The new Mauritsen and Stevens paper (discussed in the companion post by Rud Istvan) is breathing new life into Richard Lindzen’s iris hypothesis.
Basic mechanism of the iris hypothesis
The basic mechanism of the iris hypothesis is described by text excerpted from the Mauritsen and Stevens paper:
The tropical atmosphere consists of moist and cloudy regions associated with large-scale rising motion, convective storms and pronounced precipitation on the one hand, and dry and clear regions with subsiding motion on the other hand (Fig. 1). The atmospheric circulation maintains an approximate balance between radiative cooling, which occurs preferentially in the dry and clear regions, and latent heating from the condensation of water vapour in precipitating clouds. In the tropics, radiative cooling predominantly occurs in dry and clear subsiding parts of the atmosphere. The radiative cooling is balanced mainly by latent heat released in precipitating deep convective clouds (Fig. 1).
Processes that may change the balance in favour of dry and clear regions in warmer climates have been proposed to constitute a possible negative feedback not represented by climate models. This potential feedback has been termed the iris effect, in analogy to the enlargement of the eye’s iris as its pupil contracts under the influence of more light. The controversial ‘iris hypothesis’ proposes that the fraction of the dry and clear regions could increase with warming and exert a negative feedback: a larger extent of the dry and clear regions would lead to a less cloudy upper troposphere and hence an increase in OLR.
Such an effect could mitigate against climate change [by reducing climate sensitivity]. But a drier upper troposphere would also allow more solar radiation to be absorbed by the Earth and atmosphere, rather than reflected back to space by the clouds, so that the net effect of reducing high clouds is not obvious. On balance, the effect is thought to be negative. The estimate of climate sensitivity with an iris effect, however, depends not only on the rate of reduction of high-level clouds, but also on the cloud optical properties of the most sensitive clouds. If the thinnest clouds are preferentially removed, the effect on outgoing longwave radiation is stronger than that on reflectivity, and the iris effect is stronger. On the other hand, if the reduction in cloud cover affects thicker clouds more strongly, the loss in reflectivity plays a more important role, and the iris effect is less pronounced.