Earthshots: Satellite Images of Environmental Change

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» Yellowstone National Park, USA

The summer of 1988 was unusually dry in the western United States. Dry cold fronts brought high wind and lightning but no rain. Along with high temperatures, these conditions created extreme fire danger.

Yellowstone National Park encompasses about 2,221,800 total acres. By mid-July 1988, 8,500 acres had burned in greater Yellowstone. That’s not too unusual. But by the end of July, the dry conditions were quickly making the fires uncontrollable, and the fires grew to about 99,000 acres. On August 20, strong winds increased the size of the fires and 150,000 acres burned on that one day. The fires did not stop until snow came in September. In total, more than 793,000 acres, or 36 percent of the park, burned.

Yellowstone is representative of temperate mountain ecosystems throughout western North America. What is learned from the massive 1988 fires and subsequent recovery of these ecosystems can be applied to other regions.

Map of the featured area.

In these Landsat images, the old forest is dark green. In the 1987 image, this forest is occasionally broken up by lighter green grassy meadows or grassy plains. Geyser fields are pale blue and white. In a few places, a pink tinge may indicate old burn scars or dormant plants.

The August 23, 1988, image was acquired when many fires were active. Smoke obscures some of the land, showing up as blue because the image uses infrared wavelengths of light. Land just burned is dark red. Lighter red patches are less severely burned.

The band combination of shortwave infrared, near-infrared, and visible green (bands 7,4,2) highlights the changes in vegetation caused by the fires and recovery. Dark red burn scars fade over time as vegetation recovers. Grasses and wildflowers grow out of the ashes, and young trees begin to take root and grow. These light green areas start replacing the red and pink from the burn scar.

As intense as the fires were, studies showed that less than 1 percent of soils were heated enough to sterilize the soil to kill belowground plant seeds and roots. Grass actually prospered in the rich soil immediately after the fires because of the release of nutrients and the decrease in shading from shrubs and trees. Less competition with other plants for resources also allowed grass to flourish.

Even though grass flourished and saplings soon emerged, the recovery is gradual. Even in the 2011 and later images, the burned areas from 1988 can still be seen. The high elevation in this area causes a short growing season. With the hot, dry summers and cold, harsh winters, the forest will not return to its prefire conditions for decades. But recovery will continue.

A few new fires show up in the later images to show that fire continues to be a part of the Yellowstone wilderness.

This series shows the area around the famous Old Faithful geyser. Geysers and hot pools are the bright or blue spots within the dark green forest.

The vertical line on the left side of the images is the western park boundary. The patchy spots to the left of the line are logging areas outside the boundary. The 2011 image reveals another smaller burn scar just to the northwest of the airport at West Yellowstone, Montana, and the 2016 image shows another new burn scar at the top edge.

The fires scorched a lot of the park, but they did not destroy forests in one large swath. The severity of the burn varies. Unburned land is interspersed with the burned area. Dark red areas are severe burns, and lighter red areas are less severe.

Over time, the pink bare ground begins to give way to light green as vegetation returns.

As you look through this time series, see if you can identify new fires that occurred after the big 1988 fires. Even the latest image is a patchwork of undisturbed forest, areas at different stages of recovery from fire, and fresh burn scars.

Fires continue to burn in Yellowstone, and future fires are expected. Scientists have noticed that the average number of lightning-started fires has been increasing each year since the 1990s. In the near future, the western United States will likely have increasingly intense wildfires.

Besides some recovery from the 1988 fires on the west side of Yellowstone Lake, this series of images shows the effects of another fire.

Frank Island is the largest island in Yellowstone Lake. The island was set up as a protected area for ospreys that nest on the island. The public is not allowed on the island during the summer. Only a small picnic area on the southeast point is open for visitor use.

On August 8, 2003, lightning struck Frank Island. An inferno quickly engulfed nearly all of the old growth trees. Most of the forest on the island burned.

The first image in this series is from one week before the lightning strike that started the fire. The island appears deep green in the middle of the lake. In the other images, virtually no vegetation remains on the island. The green was replaced by pink, which indicates barren ground.

In natural color satellite images of wildfires, smoke often obscures the view of the ground. In the natural color image at the left, Yellowstone Lake is not even visible. Landsat uses several infrared wavelengths, or bands, of light. If some of those infrared wavelengths are used, then different details about the location of burned areas emerge.

The combination of bands we have been looking at use two infrared wavelengths (bands 7 and 4) along with one visible band (band 2, green). This combination shows freshly burned areas as deep red and vegetation as green. Smoke from fire appears blue. The infrared bands help us see the burned areas clearly, but the smoke can still get in the way.

The third image at the left uses Landsat’s thermal infrared band (band 6) and a shortwave infrared band (band 5) to cut through the smoke to see ground conditions. In this false color image, burned areas are red-orange. This view can help people see where fire might be approaching populated areas.


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