deployment by 2050 (relative to global warming by 2050 without near-term cooling): “Up to 1.5 C” (dark green)”; “Up to 0.5 C” (light green); “Up to 0.2 C”, “Up to 0.1 C” (very light green); “Unknown” (white).

8. Benefits of reduced global/hemispheric warming relative to projected cost: “High” (dark green) – Likely higher or equal to the benefit/Cost (Centigrade global cooling)/$) of SAI; “Moderate” (light green) - Likely lower than the benefit/cost of SAI due to a need for numerous applications across the globe to achieve a more modest magnitude of global cooling;  “Low” (very light green) – Likely much lower than the benefit/cost of SAI due to a need for comprehensive reforestation across the globe to achieve an even lower magnitude of global cooling; “Unknown” (white). 

9. Technological readiness level: “High” (dark green) - Based on existing applied technologies or regulatory regimes; “Medium” (light green) - Based on proven effects and detailed operational plans; “Low” (very light green) - Technological details or deployment logistics, for global scale deployment have not yet been settled; “Very Low” (light red)  - Technological details, or logistics for deployment, have not yet been formulated.   Table 2 below displays this modified 4-Level crosswalk:

   
   

Table 2: Modified 4-Level Technological Readiness Level (TRL) Crosswalk

10. Current level of scientific confidence and efficacy compared to non-use: “High” (dark green) - Not applying this method has been proven to be a current cause of significant global warming; “Modest” (light green) - Known and proven natural, ecological and theoretical analogs; “Low” (very light green) - Proven in laboratory,   past history, or through natural analog, but technology is not yet operational.

11. Projected level of additional beneficial health and ecological effects (not directly related to cooling, or more related to local rather than global cooling impacts): “High” (dark green) - Reduces greenhouse gas or particles that can be indirectly harmful to human health  (high levels of methane can increase tropospheric ozone and in some cases can be used for energy and reduced to CO2) or directly harmful to human health (black carbon), or restores nature (reforestation); “Moderate” (light green) - High local or regional cooling impacts may help to slow destruction of polar ecology, reduce tropospheric methane, slow warming of ocean surfaces, reduce intensity of storms and slow or stop dying of coral reefs.

12. Current level of policy consideration: “High” (dark green) - Has been a priority for  large-scale international agreements (methane emissions reduction); “Modest” (light green) - Has been a national or local priority for heavily affected regions (black carbon in China and India, reforestation in Brazil, Africa and China); “Low” (very light green) - Has been the subject of considerable research and modeling (SAI, MCB and marine sulfate aerosol reduction) and pilot implementation (MCB in Great Barrier Reef); “Negligible” (white) - has received very little research or policy attention. 

    
    

We then rank-ordered the ten methods, or rows, for relative importance in addressing the harm from not urgently cooling the globe by 2050 as follows; 

1. First sort based on tipping point risk reduction (E)

2. Within this sort by global deployment scale-up time (F)

3. Within this sort by (natural science) risks compared to non-use (G)

4. Within this sort by 2050 magnitude of cooling relative to no-intervention (H)

   
   

This ranking procedure produces the near-term climate cooling method (row) ordering methods displayed in table 1 above (and slide 8 of the Near-Term Cooling power point in the middle of the landing page of the HPAC website (HPAC 2025) and categorized into three groups as follows:

A.  Methods that could substantially reduce climate tipping point risk and address

      near-term harm from global warming

1. Reducing methane emissions (CCAG 2025; Soerk et al. 2025; C2ES 2021; Baiman et al. 2025 p. 4-10, 19; Króliczewska et al. 2023; Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions. 2021; Nisbet et al. 2020; Kang et al. 2019; Karakurt et al. 2012; Anenberg et al. 2012; UNEP. 2011).

2. Polar stratospheric aerosol injection (Baiman et al. 2025; Moore et al. 2025; Baiman et al. 2024  p. 15 -19; Smith et al. 2024; Smith W. 2024; Keith and Smith. 2024; Lee et al. 2023; Smith et al. 2022; MacCracken et al. 2013).

3. Global stratospheric aerosol injection (Baiman et al. 2025; Zhao et al. 2025; Mathiesen and Hiar. 2025; Hack et al. 2025; Kebrich et al. 2025; Aubry et al. 2025; Hyde, R. A. 2025; Boselius et al. 2025; Nielsen J. 2025; Chitty O. et al. 2025; Harding et al. 2024; Baiman et al. 2024a p. 15-19; Futerman et al. 2023; Irvine and Keith 2020; Smith W. 2020; Tilmes et al. 2020; Irvine et al. 2019; Smith and Wagner 2018; MacCracken M.C. 2016).

4. Catalytic reduction of atmospheric methane (Spark Climate Solutions 2026; Smith and Mathison 2024; Baiman et al. 2024a, p. 4-10, 15; Wang and He 2023; Jackson et al. 2021; C2ES 2021; Abernethy et al. 2021; Lafente and Cano-Diaz 2021).

B.    Methods likely to have a modest near-term global warming harm reduction and tipping risk reduction impact

1. Easing IMO shipping fuel regulations in the high seas (Diamond and Boss 2025; He et al. 2025; Baiman et al. 2024b; Jordan and Henry 2024; Yuan T et al. 2024; Hansen et al. 2023; Partanen et al. 2013).

2. Reducing black carbon emissions (C2ES 2025; Harmsen et al. 2020; Yang et al. 2019; Anenberg et al. 2012; UNEP. 2011; Ramathan and Carmichael 2008; Bond and Sun 2005).

3. Non-fossil fuel generated benign tropospheric aerosol injection (Smith et al. 2026; Baiman et al. 2024a p. 16-17; Li et al. 2022; Oeste et al. 2017).

4. Marine Cloud Brightening (Blue Cooling Initiative. 2026; Yu et al. 2026; Gadian 2025; Chen et al. 2025; Lee et al. 2025; Hernandez-Jaramillo et al. 2025; Baiman et al. 2024a p. 13-14; Latham et al. 2014; Latham et al. 2012; Salter et al. 2008).

   
   

C.    Methods likely to have minor to no near-term global warming harm reduction and tipping risk reduction impact

1. Reforestation/afforestation and other means of increasing ecosystem carbon dioxide uptake (Van der Spek et al. 2025; Rath and Rathi 2025; Baiman et al. 2024 p. 11; Psistaki et al. 2024; Rohatyn et al. 2023; Jayakrishnan and Bala 2023; Makarieva and Gorshkov 2007).

2. Place mirrors (or sunshades) in high-Earth orbit to reduce absorption of incoming solar radiation (Angel R. 2026; Matonti et al. 2026; Bahlmann et al. 2025; Bahlmann  2023; Baum et al. 2022; McInnes CR. 2009)

   
   

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