Sizing up cities

Summary

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More or Less: "Sizing up Cities"

BBC Radio 4, January 27, 2012
Host: Tim Harford
Notable Guests: Ruth Alexander, Rich Green (Urban Geographer), Professor Kamweng Chan (University of Washington), Dr. Chris Smith (Cambridge University/The Naked Scientists), Charlotte MacDonald

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Episode Overview

This episode delves into the complexities behind measuring and defining the world’s largest cities—a seemingly simple question which reveals profound statistical ambiguities. With the backdrop of China’s urbanization milestone (more than half its population now living in urban areas), the program investigates competing definitions of ‘city’, debunks commonly misquoted statistics about city sizes (especially in China), and explores the literal weight of the world in the final segment.

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Key Discussion Points & Insights

1. What Is the “World’s Largest City”?

(00:12–05:40)

• No Standard Definition: Ruth Alexander and expert Rich Green outline the three main ways cities are defined:

  • Municipal definition (legal city)
  • Urbanized area (built-up portion)
  • Metropolitan area

• “Each list is mixing these.” – Rich Green (01:27)

• Tokyo Tops the List, But...

  • Tokyo is generally considered the world’s largest city, with 36 million people—only 8 million in the city proper, the rest from surrounding towns and cities including Yokohama.
  • “The core of Tokyo city has a population of only about 8 million. The 36 million figure comes from adding those 8 million to the numbers living in the nearby port city of Yokohama...along with the inhabitants of 86 other towns and cities.” – Ruth Alexander (02:07)

• What About China’s Super-Cities?

  • Frequently-reported stats about Chinese cities are misleading.
  • Example: Chongqing is often called the world’s largest city (30 million), despite much of that population being rural.
  • “...what China calls a municipality or city is better understood as a province.” – (Ruth paraphrasing Professor Kamweng Chan, 03:20)
  • “Chongqing will probably be somewhere only around 6 or 7 million. A few million in that range. And compared with the 30 million, there's a huge difference.” – Professor Kamweng Chan (04:02)
  • Expert correction: Even scholars and major organizations regularly misuse these definitions.

• Shanghai as China’s Biggest City:

  • Common stat: 20 million; likely closer to 16 million, says Professor Chan.
  • “Everyone just loves to think Chinese cities are bigger than they are.” – Ruth Alexander (04:39)

Notable Quote

• “They were trying to paint a really gloom and doom picture about these mega cities, these cities that go in the range of 20, 30 million, which is not true because they are just picking up a wrong definition.” – Professor Kamweng Chan (04:59)

• Cities Over a Million in China:

  • Oft-cited figure: 160 cities, but analysis suggests only 40 or so (“There may be more now, but experts...say there's still nowhere near 160.” – Ruth Alexander, 05:25)

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2. Is the Earth Getting Heavier or Lighter?

(05:40–09:05)

• The Listener’s Question: Has Earth become heavier or lighter since the Industrial Revolution?

• Charlotte MacDonald investigates with Dr. Chris Smith (Cambridge University/The Naked Scientists).

• Basic Principle:

  • Human construction doesn’t add weight—just rearranges existing atoms.
  • "When we build something, all we're doing is rearranging atoms...doesn't matter whether it's building buildings or building people, you're still using the same matter which the Earth is endowed with." – Dr. Chris Smith (06:36)

• What Actually Adds or Subtracts Mass?

  • Adds:

    • Space dust: "We've got one source of mass coming in, dust raining in from space..." – Dr. Chris Smith (07:02)
    • Global warming: Slight increase due to energy/mass equivalence; about 160 tons/year.
    • Total added: ~40,000 tons/year.

  • Subtracts:

    • Rocket launches: Not significant, since most fall back.
    • Loss of light gases: Earth loses about 3 kilos of hydrogen/sec and also helium to space. This outweighs mass gained from space dust and global warming.
    • Total loss: Earth ultimately gets about 50,000 tons lighter each year.

• Perspective: The net loss (50,000 tons/year) is "just less than half the weight of the Costa Concordia, the Italian cruise liner that ran aground recently. But really, as a proportion of our planet, it's a tiny amount of mass." – Charlotte MacDonald (08:39)

Memorable Moment

• “Just naught point. Naught, naught, naught, naught, naught. No, no, no, no, no, no, no, no, no.” – Charlotte MacDonald’s humorous way to express how tiny the percentage of loss is (08:54)

• “1%, which, without wishing to get too technical, is a very small number.” – Tim Harford (09:05)

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Timestamps for Important Segments

• 00:12–01:27: Introduction; why defining city size is complex; Rich Green explains city definitions.
• 02:07–02:55: Tokyo’s population numbers broken down; explanation of metropolitan aggregation.
• 03:01–05:40: Chinese statistical anomalies; Chongqing and Shanghai populations; global megacity misconceptions.
• 05:40–09:05: Weighing the world; interview with Dr. Chris Smith; final numbers and perspective.

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Notable Quotes

• "Each list is mixing these."
  — Rich Green on city definitions (01:27)

• "The core of Tokyo city has a population of only about 8 million..."
  — Ruth Alexander (02:07)

• “...what China calls a municipality or city is better understood as a province.”
  — Ruth Alexander paraphrasing Professor Kamweng Chan (03:20)

• "Chongqing will probably be somewhere only around 6 or 7 million...compared with the 30 million, there’s a huge difference."
  — Professor Kamweng Chan (04:02)

• “They were trying to paint a really gloom and doom picture...which is not true because they are just picking up a wrong definition.”
  — Professor Kamweng Chan (04:59)

• “When we build something, all we're doing is rearranging atoms...you're still using the same matter which the Earth is endowed with.”
  — Dr. Chris Smith (06:36)

• "The big ones on our losses are actually gases. And if you go to high up in the atmosphere, physicists have shown that the Earth is losing about 3 kilos of hydrogen gas every second."
  — Dr. Chris Smith (08:25)

• “Just naught point. Naught, naught, naught, naught, naught. No, no, no, no, no, no, no, no, no.”
  — Charlotte MacDonald (08:54), on how minuscule the Earth’s annual weight loss really is

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Tone & Style

The episode features classic “More or Less” wit and skeptical inquiry, with Tim Harford’s signature approach of demystifying numbers and unveiling the truth behind widely-circulated statistics. There’s a light, engaging tone, especially during exchanges between Tim and his colleagues, and in Charlotte’s playful discussion with Dr. Chris Smith.

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Summary

• Defining city size is a statistical minefield, as shown by wildly varying lists and population figures for world cities, especially when comparing countries with different administrative conventions. Tokyo is typically “the biggest,” but only by global metropolitan area standards.
• Chinese city statistics are frequently misunderstood: Chongqing’s actual urban core is far smaller than often reported, while even Shanghai’s population is routinely overstated.
• The Earth's mass changes little over time: additions from space dust and energy are outpaced by the slow loss of light gases, resulting in Earth getting fractionally lighter each year—a figure so small as to be virtually meaningless in practical terms.

This episode is a must-listen for anyone curious about global statistics, urbanization, and the pitfalls of headline-grabbing numbers.

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Transcript

[BBC Announcer]

Thank you for downloading from the BBC. For details of our complete range of podcasts and our terms of use, go to bbcworldserveys.com podcasts.

[Tim Harford]

You're listening to a download from the BBC. This is more or less the mathematical icing on the cake of life. We make an edition of the program for BBC Radio 4, but this is a download of a special edition broadcast on the World Service. Hello and welcome to More or Less on the BBC World Service. We're your weekly guide to the numbers in the news and in life. And I'm Tim Harford. In this program, we make a Herculean attempt to weigh the whole world. But first we thought we'd start with a simple quantitative question to ease our way into the programme, inspired by the recent announcement that China's cities are growing so fast that for the first time, more than half the population live in urban areas, which we pondered are the world's largest cities. Ruth Alexander's here. Ruth, what's the biggest city in the world?

[Ruth Alexander]

I'm afraid, Tim, it's just not that simple. If you go on the Internet, you can find lots of candidates. Seoul, Chongqing, Shanghai, Tokyo. The problem is, it all depends on what you mean by city. Rich Green, an urban geographer from Northern Illinois University in the United States, has studied various lists of top 20 cities to work out the why they're all so different.

[Rich Green]

We tend to think of three concepts of the city. One being a municipal definition, the legal city, if you will. And a second, we call the urbanized area or the built up portion. A third is what we call a metropolitan area. Each list is mixing these.

[Ruth Alexander]

Even the most authoritative list from the United nations was comparing apples and pears, as he put it. And that's because it takes population data directly from countries and they all define their city areas a bit differently from one another.

[Tim Harford]

Ok, so it's difficult because there's no international standard. Come on, you must know which is the world's biggest city, Ruth.

[Ruth Alexander]

I do. The general consensus is that it's Tokyo with a population of about 36 million. But even that isn't a straightforward case. The core of Tokyo city has a population of only about 8 million. The 36 million figure comes from adding those 8 million to the numbers living in the nearby port city of Yokohama, which just happens to be Japan's second largest city, along with the inhabitants of 86 other towns and cities.

[Tim Harford]

Sorry, The Japanese get to claim the world's largest city because they, they add together their two largest cities and then add another 86 other places. How's that fair?

[Ruth Alexander]

Well, because in practice, if you travel between all these places, there'd be no gaps. It's just one big, humongous built up area.

[Tim Harford]

Okay, I'll let them off. So what do we know about these Chinese cities, given there's so much talk about how fast they're growing?

[Ruth Alexander]

Well, China's population statistics actually can be particularly misleading, and people do love to be misled by them. Here's Time magazine.

[Tim Harford]

Virtually overnight, Chongqing has become the largest city, not only in China, but in the world.

[Ruth Alexander]

Now, neither of those facts are true. It's not the largest city in the world or, or in China, but people often think it is. Why? Well, a man who's made a career out of correcting people on Chinese population statistics is Professor Kamweng Chan of the University of Washington. In the United States, what China calls a municipality or city is better understood as a province. He says many of the 30 million people who live in Chongqing are actually agricultural workers living in a rural setting. And he says if you try to track travel from downtown Chongqing to the outskirts, it could take you up to two days.

[Professor Kamweng Chan]

So you can see that this cannot be possibly called a city, because when we call a city, the general understanding is that we have to talk about a commuting zone. So Chongqing will probably be somewhere only around 6 or 7 million. A few million in that range. And compared with the 30 million, there's a huge difference.

[Tim Harford]

He sounds like a smart guy. Where did you find him, Ruth?

[Ruth Alexander]

Well, actually, he found us. He wrote to a friend of mine at the BBC to point out he'd made a mistake about the size of a Chinese city.

[Tim Harford]

Well, fair enough. Even the BBC makes mistakes. While we have him, what is the biggest city in China?

[Ruth Alexander]

It's Shanghai, commonly thought to have a population of 20 million. But Professor Chan thinks 16 million is actually a better estimate. He says everyone just loves to think Chinese cities are bigger than they are. He's even had to correct fellow experts at a world conference on global megacities of the future.

[Professor Kamweng Chan]

They were trying to paint a really gloom and doom picture about these mega cities, these cities that go in the range of 20, 30 million, which is not true because they are just picking up a wrong definition. So you can see that that groom and doom scenario will definitely need to be revised.

[Ruth Alexander]

Another thing that's commonly said about China is that there are more than 160 cities with a population above a million.

[Tim Harford]

And don't tell me the numbers are wrong.

[Ruth Alexander]

They're wrong. Rich Greene looked into this a few years ago and found only about 40 cities had populations of more than a million people. There may be more now, but experts like Professor Chan say there's still nowhere near 160.

[Tim Harford]

OK, well, thank you, Ruth. Well, we've sized up the world's biggest cities now for the world itself. We've had this question from Mr. J.P. hobson, who wrote to us via More or [e-mail]. has the world become heavier or lighter in the past 250 years or so since the Industrial Revolution? Good question. A little more science than statistics, but we aim to please. Charlotte MacDonald's been weighing this one up.

[Charlotte MacDonald]

It turns out that a group calling themselves the Naked Scientists have already tackled this question. They're from Cambridge University in the UK and like trying to explain science to the public. When I visited their offices, I was surprised to find they were clothed. But I started by stating what seemed the obvious, that over the last 200 or so years, humans have constructed lots of buildings and the population has exploded. Surely this has made the world heavier? Dr. Chris Smith gave an emphatic no.

[Dr. Chris Smith]

When we build something, all we're doing is rearranging atoms. And the atoms have come from one place, an ore in the Earth, and they've been assembled into molecules in another place, the bricks and mortar that make the building. And it doesn't matter whether it's building buildings or building people, you're still using the same matter which the Earth is endowed with.

[Charlotte MacDonald]

But he told me there is new matter arriving on Earth all the time.

[Dr. Chris Smith]

We've got one source of mass coming in, dust raining in from space, which either is because of asteroids that have broken up or things that never formed into a planet and it's drifting around. Then we've also got global warming. NASA have calculated that the Earth is gaining about 160 tons per year because the temperature of the Earth is going up. If we add energy to the Earth system, the mass must go up.

[Charlotte MacDonald]

So the dust and the warming amount to about an extra 40,000 tons a year. But what about the fossil fuels we've been burning? Does that offset this weight gain? Well, Chris points out that the Earth will get slightly lighter because the fuels have lost chemical potential energy. And if the energy in something goes down, then its mass must go down, in this case by five tons a year. But in the grand scheme of things, because all we're doing there is turning solids and liquids into gases, the molecules are more dispersed, but they still have pretty much the same mass. I was skeptical but he put on a little experiment, letting air into a vacuum to show that a gas does indeed have mass. Oh, there we go.

[Dr. Chris Smith]

So if you watch the scales, you'll see we're watching the numbers physically climbing. 3 grams, 4 grams, 5 grams.

[Charlotte MacDonald]

OK, but what about all those rockets and satellites we launch into space? Chris discounts this, saying most of them will fall back to Earth. But he says there is something else going on.

[Dr. Chris Smith]

The big ones on our losses are actually gases. And if you go to high up in the atmosphere, physicists have shown that the Earth is losing about 3 kilos of hydrogen gas every second. And the other very light gas that this happens to is helium.

[Charlotte MacDonald]

So when all the sums are done, albeit on the back of an envelope, Chris reckons the Earth is getting about 50,000 tonnes lighter a year, which is just less than half the weight of the Costa Concordia, the Italian cruise liner that ran aground recently. But really, as a proportion of our planet, it's a tiny amount of mass to be shedding. Just naught point. Naught, naught, naught, naught, naught. No, no, no, no, no, no, no, no, no.

[Tim Harford]

1%, which, without wishing to get too technical, is a very small number. Thank you, Charlotte. And you can download our podcast via our website, bbcworldservice.com more or less but if you're listening via good old fashioned radio in a city, in the countryside or even in Chongqing, we'll be back at the same time next week. More or Less was presented by me, Tim Harford of the Financial Times. The producer was Ruth Alexander and the editor was Richard Varden. For more information, go to the website bbcworldservice.com More or less

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