Remarks, comments and additions by individual signatories

These comments are expressed by signatories in their personal capacity. Appearance in this list does not imply that the authors or other signatories agree with these comments.


Dr. Julien Anet

   In 2010, I was given the opportunity to do my PhD either on past & future natural climate variability or on SRM. Then, I declined the SRM research offer, considering it as "no-touch" option. I still consider SRM as an extremely dangerous and hot topic. Robock et al. already showed years ago how quickly temperatures would rise once an SRM action is interrupted - deadly for many ecosystems, probably. Banning / forbidding SRM research is, however, even more dangerous. Our society needs to find solutions against Climate Change. Reaching the 1.5 °C is impossible with GHG emission reduction only. 2°C will be extremely challenging. Hence, SRM needs to be considered as a viable options - among many, many other solutions (which may be better - who knows).


Jonas Bahlmann

   I'm member of the Planetary Sunshade Foundation and pursuing a Master's thesis in the field of space-based geoengineering. My motivation is that I don't see any sufficient action regarding achieving a net-zero economy (in contrast to that a lot of greenwashing) and I think we are at a point where climate tipping points will be reached. To buy time to capture and store greenhouse gases, which is a century-scale-task, we could use sunshades in space. Yet, this technology can be used to prolong the time to pollute the air. On the other hand, fossil fuel reservoirs will come to an end this century. Therefore, the switch towards a green economy is inevitable.


Ken Baeza

   Peter Singer’s argument that our current path will inevitably lead to the need of choosing whether to use SRM or not is convincing: Making a decision in ignorance could be fatal.


Tobias Bergmans

   While researching the current applicable international legal framework encompassing Stratospheric Aerosol Injections, I realised how little this technology is currently regulated. This leaves open a lot of space for countries or even private corporations or individuals to start experimenting with it. Something which could lead to disastrous effects, for this generation, and for the generations to come (intergenerational equity).


Prof. Dr. Michiel van den Broeke

   I am strongly in favour of knowledge based policies.


Matt Brune

   I believe in a balanced approach - cautious but advancing the body of knowledge on SRM. Since the concept of the technology exists, I believe it may one day be implemented, whether multilaterally desired or not. As such, developing monitoring methods should be a peak research aim, followed closely by further physical understanding and a public education campaign.


Dr. Tais W. Dahl

   Solar radiation management is – taken on its own – a bad idea as it treats a symptom rather than the cause. Yet, SRM research is an important scientific field potentially offering a helping hand against devastating climate change. It should not be seen as a replacement for reducing GHG emissions or deliberate removal of greenhouse gasses. Yet, it may prove helpful for a period of time while emission reductions and negative emissions are being scaled up (see for example MacMartin et al., PNAS 2018). In case of a rapidly cascading climate emergency (e.g. indications of a sudden collapse of the West-Antarctic icesheet causing a dramatic sealevel rise), I could envision that SRM may be a helpful effort to avoid amplification of further warming. At least, I'm not ready to rule out that some form of SRM could prove helpful. For sure, we need research in this area before we consider implementing it. That said, we should be aware of mitigation deterrence among politicians and various decision makers. In theory, SRM research could also potentially lead some nations to down-prioritize emission reductions (or even negative emissions) and make the global warming problem bigger. That is my main concern. Finally, I really hope we will never need to implement SRM at a global scale, but if we will, I hope we will know exactly what we are doing.


Dr. Alfonso Fernandez

   I firmly believe scientific evidence must inform decision-making.


Dr. Shaun Fitzgerald

   I am supportive of furtherance of knowledge in this area, and where the research is undertaken collaboratively.


Dean Fowles

  I'm nothing more than a father. Not researching something that could be so crucial to our future is just utter insanity.


Yurong Gao 

   Today's climate models are still not perfect, and we don't know what the consequences of rashly adopting solar engineering will be. Once the damage is done, it will be irreversible. Therefore, we should be cautious and it is not the right time yet.


Morgan Godwin

   I help run an organization dedicated to exploring the feasibility of the space-based SRM option. We believe that this is a necessary consideration for any SRM discussion given the need for a wind-down strategy from SAI as well as many other positive benefits that could come from a space-based infrastructure project of this scale. We observe that most actors in the climate and SRM space are atmospheric experts and solutions tend to focus on atmospheric means, and we wish to create connections and build trust with this community.


Prof. Dr. Jessica Gurevitch

   This letter addresses a critical and urgent need for research into the relative risks of climate intervention compared to ongoing anthropogenic climate change.


Daniel J. Haxton 

   We must start large-scale experiments to evaluate geo-engineering solutions like stratospheric aerosols and iron fertilization of the oceans because we cannot control our population and consumption.  Without such geo-engineering solutions, we will not be able to stabilize the climate and our civilization will collapse due to lack of food and habitat.  Geo-engineering is our only viable strategy for avoiding the Mad Max scenario of civilization collapse.


Dr. Forrest M. Hoffman

   I personally advocate for extensive research into SRM and any other large-scale climate change mitigation scheme before considering implementation, in agreement with the US National Academies report, 'Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance'.


Dr. ir. Achim Hoffmann

   It is prudent to build the fire-engine before a fire breaks out. We must stop the global warming, not give up on 1.5°C. NetZero is currently our only strategy to achieve that, which covers the emission reduction and removal of greenhouse gases.  Performance so far shows us that this will take a lot longer than we have time. Cooling technologies such as SRM are essential to have in our back-pockets and ready to go to by NetZero the time needed. This is an essential first step towards a future deployment capability. It is important to build on this, use the momentum and drive the experimental verification work that is much needed.


Matthias Honegger

   We should pursue careful deliberation, research cooperations, and broad-based assessment including in the IPCC and in other relevant fora. How else can we build the foundations for informed, accountable and robust decisions? The time is now. 
Near-term governance questions revolve around how to best shape knowledge generation in such a way as to help create the best basis for future decisions. Without robust basis, governance cannot be effective no matter the intended outcome (use or non-use). The emphasis has to be on process and must not preclude one or the other outcome. Decisions of such nature cannot be taken in the present as future decision contexts are unknown (though serious climate impacts are to be expected), decisionmakers around the world have no access to domestic expertise for forming a position that actually reflects domestic interests. Responsible country governments should ask themselves how they could contribute to building a robust basis for governance. Carefully designing interdisciplinary research cooperations that empower research partners around the world is in my view an absolutely essential part of the answer.


Dr. Shyamal Karmakar

   Essential to keep the research open before a conclusion is made on SRM, considering only the risk rather than the opportunity that it can serve.


Anton Keskinen

   I personally agree with the message of this letter. I want to support this letter as an activist for the special reason that I find it very rare that any climate activists publicly support a balanced approach to SRM research. I believe that no one should decide on behalf of the younger generation that we shouldn't research SRM. Some SRM techniques, alongside GHG mitigation, might be required to avoid crossing some climate tipping points. We should always consider the risks associated with not researching SRM with the expected risks in researching it.


Prof. Dr. Sir Dave King

   This is a necessary intervention.


Marc Salzer Levi

   We are losing the race against climate catastrophe. We are already breaching tipping points beyond which climate catastrophe becomes irreversible and unstoppable, like a runaway nuclear reaction. Only by immediately implementing radical reduction of GHGs emissions, deep removal of GHGs from the atmosphere (back to a pre-industrial 280 ppm CO2) and refreezing the Arctic will we keep our planet safe for our children. If one of the 3Rs is missing, climate catastrophe will follow. We are woefully lagging behind on all 3 R fronts. And we have no plan(et) B.  SRM can potentially buy us time to fully implement the 3Rs. We owe it to our children to fully investigate SRM techniques. The time for action is now. We must give our children a fighting chance.


Dr. Michael MacCracken

   With my original research involving modeling of hypotheses put forth for glacial-interglacial cycling, Earth system history makes very clear that seemingly small radiative forces can cause very large changes in climate and sea level (peak glacial was -6°C or so with sea level down 120 meters!). The notion that indefinite duration of warming of even 1.5°C, much less going above that, will not lead to catastrophic consequences seems to me far too large a risk for society to take, and so I am in favor of moving quickly to initial SRM deployment aimed at keeping global average temperature from rising higher in a learn and adjust mode and as understanding increases pulling the global average temperature anomaly back to 0.5 C or so. In that global GCMs have done very well representing the effects of various forcing over the 19th and 20th centuries gives me much more confidence in projections of how SRM would work as compared to the projections of the global situation would be with warming of where world is headed.


Prof. John Moore

   Research on interventions needs to be normalized so that young peole feel confident to follow this course if they wish. We have a moral duty to consider all options since the majority of huge climate damages will on those least able to afford it, with least responsibility for past emissions, and with least say in global governance.


Dr. Monica Ainhorn Morrison

   The statements urging for more balanced and inclusive research on SRM are incredibly important, as the research up to now has reflected only certain epistemic perspectives (interests, values, concerns, research questions, etc.) on the topic. Much more research in needed that is more diversified across communities. This is key to reducing the risk associated with providing information for deliberation on the efficacy and safety of SRM that is only partially representative of the diversity of community concerns.


Prof. Dr. Andreas Oschlies

   The balanced approach regarding research is great, particularly as calls for a moratorium of deployment (OK for now) and research (not OK) appear gaining momentum. As long as there is no robust path for tackling the climate crisis, narrowing down the space of options is not a good idea.


Jeff Overbeek

   stay allowed to work on tech to potentially save the planet from the worst affects of climate change!


Prof. Dr. Rick van der Ploeg
   Research into SRM is absolutely crucial, even though it should not be the first policy tack to be considered.


Prof. Dr.  Alan Robock

   I support more research into SRM. We need to understand the potential benefits and risks. If society is tempted to implement SRM sometime in the future, it needs to be an informed decision. Certainly more indoor research (modeling, studying analogs like volcanic eruptions and ship tracks) is needed. Outdoor research needs a governance framework, which currently does not exist. And we need to know if the technology is even practical. If we wanted to implement stratospheric or marine cloud brightening climate intervention today, we could not, as the technology does not exist.


Frank Rotering

   The letter is a clear advance over previous statements by climate scientists.  It recognizes the severity of the crisis and the inadequacy of emissions reductions to solve it.  However, as an advocate for the global young (see, I must point to the following shortcomings:

1. It doesn't come close to expressing the existential URGENCY of finding a workable solution.  For a 15-year-old living in Malawi, rapid SRM deployment (particularly SAI) is the only conceivable salvation.  
2. It aims to limit global warming, but youth survival will require global COOLING.
3. It ignores the fundamental problem with decarbonization: cooling aerosols disappear as FF emissions decline.  This partially or fully cancels the gains from GHG stabilization. 
4. It falls into the governance trap. The world is far too complex and divided for broad governance to work in the time available.  SRM implementation must instead be guided by the principles of global justice and intergenerational equity.


Prof. Dr. Joan-Pau Sanchez
    I sign the letter because I mostly agree with its overall content. I certainly agree that vetoing or tabooing SRM research based on poorly understood risks entails a series of consequences which are well described in this letter. Perhaps a potential risk that needs further discussion is the possible hindering of research effort on mitigation and adaptation (i.e., dispersion of zero-sum research budgets). SRM research should thus be able to increase the amount of research effort put into relevant challenges of climate change, by bringing new expertise and research activities.


Dr. Robyn Schofield

   We have committed to a lot of global warming through carbon emissions, this is currently being masked by aerosols also from our energy system. To understand how interventions would work we need to have baseline observations of aerosols and clouds in the remote Southern ocean, Australian reef systems and land areas of the Southern hemisphere. We need observations to understand how aerosols and clouds will change our climate impacts as we transition to a zero-carbon (and zero - NOx, VOCs and sulfur pollution) energy world. That is, we need to understand the effects of the current geoengineering of our climate system through our fossil fuel combustion based energy system emissions to be confident in the response of clouds and regional climate to decarbonisation. We will then be able to be well informed about how any deliberate interventions will change aerosols, clouds and radiative forcing. An adaptation strategy that doesn't assess the risk of inaction as well as action is not as resilient as one that does.


Herb Simmens

   While I support the essence of the letter I am concerned that it does not more explicitly describe, or define the term research. Given the intense disagreement over whether field testing should be allowed, I would have preferred had the letter included field testing, in its description of research. That said it it is articulate, clear and well thought out.


Andrew Smith  

  Few seem to understand the timescales and risks over the next 20 - 50 years. Yes, we are decarbonising, but the timescales for reaching 25%, 50%, 90% of greenhouse emission reduction are a massive unknown. The climatic system in the Northern Hemisphere is heavily influenced by a frozen Arctic. Yet this is melting away remarkably quickly. Cutting down incoming solar radiation now could buy the Arctic enough time until we have reduced our GHG emissions sufficiently. If we let the Arctic melt we will have a massively destabilised climatic system - even if we fully decarbonise.


Wake Smith

   This is consistent with views I have already published.  We need open, comprehensive research on climate interventions.


Prof. Dr. Bjorn Stevens
   Although I support the initiative, my reasons for doing so differ in some respects from those expressed in the letter. Tipping points and specific temperature targets are concepts introduced to serve a political purpose, and I am hesitant to contribute to the impression that they have any particular scientific importance.  I am convinced that SRM research is important for enlarging our ability to anticipate the consequences of possible human actions, regardless of how likely or well intentioned such actions may be.  Whether, in the end, such actions are good are bad is not something science can decide, but science can contribute information that society might find useful as it forms its own opinions on these matters.


Dr. Md. Abu Syed

   Reaching mitigation commitments needed for the 1.5°C goal is no longer possible through conventional approaches. Hence, we should explore all possible scientific ways to reduce global warming. I think SRM has the right potential with some associated risks. To better understand risks we must carry on the research and experimentation with the deployment SRM options.


Prof. Dr. Guus Velders

   Actions to limite the effects of climate change are necessary and all potential options to achieve this should be investigated. 



Dr. Jonathan Wille

  Any efforts to denounce SRM research and future efforts are short-sighted and dangerous. I hope we never need to use SRM technologies, but current climate trajectories may put us in a scenario where innovative methods are needed to prevent ecological/societal collapse. If that day arrives, more knowledge, not less, will help governments reduce risks when making high-impact decisions.



All signatories, listed here in alphabetical order, signed in their individual capacity. Remarks, comments and additions by individual signatories are provided below.

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