One of the interesting fields in remote sensing science is the study of albedo, a term derived from the Latin word for whiteness. Albedo is another name for reflectivity. The albedo of a surface determines how much sunlight will be absorbed and warm the surface compared to another surface that reflects most of the light and does not change temperature.
Landsat Science Team's Crystal Schaaf Discusses Albedo, Its Importance, and How It Can Affect Climate
Those who study albedo have long known that white snow, ice, and bright sands reflect a lot of the sun’s energy back into space. Darker surfaces, like oceans and forests, absorb energy, warming the water and land. Researchers also know that humans often change albedo through such activities as deforestation and urbanization. When that reflectance is altered, it can have implications for weather and climate.
Dr. Crystal Schaaf, a professor of remote sensing in the School for the Environment at UMass Boston, understands the influence of albedo well. She’s a member of the Landsat Science Team who has spent a considerable amount of her career looking at albedo, its importance, and its impacts. She recently talked about albedo and her work—as well as that of her team of graduate students and researchers—in this conversation.
When it comes to albedo and energy, explain how energy balance works on the Earth, and the value of albedo in studying that balance.
“Well, obviously all of our energy comes from the sun, and how warm or how cold we are at the Earth’s surface has a lot to do with how much energy is being absorbed by both the Earth’s surface and the Earth’s atmosphere. So, if the Earth’s surface is changing in any way (or we are changing it), then the albedo that’s reflecting the sun’s energy that could be used to warm us up may also change.”
Tell us how such things as changes in seasons or disturbances on the landscape alter albedo over time and space.
“On an annual cycle, especially for us in the more temperate and higher latitudes, the biggest change that happens to albedo is snowfall. So, if we have a boreal forest up in Canada where the snow falls on the forest for a fairly short time, it may be quite bright and white and reflect a lot of energy. But then as the snow sifts through the canopy, it will turn back to a dark forest and will absorb more energy than it reflects. But if it’s a deciduous forest, if it’s maples and beech and all those sorts of forests, then there’s no leaves, and the snow filters right down to the ground. So, the surface there can be quite bright quite a long time after the snowfall has occurred. So, snow is the biggest impact.”
Do you have other examples?
“Certainly. Events like flooding or timbering are examples. Obviously, if you’ve got a conifer forest, a boreal forest, and suddenly it becomes a field, then the snow will become very obvious in the winter. The lush green deciduous foliage growing up after such a harvest tends to be brighter than the original dark conifer forest. All those sorts of variations play into it. In agriculture, right after the farmers till the fields, it’s dark, rich earth, which is absorbing a lot of solar energy until the plants start to grow. Then they come in as a faint green canopy, and as they get lusher and lusher, they can start to absorb more energy as well. So, it all has to do with what the surface cover is. What the structure is.”
What’s the value in understanding that information?
“We mostly care about it because we want to know what our local energy balance is. How is that going to affect our local temperatures? If it’s absorbing more energy, we’re going to have warmer temperatures in a certain location. This can impact the local weather. And as far as annual cycles of vegetation, if a lot of the energy gets reflected off the top of the canopy, it isn’t going to filter down into the lower understory, provide energy, and drive photosynthesis at the lower areas. So, it has a pretty big role ... a pretty important role.”
Give us some other examples of why variability in albedo at local scales is important for understanding how the land affects weather and climate?
“When we’re worrying about meteorology or predicting the weather, we really care about what the surface temperatures are, what the variability is in the surface temperatures. Obviously, if you’ve got a field that’s bright white because it’s covered with snow next to a dark conifer forest, the solar energy is mostly going to be absorbed in that part of that local area that has the dark forest. So, there’s going to be some real temperature changes between those two areas. That can affect how your wind patterns and surface-atmosphere mixing are going to progress. So, from a local standpoint, you do care about those things.”
Do people like agricultural producers and forest managers care about albedo?
“Agricultural producers and forest managers care about knowing what the albedo is because they’re trying to do a better job in tracking vegetation growth, vegetation change. Sometimes it’s easier for us to recognize disturbances by looking at them when there’s snow on the ground, because again, over time, the albedo suddenly become bright white. I’m also thinking about things like pest disturbances in forests. You think about out west, about the pine bark beetle. Here in the east, it’s the hemlock woolly adelgid. As they start to affect our canopies, our areas become much more open. We can see the snow better in the winter, and so we see a much brighter albedo. So, they actually act as a signal to tell us that, ‘oh, this forest is declining.’ Recovery after a fire is another example. If there’s been a big fire up in the north, for a while, that location will be very flat and open, and will show up as a bright white surface. But you can see as the canopy regrows around the edges, how the forest regrows, and recovery happens.”
Would you expect that albedo plays any kind of role in localized, climate-related events, like the wildfires going on in California, for example? Does changing albedo influence turbulence or stronger winds that drive wildfires?
“Not so much the immediate winds. But certainly, as I say, things like forests that have been impacted by disturbance and have become dried out. The drought relates directly to the fact that the understory, the soil, has gotten dry, and tinder dry. That means the albedo is higher than it should be. Reduced moisture makes triggering wildfires quite easy because there’s all that dry vegetative fuel. Also, if a forest has been infected by a pest for a long time, there’s just a bunch of old stumps still standing there, and they can just act as torches. They’ve lost their moisture content, and they show up much more brightly in albedo. So again, albedo is a way to monitor how things are changing.”
Has Landsat been a good tool for looking at albedo, and how albedo changes over time?
“Landsat is spectacular because it’s been around for a long time. We have this amazing record back over the decades now, so we actually can see how things have changed. And Landsat also has relatively fine resolution. The fact that I can now start to build albedo measures at 30 meters is a big improvement over the 500-meter gridded products from coarser resolution satellites. It gives us a lot more of this variability within a small area, so that we really can say something about how the Earth’s surface is absorbing and reflecting energy.”