The Truth Behind Climate Activist’s Favorite Model

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 “implausible” 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 scenario’s 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 IPCC’s belated recognition, one might borrow a line from Die Hard character John McClane: “Welcome to the party, pal.”

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.