Editors’ Vox is a blog from AGU’s Publications Department.
Geophysical Research Letters is proud to have provided a forum for a large number of publications focused on improving understanding of climate change, its drivers, climate modeling, future projections and expected climate impacts.
Climate change is an immediate and pressing threat to the future of our planet, its environment, and its habitability.
Geophysical Research Letters is proud to have provided a forum for a large number of publications focused on improving understanding of climate change, its drivers, climate modeling, future projections and expected climate impacts.
These publications include studies performed under the broad umbrella of the Coupled Model Intercomparison Project (CMIP) and analyses of modeling results from the various phases of this project (including CMIP5 and CMIP6). In fact, we have published more than 300 research letters related to CMIP5 and CMIP6.
Although climate models provide valuable insights into how Earth’s climate may evolve in the future, model projections are inherently uncertain due to incomplete representations of various complex processes and feedbacks, as well as the future scenarios used in these simulations. There is a large spread among the projections of specific phenomena simulated with climate models, which makes it challenging to translate climate model projections into policy.
A simple and important metric of climate change relates to how much the Earth warms in response to greenhouse gas increases. Detailed model studies consider the history of greenhouse gas emissions and radiative forcing, and several representative future emission scenarios. Additionally, metrics such as the equilibrium climate sensitivity and effective climate sensitivity have been proposed to diagnose the sensitivity of temperature change projections to various processes and their representations in climate models. Even if derived from a hypothetical scenario of instantaneous doubling or quadrupling of CO2 concentrations, they provide useful insights into the future climate impacts under more realistic future scenarios.
These metrics are typically derived from climate model simulations and are influenced by the process representations therein. A nuanced understanding of the relationship between underlying process parameterizations and model calculated climate sensitivity metrics is critical for objective interpretation of climate model projections and their improvement in future efforts. The paper “Causes of Higher Climate Sensitivity in CMIP6 Models” by Zelinka et al. [2020] is an excellent example of research that provides such insights and understanding. It was published in GRL in 2020 and has been cited more than 700 times since.
The authors evaluate the reasons for the substantial increase in the effective climate sensitivity in the latest generation (CMIP6) of global climate models.
In “Causes of Higher Climate Sensitivity in CMIP6 Models”, the authors evaluate the reasons for the substantial increase in the effective climate sensitivity in the latest generation (CMIP6) of global climate models, compared to the preceding CMIP5 and recent consensus estimates. They caution that this increased sensitivity is not statistically significant considering the spread in the response across the CMIP6 model suite.
At the same time, they highlight the need to better understand the source of this higher sensitivity. They base their analysis on a detailed comparison of CMIP6 and CMIP5 model results, in particular cloud versus non-cloud feedbacks. They demonstrate that the different results occurred because the water content and coverage of low clouds decrease more strongly with warming in CMIP6 models, causing enhanced absorption of sunlight, leading to a positive feedback that results in more warming. If the increased sensitivity were real, it would have major global and societal implications. While a positive low cloud feedback is supported by theory and observations, the stronger feedback predicted by CMIP6 models is not certain.
There is a critical need to more rigorously establish the basis for the cloud physics parameterizations and evaluate them against observations before accepting these model results. There is also a need to carefully evaluate and establish the plausibility of these higher sensitivity models by considering other processes and controls.
Zelinka et al. [2020] has motivated many in-depth follow up investigations of model physics and guided improvements in the representation of cloud feedbacks in next-generation climate models.
The authors note, for example, that some of the models with higher climate sensitivity may incorporate biased and compensatory representations of other negative feedbacks which makes them appear to match the historical record satisfactorily. Such models would obviously not generate accurate future climate projections.
The paper by Zelinka et al. [2020] has motivated many in-depth follow up investigations of model physics and guided improvements in the representation of cloud feedbacks in next-generation climate models.
On the occasion of the 50th anniversary of Geophysical Research Letters, we are proud to have hosted several rigorous and insightful manuscripts on climate change and climate modeling. These manuscripts provide testament to the dedication of the global climate science community towards advancing the enterprise of climate modeling and projections for the benefit of humanity.
Paper Citation: Zelinka, M. D., Myers, T. A., McCoy, D. T., Po-Chedley, S., Caldwell, P. M., Ceppi, P., et al. (2020). Causes of higher climate sensitivity in CMIP6 models. Geophysical Research Letters, 47, e2019GL085782. https://doi.org/10.1029/2019GL085782
—Harihar Rajaram (hrajara1@jhu.edu, 
0000-0003-2040-358X), Editor-in-Chief, Suzana Camargo (0000-0002-0802-5160), Editor; Alessandra Giannini (0000-0001-5425-4995), Editor; and Hui Su (0000-0003-1265-9702), Editor, Geophysical Research Letters