The determinations of the Habitable Zones (HZ) around stars, the orbital radii at which surface conditions are suitable for life, is becoming an increasingly important and useful concept as planet hunting missions continue to return new and updated censuses of planetary systems. One important aspect of determining the extent of the HZ around a star is the nature of clouds and hazes in the planetary atmosphere which effect the energy balance of the planetary climate. Many studies have utilised atmospheric models to estimate the extent of the HZ however they have typically been one-dimensional (1D) models which fail to accurately represent, or even completely neglect, important three-dimensional (3D) processes. Recently, 3D models have began to tackle this problem and shed new light on this interesting topic.
A recent paper by Jérémy Leconte et al. investigated the impact of increased solar forcing on the climate of Earth to place estimates for the onset of a runaway greenhouse state; the point at which thermal emission (cooling) of the atmosphere reaches a maximum and any further increase in incoming energy will lead to rapidly rising surface temperatures. Previous studies of hot climates with 1D models have suggested that the response of clouds to increased irradiation will help to resist warming and provide a negative radiative forcing. This was due to the assumption that clouds provide a net cooling of the climate and increased evaporation at the surface would lead to thicker and more reflective clouds. Cloud dynamics is, however, very much a 3D system and this new study suggests quite the opposite.
Leconte et al. found that the vertical extent of clouds is increased in their model when the solar radiation is increased. In other words the cloud tops are higher up in the atmosphere. This results in the thermal emission of clouds operating in a cooler environment and hence emit less radiation. This process enhances the warming impact of clouds. The reasoning of this increase in vertical extent is two-fold. Firstly, stronger irradiation of the atmosphere results in stronger dynamics and convection, extending the extent of the troposphere. Secondly, cloud drop formation requires the loss of latent heat during condensation. The increased infrared opacity of the atmosphere increases the altitude at which this energy can be radiated away, allowing the clouds to cool.
This is a major result in terms of understanding the important role of clouds in the energy balance in planetary atmospheres. Even though clouds were ruled out as a stabilising feature of the atmosphere, a new source of stabilisation was identified. As previously mentioned, the increased insolation strengthens the atmospheric dynamics, including the Hadley cell; a large-scale overturning of air masses in low latitudes resulting in the moist equator and the dry sub-tropics. The strengthened Hadley cell causes further drying of these sub-tropical regions and in effect forms a window of energy loss where the greenhouse effect is locally much less than the global average. This acts to cool the climate. This process could only be uncovered in 3D model and highlights the need to use models of varying complexities and abilities to study planetary climates.
The key result of this work is to extend the inner edge of the HZ around the Sun to ~0.95 AU due to the stabilising effect of these dry, sub-tropical atmospheric windows which strengthen as the solar radiation increases.
For more detailed information see the Nature paper by Leconte et al. 2013: http://www.nature.com/nature/journal/v504/n7479/full/nature12827.html