Research Staff: Maura Rabbette, Peter Pilewskie, Christopher McKay, and Richard Young

Water vapor is an efficient absorber of outgoing longwave infrared radiation on Earth and is therefore a primary greenhouse gas. Since the amount of water vapor in the atmosphere increases with increasing surface temperature, and the increase in water vapor further increases the temperature, there is a positive feedback. The runaway greenhouse effect occurs if this feedback continues unchecked until all the water has left the surface and enters the atmosphere. For Mars and the Earth the runaway greenhouse was halted when water vapor became saturated with respect to ice or liquid water respectively. However, Venus is considered to be an example of a planet where the runaway greenhouse effect did occur, and it has been speculated that if the solar luminosity were to increase above a certain limit, it would also occur on the Earth.

Satellite data acquired during the Earth Radiation Budget Experiment (ERBE) clear sky conditions shows that as the sea surface temperature (SST) increases, the rate of outgoing infrared radiation at the top of the atmosphere also increases, as expected. Surprisingly, above 300K the outgoing radiation emitted to space actually decreases with rising SST. Less energy to space implies that more energy is available to heat the surface, leading to a potentially unstable situation. This behavior is a signature of the runaway greenhouse effect on Earth. However, the SST never exceeds 303K, thus the system has a natural cap which stops the runaway (Figure 1).

Figure 1: Satellite data acquired during the Earth Radiation Budget Experiment (ERBE) over the Pacific ocean during clear sky conditions. As sea surface temperature (SST) increases the outgoing infrared radiation (OLR) to space also increases as expected until a SST of 300 K is reached, at which point the OLR starts to decrease with increasing SST. Is this the signature of the Runaway greenhouse effect? And how is the upper SST limit related to the runaway greenhouse?

According to Stefan-Boltzmann's law, the amount of heat energy radiated by the earth's surface is proportional to (T)⁴. However, if the planet has a substantial atmosphere, it can absorb all heat radiation from the lower surface before the radiation penetrates into outer space. Thus, an instrument in space looking at the planet, does not detect radiation from the surface. The radiation that it sees comes from some level higher up. The effective temperature (T_e) is the temperature of this emitting region within the troposphere, lower levels may have much higher temperatures. On earth the average temperature of the surface is 288 K, but the effective temperature is only 255 K. The value T_e = 255 K corresponds to the middle troposphere, above most of the water vapor and clouds.

Atmospheric instruments and sensors on high altitude aircraft, radiosonde and satellite platforms provide direct observations of sea surface temperatures, outgoing infrared flux to space, and atmospheric humidity and temperature profile measurements. The ERBE data is now being used to model the Sea Surface Temperature and outgoing flux to space. The aim is that this radiative transfer model will reproduce the signature of the potential runaway greenhouse effect on Earth (Figures 2 and 3). The atmospheric temperature and dew point temperature profiles are given in Figure 4. It can be seen for SST values 301 K - 303 K that much higher concentrations of water vapor were introduced into the model's atmospheric profile. The model will be a link between ERBE measurements and theory and will help us understand climate evolution and divergent climates of Venus, Earth, and Mars, as well as the inner boundary of the habitable zone in other planetary systems.

Figure 2: According to Stefan-Boltzmann's law, the amount of heat energy radiated by the earth's surface is proportional to (T)⁴ (upper line). The output from MODTRAN i.e. the modeled Top of Atmosphere Emission is also displayed (lower line). The model incorporates user defined atmospheric pressure profiles and temperature profiles based on a moist adiabatic lapse rate as well as relative humidity profiles. Together these profiles give the best modeled fit (up to a SST of 300 K) to the Top of Atmosphere Emission.

Figure 3: The radiative transfer model was then used to reproduce the signature of the potential runaway greenhouse effect on earth. For SST values 301-303 K much higher concentrations of water vapor were introduced into the atmospheric profile. As a result a turn around and decrease in the outgoing longwave radiation model was achieved (solid line through ERBE data points). Also shown is the corresponding sharp increase in atmospheric opacity (dashed line).

The significance of the observed sea surface temperature at which the outgoing longwave radiation to space begins to decrease, as well as the observed upper limit on the sea surface temperature, is that both phenomena are relevant to several aspects of paleoclimatology and astrobiology. We will use our model in an attempt to predict the upper limit of sea surface temperatures for different values of atmospheric CO₂, an objective directly relevant to understanding past and future climate states of the Earth. For example, did these same processes prevent the oceans from evaporating during past climate episodes of enhanced CO₂? Evidence suggests that even during past climates SST did not exceed 303K. The runaway greenhouse is thought to be a critical factor in defining the inner edge of the habitable zone of any planetary system, therefore an understanding of the phenomenon on earth and in our own solar system is of great importance.

On going work includes using CERES (Clouds and the Earth's Radiant Energy System) satellite data as well as ERBE data for measurements of outgoing infrared flux at the top of atmosphere. This data will be coupled with time synchronous in situ atmospheric profiles (including relative humidity, temperature and pressure) which will be used as inputs to the radiative transfer model. Another step will be to introduce various cloud layers into the model to see the effect on reproducing the signature of the potential runaway greenhouse phenomenon on Earth.

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

Figure 4: Atmospheric temperature profiles (upper right line of each pair) and Dew point temperature profiles (lower left line of each pair) for Sea Surface Temperatures (SST), 300K, 301K, 302K and 303K. It can be seen from the Dew point temperature profiles that in order to make the Modeled Top of Atmosphere Emission turn around it was necessary to introduce higher relative humidity values (typically RH=80%). At SST =301K the RH% was increased in the upper troposphere and for SST values of 302K and 303K the water vapor was increased further at lower levels in the atmosphere.