PNNL

The Science                                

The global mean temperature at Earth's surface has risen rapidly since 1980. Three of the planet’s polar regions—the Arctic, Antarctic, and Tibetan Plateau—have warmed more than the global mean, a phenomenon commonly known as polar amplification. Radiative feedbacks such as surface albedo feedback, temperature feedback, and cloud feedback contribute to polar amplification. Scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory led a study to quantify these feedbacks using historical short-term climate simulations. These simulations can reproduce the observed warming and polar amplification.

The Impact

This research is the first systematic quantification of individual radiative feedbacks over the “three poles” based on historical short-term simulations from multiple state-of-the-art climate models. The global mean net feedback in historical years 1980–2017 is estimated to be negative; this means that the net feedback decreases the historical warming. The magnitude of the net feedback is stronger than that estimated from long‐term experiments, with rapid warming driven by quadrupled carbon dioxide levels. This large magnitude is primarily due to a near‐zero global‐mean cloud feedback in recent decades. All models agree that the temperature lapse rate feedback is the largest contributor to polar amplification.

Summary

Previously, the team identified that incomplete knowledge of the evolving effective radiative forcing due to changes in greenhouse gases, aerosols, and land conditions can produce uncertainty in quantifying the feedbacks based on the historical short‐term climate simulations. This study extends that work to examine historical radiative feedbacks by analyzing a unique set of atmospheric general circulation model (AGCM) simulations. This includes simulations from the Atmospheric Model Intercomparison Project within CMIP Phase 6 (AMIP6) with known effective radiative forcing for 1980–2014 and a specifically designed CAM5 simulation with zero effective radiative forcing for 1980–2017.

The historical global mean net feedback estimated from the AGCM simulations is around −2 W m^-2 K^-1, which is about twice the magnitude estimated from dozens of long‐term warming experiments driven by quadrupled levels of atmospheric carbon dioxide. This difference is mainly caused by a near‐zero net cloud feedback for the historical time period in short-term simulations. The team also showed that the temperature lapse rate feedback for 1980–2017/2014 is the largest contributor to the amplified temperature change over the three poles, followed by surface albedo feedback and Planck feedback deviation from its global mean. Interestingly, except for a higher surface albedo feedback in the Antarctic region, all other feedbacks are similar between the Arctic and Antarctic. The largest disagreement between the CAM5 and the AMIP6 model results is in both shortwave and longwave cloud feedbacks that differ in sign as well as magnitude. This result calls for further investigation into why this uncertainty in global and regional cloud feedback exists in climate models. 

PNNL Contact 

Hailong Wang, Pacific Northwest National Laboratory, hailong.wang@pnnl.gov

Funding

This research has been supported by the U.S. Department of Energy Office of Science, Biological and Environmental Research, Regional and Global Model Analysis program as part of the HiLAT-RASM project.