NASA Satellite Now Tracks Fall Leaf Color Across the Whole Planet

NASA's PACE satellite is giving scientists a new way to follow autumn color as it moves across the globe. According to NASA's Earth Observatory, the spacecraft is the first that can measure all three key leaf pigments over the entire planet every day or two.

The agency is also asking the public to help. Photos of fall foliage taken from the ground can help researchers check what the satellite sees.

How does a satellite see fall color?

PACE carries a hyperspectral instrument called OCI, short for Ocean Color Instrument. It observes the oceans and vegetation daily across 286 spectral bands. Some of those groups of wavelengths pick up changes in leaf pigments: anthocyanins, which produce reds; carotenoids, which produce yellows and oranges; and chlorophylls, which are green.

NASA scientists compare how much red, orange and yellow a forest shows against how much green. Following that ratio over time shows where leaves are changing and when the color peaks.

Why do leaves change color at all?

Leaves turn as they lose chlorophyll, the green molecule plants use to make food, NASA explained. Chlorophyll is not stable, so plants must keep producing it, which takes plenty of sunlight and warm temperatures. When temperatures fall and days get shorter, chlorophyll levels drop. The greens fade, and carotenoids and anthocyanins get their chance to show.

What did the satellite find this fall?

NASA published maps of vegetation redness for Sept. 29 to Oct. 4 and for Oct. 5 to 11. On the maps, green areas have values near 1, meaning the satellite detects about as much red as green and most leaves have not yet turned. Light yellow and red areas show progressively more red pigment compared with green.

From Sept. 29 to Oct. 4, the satellite detected particularly high redness in parts of northern and eastern Russia, northern Alaska and northern Canada. During the week of Oct. 5 to 11, redness began to fade in New England and upstate New York, even as it spread across much of the Midwest and the Mid-Atlantic.

Some of the reddest places, including the Adirondacks in New York, the Green Mountains in Vermont and the White Mountains in New Hampshire, have many sugar maples and red maples, which turn crimson in fall. NASA said blueberries, bilberries, sedges, mosses and grasses likely add to the intense reds seen in marshy tundra in northern Alaska, Canada and Russia.

The agency also noted that Landsat 9 captured an image of intensely red foliage around Ripton, Vermont, on Oct. 3, near the peak of redness there. Poet Robert Frost kept a summer home and farm in Ripton.

Does every red patch on the map mean fall color?

No. NASA cautioned that not all the yellows and reds on the maps are autumn foliage. Crops in some farming areas show high anthocyanin levels as they age and after harvest. Clouds can also create what look like gaps in vegetation.

Stress matters too. NASA data scientist Skye Caplan said pigments can change when plants suffer from drought or insect damage. She said such changes likely help explain why PACE measures high redness in agricultural regions of Brazil, Australia and Canada that do not follow broader seasonal patterns.

What does this mean for leaf peepers and researchers?

Other satellites with hyperspectral sensors can measure foliage pigments in a similar way, NASA said, but only over smaller areas and less often. Karl Huemmrich, a vegetation scientist at the University of Maryland, Baltimore County and a member of the PACE science team, called repeated measurements of these pigments everywhere on Earth “game-changing.” He said the time series lets scientists determine peak redness and yellowness anywhere on the planet.

For people who enjoy following the season, Caplan has been posting weekly foliage maps as part of the North American Phenology Campaign, run by the GLOBE program. The maps show waves of red emerging in the high latitudes in August and moving toward the equator through September and October.

For scientists, the team hopes the data will eventually support research on several questions: how the timing of spring greening and fall leaf aging is changing, how vegetation changes affect the global carbon cycle, and how leaf color influences wildfire risk. They also want to learn whether PACE can detect early warning signs of drought, insect infestations and plant disease.

How can the public take part?

The PACE team is asking people to submit photos of fall foliage so scientists can check the satellite observations against what is actually on the ground. Caplan said more ground photos are better, and the pictures help refine how PACE reads vegetation color.

Participation through the GLOBE campaign is not limited to people near dense forests or postcard-style foliage. Scientists also want observations of shrubs, grasses, wildflowers, evergreens, crops and other vegetation.

Around Prague, based on reporting by NASA