How come sea levels are rising faster in some places than others?

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1:10Hello and welcome to CrowdScience from the BBC World Service the show that's powered by your curiosity I'm Anand Chagatia and our question this week was sent to us from a beach in Australia Hi there, it's Alison here from Sydney, Australia Today, I'm down at Cronulla, which is one of Sydney's southern beaches. It's a scorching hot day, the sky is blue, the surf is up, and I'm just one of many people down here today taking a dip in the ocean to cool off. Recently, I heard a report about oceans, which said that nations in the Pacific are experiencing sea level rise at a faster rate than the global average.

1:57But to me, that doesn't make sense. How can there be a global average? Surely all the oceans of the world are really just one big ocean. So how can parts of it be rising faster than other parts? So there you go, CrowdScience. Can you please explain how it's possible for sea levels to be rising at different rates in different locations around the world? Thanks, Alison. I have to say I'm kind of stumped by your question. Surely you're right and all the oceans are constantly sloshing into each other so that any sea level rise in one part of the world soon balances out to the rest. To understand what's going on, first let's cover some basics.

2:45What's actually causing sea level rise? So there are two key factors that drive sea level rise or let's say changes in sea level because, I mean, sea level can go down as well as up. This is Jonathan Bamber, Professor of Earth Observation and Glaciology at the University of Bristol in the UK. So thermal expansion of the oceans. As they warm, they expand. And that's about half of present-day sea level rise. And the other half is due to changes in mass of the oceans. And that is almost entirely driven by melting of glaciers and the ice sheets that cover Greenland and Antarctica. And so how much of the world's water is actually locked up in the ice caps and in glaciers?

3:33So the ice sheet covering Antarctica in particular is huge. I mean, Antarctica itself is one and a half times the area of Australia. It's a huge continent and it's almost entirely covered by ice. And so if you take Antarctica and Greenland together, that is somewhere around 80 % of the entire freshwater supply of the planet is just locked up in those two ice sheets. If you melted Antarctica and Greenland tomorrow, you would raise global sea level by about 65 metres. Whoa. I mean, that's a terrifying fact, but it just gives a sense of what all the water in the world is doing and, I guess, drives home how pressing and how sort of scary sea level changes are from climate change.

4:19That's the potential. Nobody is suggesting that those ice sheets are going to sort of dump all that ice into the oceans any time soon. But if we look at, in particular, the satellite record, we see a general increase in mass loss from both ice sheets. We'll be hearing more from Jonathan later. To answer Alison's question, we've travelled to a country in the Pacific, which is the part of the world that inspired her to get in touch. It's a place where sea levels are rising much faster than the global average, the island nation of the Philippines. Our first stop is a place called El Nido. It's a quiet tropical paradise famous for its coral reefs and pristine beaches.

5:02These are particularly beautiful, with white sand, sapphire waters and lush green cliffs rising up along the coastline. So we've just driven to the beach in El Nido, the shoreline, and there are huge cliffs and mountains surrounding us and boats everywhere. It kind of, it feels like paradise really, but for the people that are living here, it maybe isn't because just a few metres away from the shoreline, There are lots of buildings, houses, hotels, restaurants. And the sea, as we can see, is pretty close. It's getting closer. Getting closer. So right now we are looking for the changes because of climate changes.

5:44So we're going to go over there and ask the folks here about their already experience. This is our guide, Jordan. He's a park ranger helping to manage and protect El Nido's incredible biodiversity. But today he's helping us speak to locals who are directly affected by sea level rise. As we walk along the seafront, he points out some palm trees that show just how much the coastline is changing. It's not just that the water level here is increasing. Rising seas are also eroding the beach by washing away the sand. The sand has already been washed out by the ocean. I mean, these are massive, really tall coconut trees.

6:28It must be at least 20, 30 metres tall. You can see how the roots of this tree, which we can see the tendrils, they're just sticking out into the air. And before, they would have been intertwined with all of the soil and there's loads of rocks and coral that they're clumping together, but now 50, 60 centimetres is exposed by the sea. Yeah, it's because of the changes of the ocean. These exposed tree roots, which would have previously been hidden underground, are a sign of how much the landscape here is shifting, something which residents have witnessed for themselves. Not far from the palm tree, a couple of small boats are beached on the sand.

7:13One of the owners is Rami Rodriguez, who runs boat tours of the surrounding islands. I have been here for 11 years and every day I see the water sea level changes. Now it's coming closer to our homes, just like last year. When it's low tide, the water goes further away from our homes, but when it's high tide, it comes closer here. Is the water reaching your house? Yes, there are months, like last December, the water reached our house. We used to live here, at this place, but we were told we need to move further away because the water is reaching our home. But there will come a time that we also need to move again because the water is coming really closer to our homes.

7:58So you've already had to move once and you might have to move again. Would you ever consider moving inland, away from the sea? Yes, we will have no choice but to relocate further, more inland, because we have no choice because the water will really reach our homes.

8:15We walk further along the beach, past a couple of bars and houses, to a small beachfront massage parlour. Its owners, Christy and Jenny, are sheltering inside from the heat of the sun. If the water comes here inside our business, then we have no choice but to close the business because we have no other space to go. We have no other place to build our structure, our business. So we might close if ever comes a time that water will enter our business. Do you worry about the future? Definitely we are afraid, there's fear in us because we will lose our jobs not just for one day, for two days it might be for months not only that we will lose our jobs but the things here, our equipment here will be damaged so everything will be very difficult for us if ever water comes here inside our business Visiting El Nido I can see for myself how quickly sea level rise is happening and how much it's affecting the people who live here But how fast exactly?

9:17And how does that compare to the rest of the Philippines and the rest of the world? To learn more, we make our way to the University of the Philippines in Manila to meet someone who can explain the science behind rising sea levels. I am Chorina Lin Repolo. I am an assistant professor. And I am an expert on physical oceanography. But currently I'm also sitting or serving as the deputy director for research and development of the Marine Science Institute. The Philippines is a country with over 7 ,000 islands and a huge amount of coastline. Does that mean that sea level rise is a particular worry for a country like the Philippines?

9:56Yeah, so sea level rise is very important because as an archipelago, we have a lot of islands and that translates to a very long coastline. And most of our coastal communities live along these coastlines, low-lying coastal areas that really are significantly affected by even small changes in sea level. We've been traveling around the Philippines and we went to a place called El Nido and there the sea levels are rising. We spoke to local people about how it's affecting them. Do you have any figures for what the numbers are? So El Nido has a sea level rise of about nine millimeters per year and in comparison to Metro Manila, which is about 13.13 millimetres per year.

10:44And for a context, if you compare it to the global sea level, global sea level rise is about 3 to 4 millimetres per year. So those areas are experiencing like 3 to 4 times increase compared to the global sea level rise that we have. So it's not just that the Philippines is experiencing sea level rise well above the global average. even within the Philippines, there's variation. The seas around Manila are rising even more quickly than those around El Nido. But how do oceanographers like Chorina actually measure that? Here in the Philippines, we started monitoring the sea level, but more for safety, for ports, for navigation and for shipping.

11:29That started in the early 1900s, But eventually, because now we tend to experience and believe the reality that sea level change is here, it's increasing as time goes by, tide gauges becomes one instrument that we use to monitor sea level rise. And what is a tide gauge? How does it work? So a tide gauge is like, you know, it's an instrument that you install near the water. So it measures the variability of sea level. So it has like, you know, a floater. And then once it moves up and down, it measures the height. And then you can have the sea level as time goes by. In El Nido, we spoke to some people who have buildings or businesses or homes on the seafront.

12:16And they have had to move their homes back a few meters away from the sea. And that's challenging enough, I think, dealing with people in a place like that. But in somewhere like Manila, which is a huge city with millions of residents and lots of buildings, that's going to be a huge challenge, right? Just moving further back into the land. There's no more space. There's no more space moving back. I think the best way to address is to adapt. Move all your establishment away from the coastal areas. Manila is one of the world's largest and most densely populated cities. and it's very low-lying, with some areas only just above, or even below, sea level, which means that any increases are keenly felt by the millions of people who call this place home.

13:02Charina is keen to introduce us to a place that's doing work across the Philippines to protect people from the effects of climate change, the Oscar M. Lopez Centre. Hi, sir. Hi, Dr. Lascaux. I'm Anand. Anand, yeah. We've come to meet the centre's executive director, Dr. Radell Lascaux, a climate scientist and an author of several reports of the Intergovernmental Panel on Climate Change. He told us how their work is trying to help people in the country become more resilient to sea level change. A key to this is early warning. So we have beefed up our Met office with newer equipment and more people, more training, more expertise, so we can predict ahead of time if a typhoon is coming, how strong, how much rainfall.

13:50And part of our effort is to model what's happening with our seas and the rising sea levels. And we translate these models into tools that people can use. We call it C-Narios. And this is a web-based tool and people can see what could happen to their cities when sea level rises and therefore see the risks, the vulnerabilities, and the exposure that they will be subjected to with sea level rise. Thinking about the future can be scary in the times that we live in, even though we maybe have no choice but to face the reality that we're in. I'm just wondering, on a human level, does this stuff keep you up at night?

14:28Well, as a scientist who has been involved in all of this for maybe almost 30 years now, it's kind of half pool, half empty glass for me. I've seen the change over the past 25, 30 years. People are more aware. But at the same time, the reality is we're not doing enough. The science is also very clear. We will breach 1.5 degrees centigrade, the Paris Agreement, and even 2 degrees, the higher limit, is probably going to be breached. And so I think we need more political commitment by policymakers. We need more concerted global action. There is a window. It's fast closing, but still open if we do something now.

15:10But sadly, I don't see that kind of political commitment at the international level to really slow down the emissions of greenhouse gases. But we must not give up because change is possible. I have seen it. But at the same time, I am realistic enough to recognize that change may take time. But for the younger people who may be listening out there, don't give up. Keep on pushing. And I think change could happen sooner than later. Listener Alison got in touch because she heard that sea levels are rising faster for Pacific countries than the world average. So far, we've learned how hotter temperatures are driving this change.

15:50We've seen that in Manila, sea levels are rising at almost four times the global rate, and that even across the Philippines, there's huge variation. But to tackle this question, we didn't just want to speak to scientists. We wanted to meet people who are living with the reality of rising oceans. And we'll be hearing more from them next.

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17:12You're listening to CrowdScience from the BBC World Service the show that rises to the challenge of answering your science questions week in, week out I'm Anand Chakatea and in this episode we're exploring how sea levels can be rising at different rates around the world Surely the oceans all flow into each other so there's only one sea level Even though that sounds like a reasonable theory, we've seen it isn't the case. And in places like Manila, sea level rise is happening alarmingly quickly. To see the impact of this change for ourselves, we're heading across the city to a part of town near the waterfront called Parola Compound.

17:53It's an area with high levels of poverty, where recent floods have been particularly damaging. We're being accompanied there by a member of the Coast Guard, whose offices are nearby.

18:09As we wait for our escort, we take the chance to ask the Coast Guard staff if they felt the effects of sea level rise here. I am Commander James Barandino. I am the Acting Deputy Chief of Coast Guard Staff for Marine Environmental Protection. Actually, during rainy days, we're experiencing a lot of flooding and the floods are getting deeper and deeper nowadays. It's not the same as usual. And so you are noticing that sea level rise is affecting the coastlines. Is that changing the nature of the work that you do? Is that presenting new challenges or meaning that you have to respond in different ways?

18:49Nowadays, we experience a lot of rescues during rainy days and flooding. Flooding in different areas. and we're not only rescuing in the seawater but also inland. We set off from the Coast Guard offices to Parola compound. This part of Manila has always been prone to flooding but with climate change it's becoming more frequent and more severe.

19:24so we've just driven down towards the the shoreline in manila we've driven through the industrial port through some security gates we've been let through by the coast guard and lots of people with guns lots of the coast guard staff are having their lunch here but there are also some informal settlements of people who live right by the waterline and they're really being affected by rising sea levels because the tide often comes onto land anyway here and over the last few years that's getting a lot worse so hopefully we're going to be able to go and meet people who live here. We meet 44 year old Robert Gonyo who was born here and has felt the worsening impacts of sea level rise.

20:08How have you been affected by the sea level rises in the last five or ten years? During typhoons, water really rises here, but even there's no typhoon at night. When the water level rises, when it's high tide, it reaches our home. The water goes up to my knee inside our house. Does that cause a lot of damage to your house, to your property? Yes, my house has been damaged three times now and it's being washed away by the seawater. That sounds hard. Has it been difficult for you? It's really difficult because I still need to pay for the water and electricity bills and I'm just a tricycle driver here.

20:53So you grew up here, you've lived here for 44 years. Have you noticed the problem getting worse since you were a child? When you were a child, was the problem bad? When I was a kid, when there was typhoon, water really reaches our house. But what I've observed is that as I grew up, even if there's no typhoon or there's no heavy rain, water really rises through the years and really reaches our homes. Sea level rise is part of the grim reality of climate change for many people living in Manila. Even the Coast Guard have had to move offices here because of flooding. So how can the Philippines be facing faster sea level rises than the global average?

21:36Back at the University of the Philippines, Charina has an answer. So there are places where sea level is rising and some are falling. And it's because of the geographic setting of the Philippines. Philippines is highly tectonic, so our islands are moving. So if you have like a sinking land, then it will enhance the sea level rise. And if you have like uplifting islands, then it will like cancel out the sea level rise. So that's very important. Okay, so maybe a way to think about it is that even though possibly the oceans are flowing into each other, at any one piece of coastline, what's happening to the land itself is also going to change how the rising seas will balance out.

22:28Yes, that's why we don't rely much on the global average of the mean sea level rise. But what we recommend is to have like a local assessment, because every locality have different response on the sea level rise. So does that mean that there could be some places where, even though globally the seas are rising, actually in some places sea levels will be falling? Yes. So there are areas that we measured from tide gauges that are also falling, because of that. We also have very urban areas wherein there's a lot of groundwater extraction and that also contributes to the falling of the land, the vertical motion of the land that enhance the sea level rise.

23:13So if you extract a lot of groundwater, so you're removing water excessively from the land. And that way, because there's no more water underneath, so that land will tend to subside, going and falling from their original position. So there you go, Alison. Sea level rise isn't just about the sea. It's also about what's happening on land. In places where the land is sinking, sea level rise will be accentuated. This is especially true in places like Manila, where groundwater has been removed, which also causes the earth to sink down. But is that the whole story? Do the oceans themselves also behave differently in different locations on Earth?

23:59Well, it turns out they do, as oceanographer Jonathan Bamber, who we heard from earlier, explains. One of the things that is important to distinguish is the difference between what's called relative sea level rise and absolute sea level rise. Now, relative sea level is if you're at the coast, it's the change in the water level with respect to the land. And that can change because of changes in land elevation, primarily actually land subsidence. As we extract groundwater, the land drops. And so you can have no change in the absolute sea level, you know, no thermal expansion, no increase in mass.

24:43and you can get a change in the relative sea level if the land is sinking. So that would appear like sea level is, or what it would be, sea level is going up at the coast simply because the land is going down. Okay, so that's one aspect of sea level rise, you're talking about relative sea level change to land. And then is there a sort of way of just measuring just the sea level just in an absolute sense as well? So that's the other way of framing the sea level problem, which we call absolute sea level. And that is typically measured by satellites. And that absolute sea level is literally just the change in volume of the oceans and the change in mass of the oceans with respect to some reference surface.

25:29And what I mean by that, that is some fixed artificial surface. So it's almost like you've got a kind of virtual measuring ruler. Imagine always sort of sticking a ruler that you might see at a swimming pool sort of in the middle of the ocean and it's sort of fixed rather than it's on the land and the land is sort of going up and down and then giving you changes in a relative way. Exactly. What you have to think about with this ruler is that it's sort of fixed in a sense with respect to space, you know, with respect to the centre of gravity or centre of mass of the Earth. So it doesn't move in a virtual sense in space.

26:08Whereas if you had a ruler that was at the coast, for example, like a tide gauge, which is exactly what a tide gauge is, it's going up and down because the land is going up and down at that site. Using this way of measuring sea level, which ignores the effect of land movement, reveals that the oceans are rising at different rates across the planet. And amazingly, part of the reason for that is gravity. If you melt ice over Greenland and Antarctica, which is what is happening right now, and you stick it in the oceans, you actually change the gravity field of the Earth. That is the strength of gravity at different locations on the surface of the Earth.

26:53The gravity field is a function of how much mass there is in any one place. So, for example, if you fly over Mount Everest, the strength of gravity increases because you've got this huge piece of rock there. It's exactly the same with ice. And when you melt that ice and you put it in the oceans, you're spreading that mass over the whole of the oceans. And so what happens is in Greenland, and this is what's happening right now, the gravity field gets a bit weaker because the amount of mass over Greenland is going down. And what that means is the sea level around Greenland is actually going down because of the change in gravity due to that effect.