Difference climate response persistence causes warming trend unevenness at continental scales

GW exhibits differences at continental scales. We show that CMIP6 models underestimate warming unevenness, possibly leading to a biased estimation of anthropogenic influence on warming. We find that SAT changes over China more sensitive to external forcing owing to stronger long-range persistence.
Published in Sustainability

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Global warming and a series of environmental problems brought about by it are increasingly and profoundly affecting the space for human survival and development, and scientific research on global climate change provides a lot of very important decision support for national/regional/global sustainable development. However, there are still many gaps and uncertainties in the research on many aspects of regional warming. Recently, we published the research paper "Different climate response persistence causes Different climate response persistence causes warming trend unevenness at continental scales" in Nature Climate Change. The paper answers a series of long-standing concerns on climate change at the continental scale.

The paper is based on the analysis of the global land temperature dataset C-LSAT2.0, which was recently developed by the research team and included in the 6th Scientific Assessment Report (AR6) of the Inter-governmental Panel on Climate Change (IPCC), and shows that the average regional warming rates in China since the early and mid-20th century are about 1/2 to 1/3 faster than those in the United States, respectively. Some researchers in recent years have tended to attribute some of this difference to either rapid urbanization or higher GHG emissions in the Chinese region, but it is clear that this explanation lacks very strong observational and simulation evidence.

To fully understand the physical nature of the continental-scale regional warming disequilibrium, the paper systematically analyzes the external forcing ("signal") and internal variability ("noise") components of the multi-time scale variability of the mean temperature in China and the United States using different data and methods. According to the latest IPCC’s Coupled Model Inter-comparison Project phase 6 climate models (CMIP6), the response of the U.S. regional mean temperature to external forcing (mainly Greenhouse Gas (GHG) and aerosols) is significantly stronger than that of the Chinese regional mean temperature, which is the opposite of the observed findings. Considering that classical attribution tools tend to rely heavily on the performance of model simulations, it is inferred that an attribution study of such regional climate change using a poorly simulated CMIP6 model will inevitably lead to some biases and uncertainties; in contrast, the use of an integrated statistical model developed by the team itself directly and better decomposes the linear and nonlinear response components of the external forcing signal consistent with the historical observed facts of continental-scale temperatures.

Further analysis shows that this unevenness of regional warming can be better explained by the long-range persistence ("memory") of the regional mean air temperature response to external forcing. By comparing paleoclimate proxy reconstructions with modern climate benchmark observations, it is found that the long-range persistence of regional mean temperature is significantly stronger in China than in the United States, both at the millennial-scale and the centennial-scale, i.e., it contributes more to the warming trend due to external forcing (while the CMIP6 model again underestimates this persistence difference). This long-term "memory" incorporates the effects of atmospheric and oceanic coupling and is emphasized on multi-decadal and century scales, making it a key issue in current climate change studies. Considering that the climate impact of sulfide aerosols is one of the major concerns of regional temperature change studies in recent years, many researchers believe that the regional air temperature change in China is more influenced by aerosols. The paper also integrates model reanalysis data with classical fingerprint attribution methods to demonstrate that the contributions of sulfide aerosol forcing to temperature trends in two regions, China and the United States, are essentially similar. After excluding the possible influence of aerosols, the paper concludes that the unevenness of the above continental-scale warming trend is mainly due to the persistent response of regional air temperature to GHG emissions.

This paper provides ample evidence that even assuming that anthropogenic GHG emissions are well mixed globally, there are significant regional differences in the response of continental-scale mean temperatures to them, i.e., GHG forcing has a stronger modulating effect on the warming rate in the Chinese region than in the U.S. region. This also further indicates that to complete the global emission reduction tasks as stipulated in the Paris Agreement, to achieve the carbon peak and carbon-neutral goals, and to keep the global air temperature warming within a certain range by the end of this century, it is not comprehensive to rely solely on the efforts of an individual or some countries (regions), but more urgent to develop broader global cooperation, to work together, to save energy and reduce emissions, and to create a better life for mankind. We need to work together to save energy and reduce emissions to create a better life for all.

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