New measurements may answer warming queries

Washington-Scientists may soon get a firmer grasp on the impact of global warming after they obtained measurements of upper atmosphere temperatures, iron densities, and mesospheric clouds over the North and South Poles.

"Measuring temperature profiles over the poles is essential for validating

global circulation models and for providing a baseline for assessing the impact of global warming over the coming decades," said team leader Chester Gardner, a professor of electrical and computer engineering at the University of Illinois. "Until now, we were limited to measurements taken with balloon-- borne sensors to altitudes of less than 20 miles [32 kilometers]."

In collaboration with scientists at The Aerospace Corp. and the National Center for Atmospheric Research (NCAR), Gardner helped develop a more robust lidar (radarlike laser) system for measuring temperature profiles from the middle of the stratosphere (about 20 miles, or 32 kilometers, up) to the lower thermosphere at the edge of space (about 70 miles, or 110 kilometers, above Earth). The system uses two lasers operating in the near-ultraviolet region of the spectrum and two telescopes to detect the laser pulses reflected from the atmosphere.

The researchers use two techniques for determining temperature. For altitudes up to 50 miles (80 kilometers), they measure the amount of laser light reflected from air molecules to derive the temperature profile. For higher altitudes, they measure the scattering of the laser beams from iron atoms deposited in the upper atmosphere by vaporized meteors.

In June 1999, the scientists flew the lidar system over the North Pole aboard an NCAR research plane to obtain temperature and iron density measurements during the Arctic Mesopause Temperature study. Six months later, they took the instrument to the AmundsenScott South Pole Station, where it is now measuring the atmospheric temperature structure throughout the year.

"Temperature profiles obtained in the thermosphere over the North Pole on 21 June 1999 and in the menopause region over the South Pole on 27 January 2000 agreed closely with model predictions," Gardner said. "Significant departures from the model were observed during the austral fall, however. On 8 May 2000, for example, the lower mesosphere was about 20 deg C [36 deg F] warmer, and the upper mesosphere was about 20 deg C cooler than predicted."

Gardner also measured the heights of polar mesospheric clouds that formed over each of the poles during midsummer. These clouds are the highest on Earth, forming at an altitude of about 52 miles (84 kilometers). Their brightness and geographic extent have been increasing during the past four decades. Scientists believe these changes may relate to increasing levels of atmospheric carbon dioxide and methane, which in the upper atmosphere lead to cooler temperatures and increasing levels of water vapor.