- So, we do lots of climate and environment stories, and we've run up against one concept all the time, sea level rise. This probably isn't news to you, but as the planet warms the average sea level is rising, and it's threatening coastal populations around the world. But we got tripped up by one thing. Here in New York, the sea level is rising one and a half times as fast as the global average. And we thought, how is that possible? How are different parts of the ocean at different heights? So we did what we always do, we called an expert. - Yes, I'm Dr. Andrea Dutton, and I work at the Department of Geological Sciences at the University of Florida. - And instead of that call clearing things up, it got so much weirder, so we're going to use New York City as an example, and explore sea level rise. Because honestly, it made us see the whole planet differently. (electronic music) So the big picture here starts with climate change. Temperatures are on the rise, and that does a couple global things to the oceans. - As you warm up the planet, we're going to melt a lot of the ice that's sitting on land, so the large ice sheets as well as the smaller mountain glaciers that go into the ocean. And you're going to heat the ocean as a whole, which causes that water to expand, and that causes some amount of sea level rise as well. - The easy analogy is pouring water into a bathtub. Turn on the faucet and the water level in the tub rises. But don't get too attached to that comparison. - Sea level rise is not like turning on the faucet to a bathtub and watching the water rise at a uniform rate. So it turns out that if, on the surface of the ocean today, the surface is not perfectly flat, there's a topography to the ocean surface. - That's one reason you can have more sea level rise in New York, oceans just aren't flat. There are hills and valleys of water out there. Seriously, look at this topographic map of the ocean surface. There are a few reasons for this, first... - Well turns out gravity is some of the problem, right? So if you have large ice sheets, or even large masses of continents and land, right, all that mass can attract the water to it. - The Earth's mass is not evenly spread around. It's a patchwork of continents, ice sheets, and other huge masses, and those masses exert a gravitational pull on everything around them, even water. In Antarctica for example, there's so much ice that its gravity actually pulls ocean water towards it, raising the sea level around the continent. But, as the ice in Antarctica melts, its mass shrinks, and its gravitational pull weakens. Which means the sea level goes down near Antarctica, but rises up as far away as New York instead. Another big cause of ocean topography are currents. And currents are driving up the sea level in New York too. One of the dominant currents in the Atlantic is the Gulf Stream. Andrea describes it like a conveyor belt, continually moving water away from the East Coast and up across the Atlantic, where it releases its heat and sinks to form deep water. The sinking action keeps the conveyor belt moving, but melting glaciers are a factor here too. As the ice melts, lots of fresh water gets added to the current. That lowers the density of the water, prevents it from sinking as readily, and jams up the whole Gulf Stream conveyor belt. - As the Earth continues to warm, we expect this rate of water moving north to slow down, we expect the conveyor belt to slow down. And as you do that, you'll be piling up water, literally, against the East Coast of the US. - So the actual amount of water amassed on a coastline can change from place to place. But there's one other big cause of sea level rise that has nothing to do with water, and that's land. - So the thing about sea level is it's always relative to land. - If coastal land sinks, the relative sea level there will rise, and vice versa. Land can move up or down for all sorts of reasons. Shifting tectonic plates in California, for example, can cause uplift there. - So as land rises, if this is the ocean, and the land is rising up, sea level is falling along that coastline. - Whereas parts of Louisiana are sinking. The Mississippi River is dumping so much sediment there that the land is compressing under the weight of it all. The land under New York is sinking too, but it's a little more complicated. - And I anticipated this question. The best way to do this is with a picture, so I have a picture for you. (laughs) I could have gotten up with my whiteboard, but my whiteboard's a mess, so I'm not gonna do that. Okay, so. - [Cory] Andrea's picture didn't come through great on video chat, so here's a reproduction. Basically 20,000 years ago, New York sat right on the edge of a huge ice sheet. It was the end of the last ice age, and the weight of the ice pushed down on the Earth's mantle underneath it. So much in fact that it forced some of the Earth out past the edge, which pushed that land up. New York City's land sat on that peripheral bulge. - You can think of this as if, when you go home and sit down on your couch, the same thing happens, right? - [Cory] Picture what happens to the cushion when you sit on it, the middle gets smooshed, but the edges? - [Andrea] It pushes out to the sides, right, 'cause that's the only place it can go. - But, the climate actually began warming about 20,000 years ago, the ice started melting and the whole process reversed itself. - [Andrea] And then when you stand up, what happens? It relaxes again back into it's natural position. - And today 20,000 years later, the land underneath New York is still slowly sinking, so the sea level is rising. Humans didn't cause this, it's a 100,000 year process. So, there you have it, New York City's sea level is a product of warming oceans, melting glaciers, Antarctic gravity, Atlantic conveyor belts, and the world's largest couch cushion analogy. Measuring sea level rise means a complex accounting of all of these factors and more. Andrea says scientists understand these concepts really well, what they're less sure of is the future. What happens if all CO2 we put in the air heats the planet further, melts more ice, and accelerates many of these changes? Whatever happens, the Earth will change, it always does. - It's not a static, fixed ball, the planet. (laughs) It is dynamic, it is a dynamic Earth. - Hey everyone, if you like what you saw here, be sure to check out all the other videos on our Verge Science YouTube channel, where we're putting out a new video every week, and which you should subscribe to, thanks.