Podcast Summary: More or Less (BBC Radio 4)

Episode: RCP 8.5: Why did the climate change model get it wrong?

Date: December 13, 2025
Host: Tim Harford
Guest: Dr. Zeke Hausfather, Climate Scientist and Research Lead at Stripe

Overview

This episode of "More or Less" investigates how the widely used climate change scenario RCP 8.5 came to be seen as a "business as usual" prediction and why that was a mistake. Host Tim Harford and guest Dr. Zeke Hausfather unpack the origins, misuse, and eventual rejection of this model, highlighting the evolution of climate science and policy. The episode brings statistical scrutiny and plain language to a critical debate, revealing both the importance and the pitfalls of modeling the future.

Key Discussion Points and Insights

1. What is RCP 8.5? [(01:09)-(03:49)]

• RCP stands for "Representative Concentration Pathway," describing different possible levels of greenhouse gases in the atmosphere by 2100.
• 8.5 refers to the amount of radiative forcing (in watts per square meter), which reflects how much extra heat is trapped by greenhouse gases.
• Originally, RCP 8.5 was intended as a worst-case scenario—the 90th percentile of baseline cases if the world ignored climate policy.

“...the climate science community had been making a mistake for the last decade, and that mistake is that they had been conflating the worst case scenario with the most likely outcome.” — Dr. Zeke Hausfather [02:39]

2. The Role of Climate Modeling [(03:06)-(04:18)]

• Accurate temperature projections depend on assumptions about future emissions, which are hard to predict due to factors like population growth, economic development, and technological changes.
• Multiple scenarios (RCPs) were developed to cover a range of plausible futures.

“It depends on a huge amount of factors, on how much population grows, on how wealthy we get, on how fast different technologies develop and change in price...” — Dr. Zeke Hausfather [03:25]

3. Origins and Rise of RCP 8.5 [(04:18)-(06:27)]

• RCP 8.5 entered mainstream use around the early 2010s, ahead of key IPCC reports.
• It assumed unrealistically high future coal use—five times the 2010 global levels—and even projected synthetic fuels made from coal for cars.
• Early 21st-century emission trends and rapid coal expansion (notably in China) made such high-emission futures appear plausible.

“You know, global emissions that increased by a third in just the prior decade between 2000 and 2010. You know, China was building two new coal plants a day...” — Dr. Zeke Hausfather [06:49]

4. Misinterpretation as "Business as Usual" [(06:01)-(07:14)]

• Communication breakdown: The scenario was crafted as an extreme, not a baseline, but was widely used and cited as "business as usual."
• "Business as usual" became conflated with RCP 8.5 in scientific and public discussions, including major UN reports.

“That information was a little lost in translation between the energy system modelers and the broader climate science community. And so for a decade or so... there was increasing use of this high end scenario, RCP 8.5, as business as usual...” — Dr. Zeke Hausfather [06:01]

5. Why RCP 8.5 No Longer Fits Reality [(07:14)-(08:09)]

• Since 2015, trends have shifted: renewable energy has boomed, solar is now the cheapest new energy source, and global coal use has plateaued.
• Thus, the world described by RCP 8.5—dominated by coal, with cars running on synthetic petrol from coal—has become implausible.

“Obviously today, when solar is the cheapest form of new energy in the world, and almost all energy, or electricity at least, being added globally, is coming from renewables, it does seem crazy...” — Dr. Zeke Hausfather [07:19]

6. Updated Projections and Current Consensus [(08:09)-(09:12)]

• Barring drastic policy changes, emissions will likely be flat for decades, then decline, putting us between 2.5–3°C warming by 2100.
• A 3°C warmer world would still have major impacts, especially for regions with limited ability to adapt.

“Based on the estimates we have today... we'll probably have emissions be flat for the next few decades and then falling gradually by the end of the century... we end up somewhere between two and a half and three degrees warming.” — Dr. Zeke Hausfather [08:31]

7. Moving On, but Lingering Effects [(09:12)-(09:50)]

• Most serious organizations no longer use RCP 8.5 as the baseline.
• However, it still appears occasionally in research and public discourse.
• Dr. Hausfather stresses this is a natural, messy part of scientific progress.

“I see this as the natural way that science advances. It's a messy process, it is often slower than we'd like to, but it is ultimately self correcting.” — Dr. Zeke Hausfather [09:41]

Notable Quotes and Memorable Moments

• On the main modeling mistake:
  “...they had been conflating the worst case scenario with the most likely outcome.” — Dr. Zeke Hausfather [02:39]

• On how the world has changed:
  “...today, when solar is the cheapest form of new energy in the world... it does seem crazy...” — Dr. Zeke Hausfather [07:19]

• On adjusting scientific consensus:
  “I see this as the natural way that science advances. It's a messy process... but it is ultimately self correcting.” — Dr. Zeke Hausfather [09:41]

Important Timestamps

• 01:09 — Tim Harford introduces the topic and the climate model numbers.
• 02:39 — Dr. Zeke Hausfather explains conflating worst case and most likely outcomes.
• 03:53 — Explanation of what RCP 8.5 actually measures.
• 04:39 — Origins of RCP 8.5 in climate modeling.
• 06:01 — How RCP 8.5 got misused as "business as usual."
• 07:19 — Shift in global energy trends.
• 08:31 — Predicted global warming if trends continue.
• 09:41 — Reflection on scientific self-correction.

Tone and Language

Tim Harford maintains a clear, curious, and slightly wry tone, carefully unpacking complex issues in everyday language. Dr. Hausfather provides frank, jargon-light analysis, emphasizing nuance and the evolving state of scientific understanding.

Summary

This episode clarifies how RCP 8.5, once thought to represent humanity’s likely climate future, was always an extreme scenario. It became the default “business as usual” projection due to miscommunication, even as the world shifted towards renewable energy. The error—while influential—is now being corrected, highlighting both the fallibility and resilience of science, and reminding listeners that future climate risks remain serious even if the darkest projections no longer fit our path.