"Energy transition scenarios at a glance"
The IEA's World Energy Outlook 2022 (WEO 2022) sets out three scenarios: the Net Zero Emissions by 2050 Scenario (NZE), which is the most prominent WEO scenario, the Announced Pledges Scenario (APS), which assumes that governments will meet, in full and on time, all of the climate‐related commitments that they have announced, and the Stated Policies Scenario (STEPS), which analyses actual policies & measures in place [CWC, 2023].
NZE - Net Zero Emissions by 2050 Scenario (or "International Scientific Consensus" Scenario)
NZE is a normative scenario in that it is designed to achieve specific outcomes, showing what should happen in the future in order to achieve certain strategic targets: it starts with a fixed aim (limiting warming to 1.5 Celsius degree) and then works backwards to see how it would be possible to get there). In short, NZE shows a pathway for the global energy sector to achieve net zero CO2 emissions by 2050, with advanced economies reaching net zero emissions in advance of others [IEA, 2023].
APS - Announced Pledges Scenario (or "Pledges" Scenario)
Introduced in 2021, APS, an exploratory scenario, includes all recent major national announcements as of the end of August 2023, both 2030 targets and longer-term net zero or carbon neutrality pledges, regardless of whether these announcements have been anchored in legislation or in updated Nationally Determined Contributions.
STEPS - Stated Policies Scenario (or "Business as Usual" Scenario)
STEPS, also an exploratory scenario, reflects current policy settings based on a sector-by-sector & country-by-country assessment of the energy-related policies that are in place as of the end of August 2023, as well as those that are under development. The scenario also takes into account currently planned manufacturing capacities for clean energy technologies.
"Retired" IEA scenarios
"Retired" IEA scenarios are: CPS: "Current Policies Scenario" (used in the past to justify continued investment in fossil fuels); SDS: "Sustainable Development Scenario"; and NPS: "New Policies Scenario” (former STEPS name) [EP, 2021].
Differences btw STEPS, APS, NZE
While STEPS & APS see rising number of people exposed to heavily polluted air during the next decade, the NZE Scenario leads to dramatic reductions as set out in Sustainable Development Goal 3 (target 3.9). By 2050, around 2 billion fewer people breathe heavily polluted air. Figure 1 shows the "gaps" between APS & NZE. Figure 2 shows global GHG emissions & warming scenarios.
Figure 1: APS vs NZE scenario
Figure 2: Global GHG emissions & warming scenarios