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 uncertainty.

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.

1. Lindzen, Richard S., Ming-Dah Chou, and Arthur Y. Hou; 2001: “Does the Earth Have an Adaptive Infrared Iris?” Bulletin of the American Meteorological Society, Vol. 82, No. 3, pp. 417-32.
2. Lin, Bing, Bruce A. Wielicki, and Lin H. Chambers; 2001: “The Iris Hypothesis: A Negative or Positive Cloud Feedback?” Journal of Climate (accepted September 5, 2001).
3. Fu, Qiang, Marcia Baker, and Dennis L. Hartmann; 2002: “Tropical Cirrus and Water Vapor: An Effective Earth Infrared Iris?” Atmospheric Physics and Chemistry, Vol. 2, pp 31-37.
4. Hartmann, Dennis L., and Marc L. Michelsen: 2001: “No Evidence for Iris.” Bulletin of the American Meteorological Society (submitted in April of 2001).
5. Lau, K.-M., H.-T. Wu, and S. Bony; 1997: “The Role of Large-Scale Atmospheric Circulation in the Relationship Between Tropical Convection and Sea Surface Temperature.” Journal of Climate, Vol. 10, No. 3, pp 381-92.
6. Wielicki, Bruce A., et al., 2002: “Evidence for Large Decadal Variability in the Tropical Mean Radiative Energy Balance.” Science, Vol. 295, pp. 841-44.
7. Chen, Junye, Barbara E. Carlson, and Anthony D. Del Genio, 2002: “Evidence for Strengthening of the Tropical General Circulation in the 1990s.” Science, Vol. 295, pp. 838-41.
8. Harrison, Edwin F., Patrick Minnis, Bruce R. Barkstrom, and Gary G. Gibson, 1993: "Radiation budget at the top of the atmosphere." Atlas of Satellite Observations Related to Global Change. R.J. Gurney, J.L. Foster, and C.L. Parkinson, eds. Cambridge University Press; pp. 19-38.

 Evidence Against the Iris Hypothesis