Dynamic coastlines

Coastlines provide homes, livelihoods and food for a significant and growing proportion of the world's population. And when we move there, we inevitably inflict changes on the environment. But these are already highly dynamic places in their own right, constantly being remodelled by the effects of climate, sea level and events like storms. So, as he explains to Chris Smith, Cambridge University geographer Tom Spencer thinks we should see them, and our relationship with them, as a connected continuum, and plan accordingly...

Tom - Something like half the world's population now lives relatively close to a coast. So we have huge numbers of people at very high densities, very rapid increases of coastal populations, and therefore we have to start thinking about the coastal system, not as a biological or physical system, which then has some kind of human component tacked onto the end of that, but a system where you've got this intertwining of the biological, the physical and the human.

Chris - Of course coasts are extremely high energy places. You're dissipating wave energy, wind energy, and thermal energy there, aren't you?

Tom - Yes, you are. And one of the interesting things about coasts is that if they're left to behave in a natural kind of way, those coasts will shift, and they'll shift as those energy levels change and shift around. And one of the problems is when we have human populations living and working and interacting at the coast, they start to interfere with those natural processes.

Chris - And what's the consequence of that? Because if I had a house and it was falling into the sea, I'd want someone to interfere, or would I?

Tom - Yes, I think you probably would, but the difficulty is if you build a sea wall or if you build some kind of breakwater and you start to interrupt the natural movement of sediments, and of energy, you can get yourself into big problems. So I could give you a number of examples where people have moved to the coast because this, they've got a house with a beautiful beach in front of it, that back of that beach starts to erode. So they put in a sea wall and that prevents the translation of that energy to be dissipated in say, sand dunes behind the beach. That energy's gotta go somewhere. If it can't go backwards, it's gotta go down. And often what happens in that situation is the beach gets eroded away.

Chris - Of course there's an additional wrinkle, which is we also know that the climate is changing; so that, presumably, is gonna have another layer on top of what's naturally happening anyway?

Tom - Yes. And, and this is why I think working on the coast is particularly challenging. And it's complex because we have some progressive changes. So we have sea level rise, we have things like ocean warming, we have ocean acidification. But then overlying that we also have acute events, hurricanes and typhoons and storm surges. And what's interesting I think, is that the climate change debate has changed somewhat over the last few years. We used to just talk about changes in global temperature, changes in global sea level. We are now starting to talk a lot more about extreme events, about floods, about fires, about ocean heat waves. And it may be people are starting to think, well, perhaps the climate change signal is really starting to come through in the increased magnitude and the increased frequency of these extreme events.

Chris - What can we do about all these things then? Obviously, we're trying to keep the temperature down with climate change - whether we'll succeed or not, I mean, that's to be determined, isn't it - but what can we do then?

Tom - Well, you talked a little bit there about mitigation and I think if we can mitigate climate change, then we should obviously do that. But that looks like it's, it's very tough and very difficult as we know from these various COP meetings that we've had. So we really have to think about adaptation and in the kind of context that I'm working in, that means thinking about how can we work with nature So we know, for example, if we think about ecosystems as providing ecosystem services, effectively what's happening there is nature is providing those services for free. So we have a lot of talk about working with nature, nature-based solutions to climate change. And we can talk about planting mangroves, we can talk about restoring salt marshes, we can talk about extending mud flats to provide those kinds of natural buffers at the coast, which will dissipate some of this increasing energy that we are likely to get from higher sea levels and perhaps dissipate some of the energy we get from warmer oceans, which give us stronger cyclones. But the difficulty I think is that, while there's a lot of interest in that, and there are quite a lot of policy documents out there, which, you know, aim to deliver these kinds of nature-based solutions, the science is some way behind the policy. And I think one of the things that coastal scientists can do is to think about what are the design rules for working with nature? How can we take what we do know about these systems and apply those to these kinds of nature-based solutions to provide a better defense natural defense against these increasing energy levels at the coast.

Chris - The other thing I suppose that one has to bear in mind is that nature operates at the timescale over which nature operates! You can't just say to nature, I need a nature based solution tomorrow, so we really need to start today then, if we're going to go down this route, then strategists and policy-makers need to be implementing what you are saying now with 30 years in ahead in mind?

Tom - Yes, I think one of the difficulties is we, we've seen some of these nature-based solutions. I can give you examples where people have tried to recreate salt marsh for example, but they've tried to do it at too lower level and so the vegetation doesn't become established. You get left with a muddy pond. You know, there's a huge challenge here because engineering structures, of course you can, you can easily get the design criteria for a seawall of a particular height to particular lengths. And an engineer will tell you what sort of event that will protect you against. It'll give you the maintenance costs for something like that. And we don't have those kinds of rules at the present time for these natural systems. And that's what we need to move towards. And you are absolutely right. I think we have to accept that some of these things are gonna take time. But they're very, very powerful if they work. So salt marshes, for example, if you supply them with sufficient sediment, will track sea level rise if the sediment supply is, is provided. Obviously if you dam your rivers or you, you build defenses in front of your cliffs, then that sediment might not get into the the salt marsh system in the way that you'd want. So there are a lot of spatial interconnections here, which you have to think about as well. But, potentially, that's a much more sustainable solution than building a solid concrete wall, which you then need to rebuild 10, 20, 50 years later.