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Discussion about integrated assessment models and the IPCC AR6 WGIII Chapter 3

Updated: May 23, 2023

Interviewer: Antoine Rondelet, Founder @ Ganddee

Guest: Keywan Riahi, Director of the Energy, Climate and Environment Program @ IIASA


The International Institute for Applied Systems Analysis (IIASA) is an international research institute that advances systems analysis and applies its research methods to identify policy solutions to reduce human footprints, enhance the resilience of natural and socioeconomic systems, and help achieve the sustainable development goals. (More info here).

Keywan Riahi is the Director of the Energy, Climate and Environment Program at the International Institute for Applied Systems Analysis. He coordinated the chapter 3 of the Working Group III (WGIII) of the Intergovernmental Panel on Climate Change (IPCC)’s Sixth Assessment Report (AR6). Keywan Riahi also lectures as a Visiting Professor of Energy Systems Analysis at the Graz University of Technology. He is a Fellow of the Payne Institute of the Colorado School of Mines, and serves as an External Faculty Member at the Institute for Advanced Study at the University of Amsterdam. (Full bio here).


Antoine: Could you explain what is the purpose of the Chapter 3 of AR6’s WGIII?

Keywan (IIASA): Chapter 3 looks into long term mitigation pathways. Mitigation pathways are basically derived from so-called integrated assessment models (IAMs) which try to represent different greenhouse gas (GHG) intensive sectors. They look at the human system, the climate system and try to understand how human and climate systems interact, what sort of policies or measures are needed in order to reduce the human footprint on the climate system, and so how we can reduce GHG emissions. For that, you need holistic and integrated models because GHG emissions are basically coming from every activity that humans set in the economy. In order to understand the effect of different mitigation measures, you need those IAMs so that they can tell you how the system responds, how much it costs and how much emissions reductions you can achieve, given certain measures.

In the chapter, we basically look into different mitigation pathways that are consistent with different temperature change levels. So, the lower you want to limit temperature change in the future, the more rapidly you need to reduce emissions. We define that, for example, in order to limit temperature change to 1.5C with a small overshoot, we need to reduce emissions roughly by more than 40% in 2030 and continue to reduce emissions so that in 2050 we basically achieve net-zero CO2 emissions. However, there are other GHGs than CO2, and our scenarios project net-zero GHG emissions by 2070. Those pathways tell you about the required changes to the energy system which is one of the major contributor to climate change, but also what sort of other changes need to happen to agriculture systems, mobility, industry and building sectors, for example, and what combination of measures we can use to get to net-zero at the end. We have shown that if you want to go to net-zero, this does not mean that every activity on the planet needs to be net-zero. We need to reduce emissions by roughly 75 to 95%, but some residual emissions are going to stay because some sectors are very difficult to free 100% from CO2 or from GHGs in general. In order to offset those remaining emissions, we need to start to remove CO2 from the atmosphere. So in a 1.5C scenario where we need net-zero CO2 emissions by 2050, there will be some residual emissions still left which will have to be compensated by CO2 removal from the atmosphere. There are various different options to do this. Not every possibility on how to take CO2 out of the atmosphere is represented in the chapter, which focuses on the approaches included in the IAMs. Natural sinks are very important, of course. Afforestation, biochar, enhancing soil carbon, are many different nature-based solutions which have relatively little tradeoffs with other sustainability objectives. Afforestation, of course, is a very positive activity if you think about other possible side effects that CO2 removal might have. There have also been suggestions, for example, to take biomass and produce bioenergy. So, when biomass grows, it takes the CO2 out of the atmosphere. Then, when you burn it to make either heat, electricity, hydrogen, whatever you need out of it, after that conversion process the suggestion is to take CO2 out of the flue gas and then put it back into the natural reservoirs under the earth crust. This is an option that is not available at large scale, but is deployed a lot in the IAMs as a solution to get to net-zero. Still yet, it needs to be demonstrated whether it is feasible at large scale.

But now I’m lecturing beyond your question…

In this chapter, we want to understand the combination of measures and what alternative pathways are there in order to limit temperatures under specific thresholds. We also want to understand when the doors are closed and when we cannot get there anymore. One of the big conclusions of our chapter is that if we focus on what countries have pledged as part of the Paris Agreement so far and if we go that path until 2030, 1.5C is not feasible. We can still get to 2C after that, but we cannot get to 1.5C anymore.

I’m interested to understand the methodology that was followed to come up with this chapter, and the report generally. Unless I’ve missed something, I counted around 70 or so scientists who worked on this specific chapter. How did you all coordinate and what processes have you followed to write this chapter?

A really important backbone of our discussions and analysis was a comprehensive collection of integrated assessment pathways, so pathways that are developed within particular assessment models. These are all publicly available as part of an open community database. We collected more than 2000 pathways from hundreds of different groups in the world. This was done in a way that, we had a data template, with definitions of different data we would need in order to assess the pathways in a comprehensive way, we had more than 400 variables that described the future state of the energy, land, and other GHG intensive systems. Then, there was a call to the community to send us all that information into a digital database, and that database served as one of the important foundations to explore, in a comprehensive way, what it takes to go to net-zero for example, and also look at the uncertainty ranges that different models see in terms of individual measures but also in terms of the flexibility of the emissions pathways. So, having that database was very important because it allowed us to have a collection of scenarios which are comparable and which follow certain definitions. That’s the first thing that we had to achieve. Just imagine a situation where you need to assess 2000 scenarios from hundreds of studies which all follow different definitions, that’s almost impossible. So, the database was the first step.

The second step is that we had a writing team of around twenty lead authors. In our first meeting, we introduced each other as we’re all coming from different backgrounds. It’s a very interdisciplinary team. We then divided more or less different aspects of the analysis of the pathways between people who had the most critical knowledge to answer specific questions. We have a subsection, for example, which looks into the economics of the transition, how much does it cost, what are the prices etc. There you want to have leading economists who know about macro and micro economy considerations and who can assess the scenarios based on that. Then we had a section on the strengths and weaknesses of the models that are used. They are very different, so some of them are coming from the engineering perspective where basically you represent the world in terms of technologies and how much these technologies cost so that you understand the conversion chain from energy that is extracted somewhere then it’s converted, then it’s used in a certain sector, and you also understand efficiency losses, but also emissions implications along the value chain of different services etc., so that’s engineering models. Then we have macroeconomic models which represent basically the world in terms of different macroeconomic sectors which take energy as an input to economic activity, so the productivity of energy is the main focus of those models, and they have, of course, a different perspective to the problem. By using a multitude of very different economic models, optimization models, simulation models etc., we derived robust conclusions from the perspective of different methodologies. I would say this is another important asset by bringing so much information together. And then, of course, on a practical term, if you ask how we have collaborated, at the beginning we met physically all together, once in Scotland, and then once in India at the so-called Lead Authors Meetings where all scientists from the report come together, where we discuss our own chapter, but we also discuss crosscutting issues across different chapters. But then, Covid-19 happened, so we moved from meeting physically to meeting virtually. We had literally thousands of Zoom calls, I’m not sure how many, to discuss in detail and adjust things within the chapter but also many many calls to think about crosscutting issues. It was actually very challenging. I have been in this process for quite some time and doing an IPCC report with hundreds of people only electronically and only with virtual meetings was very very challenging. We were all over the world, so we had literally dozens of meetings at 2/3 o’clock in the morning because we couldn’t always ask people in Asia to participate at European time etc.

What was the group’s approach to assess specific technologies in the various mitigation pathways? The report mentions many technologies (EVs, CDR technologies etc.), and so, how did you assess them in terms of their environmental cost of production, with associated environmental externalities, in terms of their lifecycles, etc.? Were there some criteria that you used?

Different aspects of different technologies were assessed at different scales. The report is structured in a way that we have individual bottom up sector chapters where the prospect of individual technologies are discussed in detail. Technologies like solar, nuclear etc. are, e.g. discussed in the energy chapter, including lifecycle perspectives. There is also a demand side chapter which is really, really interesting, it’s Chapter 5. You should have a look specifically at that because I think many of the solutions are coming from the demand side, that’s not my chapter, but I’m co-leading a big consortium of scientists, the so-called EDITS consortium, together with colleagues from RITE in Japan and many researchers from around the world where we try to understand how we can change lifestyle, behaviors etc., how can different sectors reduce demand, but, ok, this wasn’t your question. So, we have this bottom up assessment of the technologies, individual technologies and options, and how they can be brought together for individual systems. There is a buildings chapter, there is an industry chapter, an energy chapter, land use chapter etc. Then, we bring these assessments of technologies together by looking at the technical or economic parameters of the technologies, and by looking at their individual potential to reduce emissions. This is nicely summarized in a figure in the summary for policymakers (SPM) where you also see very nicely that many of the options are already economic, and it would be cheaper to deploy rather than their fossil alternatives. We actually go beyond this, and we also look, for each individual options, what are the specific tradeoffs but also synergies with other societal objectives. The analytical framework that is used here are the so-called Sustainable Development Goals (SDG) by the UN. That’s 17 different so-called SDGs that basically define environmental, social and economic sustainability. Not all options that can lead to reduced GHG are equal, some of them can create big tradeoffs with those objectives. Think about large scale use of biomass for removing CO2 from the atmosphere, if you do that at a large scale, you will need to avoid competition with food production so that you don’t squeeze food and increase prices and reduce food security, particularly in countries where there are already risks of hunger, for example. Options can also create tradeoffs for biodiversity, for water, etc. Think about nuclear, it’s the technology with the most intensive water use among all different options. If you don’t have a lot of water, you don’t want to apply nuclear technology.

So, synergies and tradeoffs were the next things we looked into, but then, there is also an assessment of all of these options in terms of feasibility. Lots of things that look nice on paper are sometimes difficult to implement and there might be barriers. These can relate to, you know, geophysical feasibility considerations, this might be technological and economic feasibility, upscaling sometimes is very difficult, but this relates also very strongly to institutional and governance capacities of different countries. We’ve seen that the developing world need to reduce their emissions very quickly in order to meet the 1.5C objective but at the same time the question is: Are those countries able to design, implement and govern those policies and implement them as it would be needed? The feasibility assessment of the IPCC pathways actually shows that the pathways are consistent with many feasibility concerns along those other dimensions, but the biggest feasibility concern might be on the governance and institutional side to scale up the technologies and bring emissions down in the developing world. That’s where those countries need to have support. So, figuring out how one can strengthen the institutions and the governance in the developing world so that they can actually achieve rapid emissions reductions is one of the big challenges.

This is a fascinating topic. As you mentioned, there are tradeoffs everywhere. If we look at EVs for example, the emissions associated with the use of an EV in our day-to-day may be relatively low, but producing such vehicles is still relying on polluting industries, like the steel industry, plus, we need to discuss about the lifecycles of batteries etc. And this discussion can be extended more generally to discuss the cost to bootstrap the low-carbon economy. Renewable infrastructures are very much reliant on concrete etc. So, how to account for this “bootstrapping” cost, i.e. the environmental cost to bring the world from how it is today to a net-zero world? Is this bootstrapping effect all accounted for in the models?

At the moment, some of these aspects are accounted for, but definitely not all of them. Many IAMs are, as we speak, extended to also better represent not only energy needs but also material needs. I think you need to think in an integrated energy and material resource system in order to be able to understand how to respond to those questions. We are very active in this area at the moment, there is a project that we are coordinating that is called “CircEUlar” where we try to bring material experts together with the Integrated Assessment Modelling team in order to have a better representation of the material flows, or material stocks I should say, compared to energy flows to better represent that in the IAMs. So, I would say that it is a research frontier to better represent this. It’s also needed to better understand the consequences of some of the solutions like the shared economy, the circular economy and also moving from an energy intensive society based on fossil fuels to a renewable one, means we move towards a material intensive society. Those materials are also not equally distributed across the planet, so one has to be careful that one doesn’t shift vulnerabilities rather than reduce them at the end.

The Working Group III’s contribution to the Sixth Assessment Report was finalized in April this year (2022). Do you think that some of the report’s conclusions have already been obsoleted and should already be reconsidered, based on what we’ve observed this year (lots of climate disasters, considering the climate impact of the war in Ukraine, the conclusions of COP27 etc.)?

I think the biggest difference to the scenarios that the IPCC assessed in our chapter, is the change in geopolitical situation because of the war. I think that the energy crisis itself will have less impact than the fragmentation that we see because we cannot solve the climate problem without a collaborative activity across the main emitters. If it’s not global, at least we need to have regional clubs, or something like this, where people are willing to work together and there must be a minimum level of solidarity so that those solutions are also really implemented. This is actually also referred to in the report quite clearly that in a fragmented world it is not possible to limit warming to 1.5C or 2C. None of the models give you a feasible result to go to 1.5C if the world does not collaborate. This is something that is realized under the so-called Shared Socioeconomic Pathway (SSP) number 3. There are different SSPs and one is basically featuring this fragmentation. In that world, 2C or 1.5C is out of reach. On the other hand, we see that a very strong focus on sustainability, not because of climate reasons but because of other reasons, will create a lot of co-benefits and will make it much easier to achieve the climate goals. Some of those changes are already happening, particularly demand side, the main stumbling block is however the lack of trust, which is the basis for establishing joint international efforts to solve climate change.

The floor is yours, anything you’d like to mention or share with the community? (a book you’ve written? A paper that got published? Something, on climate, you’d like more people to be aware of?)

The COP negotiations showed clearly how important it is to derive solutions that are perceived as fair. In this context, our recent paper in Science which appeared during the COP, shows that in order to achieve fairness, we need to significantly scale up the finance support to the developing world. So, what we have at the moment is simply an unfair expectation about who’s covering which sort of investment, and that’s why those investments are not happening in the developing world. To give you an example, there is a fund by the UNFCCC for mitigation and adaptation of around 100 billion USD, but only 60 billion has been covered so far. In our assessment we showed that, as a minimum, you need to scale up this fund to 250 billion and if we look at many different other alternatives and ethical principles of how to allocate the investment for the future, then actually finance flows need to be about significantly above a trillion per year. As a scientist I was excited that our paper came out in Science, but at the same time also disappointed by the COP27 that no upscaling of the finance support for the mitigation in poor countries could be achieved.


Further reading:


We originally published this interview on OCRA, a climate forum and community that we started. This interview has been moved to our blog following the closure of the OCRA forum.

Ganddee interviews offer an opportunity to put some guest community members at the center stage. During these interviews we ask questions to our expert guest to better understand their work, learn from them, and understand how they contribute to the fight against climate change. We hope these interviews will serve as an invitation to expend the discussion on the forum.

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