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Diving Deeper: Sea Level Rise

Episode 34 (Dec. 1, 2011)

HOST: Welcome to Diving Deeper where we interview National Ocean Service scientists on the ocean topics and information that are important to you! I'm your host Kate Nielsen.

Today's question is...Is sea level rising?

There is strong evidence that global sea level gradually rose in the 20th century and is currently rising at an increased rate. Sea level is projected to rise at an even greater rate in this century. The two major causes of global sea level rise are thermal expansion of the oceans, which means the water is expanding as it warms and the loss of land-based ice due to increased melting.

To help us dive a little deeper into this question, we will talk by phone with Billy Sweet on sea level rise. Billy is an oceanographer with NOAA's Center for Operational Oceanographic Products and Services. Hi Billy, welcome to our show.

BILLY SWEET: Hi Kate, glad to be here. Thank you.


HOST: Billy, first I want to clarify a definition with you as I've heard of both sea level rise and coastal inundation. Do these mean the same thing?

BILLY SWEET: Well, they don't actually mean the same thing. Sea level rise, or sea level to begin with before we talk about the rise component is generally referred to in terms of a relatively longer term mean. Usually we talk on the order of about a month, so it's difficult, but when you live at the coast you see the tides going up and down each day and maybe you see the spring tides during the month where the high tides are a little bit higher, that is not really what we really want to talk about when we talk about sea level, so usually we talk about a mean level over the course of a month. So, it's something that's sort of hard to see firsthand unless you really have a measurement device that can say over the last month, here was the average height. So, in terms of sea level rise, we're talking about this sort of mean rise that the tides then play off of our storm surges, or on top of.

Coastal inundation then would be that variability that you see. You could have coastal inundation with a spring tide, you could have a storm surge that comes and inundates a certain low-lying area so it's usually sort of the wiggles, the ups and downs. In this case inundation is going to be the higher variability above the mean sea level that an observer would tend to see. And they happen over a range of frequencies. It could be something that lasts for a week, where all your high tides are just higher than normal due to external forcing or it could happen just with a strong, sudden wind storm.

HOST: Thanks Billy. So, back to our initial question...are sea levels rising?

BILLY SWEET: Yes, indeed they are rising. Of course, there are some caveats to that as well. On a global level, yes, sea level is rising and we know this from measurements of satellite where we know that on average all the world's oceans are rising. However, there is variability, spatially, so from one coast to another coast, it's not a bath tub that's just slowly rising everywhere at the same rate. There are some areas where sea level is actually dropping slightly. But those are usually very rare and it usually has to do with the land that is rising out of the water or it could be due to prolonged weather effects, which would actually make sea level appear to be dropping, but the short-term responses that when you look over 20, 30, 50 years, even in these locations, sea level is indeed rising, but you do have some year-to-year variability.

But to answer the question, yes, sea level is rising and that's generally true for the majority of the world's coastlines.

HOST: Billy, what role do tides play in sea level rise?

BILLY SWEET: Well, tides actually don't really factor in to a calculation of sea level or sea level rise. As you know, if you live at the coast, you have a high tide and then you have a low tide. And so, if you take the average of the two, you tend to get very close to where this mean sea level would be.

Tides, though, are important because as sea level begins to rise or even if we have some short-term rise, let's say over the course of a few months, you tend to have a bump up in elevation in your mean sea level due to, let's say some prevailing winds. For instance, it's been a very windy spring and the winds have been out of the northeast and so it's causing sea levels to slightly be higher, let's say along the Mid-Atlantic coast of the United States. Well, then you have a high tide, that high tide will be higher than normal, so it may come up a lot higher on your dock or it may actually start getting up to your primary dunes. And so, the tides will actually build or play upon the background sea level. So, they don't actually factor into the calculation of the change in sea level, but they make any changes in sea level become more noticeable.

HOST: Are there other types of factors, other external factors then that can cause an increase in sea level rise?

BILLY SWEET: There are other factors and they play a large part actually in many of the signals that we see. The main external factor that really drives a signal in the sea level series is vertical land motion and that varies widely across the United States. Particularly within the Gulf of Mexico and areas in the Chesapeake Bay, land is actually sinking and the sinking actually causes an apparent rise in sea level, but nonetheless it's a rise. There are parts of the country, for instance in Alaska, that land is actually rising up out of the water and this is due to the removal of ice, it could be some near glaciers that are retreating, but for the most part in the lower 48, in these areas where land is actually sinking due to compaction of sediments or it's a slow adjustment due to the removal of large ice sheets that we had 20,000 years ago at the last glacial maximum.

Another factor that's very important is circulation changes. Normally we think of sort of a mean circulation, but really nothing ever follows the mean and so if you have an increase let's say in the Gulf Stream Transport or a decrease, well this will actually reflect into a sea level response at the coast.

HOST: Billy, how is sea level rise related to climate change?

BILLY SWEET: Well, that's a great question. And I think that is really what is on the minds of many who live along the coast. When you look at sea level over the course of thousands of years, in this case, let's talk about over the last 25,000 years of so. Around 20,000 to 22,000 years ago during the last glacial maximum, sea level was down about 120 to 130 meters lower than today.

Since then, sea level has risen about 130 meters and it has actually slowed 2,000 to 3,000 years ago. And so, as the world warmed, the ocean expanded and primarily since the last glacial maximum, you had a large increase of land/ice melt, so the water went from essentially being locked in ice on the land to going to the ocean and so this can follow temperature records, and we have very good evidence showing that as it warms, you tend to get more water into the ocean, less in terms of ice on the land and you also have thermal expansion.

So, climate change, what's going to happen in the future? Well, we definitely have a very rigorous observation system right now that's measuring and it shows that temperature's been changing. As you addressed in the beginning of this conversation, that it's been shown that as temperature increases, so too will the amount of volume and mass increases within the ocean.

HOST: Billy, for folks living on the coast, what kind of impacts will they see from sea level rise?

BILLY SWEET: Well, that's really what's important to us as humans, the impacts. What can we anticipate? And it's difficult because we have sort of built our communities and built our lives to a level of the ocean that we understand and we live day to day by. So when we start talking about changes to an equilibrium per se that we have become use to, there's going to need to be some response to us, to our actions. Some of the immediate impacts, Kate, are going to be the fact that we tend to really lose our beaches because they just start backing up against structures. With that, we will start also losing our wetlands.

So, you're going to get flooding in low-lying areas, you're going to get loss of beaches, loss of wetlands, and eventually you're going to have infrastructure issues that are just going to need to be dealt with - building of seawalls or barriers, moving your underground infrastructure above ground, salt water intrusion, drinking water being affected, so there's a whole host of impacts that either are ongoing or anticipated to become much more exacerbated here in the future.


HOST: What kind of data do you need to determine if the sea level is actually rising in an area?

BILLY SWEET: That is a very good question because you need data and you need sustained measurements of the rise to be able to discern really what's going on. There's a host of equipment that you can actually pull from, one of which is satellite, altimetry, that does a global assessment, but it also can focus in on a regional to local scale. However, the global measurements are about a ten day repeat cycle, so you really don't know what's happening day to day and you also cannot discern from that measurement exactly the relative changes in sea level, what is vertical land motion doing relative to the ocean's surface.

So to make the relative measurement you need a tide gauge that make a repetitive measurement and is usually stationed somewhere on a stable area like a dock or a pier and that measurement will actually incorporate both the ocean change and the land motion.

There are some other detection mechanisms that we can then determine what the land, vertical motion, is doing and those are called a Continuously Operating Reference System, or basically it's a GPS system that's there measuring over the course of a few years to let you know vertically what your land's doing. We tend to co-locate a tide gauge with a GPS system so that we can then really get a better feeling of what the ocean is doing and what the land is doing, and so with these two pieces of information, you can really then start making sense of historically what sea level has been doing and how it's been changing on the many different time periods that we've discussed.

HOST: Billy, where in the U.S. are we seeing the greatest sea level rise?

BILLY SWEET: By far the greatest sea level rise in the United States is in the Gulf of Mexico. It's on the order of seven to ten millimeters a year throughout much of Louisiana and a little less but still on the order of four to five to six millimeters a year in much of Texas. And, that's a problem. And when you actually go to these areas and talk to the residents and the commissioners and those who live around the coast and who plan for the coastal communities and the development that will pursue, they're very aware of this, and they notice that literally they have an area that is just disappearing year to year and it's exacerbated when storms come through, but it's sort of incremental changes. It's really due in part to compaction of the Mississippi River sediments or no longer actually getting the sediments that they once had to help keep the elevation where it is - we divert a lot of this in terms of our levee systems. But it's also exacerbated in terms of extraction of oil and gas in ground water can actually cause your land to sink. And so in these areas, it's almost three to four times larger than the global rate. And so this is of much concern, especially when it comes to future considerations.

HOST: So Billy, is there anything unusual we're seeing right now with the numbers and the data across the U.S.?

BILLY SWEET: Well, I guess unusual is a relative term, but that's a good question Kate. We are actually seeing some variability that has some significance let's say, so unusual yes. Along the Mid-Atlantic and the New England coasts over the last two to three years, we've actually been seeing an increase of sea level that's well above the mean rise, so when I look in terms of the last let's say 20 or 30 years, we're at levels that are a good bit higher, 20 to 25 centimeters over many months in some places, a little less in some others. This is due to changes in some circulation patterns associated with ENSO as well as the North Atlantic Oscillation, which are these pressure systems that just tend to increase or decrease your normal westerly winds that we would have in this part of the country, but nonetheless it's elevating the water levels in critical times such as the winter when you have these nor'easters or these winter storms that can impact a coastline. So when you have an elevated sea level or a bump up in sea level for many months and over a few years, you can really tend to get these storm surges that tend to reach higher elevations than they often times would. And so this has been a problem for many of these New England towns that have been seeing a lot of coastal erosion over the last few years.

Now, a flip side, on the West Coast, along California and Oregon and some of the western states, we've actually been seeing sea level over the last 15 or 20 years has actually slowed down a bit in its rise. Now this is primarily due to a circulation sort of forcing patterns that have been going on that sea level has not been rising necessarily at the global rate over this time period. So this is sort of a suppression of the signal along the coast here due to wind and ocean circulation patterns that are expected to change here in the near future, we'll say five to ten years there's some indications that some of the patterns might actually be changing currently as we speak such that the rate of sea level rise might actually tend to pick up here over the next five to ten years so this will be something that we want to closely monitor.


HOST: Billy, what is NOAA's role in regards to measuring and responding to sea level rise?

BILLY SWEET: Well, our role within NOAA is to make the measurements and to present the information as clearly and unbiased as possible to those agencies and user groups that really need this data. Much of this, and we haven't necessarily talked about the low water conditions, but as sea level starts to rise, it affects shipping, the amount of sand you might need to dredge to keep a certain minimum clearance for ships. It's important to let's say the Army Corps of Engineers who plan these public work projects for the United States in terms of flooding and water control. Coastal communities that really need to plan 50 years in advance in terms of certain types of infrastructure, wastewater treatment facilities, electric grid, your water and sewer.

We want to make sure that we give them the very best information based upon our continuous measurements and we will let them really make the decision as to how best to proceed in the future. We will synthesize what we know in our current state, but we want to give them the tools and the resources to at least let them know where they are now in terms of land elevations to water elevations so that they can make their very best sound judgments themselves as to what might be occurring in the future based on what we know has happened in the past.

HOST: One question that I think some of us have here is, can we forecast or predict sea level rise?

BILLY SWEET: Well, that's a very important question, Kate. That is what is sort of on the mind of a lot of managers and community planners right now. What is sea level going to be in 50 years? How much higher is it going to be? So there's a few different ways we can look at this question, one of which is how high will an event be? Let's say the 100 year event, how high will this storm event be and how high will it impact? And there are statistical ways we can approach this in terms of looking at the variability of the past and projecting that into the future.

But the key question is, is how much will the baseline elevation rise such that this variability is working on top of a mean incremental rise in sea level? And the way that we approach this question is generally through computer models. What really drives sea level is temperature and heat. It melts ice and it also makes water expand. And so, we have numerous models that sort of all agree that say in their outputs, there is some range of expectation, but in general we're definitely looking at upwards of a half meter to a meter rise of sea level, let's say by 2100. There is some uncertainty with these models due to the fact that we don't really know how the big ice shelves and ice masses on Antarctica and Greenland are going to respond with the increase in global temperature. So there is some uncertainty, but the general agreement is as the earth warms the ocean will expand due to thermal expansion and ice melt and ice flows into the ocean are going to follow suit and these models, not only do they help us understand weather and currents on a near real-time basis that we've all sort of incorporated within our daily patterns, but we also use them to make these longer term projections.

HOST: What can residents do if they live in a coastal community and they have concerns about sea level rise where they live?

BILLY SWEET: That's a great question. What can you do as a resident to better inform yourself and sort of take action in response? I think the most important thing a resident can do, or a concerned citizen, is to become more familiar with what sea level has been doing. What are the patterns? What are your tide cycles? What's your tide range? How high does your spring tides actually come?

Becoming aware of what sea level does on a week to week, month to month basis versus what has it been doing over the course of decades. So once you become aware that sea level is not static, it's dynamic and it's changing and it's not only changing from month to month, but it's continuously changing and in most cases it's slowly rising.

At that point, I think making maps is something that lends itself very well to explaining the situation to others. You can start to get a feeling of where these problems are going to keep reoccurring and how you might respond as a community. So on a community scale, this kind of planning and these kinds of maps are critical to understanding where your infrastructure is in terms of sea level today and sea level of tomorrow.

HOST: Thanks Billy these are some good steps and actions for us to think about. To help our listeners get to know you a little bit more, how did you first get started in this field?

BILLY SWEET: Well, let's just say it's in my blood. I grew up in the coast of North Carolina and had explored the coastline as a kid and realized just how the ocean was changing throughout the seasons and throughout the year and it sort of had a real fundamental question as a child, what is causing these changes? And it later resurfaced I would say Kate, as I studied physical oceanography and training and trade through my experiences. But as I came to NOAA, there was an event that was really sort of eye-opening for many of us. It was a high sea level anomaly that occurred from Georgia all the way up to New Jersey where it caught a lot of coastal communities off guard because there were no coastal storms that normally cause these kinds of events. There was low-lying flooding over a few days coincided with a high spring tide, but nonetheless it was higher than normal and a lot of people started asking why? The group that I am in, we run all these tide stations around the country and people come to us and say, what is going on? And I had the opportunity to really do some research as to what were some of the physical factors involved - the winds and the currents - that were causing this phenomena and it really just sort of propelled me into the research field of sea level.

HOST: That's great, thanks Billy. So my last question for you, do you have any final words for our listeners today?

BILLY SWEET: I do. I think we're at the tip of the iceberg right now in terms of the impacts that we will most likely be seeing in the next 100 plus years. And as the younger generation having realized that at some point there's a hand off between the younger and older generation between those that really plan and sort of control let's say the fate of our existence often times in terms of the ways that we develop and the way that we respond, I think it's important to listen, to ask questions as a citizen, and I also think it's important for organizations like NOAA to continue to put out knowledge-based information tools that really allow people to make some actionable decisions in terms of how should we best respond to the changes that we are seeing and anticipated that we will see in the future because I think it's going to be a very costly and life-changing events in the next 50 to 100 years and I think the more that we empower people now in terms of what we see and through the observations that we make, I think the better off we will be in really communicating this to the groups that really have the power to make the changes necessary to adapt to these changes that are forthcoming.

HOST: Thanks Billy for joining us on Diving Deeper and talking with us about sea level rise. To learn more, please visit

That's all for today's show. We will be taking a break over the holidays so please join us for our next episode in January.