Satellite map depicts average height of forests

Using NASA satellite data, scientists led by Michael Lefsky at Colorado State University have produced a map that details the height of the world’s forests.

Although there are other local- and regional-scale forest canopy maps, the new map is said to be the first that spans the entire globe based on one uniform method.

The map depicts average height over 5km2 (1.9 square mile) regions, not the maximum heights that any one tree or small patch of trees might attain.

The work – based on data collected by NASA’s ICESat, Terra and Aqua satellites – should help scientists build an inventory of how much carbon the world’s forests store and how fast that carbon cycles through ecosystems and back into the atmosphere.

Humans release about seven billion tons of carbon annually, mostly in the form of carbon dioxide. Of that, three billion tons end up in the atmosphere and two billion tons in the ocean. It’s unclear where the last two billion tons of carbon go, although scientists believe that forests capture and store much of it as biomass through photosynthesis.

One of Lefsky’s colleagues, Sassan Saatchi of NASA’s Jet Propulsion Laboratory, has already started combining the height data with forest inventories to create biomass maps for tropical forests. Complete global inventories of biomass, when they exist, can improve climate models and guide policymakers on how to minimise the human impact on the climate with carbon offsets.

A forest canopy height map of the contiguous United States
A forest canopy height map of the contiguous US. Credit: NASA Earth Observatory. Image by Jesse Allen and Robert Simmon. Based on data from Michael Lefsky

More immediately, according to Maryland University remote sensing expert Ralph Dubayah, tree canopy heights can be plugged into models that predict the spread and behaviour of fires, as well as ecological models that help biologists to understand the suitability of species to specific forests.