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Fraser C. Lott
and
Peter A. Stott

Abstract

Although it is critical to assess the accuracy of attribution studies, the fraction of attributable risk (FAR) cannot be directly assessed from observations since it involves the probability of an event in a world that did not happen, the “natural” world where there was no human influence on climate. Instead, reliability diagrams (usually used to compare probabilistic forecasts to the observed frequencies of events) have been used to assess climate simulations employed for attribution and by inference to evaluate the attribution study itself. The Brier score summarizes this assessment of a model by the reliability diagram. By constructing a modeling framework where the true FAR is already known, this paper shows that Brier scores are correlated to the accuracy of a climate model ensemble’s calculation of FAR, although only weakly. This weakness exists because the diagram does not account for accuracy of simulations of the natural world. This is better represented by two reliability diagrams from early and late in the period of study, which would have, respectively, less and greater anthropogenic climate forcing. Two new methods are therefore proposed for assessing the accuracy of FAR, based on using the earlier observational period as a proxy for observations of the natural world. It is found that errors from model-based estimates of these observable quantities are strongly correlated with errors in the FAR estimated in the model framework. These methods thereby provide new observational estimates of the accuracy in FAR.

Full access
Chunhui Lu
,
Fraser C. Lott
,
Ying Sun
,
Peter A. Stott
, and
Nikolaos Christidis

Abstract

In China, summer precipitation contributes a major part of the total precipitation amount in a year and has major impacts on society and human life. Whether any changes in summer precipitation are affected by external forcing on the climate system is an important issue. In this study, an optimal fingerprinting method was used to compare the observed changes of total, heavy, moderate, and light precipitation in summer derived from newly homogenized observation data with the simulations from multiple climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). The results demonstrate that the anthropogenic forcing signal can be detected and separated from the natural forcing signal in the observed increase of seasonal accumulated precipitation amount for heavy precipitation in summer in China and eastern China (EC). The simulated changes in heavy precipitation are generally consistent with observed change in China but are underestimated in EC. When the changes in precipitation of different intensities are considered simultaneously, the human influence on simultaneous changes in moderate and light precipitation can be detected in China and EC in summer. Changes attributable to anthropogenic forcing explain most of the observed regional changes for all categories of summer precipitation, and natural forcing contributes little. In the future, with increasing anthropogenic influence, the attribution-constrained projection suggests that heavy precipitation in summer will increase more than that from the model raw outputs. Society may therefore face a higher risk of heavy precipitation in the future.

Free access
Lixia Zhang
,
Tianjun Zhou
,
Xiaolong Chen
,
Peili Wu
,
Nikolaos Christidis
, and
Fraser C. Lott
Free access
Zhiyuan Hu
,
Haiyan Li
,
Jiawei Liu
,
Shaobo Qiao
,
Dongqian Wang
,
Nicolas Freychet
,
Simon F. B. Tett
,
Buwen Dong
,
Fraser C. Lott
,
Qingxiang Li
, and
Wenjie Dong
Open access
Chunhui Lu
,
Jie Jiang
,
Ruidan Chen
,
Safi Ullah
,
Rong Yu
,
Fraser C. Lott
,
Simon F. B. Tett
, and
Buwen Dong
Open access
Rouke Li
,
Delei Li
,
Nergui Nanding
,
Xuan Wang
,
Xuewei Fan
,
Yang Chen
,
Fangxing Tian
,
Simon F. B. Tett
,
Buwen Dong
, and
Fraser C. Lott
Open access
Jianping Duan
,
Liang Chen
,
Lun Li
,
Peili Wu
,
Nikolaos Christidis
,
Zhuguo Ma
,
Fraser C. Lott
,
Andrew Ciavarella
, and
Peter A. Stott
Full access
Bosi Sheng
,
Buwen Dong
,
Haolin Wang
,
Mingming Zhang
,
Shuheng Lin
,
Peng Si
,
Fraser C. Lott
, and
Qingxiang Li

Abstract

Precipitation in southern China during April–June 2022 was the highest since 1961. Anthropogenic forcing has reduced the probability of 2022-like Rx30day precipitation by about 45% based on CMIP6 simulations.

Open access
Yeon-Hee Kim
,
Seung-Ki Min
,
Dong-Hyun Cha
,
Young-Hwa Byun
,
Fraser C. Lott
, and
Peter A. Stott

GCM ensembles indicate that the October 2021 South Korean heatwave was extremely unlikely to occur without human influences, which corresponds to 2060s’ new normal without ambitious greenhouse gas mitigation.

Free access
Seung-Ki Min
,
Seo-Young Jo
,
Min-Gyu Seong
,
Yeon-Hee Kim
,
Seok-Woo Son
,
Young-Hwa Byun
,
Fraser C. Lott
, and
Peter A. Stott
Open access