Cirrus clouds, a genus of thin, high and wispy clouds covering about 20% of the earth’s surface, are among the principal cloud types controlling the Earth’s radiation budget. While the warming effect of cirrus clouds in the current climate has been widely realized, the role of changing cirrus clouds in climate change and variability remains uncertain. Here we show that the amount and altitude of cirrus clouds increase in response to inter-annual surface warming, and cirrus clouds are likely toact as a positive feedback on inter-annual climate fluctuations, by reducing the earth’s ability to radiate longwave radiation to space in response to planetary surface warming.

CALIPSO level-2 1-km cloud layer products between December 2007 and February 2014 are used to quantify the cirrus feedback in this study. Using cirrus cloud radiative kernels, the magnitude of the inter-annual cirrus feedback is estimated to be 0.20±0.21W/m2/°C, which represents an important component of the cloud feedback. The positive cirrus feedback primarily results from the positive response of global cirrus cloud radiative effect to tropical surface warming, and more than half of the feedback is contributed from the tropical tropopause layer. Climate models also suggest that cirrus feedback is likely to be positive in response to both inter-annual and long-term surface warming, though the ensemble average of the models yields a smaller inter-annual cirrus feedback than the most likely value from the CALIPSO observations.