The seasonal fingerprint of climate change

The identification of anthropogenically forced climate change from observational data is challenging. Climate-change effects over the time scale of decades are relatively small compared to natural variability but become progressively larger and influential as time proceeds. Detection of an evolving forced climate signal in observational data is often based on identifying characteristic space-time patterns; this approach is referred to as fingerprint or optimal detection studies. On page 245 of this issue, Santer et al. (1) identify a previously undetected fingerprint in the mid-latitude seasonal temperature cycle of temperature sensed by satellites over the past four decades. The work adds to the rigorous evidence for human influence on observed atmospheric changes.