Hi there! My name is Eli Melaas and I am currently a postdoctoral researcher in the Department of Earth and Environment at Boston University.
As you have now learned through Season Spotter, PhenoCam images provide crucial information about the timing of spring and autumn phenology events in forests across North America. However, there are millions of acres of forests on our planet and tracking them all with cameras would undoubtedly be a challenging task! That’s where satellite remote sensing plays a critical role in helping scientists and policy makers understand how climate change is affecting the environment across continental scales.
At Boston University, our research involves developing methods to detect phenological events using satellite images captured more than 700 kilometers (roughly 440 miles) above the Earth’s surface by an instrument known as the Moderate Resolution Imaging Spectroradiometer (MODIS). In addition to having a mouthful of syllables, a spectroradiometer is essentially a very elaborate camera designed to measure the radiance of light across a set of spectral ranges or “channels” (e.g., red, green, blue, near infrared, etc.). As the Earth turns on its axis, two satellites carrying the MODIS instrument called Terra and Aqua orbit the planet from pole to pole while MODIS takes images of swaths of land and ocean.
Each day, MODIS acquires spectral data across nearly the entire Earth’s surface and data are ultimately gridded in 500 meter or 1 kilometer pixels (roughly one third the size of New York’s Central Park).
As this next figure nicely demonstrates, to the normal human eye, leaves appear to be colored green because leaves reflect slightly more green light than other visible wavelengths (far left hump).
Even more importantly, leaves may reflect as much as 40 to 50 percent of near infrared light due to their internal cellular structure. Therefore, as leaves unfold and mature during spring, the simultaneously increase of green light and near infrared light is an effective way to track leaf development. Similarly, as cell walls in leaves break down in autumn, decreases in green light and near infrared light are used to indicate senescence, or deterioration with age.
MODIS was launched into Earth orbit in 1999 on board Terra and in 2002 on Aqua. With 15 years of data now available, scientists have a lengthy record of how forests are responding to climate change and variability. To assess the accuracy of these records, ground observations are needed at an appropriate spatial scale to match the footprint of the satellite’s measurements. Until recently, most observations were limited to individual trees. However, with the advent of PhenoCams, proper cross-scale comparisons are now feasible and projects like Season Spotter are very exciting!