PNNL

The Science

While there has been a lot of effort dedicated to understanding global climate sensitivity—how much the mean surface temperature warms in response to a doubling of the concentration of carbon dioxide—regionality and nonlinearity are much less understood. As no one lives in the average climate, what really matters to societies and ecosystems is regional climate change, which can vary significantly in different geographical areas. Climate change response is not uniform, even if the forcing (what is causing the change) is uniform. In a study led by scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL), researchers constructed the linear response function (LRF), which encompasses the multivariate relationship between climate forcing in the form of heat flux from the ocean and surface temperature. Analysis of the LRF revealed the neutral modes (the patterns that are most excitable by forcing) of the climate system and their corresponding optimal forcing.

The Impact

This work advances new understanding of the coupled climate system from the perspective of a dynamical system and its neutral modes. Neutral modes can serve as a metric for fingerprinting the climate change response. The methodology developed and demonstrated in this study represents an important step towards a framework for understanding and mapping the spatial relationship between forcing and response. Future work building on the methodology could provide valuable insights on where negative forcing may be applied to cool the climate, with implications for geoengineering research.

Summary

Inquiry into the climate response to forcing perturbations has long been the central interest of climate research. But the understanding of two important aspects of climate change response—nonlinearity and regionality—is still lacking. Researchers developed a Green’s function approach to estimate the LRFs for both the linear and quadratic nonlinear response to ocean thermal forcing in a climate model, whereby the most excitable temperature modes i.e., the neutral modes) can be identified for the climate system resembling the present-day climate. The resultant leading mode of the nonlinear response is characterized by a polar amplified global cooling pattern, unveiling an intrinsic inclination of the modern climate towards cooling. Moreover, optimal forcing patterns are identified as the ones that will most efficiently excite the corresponding neutral mode patterns. The forcing-response framework developed in this study can be utilized to determine the optimal forcing patterns to inform research on solar geoengineering and to interpret regional climate response and feedback in general.

PNNL Contacts

L. Ruby Leung, Pacific Northwest National Laboratory, Ruby.Leung@pnnl.gov

Jian Lu, Pacific Northwest National Laboratory, jian.lu@pnnl.gov

Funding

The U.S. Department of Energy’s Office of Science, Biological and Environmental Research supported this study as part of the Regional and Global Climate Modeling program area. FLiu was supported by the China Scholarship Council and National Science Foundation of China (41906002 and 91858210) and the National Key Research and Development Program of China (2018YFA0605702).