Research scientists continue to add to our understanding of Earth systems, thanks to the global earth observation capacity. Waleed Abdalati, associate professor of Geography at the University of Colorado, Boulder, director of the Earth Science & Observation Center, a Fellow at the Cooperative Institute for Research in Environmental Sciences, and former NASA Chief Scientist, has a perspective that encompasses the capacity, coming capabilities, and ongoing challenges. Sensors & Systems (S&S) special correspondent Matteo Luccio spoke with Abdalati about the inspiration that is provided through Earth observation, the need to balance satellite-based sensors with in-situ observations, as well as his research work studying ice and climate change.
S&S: Who or what inspired you to go into science? Why aerospace engineering science?
Abdalati: The Apollo program inspired me, from the time I was a young kid. I used to pretend that I was one of the Apollo 11 astronauts. There was just a magic, an inspiration in there that stayed with me through grade school, into college, and onward. When Apollo 16 or 17 went up, I was in the second grade and I remember telling my teacher, “I don’t understand. The rocket is moving really fast and the moon is moving really fast and so you have to aim the rocket someplace where the moon isn’t, so that they get there at the same time.”
That was my second grade mind. “The moon has to be there by the time the rocket gets there. How do they do that?” I asked. She said, “They use all kinds of math to figure it out.” I remember thinking, “Math… I am going to learn some math!” I thought it was really cool. From that moment on, I was always fascinated by trajectories, by the movement of objects, orbital mechanics, and that kind of stuff. So, it really was the Apollo era that planted the seeds that stayed with me throughout my career.
S&S: Hopefully, the Curiosity mission will do the same for another generation.
Abdalati: I sure hope so! There’s a real magic to it.
S&S: How does your training in geography guide your work?
Abdalati: I worked as an engineer for a little while and at some point I just became more interested in what the satellites were seeing rather than the design and development of them. I worked on Earth-observing satellites early on and I thought, man, there is just a real beauty on the Earth down there that is so important to understand that I became equally as inspired in trying to understand how the Earth works. Frankly, it was serendipitous: I took a class with a person who later became my advisor who did work in the Arctic and it just looked beautiful. So, it was the perfect marriage of my aerospace engineering interest, using satellites to study the Earth, my interest and appreciation for the Earth system in all its beauty, and the intrigue and adventure of going to the Arctic and studying glaciers and ice sheets.
S&S: What have been the key advances or milestones in Earth observation in the last 20 years?
Abdalati: In a general sense, it’s been just the real recognition of the Earth as a system and the interconnectedness of the various elements. Things like dust storms in the Sahara affecting hurricanes in the Eastern United States. How those two relate in a very specific sense. My own area of research, realizing that glaciers and ice sheets—things that we used to think take hundreds or thousands of years to respond to changes in today’s climate—actually respond in minutes, hours, days, months, weeks, years, centuries, and millennia, on all time scales, but the fact that there is an instantaneous effect of today’s climate on these very old, thick, slow-moving ice sheets was a major revelation in my own discipline.
Being able to look inside hurricanes and tropical storms and understand their three-dimensional structure, where the rain is falling, why it is falling, what the vertical temperature structure looks like can help us get better at predicting their trajectories and their intensity. It’s hard to pick a few, but I’ll grab a couple more. Another one is the spatial characteristics of sea level rise. Or even not just the secular trend of sea level rise, but weekly or monthly types of variability when things like El Niño set in or we transition to La Niña. The spatial character of the ocean topography tells us a lot about the physics related to ocean circulation, the energy characteristics in the ocean, and that’s just something that you cannot adequately sample with buoys or ships. The oceans are huge. Historically, we measure sea level rise using tide gauges on the coast, but those measurements are complicated by the fact that the land in which they are planted is moving in relation to the ocean. So, you get a relative sea level. From satellites, we get a terrestrial reference frame that allows us to observe absolute sea level rise.
More than 20 years ago, it was satellites that observed the decrease in the ozone in Antarctica, the ozone hole, and ultimately led to the adoption of the Montreal Protocol and to means of correcting that. And then, finally, the shrinking Arctic sea ice is a very visual illustration of how our planet is changing and this is an area that is far removed from most people’s everyday lives, and yet it is very important to the Earth system. Still, we don’t think about it that much because we don’t see it, we don’t live it, we don’t breath it, but when the satellites show the rate at which the ice is shrinking and the spatial character and combine that with the fact that our models have difficulty capturing that rate, those are huge advances.
S&S: What have been some of the key turning points in the technology aboard the satellites?
Abdalati: I’ll go way back, to 1960. Simply being able to observe the Earth from space with the TIROS [Television Infrared Observation Satellite Program] mission. One of the really big technology advances has been interferometric synthetic aperture radar [InSAR]. The ability to measure small relative displacements over large areas, in ways we just can’t do on the ground with the same degree of comprehensive coverage, has just been tremendously powerful. To inform us about Earth deformation, uplift and subsidence in certain areas associated with all kinds of geophysical processes, such as the movement of glaciers. InSAR has been tremendously powerful.
The technology associated with GRACE, the Gravity Recovery and Climate Experiment, is also tremendously powerful. By observing the change in distance between a pair of satellites we can infer the gravitational characteristics of the Earth below the satellites and thus make assessments about the movements of mass on the Earth so that we can detect the depletion of aquifers or the filling or draining of reservoirs, the changes in ice sheets, and the movements of water masses in the ocean. Then there are the standard things like getting high resolution visible imagery. When people can look at their Earth in this way, we see it very differently.
Finally, in my own specific area of research, lidar technology has provided and will continue to provide great advances in our understanding of the Earth system. The idea that we can accurately map the topography of ice and land surfaces, or the optical characteristics of the atmosphere, using lasers with such precision is truly remarkable. This capability is enabling major science advances by improving our understanding of the structural characteristics of land, ice, and vegetation, as well as the aerosols and radiative characteristics of the atmosphere.
S&S: What will be the next big advance in terms of payload?
Abdalati: That’s a tough call, because there are so many things in the works that are at different technical readiness levels. I think that the soil moisture, active and passive (SMAP) mission, to measure the water content in the soil and its implications for ecosystem health and water availability for humans on Earth, will be a very substantial advance. I also think the next ICESat mission, ICESat-2, is a different kind of laser remote sensing. It will provide detail on ice topography and the topography in other areas, potentially vegetation structure. That, I think, will be a big advance over what we’ve done to date. Wide-swath ocean altimetry, where, rather than a pencil beam of ocean topography, we get a swath that gives us more and better information on spatial structure. There are many things in the works. I tend to focus my own research on Earth surface processes. I am not being fair to the atmospheric science community, but there are, no doubt, big things coming down the pipe there.
S&S: What are the most critical environmental variables that can be measured best (or only) from space?
Abdalati: What determines those are the spatial scales over which meaningful processes occur and their accessibility. So, ocean circulation is certainly huge. The fact that we can look at the wind vectors in the ocean from space to tell us which way surface winds are blowing and the fact that we can look at the salinity in the oceans and the ocean topography to understand the ocean circulation itself. Large scale atmospheric phenomena, such as big hurricanes and major tropical storms, are another. We have had great success flying airplanes into storms and measuring their structure, but to really get a comprehensive picture of the space-based view is incredibly powerful. You need both. You can’t do it all from space, but you can’t do it without the space-based view. The last one I will mention (although there are many) is ice cover. Greenland, Antarctica, the Arctic sea ice, the Antarctic sea ice. These are large-scale processes that are very difficult to observe any other way.
S&S: What in situ measurement by ground-based sensor networks can best complement the work of space-based Earth observation?
Abdalati: Certainly, the ocean is a big challenge and ocean buoys with sensors that capture temperature, wind, radiation, anything else we can measure, are critical. The buoy networks are essential. Using spectrometers to understand the spectral characteristics of various surface types and phenomena, so that we can interpret our space-based observations is tremendously important. Air-borne measurements of atmospheric chemistry and structure in storms and other large-scale dynamic processes are essential to help us interpret what we are seeing from space—as ground validation, if you will.
Basically, anything we can measure from the ground is a big help, because, one, it is another set of data points and, two, it is another step in robust calibration of our space-based observations. They help us understand and add meaning to what we are observing from space. Look at the range of things we’ve got flying. It becomes kind of apparent what we need to do on the ground in terms of validation. We want to have similar observing tools on the ground as we have in space, for calibration.
Then, there’s the other piece: the information that we can’t get from space. I’ll come back to oceans: for the most part, we can only observe the surface, and we rely on our understanding of the physical processes to interpret what we see on the surface and infer what it tells us about what is happening below the surface. We observe surface expression of subsurface processes. So, definitely, observations on the ground or in the sea of subsurface characteristics: what is making the ocean do what it is doing? The deployment of expendable bathythermographs, or XBTs, and various other devices, is certainly an important part of a robust observing system. On the ground, meteorological towers, stations, and the like, are essential, because different things happen as you get closer to the ground.
Often, from space we are looking at column information: we are looking at the integrated effects from the surface of the Earth to the top of the atmosphere and only with some exceptions do we examine the stratification of the atmosphere and get vertical information. So, anything that helps us validate our observations and bring insight on the vertical aspect of what we’re seeing, what’s happening in the z dimension, is absolutely critical.
S&S: How is the mix of EO satellites changing — in terms of missions, technology, public vs. private, nationality, etc.?
Abdalati: One way the mix is changing is that we don’t have a flagship mission in our future—things like the Terra, Aqua, and Aura missions, which have multiple sensors for coincident observation. The missions, now and in the foreseeable future, are more targeted. They have very specific objectives that are a little more focused and we rely on models and observations from other sensors to do the integration. Whereas with flagship missions in the Earth-Observing System some of that integration occurred by coincident observation. We have the A-Train, we have multi-sensor missions. The transition is toward single-sensor missions or, maybe, dual-sensor missions with a single primary focus.
In terms of who is doing what, there is certainly much more commercial capability out there. If you just look at the high-resolution commercial imagery and compare that to the Landsat observations, which have been a staple for scientific research over the years, that’s testimony of what private industry is capable of doing. Industry, however, will always be driven by the market, by a business model that has to close, otherwise those businesses will not be successful. There are some things for which the business model won’t close. This is partly why we have Landsat 8 and the Landsat series: because, while people may pay a good price for really high resolution data of certain areas of interest, there isn’t quite the same market for lower-resolution, consistent, global-scale coverage, yet it is necessary for science, to understand the behavior of the planet. So, I think there is a place for business, there is a place for government entities, for the kinds of research activities that NASA has historically supported.
There are also other new niches, such as small satellites that industry is pioneering that may be able to fill some of the gaps that we are starting to see. But, we have developed the Earth-Observing System, which was robust and entirely government-supported, targeted at understanding how and why our Earth is changing and what the implications of those changes would be. The systems in place now, or coming down the road, are a little more focused, looking at new things in new ways, looking at old things in new ways, and in a few cases looking at old things in old ways.
There is plenty of room for innovation and ingenuity, to fill the gap that the current budget constraints are forcing us to have. The number of satellites and the capabilities in Earth observing is something that we, as a nation, should take pride in. At the same time, it is not the grand vision that was initiated years ago, because budget reality forced choices and reductions, so it is a bit of a different landscape. Launch costs will affect that as well. We had the tragic loss of two very important missions and it’s difficult to recover from that.
S&S: What are some of the types of sensors that do not currently have much of a commercial reason for existing but are still important for science? Perhaps InSAR (interferometric synthetic aperture radar)?
Abdalati: There are certainly commercial applications of InSAR. It isn’t clear yet whether they are sufficiently robust to warrant private industry taking over developing a SAR mission. So far, we have not seen it emerge, so my feeling is that it is not. Another one is something like GRACE, the Gravity Recovery and Climate Experiment. That’s tremendously powerful for understanding our world. Who is going to pay for it? I can’t really think of anyone. If we look at something like the depletion of aquifers, whoever cares about their one system of reservoirs or their water tables will invest in local measurements. They aren’t thinking about it at a global scale. I see GRACE as something that is great for the world, but is not going to grab one company or one investor to say, “Yeah, this is a real money maker.”
There is not really a business case for things like passive microwave satellite data, the things we use to monitor the sea ice cover, in some cases the snow over land and a few other things, yet it is important for understanding weather, climate, and the Earth as a system. So, there are some things that are really important that nobody is really going to pay for except the sponsors of scientific research who are the most immediate beneficiaries of the information that they provide.
S&S: Why the trend toward single-sensor, single-focus missions?
Abdalati: It’s due to a couple of things. Some of it is risk, you spread the risk over multiple launches, because the launch has become a real issue. Another is complexity: it becomes very hard and complex to integrate several instruments on one platform, each with different observing requirements and targeted observations can be optimized for the purpose for which they were designed.
S&S: From what you’ve learned so far, how and why is the Earth’s ice cover changing and what do those changes mean for life on Earth?
Abdalati: The Earth’s ice is growing in some places and shrinking in others, but the amount of shrinking is by far greater than the amount of growing. This has impacts for Earth in several ways. One is that if you look at ice sheets—in Greenland and Antarctica, certainly in West Antarctica—the loss of ice, and the loss of glacier ice in other parts of the world, is contributing to sea level rise. Oceans are going up and the keys to the rate at which they go up and how high they will ultimately rise are locked up in the ice. So, sea level rise is one big implication for life on Earth.
Sea ice and Arctic and Antarctic ice cover play important roles. One is to help keep the planet cool, by reflecting a lot of the incoming sunlight. White ice is reflective, so it prevents the sunlight from being absorbed in the Arctic and Antarctic. As that ice starts to melt, particularly sea ice, and exposes dark water underneath, more and more energy gets absorbed by the polar oceans. Second, the presence of ice affects ocean and atmospheric circulation. There is a very big barrier to energy transfer between the ocean and the atmosphere that affects the surface circulation of both and, ultimately, the climate. Third, when sea ice forms, it rejects salt into the sea water and that salt causes density increases at the upper ocean. So, you now have cold, salty water that sinks and spreads along the bottom of the ocean and affects ocean circulation in that way—quite simply, the displacement characteristics from the density changes associated with the formation of the ice.
S&S: What is the most important thing you’ve discovered about sea ice, ice sheets, and/or glaciers that the public does not know or does not understand?
Abdalati: I think the public is becoming increasingly aware, because ice is binary: it’s there or it’s not. So, there are some very visually compelling images and information about the ice cover. We’ve discovered that they are changing faster than we ever thought possible. The movie “Chasing Ice” consists of a series of time-lapse photographs of different glaciers throughout the Earth and chronicles the retreat of the ice cover of the glacier ice. It is has been so successful in part because it is just plain visual. You see ice, then you see it go away. The same is true with satellite images. The public’s appreciation for its disappearance or sensitivity to climate has gone up, but I think what we’ve discovered most recently and what may still not be fully appreciated is how fast these changes can occur. We’ve seen glaciers flow faster than we used to think was ever possible. The sea ice is shrinking faster than what all of the models that we’ve used for climate prediction—there are about 16—predicted. It’s this rate of change that I think is something I would hope people would appreciate more.
The other is the role of ice in the climate under which human civilization has evolved. I don’t think people fully appreciate that humans have never known a time without Arctic sea ice cover in the summer. It’s always been there and the resulting ocean circulation has always been with us. So, I don’t know that people fully appreciate the potential for significant changes associated with the changes in the ice.
S&S: What do you hope most to learn from ICESat-II?
Abdalati: I hope to learn the amount by which ice sheets and sea ice are changing, because it will enable us to learn about sea ice thickness. Beyond that, however, I also hope that we acquire tremendous insights into the mechanisms driving those changes. The three things that affect the growth and shrinkage of ice sheets are how much snow accumulates, how much melts and runs off, and how much flows into the sea (the iceberg calving). Each of those mechanisms has a topographic signature that, I hope, we will be able to decipher from the ICESat-II data.
S&S: How do you measure the thickness of the ice sheets over land?
Abdalati: We use ice-penetrating radar. Sometimes, we pull the radar on a sled, behind a snowmobile. If we want to do detailed surveys in very local areas, sometime we put these radar on aircraft and we just fly transects to measure the thickness of the ice.
S&S: I didn’t think that radar could penetrate as deep as the ice can be…
Abdalati: It depends on the wave length of the radar—the longer the wave length, the greater its penetration capability—and on properties of the ice. Cold ice is quite a bit more transparent to radar than warm ice. Wet ice is opaque; you can’t really penetrate the water. Under cold conditions, like in the middle of Antarctica or the middle of the Greenland ice sheet, you can go through more than two miles of ice because this is really cold ice and it is ideal for ice-penetrating radar. In coastal areas, where the ice is warmer and where the bedrock geometry is more complicated, it gets harder. In the case of the bedrock geometry, it’s not that the radar cannot penetrate the ice, but that it gets harder to decipher the bottom from reflections off the side of channels through which the ice is flowing.
S&S: What is your current understanding of the key variables and interactions affecting ice flow, melt, and deformation?
Abdalati: As a glacier flows into the sea, it often becomes afloat. In Antarctica and parts of Greenland that floating ice exhibits a buttressing effect, it holds back the glacier. The glacier wants to flow faster, but the floating ice acts like a plug, a barrier to the flow. As oceans and the atmosphere have warmed and the floating ice has melted, many of the glaciers are speeding up, because this buttressing effect is reduced or eliminated. When we observed it—particularly when the Larsen B ice shelf collapsed and the glaciers feeding it flushed their ice into the sea—that was a major advance in our understanding. It was theoretically developed decades earlier, but never really widely accepted until we observed it.
S&S: Is there a direct, linear relationship between the melting of ice sheets and sea levels?
Abdalati: The relationship between the melting of ice sheets and rising sea levels is approximately linear. However, the relationship between discharge—like the speeding up of the glaciers—and sea level rise is non-linear. You have this pent-up energy that’s been there for centuries as the glacier has tried to flow into this ice shelf and its flow has been restricted. Once that barrier is removed, like the collapse of the Larsen B ice shelf, which can be rapid, the response is also rapid. That’s the non-linear response.