Human activities are changing the Earth and its atmosphere. Long-term records show a rise in the global average temperature over the past few decades. Other observations reveal changes in the composition of the Earths atmosphere such as thinning of the stratospheric ozone layer and increases in the concentration of greenhouse gases. Scientists do not fully understand how these changes will affect climate. Therefore, highly accurate, long-term measurements are essential for gaining a better understanding of the processes that control climate change.
The Stratopsheric Aerosol and Gas Experiment (SAGE III) is a fourth-generation satellite instrument designed to observe the long-term health of the upper atmosphere. Managed by NASA Langley Research Center, SAGE III is part of NASAs Earth Science Enterprise program of climate research. It launched aboard a Russian Meteor 3M spacecraft in December 2001 for a three year mission, a collaboration between NASA and the Russian Aviation and Space Agency (RASA). It extends a long-term working relationship between the United States and Russia to understand Earths environment.
The goal of SAGE III is to measure high-resolution vertical profiles of key components of the upper atmospherethe most important being ozone, aerosols, (suspended particles) and water vapor. These measurements will enhance our understanding of climate and how human activities influence it. This information will enable national and international leaders to make informed policy decisions on climate change.
Aerosols have many natural and human-induced sources such as smoke from forest fires, wind-blown dust, pollution or volcanic eruptions. When present in large concentrations, aerosols can reflect significant amounts of solar radiation back to space and cause cooling at the Earths surface. Depending upon the chemical composition of aerosol particles, they can absorb radiation from the sun or emitted from the Earth, causing the atmosphere to warm. Aerosols can also strongly influence atmospheric chemical processes, including those that control ozone. Because the characteristics of aerosols can vary considerably, understanding how aerosols affect climate is one of the major problems confronting atmospheric scientists.
SAGE II observed the long-term global effects of the June 1991 eruption of the Mt. Pinatubo volcano in the Philippines. The eruption produced large quantities of aerosols in the upper atmosphere. The top-left graphic from SAGE II data shows a relatively aerosol-free atmosphere before the eruption. The top-right graphic reveals that aerosols in the tropics increased by almost a factor of 100 immediately following the eruption. The bottom-left graphic shows that aerosols had spread into the Earths mid-latitudes three months later. The bottom-right graphic illustrates how volcanic aerosols slowly decreased in the atmosphere over several years. The effects of Mt. Pinatubo lingered for up to 10 years following the eruption. The global distribution of aerosols as shown in these images is one of many important stratospheric processes that SAGE III will monitor.
The SAGE III instrument will measure the distribution of aerosols from the middle troposphere through the stratosphere. For example, the SAGE II satellite instrument, the predecessor to SAGE III launched in 1984, observed dispersal of volcanic aerosols following the massive eruption of Mt. Pinatubo in 1991. These measurements were crucial in linking a decline in the globally averaged surface temperature in mid-1992 of about 1 degree Fahrenheit to the large aerosol concentrations from the volcanic eruption. Aerosols from Mt. Pinatubo also strongly influenced the observed ozone trendan effect that would not have been detected without measurements like those from SAGE II. The data provided unique insight into the complex flow of air in the stratosphere that is needed to gain a better understanding of how the upper atmosphere will respond to climate change.
In the stratosphere, ozone shields life at the surface from harmful
solar ultraviolet radiation and plays an important role in controlling
the circulation of air in the upper atmosphere. Changes in ozone
distribution, like the ozone hole that forms in the spring over
Antarctica, are also a concern to health officials because of the
possibility for increased cases of melanoma and other skin cancers,
cataracts and immune deficiencies in humans. Scientists now realize that
ozone is being destroyed over the Arctic during late winter and more
slowly over middle latitudes. A primary objective of the SAGE III
instrument is to make accurate, long-term measurements of the
concentraton of stratospheric ozone and other chemical species that
control the distribution of ozone.
SAGE measurements of stratospheric ozone extend from 1979-1981 and 1984-present. This long-term, stable data set has proven invaluable in determining the decadal trend in ozone particularly in the lower stratosphere. SAGE ozone measurements are a key element in on-going assessments of ozone trends by SPARC (Stratospheric Processes and their Role in Climate) and UNEP (the United Nations Environmental Programme). SAGE III will extend this data set through much of the next decade. (Image courtesy NASA Langley Research Center)
Atmospheric water vapor plays an important role in the Earths energy
balance, in many chemical cycles and in tracing the exchange of air
between the upper and lower atmosphere. Water vapor is the most
abundant, naturally occurring greenhouse gas and traps outgoing energy
in the atmosphere that is radiated from the Earth. Precise measurements
of water vapor by SAGE III will provide important contributions to
understanding how this process warms the Earths atmosphere. Evidence
also indicates that water vapor in the upper atmosphere is increasing.
This increase is not well understood, but it could affect climate, alter
circulation patterns and allow ozone loss in the Arctic to occur more
easily. Measurements by SAGE III will provide a crucial new
understanding of how water vapor is circulated in the atmosphere and how
it is increasing with time.
The image above shows the difference in atmospheric water vapor concentrations between January and July. During summer (July in the Northern Hemisphere and January in the Southern) the warmer atmosphere holds more water (red) than during the colder and drier winter. The data were collected by the SAGE II instrument during 1986, 1987, and 1988. (Image courtesy NASA Langley Research Center Atmospheric Sciences Data Center.)
The SAGE III instrument operates by measuring the amount of solar light as it passes through the limb or edge of the Earths atmosphere as the spacecraft views the rising and setting of the sun during each orbit. The instrument measures light at wavelenghts in the part of the electromagnetic spectrum visible to the human eye, allowing it to make very accurate measurements of aerosols, ozone, water vapor and other trace gases. The advanced SAGE III instrument will also make measurements using moonlight that will provide new observations of trace gas species that affect ozone distibution. SAGE III willl also make essential temperature and pressure measurements for helping scientists better understand climate change.
A limb measurement is taken as a satellite instrument views sunlight through the atmosphere as it ascends or descends from behind the Earth.