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

Perhaps we will begin with a little bit of unraveling of what some of these terms mean.  Understanding how climate models work, what they say and what they don’t say with what certainty, is notoriously difficult even for experts and decision makers, let alone the general public.   

Let’s start there.  What does an integrated assessment model do, and how accurate are the models we currently have both on the large and smaller scales? 

What we are talking about here are mathematical tools designed to help us understand human development, societal changes, and the way that each one affects the other and also the natural ecosystem.  So, they are mathematical tools.  As such they are simplifications of reality because if they were not simplifications, they would be reality themselves. 

The idea here is to try to capture all of the complexity of the Earth’s systems, of society, future, and socioeconomic development, so basically these are very complex tools we are talking about.  Sometimes you can run a model that is going to take days for you to find a final result, but as of today these are the best tools we have to deal with, as I said, socioeconomic development, population growth, emissions, and costs for future societies, etc.  This is what we have, and it is being used very well, and the IPCC (Intergovernmental Panel on Climate Change) relies as of today very heavily on the results from these integrated assessment models. 

 How confident can we be in how closely these models relate to reality? 

They are good if you think in the short term.  When you talk about a model like this we are talking about thousands of technologies being represented in those models, so we try to project how the cost of solar photovoltaic (PV) will evolve from today up to the year 2100.  We try to represent in that model the future costs of concentrated solar power and the future costs of electric batteries for cars.  We can say that they are pretty good to explore the future, but only in the future are we going to be able to tell whether or not they were good enough to forecast what is really going to happen.   

Again, these are the best tools we have so far.  When we look back, because basically IAMs have been used for 20 years or more, so somehow now we already have scenarios that were built 20 years ago when today was the future, and they were pretty good at anticipating what reality we achieved by 2022. 

Great.  Using those models, can you tell me a little bit about pathways?  There are various versions — an illustrative mitigation pathway, representative concentration pathway, and shared socioeconomic pathways.  Can you help untangle a little bit of this? 

Let’s talk about representative concentration pathways, or what we call RCPs.  It is a kind of metric or indicator to tell the public or even scientists what is being balanced between solar radiation that reaches the Earth’s system and how much of that radiation is reemitted by the Earth’s systems.  Basically, what we have today is that we see more radiation that we have been able to reemit simply because we have created a layer of greenhouse gases in the atmosphere that is blocking part of this reemission that would be necessary for us to keep this balance.  This imbalance is exactly what is causing global warming. 

For example, when you talk about RCPs today we are talking about an RCP that roughly is about 1.5 watts/m2.  What does this mean?  It is a metric to help us understand that each square meter of the Earth’s surface today has a light bulb with a power of 1.5 watts, which is roughly 25 percent of the power of the light bulbs we have in fridges in our kitchens and basically we are generating heat at that pace.  So as of today we have this imbalance. 

When you talk about, for example, a 1.5-degree world, we are, roughly speaking, about a world that is going to have an RCP of 1.9 watts/m2, or in the case of a 2-degree world an RCP of 2.6 watts/m2.  This is a good indicator to show how bad or how well we are doing to deal with this imbalance.  That is why it is so critical — and you probably want to talk about this later — to reach the net-zero emissions over time because only when we have net zero do we stop not being balanced but basically we freeze that imbalance at the level we are at when we reach net zero.  That is why later on we are going to need to have net-negative emissions to try to extract from the atmosphere CO2 to try to bring this RCP down again. 

What about the other pathways I mentioned like socioeconomic pathways? 

Shared socioeconomic pathways (SSPs) basically are a representation of the future given in a kind of socioeconomic storyline.  As of today, scientists have agreed in defining five different SSPs, which are different future storylines — a storyline with more equality, a storyline with more dependence on fossil fuels, a storyline with more rivalry between countries, etc. — so these are possible futures, and each possible future from a socioeconomic point of view will have an implication in terms of emissions, in terms of adaptation, and in terms of the level of mitigation we are going to need to be able to keep temperatures at any level we want. 

Finally, we have illustrative mitigation pathways.  What are these?  In the most recent IPCC report, released in April of this year, basically we reviewed or collected roughly 2000 global mitigation scenarios that were available in the literature.  But because it is too difficult to discuss 2000 different things, we decided to elect five “illustrative” mitigation pathways because we thought these five stories would be good stories to tell the public on what would be necessary to keep temperature below 1.5°C or below 2°C. 

In these five different pathways one pathway relies more on renewable energy; one pathway relies more on low energy demand; one pathway relies more on shifting pathways in terms of socioeconomic development; another pathway relies heavily on negative emissions; and finally, a fifth one relies on not doing much in the short term and later on having to do a lot to compensate for that.  We reported five storylines based on these more than 2000 mitigation scenarios we collected from the literature that would be very interesting to represent that we still have time to limit temperature to 1.5°C or 2°C, but there are different consequences associated with them.  

So it is not predictions; it’s laying out potential paths, and then we choose among them.  How do these pathways help policymakers choose the right way forward? 

As you said well, Mark, we are not making predictions here but exploring possible futures, and each one of these five different IMPs show what will be needed if you want to rely more on solar energy, for example.  In that case, chances are that you are going to need to rely less on carbon dioxide removal (CDR).  Or, if you have a future that is able to cope with lower demand for energy, this future will look different, society will have different costs for that, etc.  So this will let us show different “highways” for the future, each one with its specificities, costs, and impacts, etc. 

I realize this is now, given all these possible scenarios or pathways, going to be a reductive question, but what can we say about the likelihood of limiting global warming to 1.5°C or even to 2°C?  As the famous phrase at the climate summit went, “Is 1.5 still alive?”  Basically, are there pathways that would allow the world to stay below that level of warming with limited or no overshoot — that is, the idea that temperatures rise up and go down again? 

When we have had these discussions over the past two or three years during the construction of this IPCC report, because we had more than 2000 mitigation scenarios, we have been able to categorize by temperature.  So some of those scenarios would lead to a world in 2100 below 1.5°C,  some would lead us to 1.5°C with overshoot, some to a 2°C world, some to 2.5°C, some to 3°C, 4°C, or 5°C. 

We decided, “Okay, let’s pick a very small number of pathways” — we decided to pick 5°C — “and let’s focus on mostly pathways that limit temperature to 1.5°C with low or no overshoot.”  Three out of these five IMPs are in this category: 1.5°C with no or limited overshoot; one has a high overshoot but goes down to 1.5°C later on during the century; and only one that is able to keep the temperature below 2°C. 

Why did we decide to have three scenarios at 1.5°C, one at 1.5°C with overshoot, and one at 2°C?  Exactly because, as you said, chances are that if you do not act very soon, in the next IPCC report in five or six years’ time a 1.5°C world is probably going to be over already, so this is our last chance to still explore those scenarios and show what we need to be able to reach that goal.  If we do not take actions very shortly — as you said, 1.5°C without overshoot — there is no chance anymore; 1.5°C with overshoot also very low chance; and eventually only the 2°C world will still be possible if we do not act very soon. 

Thinking back over your years of work doing these scenarios, how have you seen the probabilities of achieving these pathways evolve?  I don’t know if you had all the pathways in the same way before to compare directly, but at what point did there used to be a greater possibility of achieving no overshoot? 

More and more we are trying to give a kind of probabilistic approach to those pathways, but as of today we are not exactly capable of saying which pathway is more likely, simply because mathematically speaking you can say that, but we cannot say what exactly different countries or governments are going to be able to do.  We say, “If you go that way as indicated by one of these specific IMPs you will have X percent chance of limiting temperature to 1.5°C; if you go that way, according to this pathway, you have X percent chance of limiting temperature to 2°C.”  This is a kind of menu of options we are offering to different governments and, provided that they are not going to follow exactly what is indicated in the IMP, what are the chances of limiting warming to a specific temperature? 

 I wanted to dig into this concept of overshoot a little more and what that means.  Obviously the more the temperature rises above 1.5°C and the longer it does so the greater the risks.  Given what you just said, to what degree do you see countries putting in place mechanisms to manage the risks of overshoot, whether a little, a lot, or not possible to come back?  Do you see that countries are beginning to tackle the reality that there will be an overshoot? 

It is very sad to say, but no.  What we have today is that all or almost all countries already have their pledges, and basically their pledges are written in their national determined contributions (NDCs), and those NDCs mostly make reference to the years 2025 or 2030.  Those countries have pledged where they want to be in 2030, and some of them now also have pledged up to 2050.  Country A may say, “I want to be net-zero CO2 by 2050;” Country B may say, “I want to be net-zero CO2 by 2060” or “net-zero greenhouse gas emissions by year X.”  So we have those pledges. 

But many studies and the IPCC report show that the pledges that we have in place today are not enough for a 1.5°C world.  It is not even enough for a 2°C world.  It is probably enough for a 3°C or warmer world. 

But the situation is even worse than that because what we see, even having pledges that are not good enough, countries are not even able to do what they promised in their pledges.  We have not only an emissions gap associated with the pledges, but we are also having an implementation gap because even the pledges or the NDCs are not being completely fulfilled as of today.  The situation is very bad. 

Do you see countries and policymakers recognizing the reality that they are going to have to prepare for a situation of, as you say, 1.5°C or probably over 2°C depending upon what happens next?  Are you seeing the psychological shift to, “Well, this is what an overshoot means, and this is how we need to start preparing for it?” 

Unfortunately, I don’t think so, Mark.  What happens here?  We are talking about governments that typically have mandates that last for four years, five years, and if you have a reelection, for eight or ten years, and those pledges we are talking about are for 2030, 2040, and 2050.  So I would say it is very easy for you to pledge something for 2050 if you know you are not going to be here or you are not going to be in charge of your country by the time that pledge is due.  That is why some pledges are ambitious, but I doubt whether we are going to be able to get there. 

What is important here is that most of these pledges are specific about the year 2030 or are specific about 2050, but what we need here is not what to do in 2030 or 2050 but how to get there.  We are really talking about a pathway because 2030 is important, but 2031 is important and 2029 is important, and most countries only focus on the specific years but don’t have strong plans behind it to justify how to get there and where they are going to go after that. 

Or to reduce the risks of a warmer world. 

Exactly.

You mentioned earlier carbon dioxide removal as one of the components of various pathways.  All the pathways that involve still holding on to 1.5°C include large-scale carbon dioxide removal to achieve net-zero and eventually net-negative emissions — in other words, removing CO2 already in the atmosphere.   

Can you say a little bit about the rising awareness of the importance, possibility, or likelihood of large-scale CDR in your opinion?  How has this debate changed over the last year or so? 

When we talk about carbon dioxide removal, we have to make clear that we have different kinds of CDR.   

We have those that we call “nature-based solutions” — reforestation, afforestation, improving pasture quality, and improving soil quality to retain more carbon.  All these are CDR kinds of things. 

The other thing is what you call a “technological CDR.”  As examples, there is direct air capture (DAC), which is to extract CO2 from the atmosphere, and what we call “enhanced weathering,” which is to capture also CO2 from the atmosphere in specific chemical materials, etc.  These are very technological ones.   

What we can say is that with any kind of mitigation pathway for a 1.5°C world, 2°C, 2.5°C, 3°C, and even 4°C, some level of CDR will always be there.  The issue is how much CDR we want and whether or not we want to go to more advanced technological-based CDR, which is extremely risky.  When you talk about DAC, we do not have this technology yet, or what we have we only have at a very small scale, and it is extremely expensive. 

If our models for some reason choose too much CDR, for sure this will leave some space open for not doing much in the short term with the expectation that later on we are going to find a technological fix to solve the problem.  That is why these different mitigation pathways, these different IMPs, rely differently on CDR. 

Out of the five IMPs that we have briefly discussed so far, one is called “negative emissions.”  This one relies very heavily on CDR.  In fact, this is a scenario that leads us to a 1.5°C world with high overshoot because it is a scenario where we continue to rely heavily in the short term on fossil fuels with the expectation that after the middle of the century CDR will be mature enough to extract CO2 from the atmosphere so that we can compensate later on for what we did wrong in the beginning.  It is very risky.  These are the tradeoffs that we have here. 

I wonder if I could dig into the concept of the risks.   

I think the one you are referring to is often known as “moral hazard” or sometimes “mitigation deterrence.”  Actually, it is breaking out in some ways into the mainstream debate as an increasing number of companies and countries are beginning to take the idea of CDR increasingly seriously.  Perhaps in the States there is a lot of attention now on this sector.  Within the various scenarios and pathways do you include the effect of this moral hazard?  That is, can you build that in — if you do more investment in CDR, will that have a knock-on effect on mitigation?  How do you model that within the integrated assessment models?  

What we have been able to do is to show the implications of using more or less of one technology on the other technologies.  These models are good for that.  Some of these IMPs, for example, almost by construction, force more CDR over time, and then we can see what the implications are of doing that in the short term for the fossil fuel sector, for the nuclear sector, etc.  But we are not able exactly to assess the moral hazard behind that.  We are just playing with numbers here.  So this is more risky, but that is cheaper if in the long term we are able to dominate that technology. 

So we are playing with numbers here, but again we are expecting in those scenarios that rely more on CDR that in the future that technology will be readily available at a cost that is acceptable for society, and that is why I say the best approach here will be one of a kind of precautionary principle.  It would be much, much better not to run the risk of realizing later on that that technology that you expect to be mature by 2050 is not mature, and then there is nothing to do anymore. 

That is why the most ambitious scenarios try to do as much as possible in the short term using technologies that we know work, that are not expensive.  In that case nature-based solutions are a key issue.  There is no reason from any perspective to increase deforestation, no reason not to promote afforestation and reforestation, be more efficient in the use of energy, and try to rely more on renewable energy.  These are all good things. 

Unless the forest is put on somebody’s food crop plants.  They might not think it was so good.  

Yes.  But again, many studies have shown that we have enough space in the world for agriculture, for food, and also for preserving or increasing our forests. 

There is another aspect to this.  I am not sure how much it is becoming mainstream, but there were some ideas — for example, that I think came from India recently — that developed countries should go faster toward net-negative emissions in order to allow developing countries a little bit longer to emit to obviously reduce poverty.  There is an element of climate justice in this argument, and even some element, as some have suggested, of “decolonizing the atmosphere” from past CO2.   

Do you see this idea emerging, and do you examine how that balance between some nations now may have a historic responsibility to go negative faster?  Are you putting that in the models in some way? 

Yes.  Most of the scenarios represented in the IPCC database rely on what we call a “least-cost solution,” meaning that if it is cheaper to reduce emissions let’s say in India, and more expensive to reduce emissions in the United States, the model will choose to do something in India and less in the United States.  This is the way those models normally work — least-cost solutions — meaning that in order to address this issue of justice, you have to think about financial mechanisms, technology-transfer mechanisms, and capacity building to allow those countries that are not prepared or do not have the funds to do that to do something that is going to be good for them but also for the world. 

Alternatively, we are running different scenarios using what we call different “burden-sharing schemes.”  We play with that as well.  We can have a world where each person in the world has the same rights to emit or each country in the world or each person in the world has the same rights to cumulative emissions over time.  Different schemes are possible. 

We play with that.  Different scenarios are available in literature to show what are the implications for different countries depending upon which burden-sharing scheme you are using.  The major conclusion on that is that all these burden-sharing schemes lead to higher global costs, meaning that even for India it would be good to go with a least-cost approach provided that we have international money flowing to those countries in order to make viable what from a perspective of a country does not make sense or is not possible. 

But that is a big “provided that.” 

 Yes.  We have scenarios that show exactly what the implications are for different countries of different burden schemes and also the costs for that country and also the costs for the global economy.  Again, this is a concern we have in those scenarios to show that, for example, some countries today rely heavily on fossil fuels because they export a lot of fossil fuels.  In a very ambitious scenario, fossil fuels do play a minor role in the future.  This will have implications for those countries.  You have to think about a kind of a financing mechanism to allow those countries to go through a just transition because they are doing something that is good for them but eventually is much better for the world.  Those models are trying to capture that as well. 

I would like to turn to another set of potential ideas that people are raising, given the huge challenges on cutting and ending emissions and CO2 removal.  There are scientists and policy experts beginning to explore the idea of solar radiation modification (SRM), the idea that you reflect a portion of incoming sunlight back into space to reduce the global temperature.  Sometimes this is called solar geoengineering, sunlight reflection methods, various ideas.  Of course, there are lots of uncertainties about this and there are potential risks of its own.  It may reduce climate risk on one level but bring new risks.   

Has there been any discussion or work done around adding these ideas into the models or into the various pathways? 

 Yes.  We are beginning to discuss that.  The representation of solar radiation management is very or almost nonexistent.  The reason for that is because these integrated models need a lot of data and a lot of information — costs, useful life, efficiency, etc.  Before we have pilot projects, before we have good data to show how those projects may perform into the future and how much they are going to cost, it is very difficult to put them in those models.  We already do that but, because the costs we have today are extremely high and most IAMs go with least-cost solutions, these technologies are never chosen.  Sometimes they are already represented in those models but they are not chosen because those models still see a possibility for a 1.5°C world without the need to rely on that or they still see a possibility for a 2°C world without relying on that. 

But for sure, as we have been discussing here, the more we continue without doing much, the more difficult it will be to reach ambitious climate targets, and for sure SRM may be in the future a possibility.  We are trying to avoid that as of today, but eventually this is not going to be possible.  That is why, even if our models are not choosing that as a solution yet, it is so important to study that issue and begin to advance on that because if in the near future we realize that 1.5°C is not possible anymore except only with SRM, we have to have technologies that are mature enough for us to risk trying to deploy them. 

You use the word “risk,” and that is often one of the big challenges here.  There is no risk-free pathway.  You are measuring one set of risks against another set of risks, and I guess with SRM you have the idea of a world with SRM and whatever side risks might come with that measured against the risks of a hotter world without SRM, which contains some things that we know but also a lot of uncertainty.   

How on Earth does one go about measuring these risks against each other?  How does a policymaker begin to say, “Well, there’s this versus this?”  How do you do that? 

Mark, we are not at this stage yet.  We still have a possibility of a world not being warmer and not being hot without the need to rely on SRM.  That is why I said this most recent IPCC report shows that the window to do that is closing.  It is not that open anymore, but it is still slightly open, so we still have a chance of reaching 1.5°C without the need to rely on SRM.  But the window is closing. 

Eventually, in the near future, we are going to need to weigh these different risks.  What is better here?  To go to a technology that is not proven yet and not fully tested yet, or we may decide not to go that route and try to explore the risks of living in a much warmer world?   

I would say that we are not at this point in time, but as I said before, I think the time is ripe for us to begin to investigate those technologies, to begin to play with that, to construct pilot projects, because eventually in the near future or in the medium term we may need to consider that possibility.  As of today, I would say most scientists think this is too dangerous and we don’t fully understand all the implications of doing that, but we have to begin to study that possibility as well. 

You have been doing this work for a long time.  We are seeing that it can be quite difficult to work in climate and to take on the magnitude of this problem and the challenges it brings and to maintain a sense of hope and enthusiasm in your own work while also not burying your head in the sand or “engaging in hopium,” which is the phrase.  How do you balance this?  What do you do to keep on top of this subject while maintaining what seems to be a fairly sunny disposition? 

 Well, Mark, let’s see.  We are the experts here, right?  Officially we are the ones who should better understand the problem and should better understand the risks.  I think we are still able or capable of having some hope here. 

But we are trying to keep the stakes high because we believe we still have a chance.  As I said, this chance is going down, the window is closing, but we still have hope to limit warming to 1.5°C without overshoot, 1.5°C with some overshoot, or eventually a 2°C world.  If we already begin to be too pessimistic about that, I think it is going to be a very bad message. 

We are not hiding anything here.  We are not trying to not show everything we know, and basically we know that it is still possible.  But we are trying to show that we are in the middle of a climate crisis, meaning that it is not enough to be concerned and do nothing.  We are trying to show that if we act very soon and act very actively, we still have a good chance of reaching where we want to be.  That is why we still keep all this hope. 

But you are right.  It is not going to last forever.  All this enthusiasm we have that our chances for success is still high.  Let’s hope that we will be able to do something here.