I was watching a report in the newspaper about coastal cities that have to build walls to protect their beaches. Here in Rio we're already seeing the effects of it, with the tides getting higher and flooding streets near the beach. What exactly is the scientific reason behind all this?
You're right, it's not actually that fast when you compare it to what it was like millions of years ago, but I agree the pace has definitely picked up a lot. The main problem is global warming. When the temperature rises, ocean water expands - it's like when you heat water from the tap and it takes up more space. Plus, the ice from the polar ice caps and glaciers is melting and falling into the ocean, actually adding more water. These two processes together account for most of the rise we're seeing right now.
The thing is, this is happening way faster than in the past because CO2 concentration in the atmosphere skyrocketed over the last 150 years or so, mainly because of fossil fuels. The ocean could handle slow changes, but when everything changes this fast, coastal cities find themselves in trouble. In Rio it's particularly annoying because you've got the entire South Zone vulnerable, plus areas like Barra that already suffer from frequent flooding. Walls and containment structures are more of a band-aid than an actual solution - ideally you'd stop the warming, but then you get into discussions that go beyond pure science.
What a lot of people don't mention is that two processes are happening at the same time: first, the water is actually rising because ice (like Greenland and Antarctica) is melting with global warming, but second - and this is important - the ground in some coastal regions is sinking, which makes the whole situation even worse.
In Rio especially, certain areas already had problems with natural subsidence, so when you combine that with the actual sea level rise, everything together becomes way more serious.
Basically the ocean is rising and the ground is sinking at the same time, which is why the streets flood much more easily than you'd expect from just the water alone.
You know that besides the warming itself, there's a lot of water being released from melting ice caps? Yeah, so while people focus on the ocean warming (which makes the water expand), there are tons of ice from Greenland and Antarctica melting and literally falling into the water, which raises the level.
Basically you combine all that stuff people are mentioning and it gets even worse when you think about the ground also sinking in some coastal zones, like here in Rio itself where there's ground subsidence that speeds up the effect! So it's not just the water rising, it's also the land sinking, and that creates a kind of pincer that makes everything way more critical for coastal cities.
The guys are catching the main details but there's one really practical thing that makes it easier to understand everything: take a plastic bottle, fill it with water and put some floating ice in it. When the ice melts (the stuff that's floating, like Arctic sea ice), the water doesn't rise because the ice was already taking up space. Now take another bottle and put a small container full of ice standing upright inside the water - when that melts, the water's gonna rise for real. It's basically the difference between the ice from land ice caps (Antarctica, Greenland, glaciers in mountains) that ADDS new volume when it melts, versus the Arctic that's already floating. And then you add the thermal expansion that the other guys mentioned - hot water takes up more space - and you've got ground subsidence that's sinking in certain places, including here in Rio. Put all that together and it's a mess for people living on the coast.
What's going on is most news reports only focus on the "warming," but really there are at least three processes happening at the same time and each one contributes something to make things worse. The pace really has picked up in the last 30, 40 years. You see it clearly here in Rio when it rains heavy combined with high tide - it floods streets of people who live near the beach in a way that didn't happen nearly as often before. This is turning into a real structural problem for the city itself.
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