The two images above show how the glacial ice in the Brabazon Range in Alaska has changed over the last 25 years. On the time scale of a glacier, the amount of ice is shrinking quite rapidly. Novatak and East Novatak glaciers, located just a few kilometers apart near the outlet of the Alsek River, are two of many retreating glaciers in southeastern Alaska. Over the last 25-years, the ends of Novatak and East Novatak have each retreated by more than a kilometer.
Snowfall in higher-elevation accumulation zones feed both glaciers. Over time, fresh snow compresses into ice that slides through alpine valleys and into a low-lying plain north of the Alsek. Meltwater collects near the ends of the glaciers, forming a sizable proglacial lake. This lake, filled with opaque, light-blue water, is drained by small streams that connect to the Alsek River a few kilometers to the south. The water’s distinctive color is caused by the presence of rock flour, a fine-grained silt formed by glacial ice grinding against rock.
This pair of satellite images illustrates how much both glaciers changed over a 26-year period and how that change affected the lake between them.
Notice how much the terminus of each glacier has shifted. Novatak retreated by about 1 kilometer (0.6 miles); East Novatak moved back by about two. Also note how part of the lake changed color. As the terminus of East Novatak retreated up into a mountain valley, it cut off a small tributary that supplied meltwater to the western lobe of the lake. With the supply of rock flour cut off, that part of the lake became dark blue. Meanwhile, the other side expanded and changed shape as East Novatak retreated.
CITE: NASA Earth Observatory image by Jesse Allen, using Landsat data from the U.S. Geological Survey. Original caption by Adam Voilan.
The images above show the Antarctic ozone hole on September 16 (the International Day for the Preservation of the Ozone Layer) in the years 1979, 1987, 2006, and 2011.
Stratospheric ozone is typically measured in Dobson Units (DU), which is the number of molecules required to create a layer of pure ozone 0.01 millimeters thick at a temperature of 0 degrees Celsius and an air pressure of 1 atmosphere (the pressure at the surface of the Earth). The average amount of ozone in Earth’s atmosphere is 300 Dobson Units, equivalent to a layer 3 millimeters (0.12 inches) thick—the height of 2 pennies stacked together.
In 1979—when scientists were just coming to understand that atmospheric ozone could be depleted—the area of ozone depletion over Antarctica grew to 1.1 million square kilometers, with a minimum ozone concentration of 194 Dobson Units. In 1987, as the Montreal Protocol was being signed, the area of the hole reached 22.4 million square kilometers and ozone concentrations dropped to 109 DU. By 2006, the worst year for ozone depletion to date, the numbers were 29.6 million square kilometers and just 84 DU. By 2011, the most recent year with a complete data set, the hole stretched 26 million square kilometers and dropped to 95 DU.
According to NASA, “The Antarctic hole is stabilizing and may be slowly recovering. Our focus now is to make sure that it is healing as expected.” The amount of ozone-depleting substances (ODS) in the atmosphere has stopped rising in recent years, and may actually be decreasing. The yearly ozone hole should continue for a while, though, as CFCs and other ODSs can last for decades in the air. Scientists found in a 2009 study that without the Montreal Protocol, global ozone depletion (not just Antarctic) would be at least 10 times worse than current levels by 2050.
CITE: NASA animation by Robert Simmon, using imagery from the Ozone Hole Watch. Original caption by Mike Carlowicz.
In early August 2014, not one but two hurricanes were headed for the Hawaiian Islands. Storms arriving from the east are a relative rarity, and landfalling storms are also pretty infrequent. The first image shows a nearly cloud-free eye in the center of a symmetrical storm; there is solid ring of clouds around the center rather than intermittent, spiral bands. Iselle was at its peak intensity at the time. On August 5, the second natural-color image was captured of both Iselle and Hurricane Julio en route to Hawaii. Note that Iselle’s eyewall had grown less distinct; the storm had decreased to category 2 intensity. The bright shading toward the center-left of the image is sunglint, the reflection of sunlight off the water and directly back at the satellite sensor.
CITE: NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response. Original caption by Mike Carlowicz.