Aeronautics and Space Administration (NASA) estimates at least 5,000 satellites orbiting Earth as of 2007. Beginning in 1979, dedicated weather satellites have gathered climate-related atmospheric data. Most commonly, measurements have been of atmospheric temperature.
Temperature data collected from the region of the atmosphere known as the troposphere—the region where most weather occurs—has not been consistent with surface temperature measurements, making it difficult to assess if human, surface-based activities were influencing the atmosphere. However, analysis of satellite measurements collected since 2000 indicates that the troposphere is warming, and at a rate faster than the warming of Earth's surface.
Historical Background and Scientific Foundations
The satellite era began on October 4, 1957 with the launching of Sputnik 1 by the Soviet Union, as part of the International Geophysical Year. Although the Sputnik series of satellites were intended more as a demonstration of the Soviet Union's prowess over the United States in space technology, the utility of satellites in the gathering and transmission of information was soon recognized. Sputnik 3, which was launched in February 1958, carried an array of instruments intended to record measurements of the Van Allen radiation belts.
The first dedicated weather satellite, Vanguard 2, was launched on February 17, 1959 by the U.S. Department of the Navy. It was designed to measure cloud cover. Technical difficulties in maintaining orbit restricted the information that could be gathered. The TIROS-1 satellite launched by NASA on April 1, 1960 proved to be more successful, gathering climate-related data for over two months. This success led to the launching of the series of seven Nimbus weather satellites by NASA and later by the National Oceanic and Atmospheric Administration (NOAA). The Nimbus
program began in mid-1984 and ran for the next 20 years.
Other satellites designed for meteorological measurements have and continue to be launched in the United States and elsewhere, such as through the French Space Agency and the European Space Agency (ESA). For example, in May 2007, the ESA's Met Op-A polar-orbiting meteorological satellite that was launched about six months earlier began transmitting data of temperature, humidity, wind speed over the surface of the Arctic ocean, ozone level, and levels of gases including carbon dioxide (CO2), nitrogen dioxide (NO2, an industrial pollutant), and methane (Ch4). The information will be useful in monitoring Earth's climate and in assessing the influences of human activities on global climate.
A satellite can be launched so that it orbits Earth at a low altitude (several hundred miles above the surface) or at a high altitude that can be thousands of miles high. Lower-orbiting satellites move at a higher rate of speed than the speed at which Earth is rotating, and so will circle the globe approximately 14 times each day. This sort of weather satellite is suited for monitoring climate conditions at various locations throughout theday,oratgiven locationsatdifferent timesduring the day.
A considerable amount of geography can be monitored by a low-orbiting satellite at any one point in time. For example, NOAA polar orbiting satellites that circle Earth at an altitude of 600 mi (965 km) can simultaneously scan the entire east coast of North America from the southern tip of Florida to Hudson Bay, and from the Atlantic coastline to Lake Superior (the most westerly of the Great Lakes).
Higher-orbiting satellites traveling at approximately 22,237 mi (35,786 km) above Earth's equator adopt what is referred to as a geostationary orbit. This means that the satellite circles Earth once every 24 hours. Since this occurs at the same pace as Earth spinning on its axis, the satellite remains poised over the same point on the planet. Such satellites are capable of constantly obtaining measurements from the half of the Earth that faces it.
Satellites do not measure temperature in the same way that conventional thermometers do, namely by the temperature-dependent upward or downward movement of liquid (typically mercury or alcohol) in a narrow chamber, or the expansion or contraction of a coiled piece of metal. Instead, the microwave portion of the light spectrum is measured using an instrument called a microwave sounding unit. Infrared radiation can also be measured. Mathematical calculations applied to the data give an indication of temperature, since heat is absorbed differently by land, water, and the atmosphere, and detected heat energy also differs. An example is the Gulf Stream, the Atlantic Ocean current that parallels the east coast of North America before veering over to pass in a southerly direction past the western coast of the United Kingdom. In a satellite image, the Gulf Stream appears darker against the lighter (and cooler) surrounding ocean water.
Software now enables meteorologists to select any portion of an image and measure the temperature to within 2°F (1.1°C) of what would be obtained by surface measurements. These determinations can be done even if the surface is obscured by clouds.
Impacts and Issues
Satellite measurements have permitted the monitoring of greater areas of the atmosphere (in particular the troposphere) than is possible using instrumentation carried aloft in a weather balloon.
Comparison of satellite temperature data collected since 1979 with data from weather balloons that has been collected since 1958 has produced different results. This has hampered the assessment of climate change. Specifically, while ground temperatures have been increasing at a rate of 2.7°F ± 0.1°F (1.5°C ± 0.05°C) per decade during the 1980s and 1990s, the temperature of the troposphere was apparently increasing by only 0.1°F ± 0.02°F (0.05°C ± 0.01°C).
WORDS TO KNOW
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
INTERNATIONAL GEOPHYSICAL YEAR: Internationally coordinated, first-of-a-kind effort to enhance scientific understanding of Earth processes, including solar activity, the oceans, the atmosphere, polar regions, and more: lasted from July 1, 1957, to December 31, 1958.
MICROWAVE SOUNDING UNIT: Device carried by some Earth-observing satellites that measure microwave-band electromagnetic radiation emitted by Earth's atmosphere in order to measure temperature, humidity, precipitation rates, and snow and ice coverage.
OZONE: An almost colorless, gaseous form of oxygen with an odor similar to weak chlorine. A relatively unstable compound of three atoms of oxygen, ozone constitutes, on average, less than one part per million (ppm) of the gases in the atmosphere. (Peak ozone concentration in the stratosphere can get as high as 10 ppm.) Yet ozone in the stratosphere absorbs nearly all of the biologically damaging solar ultraviolet radiation before it reaches Earth's surface, where it can cause skin cancer, cataracts, and immune deficiencies, and can harm crops and aquatic ecosystems.
TROPOSPHERE: The lowest layer of Earth's atmosphere, rangingtoanaltitude of about9 mi (15km) aboveEarth's surface.
VAN ALLEN RADIATION BELTS: Two regions of space near Earth where fast-moving charged particles (electrons and protons) are confined by Earth's magnetic field. The belts are shaped somewhat like huge hollow doughnuts, one nested inside the other with Earth in the center hole.
IN CONTEXT: TEMPERATURE DATA DISCREPANCY
According to the National Academy of Sciences: “One issue that had emerged in the climate change debate is whether temperature measurements are reliable. Temperature readings of the lower atmosphere taken with satellites appeared to show less warming than thermometer readings taken near the surface of the Earth. Some scientists interpreted this discrepancy to mean that global warming is not occurring and that the surface data are flawed.”
“A report released in 2000, Reconciling Observations of Global Temperature Change, examined this conflict in detail and concluded that the warming trend in global-average surface temperature observations during the past 20 years is undoubtedly real. The report spurred other groups to examine the discrepancy; they found that trends in satellite data were affected by instrument calibration and difficulties in interpreting temperature readings. Recent satellite and surface data are in better agreement, largely resolving the dispute.”
SOURCE: Staudt, Amanda, Nancy Huddleston, and Sandi Rudenstein. Understanding and Responding to Climate Change. National Academy of Sciences, 2006.
This difference was difficult to reconcile with the idea that the build-up of greenhouse gases in the atmosphere due to human activities was warming Earth's atmosphere. As a result, surface temperature readings from technologically advanced countries such as the United States tended to be favored, which eliminated large regions of the planet's surface and unpopulated polar regions. The lack of data from polar areas has been especially troubling, since available surface measurements have confirmed that these regions are warming more quickly than elsewhere.
However, a 2004 study published in the journal Nature, in which researchers re-examined surface and satellite measurements from 1979 to 2001, determined that the atmospheric measurements were skewed by the mechanics of the satellite instrumentation. More precise analysis of the data revealed that the warming of the atmosphere is indeed similar to surface warming, providing further evidence of the reality of global climate change.
Flannery, Tim. The Weather Makers: How Man Is Changing the Climate and What It Means for Life on Earth. Jackson, TN: Atlantic Monthly Press, 2006.
Fu, Q., C. M. Johanson, S. G. Warren, and D. J. Seidel. “Contribution of Stratospheric Cooling to Satellite-Inferred Tropospheric Temperature Trends.” Science 429 (2004): 55-58.
Wentz, F. J., C. Gentemann, D. Smith, and D. Chelton. “Satellite Measurements of Sea Surface Temperature Through Clouds.” Science 288 (2000): 847-850.
Brian D. Hoyle