Impact of Decadal Cloud Variations on Earth’s Energy Budget
Feedbacks of clouds on climate change strongly influence the magnitude of global warming.
Clouds play a significant role in Earth’s climate system by reflecting incoming solar radiation and reducing outgoing thermal radiation. As Earth’s surface warms, the net radiative effect of clouds also changes, contributing a feedback to the climate system. Scientists analyzed the cloud radiative effect with a climate model that simulates tropical low-level cloud cover. They showed that cloud variations induced by changes in the spatial pattern of sea surface temperature have contributed as a cooling effect in response to recent climate change. A combination of observations and Atmospheric Model Intercomparison Project simulations further confirmed this finding.
This study suggests that clouds have likely contributed to the slowdown of global warming in the 2000s and that the climate sensitivity calculated from recent trends is probably low biased. The study also provides the physical mechanism of how the spatial pattern of surface warming affects the magnitude of cloud feedback.
Feedbacks of clouds on climate change strongly influence the magnitude of global warming. Cloud feedbacks, in turn, depend on the spatial patterns of surface warming, which vary on decadal timescales. Therefore, the magnitude of the decadal cloud feedback could deviate from the long-term cloud feedback. In this study, scientists from Lawrence Livermore National Laboratory present climate model simulations to show that the global mean cloud feedback in response to decadal temperature fluctuations varies dramatically due to time variations in the spatial pattern of sea surface temperature (SST). The researchers found that cloud anomalies associated with these patterns significantly modify Earth’s energy budget. Specifically, the decadal cloud feedback between the 1980s and 2000s is substantially more negative than the long-term cloud feedback. This finding is a result of cooling in tropical regions where air descends, relative to warming in tropical ascent regions, which strengthens low-level atmospheric stability. Under these conditions, low-level cloud cover increasingly reflects solar radiation, despite an increase in global mean surface temperature. These results suggest that SST pattern-induced low cloud anomalies could have contributed to the period of reduced warming between 1998 and 2013. Additionally, these results offer a physical explanation of why climate sensitivities estimated from recently observed trends are probably biased low.
This work was supported by the Regional and Global Climate Modeling program of the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research under the project “Identifying Robust Cloud Feedbacks in Observations and Models.”
C. Zhou, M. D. Zelinka, and S. A. Klein, “Impact of decadal cloud variations on the Earth’s energy budget.” Nature Geoscience (2016). [DOI: 10.1038/NGEO2828]