Limitations of Observational Proxies

Since cloud and radiation observations are available on a global scale for only a climatically short time period (at best, ca. 25 years), our ability to observe cloud-climate feedbacks directly is limited. Thus the question arises as to whether there might be some observational proxies in the present-day climate for future cloud perturbations. The dynamic and thermodynamic forcings of clouds that occur on shorter timescales do not correspond directly to the dynamic and thermodynamic changes that are expected to occur with global warming. Therefore, neither the seasonal cycle, which is useful to constrain the snow/ice albedo feedback (Hall and Qu 2006), nor El Niño/La Niña events can be considered as good analogues of long-term climate changes for the analysis of cloud-climate feedbacks. This was confirmed by the FANGIO study (Cess et al. 1990), which found no direct relationship between cloud seasonal variations and the cloud response to climate change. Based on a study comparing the response of clouds simulated by a climate model in the case of a volcanic eruption (Mt. Pinatubo) or of a doubling CO2 experiment (Yokohata et al. 2005), it appears that volcanic eruptions may also not be considered as useful proxies for future cloud changes, despite the success of studies constraining the water vapor feedback (Soden et al. 2002). Thus, no direct observational proxy for future cloud changes has yet been identified.

A better understanding of the physical processes that control cloud-climate feedbacks may, however, help uncover new pathways to exploit existing observational datasets for the understanding of cloud-climate feedbacks.

0 0

Post a comment