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What will rising sea levels do to the world's megacities?

So you know how sometimes people are like, “this is a good news, bad news kind of a thing”? This isn’t one of those times. It’s bad news that comes with more bad news behind it. It’s a bad news sandwich.

This infographic, made for us by the nice people at Statista using Climate Central data, how sea level rises are going to affect a selection of the world’s cities. We’ve focused on the megacities – that is, those with populations of 10m or more – partly because these are where people are mostly likely to be affected in large numbers, and partly just to manage the size of the map.

The bubbles, and the numbers attached to them, represent the share of the cities’ 2010 populations that would be below sea level in the event of particular increases in global temperature. The size of the light red bubble represents the percentage of the urban population that could be submerged in the event of a 2°C increase in global temperature (a degree of increase which feels pretty much inevitable); the dark red one is how much people will be affected by a 4°C increase in global temperature (which we could probably still avoid if we tried, but let’s be honest, we probably won’t try).


The first bit of bad news is that these figures are very obviously awful. London gets off pretty lightly – but a 4°C increase in global temperature would still put 13 per cent of its population under water. In New York, which is bigger, a full quarter of its population lives in areas that may well be underwater.

But these numbers are as nothing towards some of those Asian cities. In Mumbai and Calcutta it’s half; in Shanghai, a city of 23m people, it’s more than three-quarters.

In 2010, Dhaka, the capital of Bangladesh, contained an estimated 32m people; in the event of the sort of sea level rise that’s expected to follow a 4°C increase in global temperature, 38 per cent of them will be underwater.

In other words, what this map shows is that tens of millions of homes are very possibly going to be underwater at some point before the century is out.

Now you may be thinking, well, could be worse. Okay, Asia’s in real trouble here (sorry Asia) but much of the rest of the world is getting off relatively lightly.

Ha. No. Remember, this graphic only shows cities of 10m or more. Some of the world’s megacities are relatively safe from sea level rises – Moscow and Kinshasa are both well in-land; Sao Paulo isn’t, but it is a long way above sea level.

But a lot of other large cities are excluded because they’re not quite big enough to make the cut. A 4°C increase in global temperature would put 26 per cent of San Francisco’s population under water. In Tampa, Florida, it’s 40 per cent. Barisal, a Bangladeshi city of 7m people, would see 88 per cent of its population submerged by the same increase in temperature.

Perhaps the most terrifying of all, though, are the figures that relate to the Netherlands, a name which literally means “lower countries”. In the event of that 4°C increase in temperatures, 98 per cent of the populations of Amsterdam and the Hague will be below sea level.

Now – below sea level doesn’t mean “inevitably underwater”, of course. There are mitigation measures cities can take.

But one of the most important mitigation measures is almost certain to be “moving a lot of people to somewhere else”.

We’re in trouble, is basically what we’re saying here.
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