A Possible Thermal Instability in the Greenhouse Effect on Earth and Its Implication for Other Planets

Maura Rabbette and Peter Pilewskie

Over most of the Earth, as the surface temperature increases the rate of outgoing longwave emission to space increases at a rate less than the third power of surface temperature. This is evidence of the atmospheric "greenhouse effect." However, there is a unique location on the planet, in the tropical western Pacific known as the "warm pool," where the outgoing infrared radiation to space decreases with increasing surface temperature. Since infrared emission at the surface increases at a rate proportional to the third power of temperature there appears to exist a strong radiative imbalance over the tropical Pacific warm pool. With emission to space falling, even as surface emission is rapidly increasing, more energy is available to heat the surface, leading to a potentially unstable "runaway greenhouse." This phenomenon occurs when sea surface temperatures (SST) exceed 300 K (figure 1).

Using satellite observations, surface and atmosphere in situ data, and atmospheric radiative modeling, we are investigating the strong coupling between ocean warming and radiative feedback processes. Our primary objective will be to understand why the outgoing longwave radiation to space reaches a maximum when SST is 300 K and decreases when SST exceeds 300K. This is an inherently unstable process which we will examine in detail and we expect to determine the significance of an SST of 300 K.

If this phenomenon occurred on a global scale there indeed would be a "runaway greenhouse," the dominant process in the planetary evolution of neighboring Venus. The runaway greenhouse is a factor in determining the habitable zone of extrasolar planets. Planet habitability is a principal topic in Astrobiology, a field for which Ames has major responsibilities.

The first step in the process involves combing the vast oceanic and atmospheric data archives to retrieve measurements acquired with instruments and sensors on various platforms including buoys, ships, radiosondes, high altitude aircraft and satellite platforms. Part of the initial process required the development of software to download and organize days, weeks and many years worth of data, which is then subdivided into the correct spatial grids. The retrieved global data sets give both spatial and temporal observations (including daily, weekly and monthly averages) of sea surface temperatures, top of the atmosphere outgoing infrared flux to space, atmospheric humidity and temperature profiles. One way that we will explore the link between ocean warming and the radiative feedback is by extracting various correlations between these variables (figure 2).

The region of interest covers a large area of the Pacific ocean: longitude 150°E to 110°W, latitude 35°S to 35°N, subdivided into 2.5° x 2.5° grid boxes. We will focus on this region because it includes the Pacific Warm Pool where the highest sea surface temperatures occur. The longwave flux leaving the Earth is recorded by the Clouds and the Earth's Radiant Energy System (CERES) instruments onboard a number of Earth observing satellites. The use of CERES data is another unique aspect of this project because it is a relatively new data source acquired with the most advanced earth-observing radiometric sensors to date. The information extracted as a result of using this data will add a new dimension to this research area.



Figure 1: CERES mean Clear Sky Upward Longwave Flux (Wm-2) at top of atmosphere versus Sea Surface Temperature (K) for August 2000. Pacific Ocean region, latitude: 36 S to 14 N, longitude: 153 E to 240 E.






Figure 2: Sea Surface Temperature (blue), Clear Sky Upward Longwave Flux (red) and Absolute Humidity at 300 mbars (green) for a 2.5o by 2.5o grid box centered on lat: 14 N, lon: 161 E over a time period of 17 months, March 2000 to July 2001.

Collaborators: Christopher McKay, SST

Richard Young, SST

Point of Contact: Maura Rabbette, (650) 604-0128, mrabbette@mail.arc.nasa.gov