Why are rising sea levels a problem?

06 November 2018

Question

Why are rising sea levels a problem?

Answer

Chris on Twitter asked "why are rising sea levels and melting ice caps a problem?" Chris Smith put this one to climate scientist James Pope from the British Antarctic Survey...

James - People think about the obvious, about the inundation of coastal towns and cities, but some of the less well discussed issues are around freshwater sources. So there are three types of ways we get fresh water. If you live somewhere like the Himalayas, you get it from monsoon rains and glaciers. If you live somewhere like the UK, you get it from reservoirs or aquifers. Aquifers are water storage underground in permeable rocks. What’s possible is that near coastal regions, if the sea level rises high enough it can actually flow and spill over from the sea into the aquifer, and essentially poison the aquifer by introducing a saline component, making it no longer suitable for freshwater.

Other issues, my mum lives up in Southport, and they have a lot of salt marshes up there, so that’s land that is at or just below sea level that gets inundated by the brackish water, so saline-fresh water mix, so you get an entirely different type of eco structure there. But obviously, if that inundated rice paddys, or standard arable lands here in the UK, you just lose the ability to grow crops in that are. But it's always worth saying that sea level rise is probably the least of our issues in terms of climate change. Under its most extreme scenario, we are looking at only a maximum of one metre, by 2100, sea level rise. Because the ice sheets, so that's the bits of ice that's actually on ground currently, it just takes a long time to melt. I mean if you put some ice cubes on the desk right now, they take a long time to melt and ice sheets take even longer, you’re talking several thousands of years even to get some of the more vulnerable ice sheets to completely collapse, so it's probably not a huge issue immediately.

Chris - So it's not so much the actual water movement that is the issue, it's what goes with it. It’s the mechanism that's driving the ice to melt that’s causing a rise in global temperatures, that's going to have other knock on consequences directly for the inhabited land area, rather than just a direct impact of sea level rise.

James - Yeah I would suspect that most coastal regions would see impacts from things like extreme weather phenomena. You could argue that even half a metre rise makes a coastal area more susceptible to storm surges from hurricanes or even just bad weather like we see here in the UK. So it’s not completely write-offable, but certainly it’s not some sort of dystopian flooded future.

Chris - And how much sea level rise are we looking at, probably? You said the worst case scenario might be a metre or so, but how much are we looking at, probably, if we go following the trajectory we appear to be on? And let's assume we end up with about a 1.5 degree rise across this century. What sort of sea level rise is that going to translate into?

James - So the 1.5 degree rise, the recent report that the Intergovernmental Panel on Climate Change produced on this, they estimated it would be somewhere in the region about 0.6, with an uncertainty range of about 0.3 to 0.7 of a meter. So there's an element that we're sort of committed to a certain extent. But I think from memory, a lot of that is mainly due to just the warmer planet means that actually the size of the ocean just expands, the water expands in size, so it’s thermal expansion less than actually the melt of ice sheets and glaciers.

Chris - Because the poles have melted in the past haven't they in Earth's history? We have had a planet in the past where there was no ice at either pole.

James - Yes. And conversely when we had the last glacial maximum, so about 20,000 years ago, sea level was as much as 120 meters lower. So in some respects you've had a hundred and twenty meters of sea level rise. We could, if we melted everything that was left, get another 70, but it would take an awful long time to melt everything that was left.

Chris - So that's sort of reassuring and not reassuring, all at the same time.

James - I guess it depends where you live...

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