What do you learn about lakes by looking at them from space?

One of the great benefits of going to space is being able to look back on Earth. Our understanding of the Earth system and humanity’s interaction with it are greatly informed by space-borne sensors. The Earth system is exceedingly complex and is made up of various subsystems. One area where spaceborne sensors can lend insight is limnology, which is the science that studies inland bodies of freshwater. To learn more about what one can learn about lakes by looking at them from space, we spoke to Moritz Lehmann. He is a Senior Scientist at Xerra, a New Zealand-based institute that specializes in Earth observation. 

What have you learned about New Zealand’s lakes through satellite-based remote sensing?

New Zealand has over 3,800 lakes. One of the most interesting things that I have learned from looking down at them from space is that we can find an almost global range of lake types in this relatively small country. Our lakes are diverse in their origins. They have been created by glaciers, volcanic activity, rivers, and wetlands. This diversity creates huge ecological diversity and makes Aotearoa a great place to study limnology, which is the science of lakes. It also makes the country a great place to develop remote sensing applications. From space, the diversity of lakes is visible by the colour of the water. The colours range from bright turquoise to deep blue, green, brown, and even red. This makes for some really beautiful imagery. 

How valuable is space-based observation for learning about the Earth system?

Space-based platforms allow us to observe large areas of the Earth’s surface in one go, to repeat these observations almost daily, and to go back in time through decades of archived satellite images. It is important to realize that remote sensing is not about taking pretty pictures. We use remote sensing to make measurements in actual geophysical units. We actually measure the number of photons reflected by the surface and can use that to calculate the temperature of the surface water, the amount of biomass in a forest, and the types of minerals that rocks are made of. We can measure the concentrations of greenhouse gases such as methane and carbon dioxide in the atmosphere. This allows us to continuously take the pulse of our planet and to test our models of the Earth-atmosphere system. These models, in turn, are essential tools in combating climate change. 

In my own field of aquatic remote sensing, satellite observations have revealed a global increase in algal blooms in lakes over the last 40 years. Here in New Zealand, we have been able to show that the water clarity of the Rotorua lakes improved over the last 18 years. Without satellite observations, such studies would have required immense efforts to collect data in the field. 

What are some interesting areas of research for individuals considering working in Earth observation?

The cool thing about space-based Earth observation is that there is currently lots of momentum in the development of new sensors and the satellites that carry them. Related to this is the growth of applications using the data produced by the sensors. As new space companies emerge and mature, there are opportunities to develop optics, electronics, software, and communication tools for spacecraft.

As an environmental scientist, I am really excited about using data from new types of sensors that open new ways of seeing our planet. Space-based lidar, for example, will provide a 3-D view of the upper ocean. The MethaneSAT mission, on the other hand, will pinpoint sources of one of the most potent greenhouse gases globally. 

I work a lot with hyperspectral data. Hyperspectral sensors see reflected light at hundreds of colours, instead of just three or six as most existing sensors do. As an analogy, your kid may be handed a budget box of red, green, and blue crayons in a restaurant to pass the time. Compare the result with a drawing that you can make with the deluxe box of 64 colours. You can draw different kinds of plants, draw rocks with different mineral content, or differentiate nuances in the blue hues of clear glacial lakes. A number of hyperspectral satellite sensors are due to launch soon and will provide data to answer questions with new levels of detail.