A Deep Dive into the IPCC Workgroup III Mitigation Report

Assessment Reports from the Intergovernmental Panel on Climate Change (IPCC) are essential touchstones for policymakers and stakeholders, providing a periodic synthesis of what we know about climate science, impacts, and mitigation and adaptation options. The UNFCCC Global Stocktake and UNFCCC deliberations are guided in part by the findings of the IPCC Sixth Assessment Report (AR6).

AR6, finalized in March 2023, consists of the reports of Workgroup I (Science), Workgroup II (Impacts, Adaptation and Vulnerability), Workgroup III (Mitigation) and the Synthesis Report, and incorporates findings from three IPCC Special Reports (Global Warming of 1.5°C (2018), Climate Change and Land (2019) and a third on the ocean and cryosphere (2019)). With COP28 at hand, this piece presents a deep dive on the findings of the Workgroup III (WGIII) report and the mitigation portion of the Synthesis Report.

Workgroup reports are typically 2,000 to 3,000 pages long, which is why the IPCC prepares a Summary for Policymakers (SPM) for each report (and for the Synthesis Report), typically 30 to 50 pages long. An SPM is very challenging to write. It requires boiling down thousands of pages to key findings and using language that is understandable for readers who are not scientists or experts. An SPM is difficult to write for a second reason, too: All 192 countries that participate in the IPCC must approve the final language. Government representatives review and comment on drafts, and then meet to hammer out final disagreements on text. No SPM can summarize fully a report of thousands of pages, and the SPM for AR6 WGIII is no exception.

I served as an Expert Reviewer for AR6 WGIII, and for years I have immersed myself in the burgeoning body of studies on net-zero pathways, including previous IPCC reports. In my view, the full WGIII Mitigation report is a comprehensive and objective assessment of how nations can collectively keep warming to 1.5 or 2 degrees C (2.7 or 3.6 degrees F). It includes: emission trends, drivers and scenarios; estimates of emissions reductions and costs for various options; global and regional analysis; more in-depth treatment of demand-side options than previous reports; discussion of equity, financing and enabling conditions; and more. There is no shortage of elements to praise in the full report. However, the SPM is missing some key findings that are essential to understanding what is needed to limit global warming.

Given that some policymakers, journalists and stakeholders do not venture beyond reading an SPM due to the length and complexity of full IPCC reports, this article presents key findings that deserve highlighting and are essential to meet the IPCC’s goal of informing policymaking.

What’s Missing from the IPCC’s SPM on Mitigation

Answers to two questions are missing from the SPM, and they’re key to understanding what it takes to keep warming to 1.5 or 2 degrees C:

• What are the key types of primary energy that must replace fossil fuels, and how fast must they ramp up over the next 30 years?
• With electricity playing a huge role in decarbonizing all sectors of the global economy, what mix of technologies can deliver greatly expanded electricity supplies while also dramatically reducing emissions?

Answers to both of these questions in the IPCC full report involve quantitative depictions of the primary energy mix and electricity generation mix in 1.5 or 2 degree C scenarios. Such quantitative depictions are featured prominently in the mitigation scenarios published by the International Energy Agency, International Renewable Energy Agency, and other organizations for both global and national studies. But readers cannot find these in the WGIII SPM, despite their salience to policymakers as they guide the transition to net-zero economies. Instead, they must dig deep into the full WGIII report to get even a partial picture of these key elements of the 1.5 and 2 degree C mitigation scenarios.

How the IPCC Depicts Mitigation Scenarios

To set the stage, one needs to understand how the IPCC compiles and depicts mitigation scenarios. In AR6, authors examined over 1,200 mitigation scenarios from the published literature, selecting 541 that were consistent with limiting warming to 1.5 or 2 degrees C with no or limited overshoot.ⁱ In one type of depiction, the authors applied a statistical lens to key indicators in these scenarios. They calculated median values of indicators such as total GHG emissions, coal use, solar generation or carbon dioxide removal across a group of scenarios. They also calculated interquartile ranges (25th to 75th percentile) of the indicators, and 5th to 95th percentiles, to depict the variation across scenarios in their results.ⁱⁱ

The second type of depiction was to select five individual scenarios as Illustrative Mitigation Pathways (IMPs) and adopt a storyline for each. These included: a scenario with high use of renewables (IMP-Ren), one with high use of negative emissions (IMP-Neg), and a scenario with very low energy demand (IMP-LD).

A Quick Tour Through the SPM

The SPM is structured in this way: Sections A and B provide an introduction and assess recent developments and current trends in emissions and mitigation options. Section C is the core presentation of mitigation scenarios and options and is the focus below. Section D covers linkages between mitigation, adaptation and sustainable development. Section E covers institutional design, policy, finance, innovation and governance arrangements.

Section C is aptly titled “System Transformations to Limit Global Warming.” It begins with a discussion of net global GHG emissions and net global CO2 emissions, showing the trajectories that these emissions need to follow to limit warming to 1.5 or 2 degrees C. The section goes on to present very granular, quantitative details on how fossil fuel use would need to steeply decline in order to limit warming to 1.5 degrees C (with no or limited overshoot) or 2 degrees C, noting that the global use of coal, oil and gas in 2050 needs to decline by about 95%, 60% and 45%, respectively, compared to 2019 (median values for 1.5 degrees C). The text also gives the interquartile ranges and the 5th to 95th percentiles.

However, there is no comparable quantitative discussion of what primary energy sources replace fossil fuels.

A key figure here is SPM.5, which throws some limited light on the mitigation scenarios. Parts a. through d. of this figure show trajectories for net global emissions of all GHGs, CO2, methane and nitrous oxide, respectively, for 2000-2100 in the 1.5 and 2 degree C scenarios, along with the trajectories for the five IMPs. Figure SPM.5.d and SPM.5.e present some complex depictions of sectoral reductions in the energy, buildings, industry and transportation sectors that would take the global economy to net-zero CO2 emissions and net-zero GHG emissions (while also showing the roles of reductions in non-CO2 gases and the role of land useⁱⁱⁱ). However, Figure SPM.5 gives no information on the types or quantities of energy that replace fossil fuels used in buildings, industry, transportation, etc.

Section C.4 focuses on the energy sector, listing the key steps necessary for net-zero CO2 energy systems, which include:

• Increase energy efficiency and conservation;
• Move to zero-carbon electricity generation;
• Electrify end-uses where feasible;
• Produce low/zero-carbon fuels (e.g., hydrogen) and sustainable biofuels;
• Minimize use of unabated fossil fuels, and deploy CCS where feasible; and
• Use CDR to counterbalance remaining emissions.

The text also notes: the encouraging decreases in the costs of renewables and storage; the need to leave large, known quantities of fossil fuels unburned; the importance and cost-effectiveness of reducing methane emissions from production and transport of fossil fuels; and the roles of CCS and CDR in mitigation scenarios. However, Section C.4 gives no quantitative information on primary energy sources or electricity generation mix in the 1.5 and 2 degree C scenarios.

The remainder of Section C contains rich, detailed discussions of mitigation options in these categories: industry, urban infrastructure, buildings, transport, AFOLU (agriculture, forestry and land use), demand-side mitigation and CDR. Each of these subsections contains a unique mixture of discussion of technologies, design and practices, efficiency, electrification, etc. Each presents some unique quantitative depictions of mitigation options in terms of tons and/or cost.

Section C ends with a discussion of near-term (2030) mitigation options and mid-term (2050) impacts on GDP of the 1.5 and 2 degree C scenarios. Two highlights: By 2030, global emissions could be cut in half taking steps that cost $100/ton of CO2e or less; and by 2050, global GDP is a few percent lower in compared to scenarios without mitigation beyond current policies. (This projection does not account for the GDP benefits of avoided climate change.) Near-term options are summarized in Figure SPM.7, providing a list of 43 mitigation steps along with a depiction of the tons of potential emissions reduction and the cost per ton for each. The text notes that the mitigation potentials are assessed independently for each option and are not necessarily additive (i.e., they do not collectively form a mitigation scenario).

In sum, Section C presents to the reader:

• Qualitative descriptions of the elements of a net-zero GHG economy
• Needed trajectories for GHG, CO2, methane and nitrous oxide emissions
• Trajectories for emissions from various sectors, and some related details on policy options, tons and costs.
• Very detailed, quantitative projections from the 1.5 and 2 degree C scenarios on how fast the use of coal, oil and gas must decrease between now and 2050.

However, Section C provides no quantitative depictions of:

• Primary energy sources that must replace fossil fuels
• The electricity generation mix needed to power a low-carbon global economy

A Deeper Dive

The full WGIII report provides useful findings on what types of primary energy must replace fossil fuels and how fast they must grow between now and 2050. The information can be found in Table TS.2 (Technical Summary, p.71), excerpted below.

This table gives key characteristics of the 1.5 and 2 degree mitigation scenarios, with the characteristics listed in the left-hand column, and five categories of scenarios across the top row. In the rows highlighted in red, we see the needed percentage decreases in primary energy from coal, oil, and gas for 2030 and 2050 relative to 2019 (median values across scenarios plus interquartile range). The first column in the red section points to the 2050 values. These are the values that appeared in the SPM (cited above): for the most stringent 1.5 degree C scenario, global use of coal, oil and gas in 2050 must decrease by 95%, 60% and 45%, respectively, compared to 2019 (median values). And just below are values for what replaces fossil fuels: nuclear, modern biomass and non-biomass renewables (solar, wind, hydro, geothermal, etc.). These three major primary energy sources increase by 90%, 290% and 725%, respectively, by 2050 (highlighted by green arrows), and by 40%, 75% and 225%, respectively, by 2030 (all median values). Interquartile ranges are quite large, and needed growth is generally slower as one moves from left to right in the table, going from 1.5 degree C scenarios to less stringent 2 degree C scenarios.

The major portion of these increases in low- and zero-carbon primary energy would be in the form of electricity, applied directly to end-uses and to assist decarbonization of many sectors of the global economy. The remainder would be in the form of heat and liquid and gaseous fuels.

The full report also provides findings on the mix of generation technologies that can deliver the needed electricity supplies with dramatically reduced emissions. The report consistently cites four sources:

“Nearly all electricity in pathways limiting warming to 2°C (>67%) or 1.5°C (>50%) is also from low- or no-carbon technologies, with different shares across pathways of: nuclear, biomass, non-biomass renewables and fossil fuels in combination with CCS. (p.84)^iv”

Chapter 6 (Energy Systems) provides the most detailed findings on mitigation options in the energy sector, including the pivotal electricity sector, which is projected to deliver on the order of 60% of all end-use energy by 2050. The key depiction of the generation mixes in the 1.5 and 2 degree C scenarios is in Figure 6.30 (p.688) below.

The figure shows shares of three categories of electricity generation — solar and wind, CCS and nuclear — as percentages of global electricity production.^v Each bar represents a category of 1.5 or 2 degree C scenario with the usual median, interquartile, and 5th/95th percentiles; and four decadal snapshots are presented.^vi In 2020, solar and wind generated a little under 10% of global electricity. The scenarios indicate the need for rapid growth for these variable renewable sources, growing to about 40-80% of total electricity by 2050. This is roughly consistent with the 725% growth (median value) in “nonbiomass renewables” cited above from Table TS.2.

In 2020, CCS facilities consisted of a limited numbers of demonstrations around the globe, and hence are represented as generating zero electricity. The scenarios show a wide range of possible shares for power plants with CCS (both fossil- and biomass-fueled) in the future, with median values growing by 2050 but remaining in the range of 5-10% of total generation. The interquartile ranges are from near zero to about 20-25%.

Nuclear power generated around 10% of global electricity in 2020. Here, too, the scenarios show a wide range of growth. Median values cluster around maintaining approximately a 10% global share, with an interquartile range of roughly 5-20%. Note, however, that total electricity production would grow sharply over the next 30 years, roughly doubling in the 1.5 degree C scenarios. This means that total nuclear generation would have to double by 2050 to maintain a 10% global share. This is roughly consistent with the 90% growth (median value) in nuclear primary energy cited above from Table TS.2.

Generally speaking, the 1.5 and 2 degree C scenarios have both CCS and nuclear power playing supporting roles to global electricity supplies that are likely to become predominantly variable renewable, but not 100% variable renewable (though the generation mix of an individual country might approach 100% or be far less, depending on local resources, preferences and policies). This supporting role for “clean, firm” electricity, such as CCS, nuclear, hydro and geothermal is consistent with other findings in Chapter 6 that appear in Box 6.8: “100% Renewables in Net-zero Energy Systems” (pp.675-676):

“High wind and solar penetration involves technical and economic challenges due to their unique characteristics such as spatial and temporal variability, short- and long-term uncertainty, and non-synchronous generation...These challenges become increasingly important as renewable shares approach 100%.... Energy systems that are 100% renewable (including all parts of the energy sector, and not only electricity generation) raise a range of technological, regulatory, market, and operational challenges that make their competitiveness uncertain.... Integrated assessment and energy systems research suggest large roles for renewables, but energy and electricity shares are far from 100%, even with stringent emissions reductions targets and optimistic assumptions about future cost reductions.”

These findings are consistent with the conclusions in a recent paper authored by 16 staff from the National Renewable Energy Laboratory (NREL):

“Significant unanswered questions remain regarding moving toward or achieving 100% RE at a national scale for all hours of the year. There is no simple answer to how far we can increase RE penetration before costs rise dramatically or reliability becomes compromised…. Reducing the costs of low-carbon generation in the electric sector, potentially by keeping non-RE options (including CCS and nuclear) available, enables electrifying and thus decarbonizing other sectors, reducing economy-wide carbon emissions.^vii”

Other sources of clean, firm power include hydro, biomass, geothermal and hydrogen-fueled turbines, all of which could also contribute to reliable, affordable power grids with high penetration of solar and wind (aided also by integration strategies such as storage, demand management, and expanded transmission). Nuclear, CCS and these other sources of clean, firm power all bring benefits, risks and concerns of various types. Nuclear and CCS are controversial in some countries, however, many other countries include one or both in their long-term strategies submitted to the UNFCCC.

The appropriate electricity generation mix should always be the subject of vigorous and fact-based debate, and account for the risks that various technologies pose, and whether and how they can be managed effectively. The generation mix will likely vary widely by country, depending on a country’s resources, its interconnection to wider grids, and other country-specific conditions.

Achieving the goals of the UNFCCC will require a broad portfolio of zero- and low-carbon energy sources, as well as carbon removal measures. And that portfolio will vary across countries. This is a key finding of AR6.

ⁱSee IPCC WGIII Technical Summary, Box TS.5 (p. 77) for details on the C1-C3 scenarios and the Illustrative Mitigation Pathways.

ⁱⁱThis statistical depiction does not imply the likelihood of a scenario happening but rather provides information on the range of modelling outcomes.

ⁱⁱⁱMore formally: reductions from Land Use, Land Use Change and Forestry or LULUCF.

^ivSee also p. 299, p. 332, p. 342, p. 354.

^vThough not explicit in the text, the CCS category appears to represent both fossil and bioenergy with CCS.

^viIn this figure, scenarios C1-C4 are depicted.

^viiDenholm et al, “The challenges of achieving a 100% renewable electricity system in the United States.” Joule. 2021