Eyes on Earth Episode 25 - Rising Seas

YOUNG: 

Hello, everyone. Welcome to this episode of Eyes on Earth. Our podcast discusses our ever-changing planet with people here at EROS and across the globe. We use remote sensing to monitor and study the health of earth. I am your host, Steve Young. Todayís guest is Dean Gesch, a research physical scientist who leads the coastal changes and impacts focus area in the Science and Applications Branch here at EROS. He and his colleagues at EROS produce coastal elevation models used by government agencies to project the impacts of flood inundation from sea level rise and storm surges. Their work marries topography, or land elevation with bathymetry, which is near shore water depth into what are called topobathymetric digital elevation models or tbdmís. The reality is as hurricanes continue to unleash storm surges from Texas to the Gulf of Mexico and up the eastern seaboard, and as sea levels continue to rise in the Pacific Ocean and exacerbate tidal flooding on low lying islands in the atolls,  decision makers need accurate elevation measurements to be able to model impact and enact measures to help protect the millions of people living in these areas. Dean and his colleagues provide that accurate elevation information. Welcome, Dean.

GESCH: 

Thank you.

YOUNG: 

How do we know sea levels are rising?

GESCH: 

Well, like a lot of physical processes that we observe itís done by observation, different measurements. Typically, a way that sea level rise is measured is with long term tide gauges. 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. We track changes in sea level, the mean sea level with these tide gauges. There are 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, then you can track how the sea level is changing over time. We also use satellites, since the early 1990ís. There is a type of satellite called a satellite altimeter, which uses radar technology to measure very precisely the height of the ocean. So, there is 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 sea level is changing, mostly rising in different locations around the earth.

YOUNG: 

So, to the degree that sea level is rising in different places, whatís causing those rising levels? Where are those rising levels coming from? 

GESCH: 

Well, thereís a number of different factors, but we can summarize it probably in just two. 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 is floating on the ocean, like in the Arctic area, when that melts that doesnít change sea level. It would be analogous to having a ice cube floating in a glass of water. And, when that ice cube melts it doesnít change the level of the water in there. So, that is the distinction we need to make. But, ice melt that is on the land surfaces is a big component that changes sea level. And, then the other big one is what we call thermal expansion. And, that is simple. Remember back to your maybe elementary school science classes where liquid expands when it heats up. Thatís exactly whatís happening. A lot of the increasing heat in the climate system ends up in the ocean. And, as the ocean warms it expands and that raises water levels. So those are the two main components.

YOUNG: 

So, is sea level rise and the rising levels uniform across the planet or not?

GESCH: 

No, they are different. The term that we use is, and Iíve used it already, is mean sea level. Global mean sea level is somewhat difficult to define and it really doesnít exist. It is different depending on where you are on the global surface of the earth. That can be averaged across but 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 to be higher than the global average sea level. Another place is on the northeast Atlantic coast of the U.S. Thatís another area where the sea level would be rising faster than it is on the global average. And thatís due to a couple different things. Ocean currents 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 land surface is actually subsiding or sinking. Itís the term we use is subsidence. In other places the land is actually rising. So, we use a term called relative sea level rise. So that is relative to the land surface. So, where the land is sinking and the ocean is rising, you get effectively more sea level rise. Where the land is rising, the ocean might be rising as well but it will effectively seem like a smaller increase or even a decrease in sea level if the land is rising faster than water. Yes, so there are great variations around the globe.

YOUNG: 

So talk about some of the dangers posed to coastal communities and people living in those communities as sea levels rise as high tides become even higher and the storm surges increase. What are some of the dangers posed to those coastal communities? 

GESCH:

A simple way but a good way to think of it is, you have a higher platform if you will. So, storm surges, those waves and higher water levels, if that starts on a higher platform, and in this case the platform would be the base ocean level or the mean sea level. So, if that is higher, then anything thatís on top of that is also going to be higher. So, as those waves propagate inland, they are going to reach further inland than they had historically. And the damaging effects of that erosion, shoreline erosion, that sort of thing, flooding, anything that is associated with that is going to take effect further inland than it wouldíve because of the base platform being higher. High waves, again, they can reach further inland if they are on that higher platform. And then there is also a term thatís got a lot of attention in the scientific community lately and itís called nuisance flooding. A interesting term that is associated with that is sunny day flooding. So, this is when there is not a storm. And thatís just the noticeable increase in high tide level. So, thereís things called king tides. At certain times of the year, when the tides are generally higher. But again, if the base platform, if you will, of the ocean is higher, then those high tides are going to be much more noticeable. And that has been called nuisance flooding or sunny day flooding. And thatís being observed quite a bit more than it has in the past.

YOUNG: 

You and your colleagues here at EROS and at CoNED, which is the Coastal National Elevation Database, map coastal elevation. How do you do that? What are the tools you use to map coastal elevation?

GESCH: 

We use satellite technology, both satellite imagery and other types of satellite data, radar data. We also use airborne or what we call aerial, or an instrument that is carried by an airplane called a lidar that is a laser instrument that measures elevation very precisely. Thatís a big one that we use. Lidar can also be used to measure the depth of near shore waters in specific types of lidar. And 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 either use a camera to take pictures, photography and translate that into elevation data. And now drones are starting to carry lidar instruments as well. So we use a whole suite of those instruments to measure and map coastal topography.

YOUNG: 

When you talk about near shore elevation or under the water, a lidar or different tools can see through the water to the bottom? 

GESCH: 

Yes, yes. Just like measuring the elevation of the land or the topography right next to the water, there are several tools. The traditional way that thatís been done is from boat surveys, what we call hydrographic surveys. So, that will be sonar, you know an instrument that sends out sound waves and measures depth. And we still use that type of data extensively. When you get very close to the water line, the shoreline, very shallow areas, that becomes more difficult for those instruments to use. And thatís where these other remote sensing, area remote sensing instruments like you mentioned, bathymetric lidar. Visible imagery, photography and satellite imagery to measure those depths as well. The lidar is whatís called an active instrument, so it sends out a laser pulse and then measures the response from that. The imagery, satellite imagery and aerial photography, whether thatís from an airplane or a drone, is whatís called passive. So that measures the reflectance, the sunlight reflectance. And certain bands or wave lengths of the spectrum, that can be measured with those photographs and those satellite imagers and they can be processed to depth information. 

YOUNG: 

How accurate is all of it?

GESCH: 

It varies. It can be highly accurate. The lidar would be the most accurate and then also the sonar, the boat. A little less accurate would be the imagery from satellites that are used to derive that. Itís on the order of a few feet in best cases from the satellite platforms, and then on the order of several inches or a foot vertically from some of those other sources.

YOUNG: 

I think a key challenge in your line of work is something called vertical uncertainty. What is that and how do you try to address that in the work you do? 

GESCH: 

Right. Vertical uncertainty, itís one of those terms that can get a little tricky, ìuncertaintyî. Sometimes when you say, ìuncertaintyî that means you know, that can be interpreted as saying, ìwell, weíre just guessingî, because thereís a lot of uncertainty. In the scientific realm, that is not the case. Uncertainty, thereís error associated with any scientific measurement. Now, we do the best job we can to reduce those errors. And the instruments keep getting better and better. But the term ìvertical uncertaintyî is maybe just a fancy term for the error that is associated with any scientific measurement. So, really the whole idea there is that the data we are using, make sure itís appropriate for the application. Thereís many different scales of mapping and scientific study from remote sensing data, some very local. So, there you would need very high resolution, high spatial resolution and very high vertical accuracy. If you are trying to cover much more broad areas, you can get by with less resolution and less accuracy. So the idea there is that as we map the coastal topography and the bathymetry, if we are trying to look at very small increments of water level increase, in this case sea level rise or storm surge flooding, that we have very high, highly accurate elevation data so that we can model those very accurately. 

YOUNG: 

It seems like you folks at EROS have been enlisted to assist in sea level rise work at some fairly exotic locales, places with names like Majuro, Pohnpei, even Hawaii recently. Give us an idea of the kinds of things you are assisting with when you go to places like that?

GESCH: 

Itís been great working in some of those places. One of the benefits of working in coastal areas, and as a geographer, being able to travel and work in those places is just tremendous. But those are places that are on the forefront of climate change impacts. Island locations, that gets a lot of attention in the press, small island nations and vulnerable areas, again to sea level rise. Majuro is an atoll in the republic of the Marshall Islands in the central Pacific. And, what we were doing there is working with a number of local groups to produce inundation maps. So, under a specific water level increases that again could be from storm surge or sea level rise itself or more likely a combination of the two. So, the mapping thatís been done of topography and bathymetry in a global sense is not detailed enough in that area of the very low-lying islands. So, what we did in Majuro was used a drone-based approach, again UAS, or unmanned aerial systems and a lot of ground survey to collect very detailed, very accurate elevation information, and then process that into inundation maps, risk maps of inundation at various levels. So that was a successful project there. Pohnpei is in the federated states of Micronesia, also in the central Pacific and a little bit different there. Itís a high relief island, of fringe by mangrove forests, which are forests that grow in the tidal areas around the island, but also very susceptible to effects of sea level rise. Mangrove forests are in tune with the tidal range and as sea level rises, they are adapted to growing in saltwater environments but are still sensitive to that. There is a wide range of species. So, the work we did in Pohnpei, again looking at how sea level rise might affect those, the resiliency of those forests. They are very important ecologically around that island. And, again it is all tied to elevation. Very accurately measuring elevation along the shoreline, and that it is the expertise weíve brought to that. And then our latest work in Hawaii that you mentioned, that is a coastal inundation assessment for an area, very valuable cultural resources. That was work that was done collaboratively, is being done collaboratively with the National Park Service. They are very interested again in 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 and that sort of thing.

YOUNG: 

You get the sense sometimes that as sea level continues to rise, that some of these low-lying islands in the atolls will simply disappear at some point in time. Is the information you are providing, the elevation data you are providing, does that get used for any kind of efforts to protect these islands or to protect these cultural sites? What do they do with the data that you provide?

GESCH: 

What this shows is, if nothing is done, if there is no mitigation thatís done, if thereís no planning and the sea level rises, happens like it is projected to, what would be the effect of that? So, that helps planners on the ground and in those locations look at vulnerable places and say, ìOk, if we do nothing, this is the projection of what could happen, erosion along the shoreline, damage to these either natural or cultural resources.î 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 sea wall here or if we built other protective measures or we move some of those resources if theyíre movable. That sort of thing, so thatís what we call mitigation planning or adaptation planning. Saying, ok, this is whatís projected to happen, in our best knowledge of the projection to sea level rise. And then, what can be done to mitigate those damaging effects and plan to preserve these valuable cultural and natural resources.

YOUNG:

A lot of your tbdems are created through a group youíre associated with here, the Coastal National Elevation Database project or CoNED. Are there other groups out there that produce digital elevation models, or is the work we do here at EROS unique at all?

GESCH:

Iíd say itís unique. There are other groups that produce these. A lot of these models are done on a very local scale, so youíll see scientific publications or studies where theyíre done for a specific location. The scale is one thing where weíre a little unique. We do it over large areas, not national yet, but very large regional areas. Our friends at NOAA do very similar work. One group at NOAA produces these models for tsunami impacts, whatís called tsunami run up, the run up of tsunami waves. Thatís mostly limited to urban areas, which makes sense. Itís where people live. So they do that, itís a limited resolution and limited to the area. We work at a much higher resolution, weíre at about a 1 meter resolution, spatial resolution, so about 3 feet. But again, itís a collaboration. They use some of our data, we use some of their data. NOAAís very good with hydrographic survey data, this bathymetry data off shore, so we use a lot of their data, thereís a lot of similarities where our models cross, but theyíre made for different purposes.

YOUNG:

What does the future hold for the work of you and your colleagues? Will you be going to more exotic locales? Have you got big projects coming up? What can you tell us?

GESCH:

We certainly hope to be going to more exotic locales, but those are hard to predict. Nothing in that category, but we do have some big efforts coming up. Thereís been a significant amount of lidar data collected over coastal Louisiana, inland. We have an existing CoNED model there, itís one of the first ones we actually did, but itís almost a complete redo, and that will be incorporated into the new version of the CoNED elevation model. Itís really important for whatís known as the master plan. Thereís a group there that weíre collaborating with, CPRA, that stands for the Coastal Protection and Restoration Authority, itís a state agency in Louisiana. And theyíre the ones that are charged with protecting the coast, restoring the coast, planning for resiliency of the coast. Itís a highly dynamic shoreline, itís well-documented, the damages that hurricanes and sea level rise have had along that coastline. So a key component of their state master plan, which is an effort that happens every five years, is this very accurate elevation model, and thatís whatís being produced in this project. Weíre the main producer of that data. And that also includes what we call validation, the checking, the quality checking and documentation of, again the uncertainty. Another thing that weíre looking at in the future is along the East Coast. Weíve done quite a bit of CoNED work along the East Coast, but more the northeast coast. So thereís an effort that weíll be starting up this year, maybe next year. This is based on hurricane impacts, supplemental funding that Congress allocates, so from the Carolinas down to the tip of Florida, basically the Atlantic Coast, so the southeast Atlantic coast, CoNED models will be developed, high accuracy coastal elevation data, will be developed for that area. And thatís part of the COSMOS, the coastal storm modeling system, this is really the first time that that system is going to be developed and deployed on the East Coast. So big effort there, weíre looking forward to that. And then the last thing that our future holds is a related project in Iíll call it inland bathymetry, weíve done quite a bit of work, as weíve been talking about today, on the coast, but the water depth information, merged with the adjacent land information is just as important for inland areas. And by inland I mean away from the coast. So that would be along rivers or lakes or reservoirs. Thereís a concept that the USGS is developing called the National Terrain Model. Weíre quite involved with that, doing some initial research and development and testing to figure out how to best map that inland bathymetry and merge it with the topography data for a seamless elevation model across the U.S.

YOUNG: 

Weíve been talking to Dean Gesch, a research physical scientist at EROS who is heavily involved in the work of producing coastal elevation models and looking at coastal changes and impacts brought about by sea level rise. Thanks for joining us, Dean. 

GESCH: 

Youíre Welcome. My pleasure.

YOUNG: 

We hope you come back for the next episode of Eyes on Earth. This podcast is a product of the US Geological Survey, the Department of the Interior. Thanks for joining us.