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Projects: Observing EarthThe Earth’s Changing Environment as Seen From Spaceby Michael D. King Earth’s land, oceans, and atmosphere are being watched from space. Earth-orbiting satellites, representing a technology only 50 years old, have transformed our understanding of the planet’s natural variability and enabled us to see how our planet is being modified by human activity. This is a perspective on Earth that previous generations could only dream about. The era of remote sensing began in the late 1960s and early 1970s, when instruments on Earth-oriented spacecraft first observed our planet from above. Without regard to political boundaries and human-created structures, these observations opened the public’s eyes to our precious planet, as well as to its vulnerability and susceptibility to natural and anthropogenic development. Consider, for example, that land and ocean surfaces, like the semipermanent ice caps and seasonally varying polar sea ice, often change dynamically, with sometimes strikingly large short-term changes on top of seasonal and interannual variations. Glaciers and other forms of land ice generally change slowly, creeping up or down their tracks, but sometimes they collapse unexpectedly. Can we notice such phenomena while standing on the ice? Of course we can. From the vantage point of space, however, using the extraordinary technological developments of the last decade, we can observe changes in Earth’s environment that are simply not visible, or at least not practically observable, from ground-based, shipborne, or airborne instruments. The Earth-observing satellites of today have evolved in capability from the earliest Earth-orbiting satellites, and they are monitoring and helping us understand the many changes that are rippling through our planet’s climate system. In this article, I will explain some of the ways we watch from space as our planet changes below our “eyes in the sky.” Precipitation—Rain and Snow
Precipitation varies spatially, and there is also considerable variability over the years, which is associated with irregular tropical patterns such as El Niño, La Niña, and the monsoon circulation on the Indian subcontinent. Space-based technologies allow us to track these phenomena worldwide without relying on local rain gauges, and all nations of the world use satellite data to monitor drought, extreme rainfall, floods, and climate variations. Lightning Strikes Figure 2 shows the annual average lightning flash rate, based on satellite observations between 1995 and 2003. These data reveal that the Congo basin of central Africa is the lightning “hot spot” of the world, with a rate of 158 flashes per square kilometer per year, and that lightning is more prevalent in the summer months, owing to deep convective cloud activity. Overall, there is very little lightning north of the east-west band of mountains that extends from Europe across Asia. These mountains serve to limit the poleward flow of tropical moisture.
The Ozone Hole Since 1978, satellites have monitored the ozone hole’s areal extent, depth, and dynamic evolution (see Figure 3). They are crucial in monitoring ozone recovery and the yearly variability expected following the Montreal Protocol, which restricted the emission of ozone-depleting substances. Upper-atmospheric ozone is expected to recover to its 1980 level by about 2070. During the annual peak in ozone hole size, in late September and early October, the ozone hole has attained a size of more than 25 million square kilometers in some recent years, exceeding the area of the entire North American continent.
Constituents of Earth’s Atmosphere Figure 4 shows the annually averaged tropospheric NO2 for the continental United States for 2006, clearly revealing the high concentration of NO2 along the Interstate 95 corridor of the northeastern United States, as well as above numerous cities throughout the country. There is also a high concentration of NO2 along the Ohio River Valley and the Four Corners region of the Southwest, associated with coal-fired power plants.
When plotted globally, the daily satellite data reveal that the eastern United States and Europe (especially the Po Valley of Italy) see a large reduction in NO2 concentration on Sunday, reflecting the day of rest in predominantly Christian nations. In contrast, the Islamic countries of the Middle East exhibit their lowest NO2 concentration on Friday, while in Israel, the lowest NO2 concentration occurs on Saturday, the Jewish Sabbath. This day-of-the-week signal, clearly evident in Figure 5, follows as a consequence of the short lifetime of NO2 in the atmosphere and the religious preferences of people in different regions, which are reflected in the use of transportation conveyances.
Rising Sea Level Sea level, which rose in the late 20th century by approximately 3.1 millimeters (0.1 inch) per year, is not uniform around the globe. Space-based radar altimeters, orbiting Earth continuously since 1992, have allowed us to monitor the spatial and temporal patterns. This would not be possible using only tide gauges located along continental shorelines and a few islands in the open ocean. Figure 6 shows the spatial distribution of sea level change from 1993 to 2006. It reveals that the rise is greatest in the sensitive tropical western Pacific, with its high concentration of low-lying island nations. The change in sea level arises primarily from thermal expansion as the ocean’s surface warms and cools.
Earth’s Surface Temperature Figure 7 illustrates the temperature “anomaly” for January 1–24, 2006 in contrast with the average for that period from 2001 to 2005. In January 2006, the U.S. news media focused on the unusually warm winter in the eastern United States and the abnormally cold winter in Moscow. Figure 7 greatly broadens the discussion, showing the global picture and indicating that the land was more than 10 degrees Celsius (18 degrees Fahrenheit) colder than normal throughout eastern Europe and much of the Russian Federation, as well as in southern Africa, western Australia, and Alaska. The abnormal warmth experienced in the eastern United States extended well into Canada and also was exhibited in east Africa, Tibet, and northern Australia. Observations such as these rely on thermal emissions from the land surface and can be obtained for multiple times of day. They are widely used to forecast crop productivity and to assess drought.
Fire
On a global basis, most fires occur in Africa, primarily as the result of clearing land for agriculture, a practice dating from at least 100,000 years ago. Archaeological evidence suggests that fire might have been exploited in eastern Africa as early as 1.6 million years ago. Although widespread, especially in the tropics, fires are largely set during the “dry season.” The widespread fires seen in Figure 8 in sub-Sahelian Africa occur primarily in December, January, and February, whereas the fires in southern Africa (Angola, Zambia, Mozambique, and Malawi) occur primarily in August, September, and October. Globally, fires in North America are few but intense, and they often are suppressed when they approach residential areas or population centers. Although some of these fires are set by humans, most are started by lightning. Again, satellites provide a uniform and border-independent way of assessing the transformation of the landscape. Changing Land Use
Earth-orbiting satellites enable us to observe our changing environment in
ways that are impossible to accomplish from the ground. The land, atmosphere,
ocean, and cryosphere are readily observable from space, often at less expense
and effort than using land-based methods. This short article cannot address
all the phenomena we monitor from space. Other phenomena not illustrated in
this article include Satellite imagery and data analysis can show more readily than any other means the large-scale state of the Earth-atmosphere-ocean system, its many dynamic processes, and the evolution of conditions worldwide. Our planet is changing, and only from space can we monitor it effectively. Michael D. King is senior project scientist for the Earth Observing System at NASA Goddard Space Flight Center. |
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