Heating up

Summer is now over (at least for those of us in the Northern Hemisphere) but in New England the weather remains mostly warm and dry, exacerbating the drought conditions of the past months that have, in several areas, produced some of the lowest crop levels in years. Such conditions bring to mind the ongoing worldwide efforts to understand global warming and its potential consequences for the environment.

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Summer is now over (at least for those of us in the Northern Hemisphere) but in New England the weather remains mostly warm and dry, exacerbating the drought conditions of the past months that have, in several areas, produced some of the lowest crop levels in years. Such conditions bring to mind the ongoing worldwide efforts to understand global warming and its potential consequences for the environment. According to the U.S. Environmental Protection Agency, global mean surface temperatures of the Earth have increased 0.5°F to 1.0°F since the late 19th century. The agency suggests that the average global surface temperature could rise another 1°F to 4.5°F (0.6°C to 2.5°C) in the next 50 years, and 2.2°F to 10°F (1.4°C to 5.8°C) in the next century.

As we move into the winter months NASA plans to give global-warming research a boost with the launch of a significant new tool. The Ice, Cloud, and Land Elevation Satellite (ICESat) shown on the this month's cover, carries the Geoscience Laser Altimeter (GLAS), which is intended to measure ice sheet mass balance, cloud and aerosol heights and optical densities, height of vegetation, and land topography (see p. 95). If all goes well, measurements from the ICESat will contribute to a more comprehensive assessment of global warming and its consequences by clarifying, for example, if the ice caps actually are shrinking.

Another contribution to fully characterizing the environment is the ability to assess cloud cover, which has a significant impact on climate. Measuring and monitoring parameters like altitude, thickness, and particle size of clouds is crucial, not only in climatology but also for aviation. A relatively recent breakthrough in cloud lidar observation involves detecting and processing the "off-beam" lidar return signal, which provides information about cloud thickness and density not available from standard lidar (see p. 101).

Light-based remote sensing techniques such as lidar rely on characterizing the effects that the area being probed has on a transmitted laser pulse. Tunable sources are a crucial aspect of modern lidar because they increase system utility without requiring multiple sources. One of the latest tunable lasers to emerge is based on chalcogenide (see p. 75). Detectors are the other half of the sensing equation and, like sources, come in many shapes and sizes. The intrinsic gain of avalanche photodiodes makes them uniquely suited to certain infrared applications (see p. 89).

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Stephen G. Anderson
Associate Publisher/Editor in Chief
stevega@pennwell.com

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