Climat sensitivity and observed negative feedbacks

Résumé: There has, for some time, been substantial evidence that current models are exaggerating climate sensitivity. This evidence is based on observed transient response to sudden climate events (volcanoes and regime change), as well as on observed changes in OLR (outgoing longwave radiation) observed from space. This evidence will be reviewed. These results are consistent with the need in current models to cancel most greenhouse forcing in order to replicate the observed behavior of global mean temperature. In addition, a specific mechanism has been proposed to account for the lower sensitivity suggested by observations: namely the Iris Effect wherein upper level cirrus in the tropics decreases in response to increased surface temperature. This mechanism will be reviewed as will recent work which uses ground based radar and TRMM instruments to show that the mechanism functions throughout the tropics. In addition we will review new analyses (by a Korean team) of the radiation budget that clearly demonstrate that the primary impact of tropical cirrus is in the infrared as required by the Iris Effect. -------------------------------------------------------------------------------- Séminaires généraux de l'Institut de Physique du Globe de Paris --------------------------------------------------------------------------------