字幕表 動画を再生する 英語字幕をプリント Welcome to Calipatria, a desert town near California's Salton Sea. This is the city with the lowest elevation in the western hemisphere, 55 metres below sea level. And here, we've also got a visual demonstration of just what that means: because that white spindly thing is a flagpole, and the American flag up at the top is flying above sea level. If it wasn't for all the mountains and hills between here and the Pacific Ocean, then we'd be wet, but that flag would stay dry. Maybe. We're dry because no rivers drain into this basin. The Salton Sea over there only exists because some engineers screwed up in 1905 and diverted a good bit of the Colorado River in there for a couple of years. That water is steadily getting saltier and more polluted as it slowly evaporates, and California's got to deal with that over the next couple of decades. But how do we know we're 55 metres below sea level? What is sea level? In theory, it is the average height of the sea around the Earth. But there are some problems with that. Problem 1: the Earth isn't a sphere. I mean, it's vaguely a sphere, don't get too excited, Flat-Earthers. It's a bit squashed. The force of its own spin makes it bulge by a few kilometres around the equator. And when you're defining height above sea level in metres, that's a lot. So geographers solved that by defining the Earth as an oblate spheroid. Basically a smooth squashed sphere, just in fairly precise mathematical terms. But then there's problem 2: the Earth isn't one uniform lump of rock. Bits of the mantle are more dense than others, so gravity isn't quite the same everywhere on Earth's surface. Which means there's a bit more water sitting over those bits. And by a bit more, I mean anything up to 80 metres more. These aren't small amounts, that could be the difference between this city being above and below sea level. And to complicate things even more, mountains are big enough that their gravity attracts the ocean, just a little. So during the 20th century, as scientists and Cold War governments started to need a global reference system for spacecraft, and missiles, which were basically the same thing back then, they spent a long time measuring Earth's gravity. The result is the Earth Gravitational Model, megabytes in size, which asks, in purely mathematical terms: if this land wasn't here, but its gravity was, and if there was exactly enough water to fill in the gaps... where would all that water sit? And how far away would it be from that theoretical smooth squashed sphere? The model was finished in 1996. It is available to this day on a web site that seems unchanged since 1996, and it's what your phone and GPS use. There are more recent, more precise models, but they are hundreds of megabytes in size, and everyone's agreed that now we've got a standard, let's not change it and confuse everyone. Also, we'd also have to update for sea level rise, whether it's best-case centimetres or worst-case metres. So saying we're "55 metres below sea level" is mostly just symbolic. But so are many things in the world, and I'd like to think we still put some trust in them.