By Roger Pielke Jr.
Tuesday, June 30, 2026
Major energy and climate decisions of the past 15
years—what infrastructure to build, what risks financial regulators require
banks to stress test against, how the projected costs associated with the
emissions of a ton of carbon dioxide inform regulations—rested on a foundation
that is now officially obsolete. Scenarios shape nearly everything in
climate policy. But very few people outside a small technical community
understand why they matter or who controls them.
A reference scenario—also known as a baseline or business
as usual—is used to project where the world is headed unless we choose to
change course. Other scenarios build off the baseline to illustrate how the
world might look if decision-makers implement policies that alter that expected
trajectory. The differences between the baseline and the policy scenarios are
typically used to assess the costs and benefits of alternative courses of
action.
In April, the only three officially recognized baseline
scenarios in international climate research—RCP8.5, SSP5-8.5, and SSP3-7.0—were
judged by the international scientific committee that selects and prioritizes
scenarios to be “implausible” due to their overprojection of future emissions
of carbon dioxide from the burning of fossil fuels. While the fact that our
expectations for future emissions have come down is welcome news, the
consequences of the scenarios’ retirement will be profound, and that is the
focus of today’s column.
A climate scenario does not forecast the future. It is a
structured “what if” exercise: explicit, internally consistent assumptions
about variables like population growth, economic development, how we produce
and use energy, and the extent to which greenhouse gas emissions and other
outcomes result from human and natural factors. The results of that exercise
are inputs into earth system models, which project how temperatures, sea
levels, and extreme weather might change in the future. Policymakers, regulators,
and courts then act on those projections and the policy and economics work that
builds from them.
A small network of researchers, operating under the
Coupled Model Intercomparison Project (CMIP)—an initiative of the World Climate
Research Programme—decides which scenarios the modeling groups use and how
they are prioritized in research. Once a scenario is developed and prioritized,
it is then used in almost all projection-based research that is analyzed by the
United Nations’ Intergovernmental Panel on Climate Change (IPCC). National
climate assessments, central bank stress tests, lawsuits, and infrastructure
design standards can depend to a significant degree on this research and the
IPCC findings.
But everything changed in April, when
the CMIP published a new scenario framework, known as CMIP7, for the next IPCC
assessment report. RCP8.5, SSP5-8.5, and SSP3-7.0—the three scenarios that
served as the body’s official reference scenarios across two IPCC assessment
cycles spanning almost two decades—are gone, having been judged to be no longer
consistent with the real world, especially in their energy technology
assumptions. Here are five of the most significant implications.
Climate projections need recalibration.
National climate assessments in the United States, United
Kingdom, Germany, Canada, Australia, Japan, and the Netherlands all
used either RCP8.5 (Representative Concentration Pathways) or SSP5-8.5
(Shared Socioeconomic Pathways) as their reference scenario. So did the World
Bank’s Climate Change Knowledge Portal, which supplies climate projections to
more than 100 client countries. The reference scenarios were the basis for more
than a decade of climate impact projections; those projections informed
planning assumptions embedded in national energy and infrastructure policy.
The primary driver of the overprojection was a coal
assumption. RCP8.5 assumed a five-fold expansion of global coal use—more coal
than the planet’s proven reserves contain—and assumed commercial deployment of
coal-to-liquid technology to replace oil at a scale never achieved anywhere on
Earth. The now-retired reference scenarios projected about 5 degrees Celsius
global temperature increase by 2100.
Initial projections of the new medium scenario suggest
about 2.5 to 2.7 degrees Celsius of warming by the late century. That gap
between 2.5 and what we previously expected—about 5 degrees Celsius of
warming—is huge. Recalibrating national assessments to reflect actual
trajectories will be both important and difficult.
Scenarios used in policy need explicit plausibility
evaluations.
The retirement of the implausible reference scenarios
exposes a structural problem in climate research intended to inform policy: For
15 years, the scientific community ran climate projections against scenarios
whose energy assumptions failed a basic plausibility test, with no systematic
process to catch or correct what we have known is a major flaw in scenario
design.
The committee attributes
the retirement to “trends in the costs of renewables, the emergence of climate
policy and recent emission trends,” framing implausibility as something that
developed over time. That framing reverses the actual history, which was well documented by
Justin Ritchie and Hadi Dowlatabadi almost a decade ago.
Reasonable people can disagree about where to set a
high-end stress test scenario to explore the robustness of policies and
infrastructure in the event of low-probability, high-impact outcomes. Different
scenario users will have different risk tolerances and other perspectives on
how to create a “worst-case” scenario. However, the CMIP7 high scenario—an exploratory scenario at the top
of their scenario set—still assumes a massive increase in coal consumption,
which may once again be implausible. Whether anyone checks the scenarios for
plausibility remains as opaque as ever.
The economics of climate impacts need to be
reconsidered.
The longtime reference scenarios’ retirement forces a
reconsideration of the projected impacts of climate change. Damage projections
rely on warming projections, which are built upon reference scenarios.
Projected damages will necessarily be substantially lower
under realistic scenarios, all else equal—a world warming by 2.7 degrees
Celsius will produce a different outcome than one rising by 5 degrees Celsius.
The quantity of emissions that require mitigation is also much lower than the
old baselines assumed, with corresponding implications for the projected costs
of deep decarbonization from the projected baselines. Similarly, social cost of carbon estimates embedded in federal
regulation and damage functions in financial risk frameworks need revision.
The Paris Agreement targets should be revisited.
Another consequence of the new set of CMIP7 scenarios is the elimination of the scenario consistent with the Paris
Agreement’s stretch goal of limiting warming to 1.5 degrees Celsius above
pre-industrial values by 2100. The World Climate Research Programme confirmed last month that its new, low-emissions scenario
no longer keeps warming below 1.5 degrees Celsius.
More actionable targets exist. For instance, I’ve long
argued that a global coal phase-out agreement—including specific dates,
verification mechanisms, and real enforcement—holds greater promise for climate
policy progress than an aspirational temperature target. Coal accounts for more
than 40 percent of all energy sector carbon dioxide emissions. As of 2023, 84 countries had plans to move away from coal, but together
they accounted for only about 30 percent of current coal-fired generation. A
framework built around coal elimination would be verifiable, negotiable across
countries with very different coal dependencies, and directly tied to the
emissions reductions that actually move the temperature numbers.
Regardless of what new targets are ultimately agreed
upon, it is now clear that the world needs new goals in international climate
policy.
Expect legal challenges.
The reference scenarios may have been initially created
with scientific research in mind, but they quickly took on important roles in
many policy settings. The Network for Greening the Financial System, an
international coalition that coordinates climate stress-testing for more than
140 central banks, originally built its “Hot House World” physical risk
scenario to be consistent with RCP8.5. That scenario has been used
in stress testing at the European Central Bank, the Bank of England, the
Reserve Bank of New Zealand, the Bank of France, and the U.S. Federal Reserve.
Any regulation, court judgment, or law grounded in the
now-retired scenarios is vulnerable to a court challenge, and that challenge
has a basis in mainstream, consensus science.
The larger lesson.
Policies that hold up across a range of futures make more
sense than policies optimized or even based upon a single scenario. Robust
decision-making does not require alarming or extreme projections. Prediction is
a hard business, but durable energy policy can account for what we do not know.
And holding tight to implausible scenarios risks compromising not just
scientific integrity but also the trust in scientific institutions that durable
climate policy requires if it is to survive over the many decades needed to
achieve deep decarbonization.
The CMIP made the right call in helping the world to move
past implausible scenarios. The difficult work of adjusting projections,
renegotiating targets, and working through legal and regulatory consequences
now falls to everyone else.
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