Cirrus Clouds and Their Response to Anthropogenic Activities

Cirrus clouds are ubiquitous, long-lived, high-level ice clouds that exert a considerable global radiative effect on the climate system. This review assesses recent observational and modeling evidence of how anthropogenic activities might affect cirrus. Changes in physical properties and chemical composition of liquid aerosol particles will unlikely affect cirrus significantly, but anthropogenic influences may occur through changes in heterogeneous ice nuclei. Two main uncertain factors contribute to the current inability to constrain background cirrus formation: small-scale variability in dynamical forcings that drive ice nucleation parameterizations and the ability of airborne particles to act as efficient heterogeneous ice nuclei. These uncertainties keep us from drawing robust conclusions about anthropogenic influences on cirrus. Microphysical and macrophysical representation of cirrus in global models must first be advanced before we can predict changes in climate with fewer uncertainties. Some climate intervention studies suggest a potential cooling effect of deliberately perturbed cirrus, but at the risk of modifying precipitation inadvertently.