In late April, the United Nations Intergovernmental Panel on Climate Change acknowledged that Representative Concentration Pathway 8.5, better known as RCP8.5, was an āimplausibleā emissions scenario.
That admission should have caused a major reckoning in climate science and policy circles. For more than a decade, RCP8.5 was frequently presented as a business-as-usual pathway, one in which global temperatures could rise by more than 4.3 degrees Celsius by 2100. Those projections were then used to support warnings about severe economic, environmental, and social consequences.
RCP8.5 also became an influential tool in climate negotiations. Governments were repeatedly urged to adopt treaties, regulations, and energy restrictions based in part on the assumption that the world was headed toward the scenarioās extreme emissions levels unless fossil fuel use was rapidly reduced.
The problem is that doubts about RCP8.5 were not new. Many researchers had warned for years that it was highly unlikely and should not be treated as the expected future. Even so, the scenario continued to appear throughout peer-reviewed studies, media coverage, policy debates, legal arguments, and public campaigns. It was often discussed less as a distant worst-case possibility and more as the likely result of failing to impose aggressive climate policies.
The Heartland Institute criticized RCP8.5 as early as 2017, arguing that the scenario was not merely unlikely but potentially impossible. A more detailed critique appeared in the 2019 publication Climate Change Reconsidered II: Fossil Fuels.
So, in response to the IPCCās belated recognition, one might borrow a line from Die Hard character John McClane: āWelcome to the party, pal.ā
Better late than never.
Unfortunately, the admission came only after RCP8.5 had already shaped an enormous body of research. By early 2020, researchers were reportedly publishing about 20 studies per day that relied on the scenario. Some estimates suggest that more than 8,500 papers using RCP8.5 appeared during an 18-month period.
There were strong reasons to question the scenario from the beginning. One of the most obvious was its assumption that global coal use would increase roughly fivefold by 2100. Critics argued that the world may not contain enough economically recoverable coal to support that level of consumption.
The scenario also relied on population estimates far above current projections, including the possibility that the global population would reach between 12.6 billion and 14.5 billion people. There is little evidence that humanity is presently on track to reach those levels.
Despite these concerns, RCP8.5 remained deeply embedded in climate research and public policy. What is now being treated as a new realization was, according to critics, understood by many people involved in the debate years ago. Instead of being retired or clearly labeled as an extreme hypothetical case, the scenario became the foundation for studies, laws, regulations, financial decisions, court cases, educational materials, and public messaging.
This approach reflects a broader concern about the relationship between science, advocacy, and politics. The late climate scientist Stephen Schneider once described the tension scientists face when trying to communicate complicated risks to the public. Scientists, he said, are expected to explain doubts, qualifications, and uncertainties honestly. At the same time, those seeking political action may feel pressure to simplify their message, emphasize frightening outcomes, and avoid discussing uncertainty in order to attract public and media attention.
That tension becomes dangerous when advocacy begins to overshadow scientific restraint. Science depends on openness, skepticism, and a willingness to acknowledge uncertainty. When worst-case possibilities are presented as probable outcomes for political purposes, public trust is weakened.
The American Energy Institute, led by Jason Isaac, recently launched a website called They-Knew.com. The site is intended to create a public record of how RCP8.5 was used by governments, advocacy groups, educators, courts, financial institutions, and other organizations.
According to Isaac, researchers had publicly identified RCP8.5 as unrealistic years ago, including in major scientific journals. Yet institutions continued to rely on it. He argues that the continued use of the scenario, even after its weaknesses became widely known, represents a major failure of scientific integrity.
The website also identifies organizations and public figures that used RCP8.5 to justify laws, regulations, and economic policies. Critics blame those policies for lost jobs, increased public debt, wasted resources, higher energy costs, and reduced consumer choice.
Perhaps the most lasting damage, however, was psychological. RCP8.5-based projections were often communicated to children as though they described an almost certain future. Many young people were left believing that the world was approaching environmental collapse because of their familiesā energy use. That messaging contributed to anxiety, guilt, anger, and fear about the future.
Now that RCP8.5 is increasingly recognized as implausible, institutions that relied on it should reconsider the conclusions they drew. Research based heavily on the scenario should be reviewed, and its assumptions should be clearly disclosed. Policymakers should also examine whether treaties, regulations, and programs were justified by realistic projections or by an extreme scenario that was never likely to occur.