Reconciling the Differences
|Currently, both Lindzen and Lin stand by their findings and there is ongoing
debate between the two teams. At present, the Iris Hypothesis remains an
intriguing hypothesis—neither proven nor disproven. The challenge facing
scientists is to more closely examine the assumptions that both teams made about
tropical clouds in conducting their research because therein lies the
The next step in resolving the Iris debate will be to use satellite-based
measurements of clouds’ physical properties—like cloud altitude, fraction,
thickness, and ice/water particle phase. Sensors on the TRMM and Terra
satellite missions routinely measure these cloud physical properties, which
scientists will match in time and space with CERES’ new measurements of
radiant energy fluxes. This matching will then allow scientists to determine
whether the Iris Hypothesis works for a wide range of ice cloud conditions. The
Moderate Resolution Imaging Spectroradiometer (MODIS), aboard Terra, has a
channel uniquely sensitive to cirrus clouds. By discreetly measuring radiant
energy at a wavelength of 1.38 micrometers, MODIS can precisely measure thin
cirrus clouds even when they occur over bright low clouds, providing scientists
their first ability to examine the Iris Hypothesis in the context of multiple
cloud layers. MODIS can also determine whether clouds are made up of ice
particles or water droplets.
Historically, it has been very difficult for scientists to measure clouds’ properties in multi-layer cloud formations using passive remote sensors. In 2004, scientists grappling with this problem will gain one of the most powerful tools ever developed for studying multi-layer cloud properties on a global scale. In that year, NASA will complete the launch of its “A-train”—a series of satellites that fly in formation around the Earth. In the lead will be the Aqua spacecraft planned for launch this spring (2002), carrying both the MODIS and CERES sensors. Following Aqua will be the Calipso and Cloudsat missions for making vertical profiles down through cloud layers nearly simultaneous with MODIS and CERES observations. These new tools will enable scientists to explore clouds on a global scale in ways only dreamed of in the past. Perhaps then these arbiters of the Earth’s energy will finally reveal their secrets.
New data products from NASA’s most recent satellites will
help scientists resolve the controversy surrounding the Iris Hypothesis.
The top image at left shows cloud phase for February 2002 from the Moderate
Resolution Imaging Spectroradiometer (MODIS Science Team). Yellow indicates cold, high-altitude ice clouds,
cyan corresponds to water clouds, and magenta shows undetermined (possibly ice clouds over
water) clouds. The lower image shows cirrus clouds. Dark colors indicate no cirrus clouds, and
white indicates cirrus clouds reflecting 30% or more of the incoming sunlight. Notice how the
areas of ice clouds in the tropics (in the top image) correspond to cirrus clouds (bottom image).