C2GTalk: An interview with Thelma Krug, Vice Chair, Intergovernmental Panel on Climate Change

Should the world consider solar radiation modification, in light of the latest IPCC findings?

31 May 2022

How could Solar Radiation Modification research better serve developing countries?

Why is governance needed for Solar Radiation Modification?

Why is carbon dioxide removal unavoidable?

What are the risks of temperature ‘overshoot’?

This interview was recorded on 6 April 2022 and is also available with interpretation into中文,EspañolandFrançais.

More research and better governance is needed to help developing countries make decisions about solar radiation modification (SRM), says Thelma Krug, Vice-Chair of the Intergovernmental Panel on Climate Change (IPCC) during a C2GTalk.

SRM has come into focus due to a likely overshoot of the 1.5C warming goal, which would bring increasing risks to people and natural ecosystems, and – depending on the length and extent of overshoot – potentially irreversible impacts. 

Thelma Krug is a former researcher at the Earth Observation Coordination at the National Institute for Space Research in Brazil, under the Ministry of Science, Technology, Innovation and Communication (MCTIC). She was elected Vice-Chair of the Intergovernmental Panel on Climate Change (IPCC) for the Sixth Cycle of Panel (October 2015 – October 2022), after having been co-chair of the IPCC Task Force on National Greenhouse Gas Inventories from 2002 until 2015.  

She holds a PhD on Spatial Statistics from the University of Sheffield, UK.  She has been Deputy National Secretary at the Secretary on Policies and Programs of Science and Technology at MCTIC; National Secretary at the Secretary on Climate Change and Environmental Quality from the Ministry of the Environment (MMA) and Director of the Department on Policies to Combat Deforestation under the Secretary of Climate Change and Forests at MMA. For more than 15 years she represented Brazil in the negotiations at the United Nations Framework Convention on Climate Change (UNFCCC), with particular focus on issues related to land use, land-use change and forestry (LULUCF); research and systematic observations; and reporting guidelines. Her main areas of interest are climate change and the role of deforestation, forest degradation and land-use change; REDD+; and national greenhouse gas inventories.  

Below are edited highlights from the full C2GTalk interview shown in the video above. Some answers have been edited for brevity and clarity.

What is the role of a Vice-Chair of the Intergovernmental Panel on Climate Change (IPCC)?  

The Vice-Chairs are also part of the IPCC Bureau. This Bureau is elected at every beginning of our cycles. A cycle lasts for six-seven years. This one started in 2015 and will end in 2022. All members of the Bureau will be elected, 34 members representing regions, so we have a good representation for all six regions of the IPCC. 

Basically, the Vice-Chairs provide the Panel with the scientific backing-up that helps them to make decisions. We also help in administrative issues. We also support some of the working groups that are created, like for instance IPCC is devoting a lot of attention to gender issues during the cycle just to understand a little bit better how we can stimulate the participation of more women in the work of the IPCC. 

Our work as Vice-Chairs is very much intensified during the approval sessions of the reports when we are asked to facilitate several hurdles. The difficult issues to negotiate are basically in the hands of the Vice-Chairs to help.  

How would you characterize the state of the climate debate and action worldwide both in light of the COP26 meeting, recent IPCC reports, and other recent developments? How far is the world still off-track to limit global warming to 1.5°C? 

First, Mark, I would like to say what is backing my answers are the reports of the IPCC, so I am very much supported by the scientific findings in the IPCC reports.  

For this cycle the contributions of the three working groups have already been approved, which is really great, because now we have all three reports ready that bring a lot of contributions both to the physical science of climate change — adaptation, impact, and vulnerability — and now this last report, approved just a couple of days ago, brings us into the “solutionist phase,” as I like to call it, on mitigation of climate change. 

The IPCC has a cutoff date for the literature it assesses. As you know, the IPCC doesn’t carry out its own research, but it assesses the literature on climate change in all its aspects. For this last report, for instance, 14,000 pieces of scientific literature have been assessed, which is really amazing. My comments will be very standard. 

The cutoff date was October 2021, which was before the COP26, so we didn’t have the possibility to assess the effects of the COP26 in the literature as well. But I will mention a couple of things that might lead us to think there have been some advances at COP26. 

What I thought would be interesting to bring to this discussion is where we stand now in terms of emissions and emission trends. This is the latest estimate that we have in the report of a couple of days ago.  

The news is not very good because in 2019 we had approximately a 12 percent increase of the global anthropogenic net emissions, greenhouse gas emissions, relative to 2010. This also represented 54 percent in relation to 1990. If we take the average of this global net anthropogenic emissions, from the period 2010–2019, this average has been the highest on the record when compared to 2000–2010. 

These data are quite worrisome because one might think that nothing is being done, which in fact is not absolutely correct because the world is just not on-track to limit the global warming to 1.5°C. But this doesn’t mean that the world is frozen in terms of taking actions to combat climate change. 

I think that it is not just being quick enough in those actions. Usually we have had policy measures that have helped to enhance energy efficiency, have helped to reduce rates of deforestation, have helped also to accelerate the deployment of renewable energy. 

This is I think the message to answer your question: We are not on-track, we are not being quick enough to be on a pathway that limits global warming to 1.5°C; however, we see that several policies and laws have been implemented and have been helping to reduce what could be an even more problematic increase in the greenhouse gas emissions. 


How serious would the consequences be of overshooting 1.5°C? Is there something special about that number, or are we just talking about increasing risks in general; and what are the risks that you see of overshooting that goal? 

First, when we talk about overshoot I think that we should also explain what overshoot means. 

The way we at IPCC look at the overshoot is it means that we are temporarily, not permanently, exceeding this 1.5°C increase in the global average temperature above pre-industrial levels. We look at the 1.5°C as a long-term target. This means that the 1.5°C might be exceeded for up to several decades, but that does not necessarily mean that the 1.5°C has definitely been crossed on a permanent basis.  

It is still possible, though extremely challenging, to return to 1.5°C by the end of the century. Technically it is possible to temporarily exceed 1.5°C and bring the temperature back down — by relying on carbon dioxide removal (CDR) methods, for instance, which would remove billions of tons of CO2 out of the atmosphere and then would store it in geological sites, oceanic, land, and so on. However, the more we exceed this 1.5°C, the more difficult and the more expensive it will be to return to below 1.5°C. There is also a limit on how much these CDR methods can achieve, so they cannot compensate for any degree of overshoot. 

Some scenarios look at today’s pathways that limit warming to 1.5°C via temporary overshoot and typically these scenarios lead to a temperature peak of 1.7°C around the middle of the century before they return to 1.5°C at the end of the century. So, for the period of the overshoot there would be a greater risk of the climate impacts associated with the higher temperature levels — for instance, on natural ecosystems, food and water security, extreme weather — and certainly if the period of the overshoot is longer.  

Some of these impacts could be irreversible, for instance on certain ecosystems with low resilience — such as polar, mountain, coastal ecosystems — or by accelerating sea level rise. The risks to humans increase, including the infrastructure, low-lying coastal settlements, and so on. 

So overshooting, depending on how long it lasts and how high it is, can have irreversible impact. 


Can you say a little bit more about that? Is this the same as the idea of a tipping point or planetary boundaries? And what do you mean by irreversible; what then happens? 

Irreversibly could also mean that we are not going to be able to stop those things. When we talk about tipping points, a tipping point is a little bit more challenging to respond because it means the point at which one system changes to another one and cannot come back anymore. 

There is a risk I believe we are talking here, in particular when we think about the polar and the mountains is something that has impacts that are hard to revert back, so I would say that sometimes you have these impacts that are still able to revert back. It depends on the time you are considering, but I would say that. 


You mentioned carbon dioxide removal. I know the latest report said that deployment of carbon dioxide removal to counterbalance hard-to-abate residual emissions, the ones that you cannot not emit or find it hard to emit, is unavoidable if net zero emissions are to be achieved.  

At the same time there are enormous governance challenges with carbon dioxide removal. What in your view are the key challenges to attaining the kind of levels of carbon dioxide removal that might be needed now that are unavoidable? 

As you said, CDR is unavoidable to really limit global warming or decreasing the average global temperature at any level. We have to get to net zero CO2 and greenhouse gases as we are expanding, although these are much more complicated, to have for instance methane emissions dropping to zero. But anyway it’s unavoidable that you have to have net zero CO2  and greenhouse gas if you want to limit warming to a certain level. 

However, you do have challenges to that because some sectors — for instance, if we talk about aviation, if we talk about industry, these sectors have difficulties. It’s hard to abate some of the emissions either because it is too expensive or because we don’t have alternatives that could let them have zero CO2 or greenhouse gas emissions. 

CDR is unavoidable in the sense that it can compensate for some of these hard-to-abate CO2 emissions and greenhouse gas emissions. This is why it’s unavoidable. We don’t expect that all sectors will be able to zero their emissions, but we have to find a solution. 

When we talk about CDR we are talking about a range of matters with different implications, with different potential side effects that you can have from each one. Obviously, when we are talking also about the side effects, it depends very much on the scope, it very much depends on the scale of implementation. 

In the pathways that the IPCC has assessed they mostly relied on afforestation, reforestation, reliance on bioenergy, with and without carbon dioxide capture and storage; it also relied on sequestration by the soil, direct air carbon capture and storage (DACCS), but these last two remain smaller in scale. 

Even if you think about afforestation and deforestation, about which we know a lot because every country implements that, the issue is the scale on which you would be doing this. Depending on the scale of afforestation and deforestation that you are going to implement, you could potentially have food and water insecurity; you could also have a conflict on the land, especially for the indigenous peoples, the local communities. So you have also side effects because you are changing the albedo on the land. 

But also basically the afforestation and deforestation could, for instance, enhance the capacity for removing the CO2 from the atmosphere; but you might have other CDR options or measures, such as weathering enhancements and ocean fertilization, for which we don’t have as much knowledge as yet, so they still require research and development so you can know better the risks. 

There are several alternatives and each one has a different mitigation potential, each one has a time element of permanence, and each one has a different cost. IPCC assesses all of these characteristics for these different options so that one would be more or less confident about the maturity of these methods to be applied and in particular at large scale.  


You’ve mentioned helpfully the difference between nature-based approaches to carbon removal and more technological ones. Do you think that there is an inherent preference by people for nature-based solutions? Is there a sense that it would be better to do this with nature, or maybe does that also create a risk of relying too much on nature and putting nature and technology into opposition which may not actually exist? I’d be interested in your thoughts about nature and technology and how we navigate those different ideas. 

To me it’s unquestionable — and I think the IPCC makes that very clear — that land-based mitigation options and adaptation options as well are very critical at this stage for us to limit global warming. 

What I believe — and again the IPCC reinforces that point — is that we need to use all the potentials that we have. We are going to have to rely on technologies that are maturing. We have to rely on already matured technologies and also land-based approaches — rehabilitation of degraded land, restoring degraded land, reducing deforestation, agroforestry — so you have a range of possibilities that are land-based and which are very much associated with what has been called broadly “nature-based solutions.”  

As we have in a footnote in the Working Group II Report, it is not a term that is internationally accepted and there is a debate in relation to nature-based solutions, in particular because of the potential misunderstanding that it could resolve the climate change problem. It has to be understood as being part of the solution, but it doesn’t replace, absolutely not, the huge effort that has to be made in the other sectors, in particular in the energy sector. 


There has been concern in the past stated by civil society and various groups that just talking and thinking and planning on carbon dioxide removal could create a moral hazard: the idea that we could clean up the atmosphere might reduce the impetus to not put CO2 in the first place.  

How do you view that idea of moral hazard? First of all, is it fair? Second, is there a way of pursuing carbon dioxide removal approaches that reduces that potential moral hazard? 

I think there is. You are absolutely right that there is the impression that when you have these alternative technologies or technologies that are not reducing the emissions, so you continue to emit, but you have a way out when you are using these technologies to remove the CO2. Removing the CO2 is something that we know a lot about, but removing greenhouse gas is something that is still being investigated; it is not mature enough for us to say that we have technologies or methods to do that. So one can really say, “Well, then countries could continue to emit because there are alternatives to bring these emissions back somewhere.”  

But is there a way we can get around this perception of the moral hazard? I would say there is. In particular, if the governments bring up-front their targets to reduce emissions, so if there is an absolutely transparent way to say, “This is what we are going to do and we are implementing all these mitigation strategies here,” that could give some level of comfort I would say because then governments could say and transparently demonstrate that CDR would be used only in those cases where you have difficulties to abate the emissions. 

All this transparency, obviously backed up by the trust that we have to have, would possibly alleviate this issue of moral hazard. 


Maybe we can move on to another set of approaches which also has raised this issue of moral hazard, called solar radiation modification (SRM). This is the idea that you might find methods to reflect more sunlight into space and reduce temperatures like that. 

The IPCC has touched on this in its reports, highlighting that they also could bring new risks and there is a lot of uncertainty. At the same time they are there as potential ways of reducing risk. 

Have we reached a moment, do you think, where the world needs to seriously consider these approaches, and how might it do that? 

If we look at the stage of solar radiation modification today, basically most of what we know comes from modeling approaches, but we don’t have too many outdoor experiments. We have some because as with CDR you also have different types of SRM. 

 The most understood one, or the most distorted one, is the stratospheric aerosol injection (SAI), but there are other approaches also in terms of in particular on the sea, on the clouds, changing the albedo so that you would not have all the incoming radiation going especially to protected corals, for instance. We have small-scale ongoing experiments localized for the modified cloud brightening. Some of these things go at small scale, I would say that they are demonstrations, but SAI is the one that has to be deployed globally. 

Considering what we know today, we know that SAI can have a good impact in terms of reducing some of the hazards of the climate change, basically the rise in the temperature, changing the precipitation, extreme events. We know that from modeling studies. 

However, we don’t have too many studies that demonstrate what are the risks and the impacts on health, on crop productivity, how it could affect the ecosystems. There is a lot still to be done to understand better these potential risks to people. 

If you ask is it not time for us to start thinking of that, I think that research is always good. We will not wait until we have the situation where you say, “Well, my goodness, how do we cope with this problem of increased temperature in particular in overshoot scenarios?” When you have these overshoot scenarios, you are going to have increasing temperature. You could cope temporarily with these SRM methods, in particular SAI, as I said, because it’s doable. 

What are the issues that we see that are recognized in the IPCC reports? Most of the research that has been carried out for these modeling exercises or small experiments did not involve many developing countries. We know that SAI, for instance, could have implications on precipitation patterns that would potentially affect some regions, in particular the most vulnerable regions that do not have too much capacity to adapt. 

This lack of a more universal engagement in the use studies or in these research projects might impair the ability of developing countries to use those results also to guide policies that might be needed in the future.  

We see that there are still several issues to be resolved in relation to SRM. 


First of all, you talked about the need to learn more and research. There have been also groups who have opposed research altogether because I guess they just think it’s sending this debate in the wrong direction; there are issues obviously again of moral hazard, and then some very practical concerns about the ability to consult and whether they think you could ever do so fairly. 

How do you feel about this opposition to research and how do you answer? Again this may come down to trust as well: Can you trust the people who would do the research to do this properly? 

How would you answer those concerns and how would you address them, and suggest how you might go about research in a way that can address those fears of taking us in the wrong direction? 

I think that results also because of the lack of knowledge that we have in relation to this. There has not been, for instance, international governance that could help alleviate this or could help us initiate in a broader way the study or the evaluation of SRM. 

We have pieces here and pieces there, like in the CDD that says some research should not be carried out; but we don’t have something really profound in terms of governance that would lead us — in particular for the SAI, which is global and can have distributional impacts that are unevenly distributed, as I mentioned — so you would not have equity in that sense. 

The fact is that we do not know much. It’s interesting to note that some studies say that when you talk about SRM in the developing countries they do not object as much as the developed countries, possibly because they are the ones who are feeling more the effects of climate change because they do not have as much ability to adapt to all these impacts and extreme events. 

Some people also do not know anything about it. There have been a lot of questionnaires and surveys in that direction. Most people do not know what this is. If somebody comes and asks, depending on the way you ask the question, it might give the impression that “Wow, yes, it is going to alleviate the temperature, it is going to be helpful for my agriculture.” The whole story is not there, so it’s half of the way. 

It is very difficult, Mark, at this stage because there is so little knowledge, in particular in developing countries, and we do need to convey better, to communicate better these alternatives.  

Obviously, there is an intrinsic difference between CDR and SRM. Before they were called “geotechnologies,” they were put in the same package. But we know that CDR has a mitigation potential, according to the definition of the IPCC; however, the SRM does not. That makes a big difference. You are just trying to block out part of the radiation to alleviate some of the impacts of climate change; but it is not resolving the issue, it is not resolving ocean acidification. 

There are many implications of SRM that I think need to be studied a little bit further and I welcome the research on some of these. 


Science can tell us a lot of things, can test assumptions, and can then help us make decisions, but of course people are also driven by many other considerations — economic, political, cultural, ethical, religious. I know that sometimes the debate around SAI is framed by some in terms of “playing God,” that the relationship between humanity, nature, and the cosmos has a lot of resonance in different cultures. 

How does a policymaker now having to make a decision whether to go ahead and pursue some of these approaches balance all these different concerns? If ultimately somebody’s belief is that this is wrong for various reasons other than science, how does the public debate go about addressing this whole balance of different things to consider as well as the science? 

This is why I think it’s important to have some sort of governance for SRM — not necessarily under, let’s say, United Nations Framework Convention on Climate Change — but it could  have a different governance that would allow governments to understand better the implications of this. 

The fear that might exist is that some governments might launch the SRM unilaterally or minilaterally that could have implications for other countries. That’s the issue. Especially for SAI there needs to be a consideration for that. And broad engagement because, as I said, this is something that has to be undertaken globally. 

Another issue that I find important to raise is that SAI has an undesirable effect when you prematurely terminate it. We call it “termination effect.” We say that once the SAI is implemented it would need to continue to be implemented for approximately 100 years. So there needs to be a long-term commitment because if you terminate the SAI abruptly, that could have the opposite effect; it would even warm very rapidly the temperature on the ground. 

It is going to be a difficult decision, and this is why I think that there needs to be a forum where all this has to be addressed before it could be deployed. 


From your experience in this field over the past years, do you think that it is possible to build, maybe not perfect of course, but a broadly fair, inclusive form of multilateral governance that isn’t necessarily dominated by the most powerful or the richest? DO you think it’s possible to do that? 

That is my own sentiment on this. If we look at the report of the IPCC, it is bringing more and more of the social sciences into the consideration of its findings. It is not too much problem-centric as it is to being the best or to give that impression. 

We talk much more about equity. We talk much more about reducing inequalities. We talk much more about a path of development that is resilient to climate change. We talk about international cooperation. We talk about issues that bring in all these elements that bring in developing countries close to each other.  

It is inevitable that the developed countries have to rely on the developing countries to implement the mitigation options and the adaptation options in an integrated way and at the same time, to ensure there is equitable addressing of climate change, we need to pursue the Sustainable Development Goals and so on. This is a more holistic vision of how the world is expected to progress when you are addressing climate change. 

So if — and I think that this is becoming increasingly more important — we are to limit global warming to 1.5°C above pre-industrial levels, this will necessarily have to engage a much larger compromise from the developed countries in particular in terms of financing the investments, not only from the developed governments but you have many other private-sector sources that have to have the confidence of where they are putting their money. It is sort of a risk management type of thin. So, to respond more concretely, I have to have hope in the world. 


Let me end on that note. There is such a difficult balance in climate communications and generally in addressing this topic between knowing and accepting how challenging the situation is and maintaining that sense of hope and agency. You see it in every climate communication, including in the communications around this latest IPCC report, the balance of “It’s really difficult, it’s really serious; however, there is still a way,” and so forth. 

How do you personally, having been in this a long time and seen ups and down, maintain a sense of hope, agency, optimism, while at the same time being fully aware of the deep gravity of the situation we are in?  

Having been at the IPCC for such a long time with the inevitable proximity to science and the way the science of the IPCC is produced, it is produced over years of voluntary engagement of authors, years of engagement of the technical support unit, everybody that is engaged on this exercise — voluntarily, so everyone, every single person in the IPCC is voluntary but the secretariat, twelve people, lawyers and so on, communication people and so on — when you see this, people devoting five-six years of their lives to science, this must give the world the message there is hope. 

They are bringing the science as it is conveyed by an enormous amount of scientific literature from all over the world so that we are not saying that we are now biased by having only the scientific literature from the developed world. That’s not the case, and the IPCC has procedures to avoid that. 

That’s my first consideration. 

Second, as I mentioned, I see that the world is moving. If we look at the Nationally Determined Contributions (NDCs), they are not quite there; but when you look at the regional ones since 2015, and then you look at the NDCs that were submitted before COP26, you see that there has been progress that has an impact on your capacity to reach 1.5°C or below 2°C. There has been gradual movement in that direction, and hopefully those pledges will be implemented in full.  

When we are talking about this, we are talking again about the need for international cooperation and engagement of the private sector, NGOs, also the civil society, to help put together the research needs, help to communicate climate change. 

IPCC cannot communicate globally. It does universally through its reports, which are very dense. The civil society helps us to convey in a simpler language the complexity of everything that is in the report, all the findings. 

When I look at this from a holistic point of view, the private sector is doing a lot already. Also the banks are coming up and trying to support initiatives from countries both in mitigation and adaptation. I think the countries will learn, and will have to learn quickly, that this financing will come more when you can indicate your needs with transparency, showing your ability to implement based on the science. 

So there is a lot out there, Mark, that makes me believe that we are moving into a transformation. You need to transition. The transformation, unfortunately, doesn’t happen with the velocity that we need.  

However, when we say, as IPCC said in this last report, if we have mitigation opportunities, technologies, and measures that if implemented would lead us to halve the emissions by 2030 — okay, it is very nice to say that; however, it also says that these implementations are not easy. 

Obviously, implementing this mitigation strategy that is there technically and has a great potential is also facing different types of barriers. These barriers are regionally specific or even locally specific, so you can have financial barriers, social/cultural barriers, environmental barriers, institutional barriers. Many things need to be overcome before you will be able to fully have the technical potential of these mitigation actions. 

This is why I think the partnerships, bringing all these partnerships, enhancing cooperation among countries, having this inclusive governance that makes it much easier for governments to be able to implement and develop policies that are going to be understood and make it easier to implement. 

This is my hope, Mark, and this is why I still think that this is giving me the strength to believe that this transformation will occur more rapidly than it would otherwise if we were not in the emergency of climate change as we are. 

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