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Climate Action Epistemology Part 2: Mitigation Modeling In The IPCC Mitigation Report

March 30, 2023

Ed. note: You can find Part 1 of this series on here.

This is the second article in a six-part series examining the 2022 IPCC mitigation report (working group III).

When the IPCC mitigation report comments on the possibilities and likely effects of different emissions reduction strategies, it usually relies on quantitative integrated assessment models (IAMs) to do so. The authors review findings from over 100 models that have produced over 1,000 climate action scenarios. IAMs combine assumptions about the economy, technology, and climate policies to generate emissions projections and explore different types of futures. Models are an important planning tool, but we need to evaluate the assumptions they’re built from, because inputting an unsound picture of the world can lead us towards an inadequate response to the climate crisis. The report acknowledges that global emissions pathways “have to be assessed with the careful recognition of these assumptions.” However, on the same page the authors also state that the “IPCC is neutral with regard to the assumptions underlying the scenarios in the literature assessed in this report, which do not cover all possible futures.” This begs the question: who evaluates the feasibility of the assumptions behind these IAMs if not the IPCC?

Thankfully, some researchers do take on this task, and perhaps no one has done more to expose various implausible assumptions behind many IAMs than climate scientist Kevin Anderson. In a recent paper, he and other scholars point towards the approach taken with today’s climate action models as one of nine reasons why emissions haven’t been seriously addressed after 30 years of study and international negotiations. One limitation they note is models’ reliance on mainstream (neoclassical) economic theory, which tends to reject mitigation actions that could disrupt the current consumption-maximizing economy. Another is models’ increasing reliance on carbon dioxide removal (CDR) strategies that envision drawing large amounts of carbon out of the atmosphere, though the technologies involved remain unproven and face significant barriers to scale. The major issue is how highly questionable assumptions like these tend to delay serious climate action by making marginal rates of change seem legitimate and suggesting that technological innovation will do all of the heavy lifting.

Let’s consider a few implications of some mainstream modeling practices. We know that models increasingly incorporate currently non-existent negative emissions technologies (NETs). They also tend to be global in scope and fail to incorporate international equity—the idea that wealthier nations should decarbonize earlier than those with fewer resources. The result is that the timeframe they suggest for reaching net zero emissions is much longer than what many nations must achieve to meet consensus climate goals. The IPCC report says that aiming for a 1.5°C warming limit requires the world to achieve “50% CO2 reductions in the 2030s, relative to 2019, then reduce emissions further to reach net zero CO2 emissions in the 2050s. Pathways limiting warming to 2°C (>67%) reach 50% reductions in the 2040s and net zero CO2 by 2070s.” Anderson and his colleagues show that when excluding NETs and taking equity into account, wealthy nations must actually decarbonize their energy system between 2035 and 2040—up to 30 years before the global picture suggested by mainstream models. That entails historically unprecedented rates of emissions reductions above 10% year after year, at least double the rate that supposedly “climate progressive” nations are considering.

One of the key assumptions in any model is how it represents the main emissions drivers, economic and population growth. Recall the IPCC’s acknowledgement that high levels of growth are the very conditions that could make it impossible to keep warming below 2°C and even threaten a 5°C increase, which would render large parts of the planet uninhabitable. Across the models assessed by the report, the population is expected to grow from 7.6 billion in 2019 to 8.5-9.7 billion in 2050 (growth of 10-30%) and to 7.4-10.9 billion in 2100 (a wide range from a slight decline to an increase of 40% compared to 2019). However, these figures could be a significant understatement, as the authors observe that the UN’s population projections “include considerably higher values for both the medium projection and the high end of the range.” The economy is assumed to grow 2.5-3.5% annually up to 2050 and then around 1-2% to 2100. The economy is therefore expected to “at least double” in size between 2020 and 2050 and continue expanding throughout the century. There is little discussion about whether these growth assumptions are compatible with maintaining a stable climate.

The idea that we can unlink the longstanding connection between economic growth and emissions growth, called “decoupling,” is behind the assumption that the economy will continue to expand. Relative decoupling occurs when our mitigation efforts cause emissions to increase at a slower rate relative to economic growth. What we’re banking on is absolute decoupling, which is when emissions stand still or even decrease while the economy grows. Interestingly, in different chapters of the IPCC report, statements about the possibility of decoupling become more contextualized and critical:

  • Chapter 1: “Recent evidence shows countries can grow their economies while reducing emissions,” the report states assertively. Without further context, it sounds like decoupling is a viable strategy for keeping warming to tolerable levels alongside continued economic growth.
  • Chapter 4: A few chapters later, the report is much more cautious about whether decoupling has actually occurred, which throws the initial assertion into question. “While some literature indicates that absolute decoupling of economic growth and [greenhouse gas] emissions has occurred in some countries, a larger systematic review found limited evidence of this.”
  • Chapter 2: “Absolute decoupling is not sufficient to avoid consuming the remaining CO2 emission budget under the global warming limit of 1.5°C or 2°C and to avoid climate breakdown. Even if all countries decouple in absolute terms this might still not be sufficient and thus can only serve as one of the indicators and steps toward fully decarbonising the economy and society.” Here a more fundamental point is added. Even if absolute decoupling has been achieved, the magnitude is far from what would be needed year after year until emissions reach zero. The main question is not whether any level of absolute decoupling is possible over any period of time, but instead whether enough decoupling is possible over a long-enough period of time to allow economies to grow as expected while reducing emissions at sufficient rates to avoid extremely dangerous warming. Assumptions of continued growth in IAMs are based on the idea that the answer is “yes.”

Chapter six observes that IAMs tend to “overestimate the contributions by energy efficiency,” which has historically been the main technological counterweight to rising emissions (i.e. larger than the effect of replacing fossil fuels with renewable energy technologies). Achieving deep emissions reductions alongside economic growth would rely in part on increasing energy efficiency. However, the same chapter later states that “Industry has seen major efficiency improvements in the past, but many processes are now close to their thermodynamic limits.” This raises a number of questions. We can ask whether such limits are well-represented in models. If they’re not, that could be one of the reasons why models often expect more efficiency gains than are realized. A bigger question is why thermodynamic limits aren’t recognized and discussed more broadly. That could entail looking into the limits of efficiency not just in industry but across all sectors, down to the very foundation of our economy, which is bound by these same limits and therefore cannot grow forever. If IAMs recognized thermodynamic limits in a fundamental sense—which would be the case if instead of relying on neoclassical economic theory they drew from ecological economics—then assumptions about a doubling or tripling of the economy across this century couldn’t pass without serious scrutiny.

Despite claiming neutrality about the IAMs’ input assumptions, the IPCC report does attempt to assess the feasibility of the resulting mitigation scenarios (i.e. model outputs) in chapter three. The authors write that “Mitigation pathways are associated with significant institutional and economic feasibility challenges rather than technological and geophysical feasibility challenges.” Yet when looking at their assessment in 2050 and 2100, geophysical challenges are expected to grow and become at least as significant as the near-term economic obstacles. Their assessment also anticipates very few socio-cultural feasibility constraints, which only seems possible if the transition doesn’t fundamentally conflict with cultural pillars like consumerism, the unbridled pursuit of personal wealth, and extreme economic inequality. Warnings about the feasibility picture produced by IAMs appear throughout the report:

  • Chapter 4: There may be unacknowledged or unrecognized economic or social trade-offs that complicate rapid emissions reductions. “While the technology elements of accelerated mitigation pathways at [the] national level are generally well documented, studies of the economic and social implications of such pathways remain scarce.”
  • Chapter 6: There could also be overlooked technological, political, or cultural barriers. While IAMs have underestimated the cost declines of key renewable energy technologies like wind turbines and batteries in recent years, which should help make the transition faster, “they tend to be too optimistic regarding the timing of action, or the availability of a given technology and its speed of diffusion. Furthermore, some technological and economic transformations may emerge as technically feasible from IAMs, but are not realistic if taking into account political economy, international politics, human behaviours, and cultural factors.”
  • Chapter 16: Ecological barriers may also not receive enough scrutiny. Processes of technological change occur within societies and ecosystems, and must be modeled with special attention to their effects on these systems. “Simplifications of complex interactions between physical and social systems and incomplete knowledge of the indirect effects of technological innovation may systematically lead to underestimation of environmental impacts and overestimation of our ability to mitigate climate change.”

Constraints within essentially all feasibility dimensions are mentioned as possible complications for the validity of model outputs. This is an issue whenever journalists, policymakers, and the public believe that the message emerging from a model is conclusive. The amount of complexity involved, and the long timeframes over which solutions are explored, mean that there is so much we cannot know. We don’t sufficiently acknowledge the uncertainty involved in projecting the future, particularly when modeling rapid transitions. The IPCC authors point out that “there is often limited discussion of uncertainty and of its implication for hedging strategies in the accelerated mitigation pathway literature.” If we aren’t clear about how much uncertainty surrounds the model’s outputs, then we may not plan for potentially significant obstacles to the transition.

As we have seen, the assumptions from which IAMs are built, if not sound, have massive implications. They would suggest that all human societies can continue growing and increasing their consumption, and can reduce fossil fuel use at a slower pace than is actually necessary to avoid climate breakdown. IAMs may also lead us towards particular emissions reduction strategies without sufficiently considering the obstacles they face. Thus despite the report’s discussion of feasibility, it appears that an excessive reliance on models and an insufficiently critical approach to their recommendations may prolong today’s high emissions rates and make dangerous climate change more likely.

There are ways to reduce the likelihood of inadequate climate planning. One key is to think seriously about reducing our demand for energy and materials rather than assuming it will continue growing. The potential impacts of reducing demand and shifting towards more sustainable societies, considered for the first time in some models cited by this report, emerge as vital for increasing the ecological and technological feasibility of emissions reductions. “Many challenges, such as dependence on CDR, pressure on land and biodiversity (e.g., bioenergy) and reliance on technologies with high upfront investments (e.g., nuclear), are significantly reduced in modelled pathways that assume using resources more efficiently (e.g., IMP-LD) or that shift global development towards sustainability (e.g., IMP-SP).” These benefits are perhaps gained in exchange for greater challenges to political feasibility, but these challenges may be easier to overcome than constraints imposed by physical limits and ecosystem collapse. Given that demand management and sustainability strategies appear crucial to our mitigation plans, we should ask whether other crucial details are currently left out of mainstream IAMs.

Another key is to think about what place models should have in our planning processes. What questions can we attempt to answer with them? What are their limits? It appears that climate policy may currently rely too heavily on IAMs. It’s vital that we consider multiple sources of information to get a more holistic and accurate picture of reality, including analyses of real-world mitigation efforts, studies of relevant issues not acknowledged in the model (e.g. the likely climate impacts of growing emissions), history, theory, and forums for deliberation.

We also need to ensure that the models we use are built from sound assumptions, and that society understands how to interpret their findings. Reading through the critiques from academics and the subsequent responses from modelers is an important part of that process. But who does that? Activists must be aware of these critiques and bring this information to the public to have any chance at informed decision-making. That way we’ll all have a stronger understanding of what action at the scale of the crisis entails, and be able to evaluate policymakers’ climate plans.



Teaser photo credit courtesy of Unsplash.

Aaron Karp

Aaron Karp

Aaron Karp is an activist writing a book about why our ecological crises demand economic and cultural transformation, not just an energy transition, and how the climate movement can lay the groundwork for these changes. He writes at and tweets @LimitsLiberate

Tags: energy efficiency, fossil fuel greenhouse gas emissions, Integrated Assessment Models, IPCC reports, powering down