More or Less: Numbers of the Year 2025 (BBC Radio 4)

Date: December 27, 2025
Host: Tim Harford

Episode Overview

In the annual tradition of More or Less, Tim Harford and a panel of expert guests reflect on some of the most significant and telling numbers to define 2025. The episode explores major global and scientific events through their numerical markers, including US tariffs at historic highs, explosive growth in China’s solar energy, and the cosmic curiosity of interstellar visitors to our solar system. Each featured expert presents their “number of the year”, accompanied by clear explanations that reveal how these numbers shape our understanding of politics, economics, energy, and the universe.

Key Discussion Points & Insights

1. U.S. Tariffs at Historic Highs

Segment: [01:10] – [05:13]
With: Thomas Sampson, London School of Economics

• Peak Tariff Rate:

  • The average U.S. tariff rate peaked at 28% in April 2025, reflecting President Trump’s “reciprocal tariff” policy.
    • Thomas Sampson (02:23): “My number of the year is 28%, which was the peak in the average U.S. tariff rate following President Trump's reciprocal tariff announcements in April.”

• Tariff Fluctuations and Methodologies:

  • By October, the average effective tariff fell to 18%, still much higher than the 2% of January 2025 and the highest since the 1930s.
  • Calculating “average” tariffs is nuanced:
    • Trade-weighted average (using 2024 import shares) vs.
    • Actual revenue average (dividing tariff revenue by imports—yielding just 9.8% in July).
    • Thomas Sampson (03:23): “There isn't a single tariff rate that the US imposes on imports... There are thousands of individual tariffs... The way that this number is computed is by averaging across products, but using import shares in 2024 as weights.”

• Interpretation:

  • The lower actual revenue average is partly due to behavioral changes (e.g., less French cheese imported after new tariffs) and the lag in tariff collection.
    • Thomas Sampson (05:07): “...at least so far, it's not collecting quite as much tariff revenue as one might have expected.”

2. China Surpasses a Solar Milestone

Segment: [05:20] – [08:05]
With: Hannah Ritchie, Researcher and Writer

• The Terawatt Threshold:

  • In 2025, China surpassed 1 trillion watts (1 terawatt) of installed solar power.
    • Hannah Ritchie (05:20): “My number of the year is 1 trillion. So that's the number of watts of installed solar power that China surpassed this year, which is equivalent to what we'd call one terawatt.”

• Contextual Power Comparison:

  • That much solar can produce about 1,800 terawatt-hours annually—almost equal to the entire electricity use of India.
    • Hannah Ritchie (05:45): “1 trillion watts of solar power produces about 1,800 terawatt hours. To give some context... India in total uses about 2,000 terawatt hours.”

• Significance for Climate:

  • Solar’s growth is rapid and cost-reducing, but:
    • China’s total electricity demand is also surging—equivalent to adding “a UK sized grid every single year.”
    • For the first time, renewables are meeting all new electricity demand, marking a pivotal shift.
      • Hannah Ritchie (07:05): “This looks like it could be the kind of first year where it's basically built enough to cover all of that additional electricity demand.”

• Fossil Fuels Still Dominate:

  • Despite expansion, solar accounts for about 10% of China’s electricity. Coal, though declining, remains predominant.
    • Hannah Ritchie (07:39): “Solar is around 10% of electricity demand... but it's important to note that... China's electricity mix is still relatively dominated by fossil fuels and primarily coal.”

3. A Third Interstellar Visitor

Segment: [08:05] – [09:54]
With: Kathryn Heymans, Professor of Astrophysics, University of Edinburgh/Astronomer Royal Faculty, Scotland

• Discovery of 3I Atlas:

  • The number “3” marks the discovery of the third interstellar object (“3I Atlas”) to pass through our solar system.
    • Kathryn Heymans (08:14): “I have chosen the number three to celebrate the discovery of the third interstellar object in our solar system.”

• Why It Matters:

  • Interstellar comets provide a rare glimpse into distant regions of our galaxy—astronomers rely on such visitors for clues about other star systems.
    • Kathryn Heymans (08:43): “We rely on these interstellar visitors to bring with them a memory of where they've come from.”

• Origin and Age:

  • 3I Atlas was identified by tracing its path: it arrived from another star in the galaxy, not merely the edge of our own solar system.
    • The comet is astonishingly old, at over 7 billion years—making it 2 billion years older than our solar system.
      • Kathryn Heymans (09:42): “We think it's come from a very old part of the galaxy, which means that it is over 7 billion years old, which makes it 2 billion years older than anything else in our solar system.”

Notable Quotes & Memorable Moments

• On Tariffs:

  • Thomas Sampson (02:23): “My number of the year is 28%, which was the peak in the average U.S. tariff rate following President Trump's reciprocal tariff announcements in April of this year.”
  • Tim Harford (03:19): “But calculating this average is not as simple as you may think.”

• On China’s Energy Transition:

  • Hannah Ritchie (07:05): “It's adding about a UK sized grid in every single year to its electricity demand.”
  • Hannah Ritchie (07:39): “...still, China's electricity mix is still relatively dominated by fossil fuels and primarily coal. Although this share... has been decreasing over time.”

• On the Third Interstellar Object:

  • Kathryn Heymans (08:43): “We rely on these interstellar visitors to bring with them a memory of where they've come from.”
  • Tim Harford (09:54): “That's it for your numbers of the year. We'll be back with more in 2026. And until then, goodbye.”

Timestamps for Important Segments

• Introduction & Context: [01:10] – [01:54]
• U.S. Tariffs: [02:23] – [05:13]
• China’s Solar Boom: [05:20] – [08:05]
• Third Interstellar Object: [08:14] – [09:54]

This episode continued the tradition of using data to illuminate trends and global events, with a clear, accessible approach to some of the year’s most significant numbers. The guests provided insight not just into what these numbers are, but why they matter—and how to interpret them in a rapidly changing world.