As sea levels rise around the planet, as storm surges threaten islands and coastal communities, decision makers face a lot of questions about how to protect their citizens and their landscapes.
Staff at the U.S. Geological Survey’s (USGS) Earth Resources Observation and Science (EROS) Center can provide them some important answers.
Understanding the impacts of rising sea levels requires accurate elevation information both beneath the water near shore, and along nearby coastal landscapes. Few agencies provide that broad-scale, regional elevation data as accurately as EROS does.
Research Physical Scientist Dean Gesch, the principal investigator for the Coastal Changes and Impacts focus area within the Integrated Science and Applications Branch (ISAB) at EROS, talks about his agency’s elevation expertise, and about sea level rise, in this conversation.
How do we know sea levels are rising?
“Like a lot of physical processes that we study, it’s done by observation and different measurements. Typically, a way that sea level rise is measured is with long-term tide gages. Most people are probably familiar with the tide as it comes and goes, high tide, low tide. In between there is something we call mean sea level, or the average tide level. It averages out those high tides and low tides. So, we track changes in sea level, the mean sea level, with these tide gages. They’re fixed measurement stations that are on the shoreline, and they measure very precisely the water level. If you do that over a long time period, you can track how the sea level is changing over time.”
How do satellites help measure average tide levels?
“Satellites have been used since the early 1990s. There’s a type of remote sensing instrument called a satellite altimeter, which uses radar technology to measure very precisely the height of the ocean. So, there’s a satellite record which is then compared to and correlated with the tide measurements, and those are the main two ways that we know that sea level is rising or changing.”
What do we know about why sea levels are rising?
“There’s a number of different factors. Primarily, melting of land ice is the big one. So, that would be Antarctica and Greenland, two very large land masses that are covered by polar ice caps. One thing to note is that sea ice, or ice that’s floating on the ocean like in the Arctic area, when that melts, that doesn’t change sea level. It would be analogous to having an ice cube floating in a glass of water. When that ice cube melts, it doesn’t change the level of the water in the glass. So, that’s a distinction we need to make. But ice melt that’s on land surfaces is a big component. That ice melts and drains into the ocean. That changes sea level. And then the other big one is what we call thermal expansion.”
Thermal expansion. What’s that?
“If you remember back to maybe elementary school science classes, where liquid expands when it heats up. That’s exactly what’s happening. A lot of the increasing heat in the climate system ends up in the ocean. As the ocean warms, it expands, and that raises water levels.”
Is sea level rise uniform across the planet?
“Global mean sea level is somewhat difficult to define, and it really doesn’t exist. It is different depending on where you are on the global surface of the Earth. There are large regional differences. For instance, in the Central Pacific Ocean, a lot of the projections for increasing sea level the rest of this century show that area to be higher than the global average sea level. Another place is on the northeast Atlantic Coast of the U.S. That’s another area where the sea level would be rising faster than it is on the global average.”
Why would it be rising faster those places?
“A couple of different things. Ocean current is a big one that has an effect. Ocean currents vary around the surface of the Earth. Gravity variations throughout the globe ... that changes the height of the water, the height of the oceans. And then, another one is the land surface itself. In some places, the land surface is actually subsiding, or sinking. The term we use is subsidence. In other places, the land is actually rising. We use a term called relative sea level rise. So, that’s relative to the land’s surface. Where the land is sinking, and the ocean is rising, you get effectively more sea level rise.”
What are the dangers to coastal communities and people where sea level rise is occurring?
“Maybe a simple way—but a good way—to think of it is, you have a higher platform, if you will. If waves and higher water levels ... start on a higher platform, and in this case that platform would be the base ocean level or the mean sea level, then anything that’s on top of that is also going to be higher. So, as those waves propagate inland, they’re going to reach further inland than they had historically. The damaging effects of that erosion, shoreline erosion, flooding ... anything that’s associated with that is going to take effect further inland than it would have because of the base platform being higher.”
Is sea level rise noticeable if there aren’t storms and surges?
“There’s a term that’s gotten a lot of attention in the scientific community lately, and that’s called nuisance flooding. An interesting term associated with that is ‘sunny day’ flooding. This is when there’s not a storm, and it’s just the noticeable increase in the high tide level. So, there’s things called king tides certain times of the year when the tides are generally higher. But again, if the base platform of the ocean is higher, then those high tides are going to be much more noticeable.”
Talk about how you map coastal elevation?
“We use a variety of tools, or a lot of different techniques, at kind of all different levels. We use satellite technology, both satellite imagery and other types of satellite data, such as radar data. We also use airborne, or what we call aerial. So, again, aerial photography or an instrument that’s carried by an airplane called a lidar. That’s a laser instrument that measures elevation very precisely. That’s a big one that we use. Specific types of lidar can also be used to measure the depth of near-shore waters. Then, we also do traditional ground survey for very detailed points. And then lately, what we would call UAS, Unmanned Aerial Systems, or commonly known as drones. So, those are small platforms that again would use a camera to take pictures, photography, and translate that into elevation data. And now drones are starting to carry lidar instruments as well.”
How do satellites 400-plus miles in the air accurately capture elevation data?
“Yes, Landsat is used, and we’ve used that on some of our projects to derive this near-shore bathymetry. We also have used some commercial satellite WorldView imagery. Digital Globe is the company that flies those satellites. So yes, they’re several hundred miles in the air. Yet, they’re still relatively high-resolution images. The WorldView imagery is much higher resolution than Landsat, but both of those have capability to see through the water to measure reflectance in clear water. That’s the other constraint, is that you have to have relatively clear water to be able to see the bottom and get that reflectance from the bottom.”
You and your colleagues have done a lot of work in places with names like Majuro, Pohnpei and Hawaii, measuring elevation. Why?
“Those are places on the forefront of climate change impacts. Island locations that get a lot of attention in the press. These are small island nations and vulnerable areas again to sea level rise. So, Majuro is an atoll in the Republic of the Marshall Islands in the Central Pacific. What we were doing there was working with a number of local groups to produce inundation maps. Under specific water level increases that, again, could be from storm surges, sea level rise itself, or a combination of the two ... we know the mapping that’s been done of topography and bathymetry in a global sense is not detailed enough in that area. They’re very low-lying islands. So, what we did in Majuro was use a drone-based approach, again, UAS or Unmanned Aerial Systems, and a lot of ground survey to collect very detailed, very accurate elevation information. We then processed that into risk maps of inundation at various levels.”
And in Pohnpei?
“Pohnpei is in the Federated States of Micronesia, also in the Central Pacific. Our work was a little bit different there. It’s a high relief island fringed by mangrove forests, which are forests that grow in the tidal areas around the island, but also are very susceptible to effects of sea level rise. Mangrove forests have adapted to growing in salt-water environments but are still sensitive to sea level rise. There’s a wide range of species, so the work we did in Pohnpei was again looking at how sea level rise might affect the resiliency of those forests. They’re very important ecologically around that island, and again, it’s all tied to elevation, so we were very accurately measuring elevation along the shoreline.”
What about your work in Hawaii?
“That is a coastal inundation assessment for an area of very valuable cultural resources. That is work that is being done collaboratively with the National Park Service. They’re very interested in, again, how these very valuable cultural resources, archaeological sites at this particular location on the big island of Hawaii, are going to be threatened as sea level increases, and as storm surges increase.
How does the elevation information you provide get used?
“Our work helps planners on the ground and in those locations look at vulnerable places and say, ‘OK, if we do nothing, this is the projection of what could happen.’ Erosion along the shoreline. Damage to these either natural or cultural resources, that sort of thing. But then also, going along with that is, ‘OK, if this is the kind of damage we would expect or the kind of impact we would expect, what would happen if we built a seawall here? Or if we built other protective measures or moved some of those resources if they’re movable?’ That’s what we would call mitigation planning or adaptation planning.”
How do other government agencies here in the U.S. use your sea level rise models?
“I’ll give you an example. We do a lot of collaborative work with NOAA (National Oceanic and Atmospheric Administration). Most folks are probably familiar with the Weather Service part of NOAA, but they do quite a bit of research and operational work along shorelines and along coastlines. These high accuracy, high resolution elevation models we produce are used by NOAA in one of the online tools they have called a Sea Level Rise Viewer. So, it’s a map tool that allows a user to very easily zoom into an area and see what potential impacts of sea level rise are. So, the real core to that is having highly accurate elevation data.”
Is the coastal elevation modeling we do here at EROS unique?
“I’d say it’s unique, but there certainly are other groups that produce these. A lot of those models are done on a very local scale, so you’ll see scientific publications or studies where they’re done for a specific location. The scale is one thing where we’re probably a little bit unique. We do it over very large areas. Not national yet, but very large regional areas.”