Clearing Clouds of Uncertainty
Scientists in the Cloud Processes Research Group at Lawrence Livermore National Laboratory, with colleagues at Colorado State University and the UK Met Office have reviewed progress on quantifying, understanding, and observationally constraining cloud feedbacks through the five Intergovernmental Panel on Climate Change (IPCC) Assessment Reports. The authors note that continued progress in this area is necessary, but, given the strides that have been made since the first report in 1990, optimism is warranted.
Despite a persistent large inter-model spread in climate model simulations of cloud feedback, substantial progress has been made in this area, culminating in the conclusion that cloud feedbacks are “likely positive” in the most recent IPCC report released in 2013. This conclusion has subsequently been strengthened by high-resolution modeling experiments as well as observational analyses of co-variability of clouds and their environment. Nevertheless, much work remains to observe and more fully understand the many relevant processes, to further improve cloud simulations, and to further narrow the range in estimates of cloud feedback. Meeting these challenges will require continued theoretical, observational, and modelling advances.
Clouds play a crucial role in Earth’s energy budget by strongly reflecting solar radiation and modulating the emission of thermal radiation to space. The change in cloud radiative properties with climate warming – the cloud feedback – represents an important and uncertain climate feedback. It is the dominant source of inter-model differences in climate sensitivity – the amount of warming resulting from doubling atmospheric carbon dioxide. This issue has therefore been featured prominently in all five IPCC reports, though with much more nuance recently than in earlier reports. In this commentary commissioned by the editors of Nature Climate Change, the authors highlight key developments in the evolution of the community’s understanding of cloud feedbacks, including observational advances, development of theoretical bases for specific cloud feedback components, improvements in modeling, and advances in diagnosing and quantifying feedbacks. They also discuss recent developments that have strengthened the IPCC’s recent assessment that the overall cloud feedback is “likely positive”, while highlighting areas in particular need of better observations and understanding to continue making progress.