Blog: The New IEA Report Shows How We Are Flattening the Curve of Future Emissions

The new World Energy Outlook 2021 from the International Energy Agency (IEA) shows how the world is making progress on clean energy and emissions and provides further evidence that very high future emissions scenarios are increasingly unlikely.

In late 2019 we published a provocative analysis arguing that “a 3C world is now business-as-usual.” This was followed by an article in the journal Nature pointing out that emissions scenarios in which the emissions double or triple this century due to dramatic expansions in global coal use are increasingly improbable in a world of rapidly falling clean energy prices. In both these analyses, we used the IEA’s World Energy Outlook to suggest more likely pathways given policies in place today and near-term commitments by countries. The IEA’s results are in line with a wide variety of studies published over the past two years – as we showed in another recent analysis.

These results have been influential, leading more researchers to focus on assessing climate impacts associated with more intermediate scenarios consistent with current policy outcomes, and leading the Intergovernmental Panel on Climate Change (IPCC) 6th Assessment Report (AR6) to conclude that “the likelihood of high emission scenarios such as RCP8.5 or SSP5-8.5 is considered low in light of recent developments in the energy sector.”

The publication of the new IEA World Energy Outlook 2021 provides an opportunity to revisit these analyses – both to update them with the latest IEA scenarios and to make them consistent with the assessed warming ranges and climate sensitivity values from the IPCC AR6.

Figure 1, below, shows the IEA’s scenarios that are consistent with current policy outcomes (CPS), near-term stated policies such as 2030 Paris commitments (STEPS), and the announced pledges scenario (APS). The IEA retired their CPS scenario after their 2019 report, arguing that the world was moving too quickly for a current policy scenario to be of much use. In the 2021 report, they added the APS scenario for the first time, reflecting the fact that countries representing over 66% of global emissions have now announced pledges to reach net-zero emissions in 2050 or 2060. The IEA has extended its scenarios out to 2050 in the 2021 report, a decade past the 2040 values included in prior reports.

Figure 1: Historical fossil CO2 emissions (black) from the Global Carbon Project, along with 2019 CPS, 2020 STEPS, and 2021 STEPS and APS scenarios from the IEA World Energy Outlook.

The new STEPS 2021 scenario has a number of interesting aspects compared to STEPS scenarios in prior years. It shows a greater near-term rebound in emissions relative to the STEPS 2020 scenario, reflecting more optimistic economic recovery and slower near-term decarbonization projections as the COVID-19 pandemic abates.

STEPS 2021 has global emissions surpassing their prior 2019 record in 2022 and peaking in 2026 before entering a long and slow decline. Emissions in 2040 are projected to be modestly lower – by around 1 GtCO2 – than in STEPS 2020. The APS 2021 scenario, by contrast, has global emissions never recovering to their pre-pandemic 2019 levels, and falling by nearly 40% by the year 2050.

Figure 2, below, shows how these IEA scenarios compare to the five emissions scenarios associated with Shared Socioeconomic Pathways that were used in the recent IPCC AR6.

Figure 2: As with Figure 1, but including Shared Socioeconomic Pathways emissions scenarios from the IIASA SSP Database used in the CMIP6 models and featured in the IPCC 6th Assessment Report.

Here we see that the old CPS 2019 scenario is modestly above the intermediate-emissions SSP2-4.5 scenario in 2040, while all the STEPS scenarios are notably below it. The new APS 2021 scenario ends up nearly identical to the SSP1-2.6 scenario. None of the IEA scenarios project future emissions remotely close to the high-emissions SSP3-7.0 or SSP5-8.5 scenarios. While these sort of current policy and stated policy scenarios represent neither a ceiling nor a floor on possible future emissions, they do make it clear that – at least today – the world is no longer headed toward a very high emissions future.

These emissions scenarios can be translated into end-of-century warming outcomes and uncertainties. To do this, we utilize the IPCC AR6 assessed warming ranges for each of the SSP scenarios. To calculate the warming in various IEA scenarios, we extend them through 2100 by assuming flat emissions after 2040 for scenarios in which emissions are not declining (CPS 2019, STEPS 2019, and STEPS 2020). For the new 2021 IEA scenarios, we follow the approach the IEA uses and assume emissions continue their trend after 2050 (based on the trend over the 2040-2050 period). We use the total cumulative response to emissions (TCRE) mean, 5th, and 95h percentile ranges from the IPCC AR6 and calculate the difference between each IEA scenario’s cumulative emissions and the SSP2-4.5 scenario to calculate the difference in warming between that scenario and SSP2-4.5. This approach effectively uses land use, non-CO2 greenhouse gas, and other climate forcings from the SSP2-4.5 scenario to expand the CO2-only values in the IEA scenarios.

The results are shown in Figure 3 below, which includes the cumulative fossil CO2 emissions in each scenario, the mean expected warming in 2100 relative to preindustrial levels, and the 5th to 95th percentile uncertainty range in warming. Note that these uncertainty ranges do not include uncertainties in carbon cycle feedbacks.

Figure 3: Cumulative fossil CO2 emissions between 2020 and 2100 in each SSP and extended IEA scenario, along with the mean expected warming in 2100 relative to preindustrial levels, and the 5th to 95th percentile uncertainty range.

We project that the new APS 2021 scenario would result in approximately 2.1C warming by 2100, and the STEPS 2021 scenario would result in 2.6C warming – effectively identical to the IEA’s own future warming projections associated with these scenarios, despite a very different approach used. The uncertainty ranges shown here are a bit narrower than those in the IEA report, reflecting the narrower range of TCRE uncertainties in the IPCC AR6.

All the STEPS scenarios project future warming broadly similar to the intermediate SSP2-4.5 scenario. None of the new IEA scenarios show mean expected warming anywhere close to the levels found in the high-emissions SSP3-7.0 and SSP5-8.5 scenarios. At the same time, however, uncertainties in climate sensitivity are large enough that outcomes of close to 4C warming are still possible under the STEPS scenarios. This reinforces the argument we have made many times before: the world has been making real progress away from high emissions outcomes that seemed more plausible a decade ago; at the same time, there is a long way to go to meet Paris Agreement goals of limiting warming to well-below-2C.