Tropical anvil clouds and climate sensitivity

The surface temperature of Earth is being increased by human activities, principally by the release of greenhouse gases (1). Future warming will depend upon the rate at which greenhouse gases are released and the sensitivity of Earth’s surface temperature to those increased greenhouse gases. An often used metric of the sensitivity of Earth’s climate is the equilibrium climate sensitivity, the amount of global average surface warming that is the steady, long-term response to a doubling of carbon dioxide. The equilibrium climate sensitivity remains uncertain by about a factor of two, despite decades of study using evidence from basic theory, instrumental observations, paleoclimatic data, and global climate models (2). Global climate models indicate that a large contributor to uncertainty in climate sensitivity is the strength of cloud feedbacks. Cloud feedback is a response of cloud structure or amount to warming, which then alters the energy balance of Earth, which causes an additional change of surface temperature. An advance was made in the most recent report of the Intergovernmental Panel on Climate Change, which concluded that cloud feedback is likely positive, meaning that the response of clouds to climate change acts to increase the magnitude of the surface temperature change (3). This consensus is based in part on the development of basic physical understanding of why high clouds get higher (4) and low clouds decrease their area coverage in a warmed climate (5, 6). In PNAS, Bony et al. (7) propose a basic thermodynamic mechanism that may cause the temperature profile to become more stable in the upper troposphere when the Earth warms. They expect this stabilization to cause anvil cloud area to decrease in a warmed climate, although they conclude that the effect of this anvil area reduction on cloud feedback is uncertain. An understanding of the role of cloud … [↵][1]1Email: dhartm{at}uw.edu. [1]: #xref-corresp-1-1