Numbers of the year 2025

Summary

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

Date: December 27, 2025
Host: Tim Harford

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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.

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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.”

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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.”

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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.”

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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.”

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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]

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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.

Transcript

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[Tim Harford]

Thanks for downloading the More or Less podcast from the BBC World Service and our annual tradition of taking a look back at the numbers of the year with me, Tim Harford. As numbers go, 2025 has been a big year for them, from climate temperature records and the 267th Pope to and unrelated, the viral question of whether 100 men could take on one gorilla. So we've gathered together some friends of the programme to give us their top numbers of the year. First up, we look to the United states, where in 2025 White House wings came down and global import tariffs went up. Way up. President Trump's big, beautiful year of tariffs certainly had its fluctuations. The Supreme Court is even reviewing their legality as we speak. The level of tariff imposed varied from country to country, industry to industry. So how do you choose just one number to sum up tariffs imposed across all countries? I'll leave that job to Thomas Sampson from the London School of Economics.

[Thomas Sampson]

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]

Finding this overall average tariff rate provides a good indicator for trying to work out what's going on with America's trade policies and economy. But as Thomas says, 28% was the peak. The number has actually fallen since April.

[Thomas Sampson]

Now, some of those tariffs never ended up being implemented, so the Average effective tariff rate by the end of October had declined to 18%, according to the Yale Budget Lab. But that's still substantially higher than the 2% level in January of this year. And the current 18% rate is still the highest average U.S. tariff rate since the mid-1930s.

[Tim Harford]

But calculating this average is not as simple as you may think.

[Thomas Sampson]

There isn't a single tariff rate that the US Imposes on imports from different countries and of different products. There are thousands of individual tariffs on each individual product coming from each different country. And the way that this number is computed is by averaging across products, but using imports shares in 2024 as weights. So if the US imported a lot from China in 2024, we give a bigger weight to tariffs on imports from China than on other countries.

[Tim Harford]

Taking the average this way, it peaked at 28% in April. This reflects the full cost consumers would have paid if, if the tariff had been imposed on last year's pattern of imports. There is, however, another way we could get to an average, because that pattern of imports will change in response to the tariff itself. When a big tariff is imposed on, for example, French cheese, American consumers will buy less French cheese. So an alternative average tariff looks at the tariffs collected in this new fromageless world.

[Thomas Sampson]

Another way to try and compute the same number is to look at how much tariff revenue the US Actually collects and divide that by the total value of imports to get an average tariff rate based on revenue collected. When we do that, and the latest data I have for that is from July, the average tariff rate is 9.8%.

[Tim Harford]

That's about 1 third as big, much lower. Why? Partly because, well, that French cheese effect, but also because some of these tariffs just aren't being collected yet.

[Thomas Sampson]

And so, at least so far, it's not collecting quite as much tariff revenue as one might have expected.

[Tim Harford]

Thank you, Thomas Sampson. Now, from percentages to wattages, I am.

[Hannah Ritchie]

Hannah Ritchie, and 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.

[Tim Harford]

A terawatt sounds like a big number. But looking at installed solar power can also be a bit misleading when we consider how much power is being used. So first, why does Hannah think it's an important number?

[Hannah Ritchie]

I think it really signifies how quickly solar power has been built out globally, but actually how much China is now dominating and really rapidly moving on solar power. Now, 1 trillion watts of solar power produces about 1,800 terawatt. Hours. But to give some context of how big that number is, India in total uses about 2,000 terawatt hours. Right. So the solar output from that 1 trillion watts of solar is almost equivalent to the total electricity generation of the whole of India from all sources.

[Tim Harford]

Suffice to say that if you've installed almost enough solar capacity to run India, the world's most populous country, then you have installed a lot of solar. Solar photovoltaics have followed a learning curve. The more the world makes solar panels, the cheaper they get. And of course, the cheaper they get, the more people want to buy them. There's a huge boom in solar panels going on roofs and huge solar farms across China soaking up those rays. However, China's electricity demand is also growing. So can they build renewables fast enough to meet additional demand for electricity and also push coal and fossil fuels out of the electricity mix?

[Hannah Ritchie]

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. I mean, to give some context on how much China's electricity is growing, it's adding about a UK sized grid in every single year to its electricity demand. And actually the fact that it's now meeting all of that additional demand from new solar and wind actually speaks to the fact of how much and how quickly they're now deploying these sources.

[Tim Harford]

So is China getting most of its electricity from solar energy?

[Hannah Ritchie]

Solar is around 10% of electricity demand, so more of that comes from wind, some more of that comes from nuclear and hydro. But it's important to note that still, China's electricity mix is still relatively dominated by fossil fuels and primarily coal. Although this share of electricity coming from coal has been decreasing over time as these new sources have increased.

[Tim Harford]

Is it a bird? Is it a plane? Is it an alien spaceship? No, it's our final number currently winging across our solar system.

[Kathryn Heymans]

Hi, I'm Kathryn Heymans. I'm a professor of astrophysics at the University of Edinburgh and the Astronomer Royal Faculty, Scotland and I have chosen the number three to celebrate the discovery of the third interstellar object in our solar system.

[Tim Harford]

That interstellar object is 3. I atlas a comet. Need a reminder of what a comet is? Well, NASA defines one as a cosmic snowball. Frozen gas, rock and dust.

[Kathryn Heymans]

As astronomers, we can't send probes outside our solar system to see what life is like outside in the rest of our galaxy. And so we rely on these interstellar visitors to bring with them a memory of where they've come from. And so this is why Astronomers have been so excited and mapping the path the journey of three I atlas as it goes through our solar system so they can learn more about what things are like across the galaxy.

[Tim Harford]

It's only the third we've ever discovered which which has come from outside of our solar system. It was first spotted in July of this year.

[Kathryn Heymans]

Astronomers looked at its trajectory and they realised that it hadn't come from just the outskirts of our solar system. It had come from another star within our own galaxy.

[Tim Harford]

Seeing as we can't ask the comet its age outright, that would be rude. We use good scientific know how to work out how long it's been around.

[Kathryn Heymans]

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.

[Tim Harford]

That's it for your numbers of the year. We'll be back with more in 2026. And until then, goodbye.

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