About Weather Attribution Center Japan (WAC Japan)

Weather Attribution Center Japan (WAC Japan) assesses how human-induced climate change as well as natural climate variability affect the likelihood and intensity of extreme weather events across Japan, and shares these findings with the public.

WAC Japan is a science-based, independent, and neutral brand led by experts in meteorology and climate science. Its stance follows the Intergovernmental Panel on Climate Change (IPCC), namely, to provide policy-relevant but not policy-prescriptive scientific information that supports policymaking and decision-making, and to help the public better understand the nature of extreme weather events and how they are influenced by global warming and natural climate variability. WAC Japan also works with weathercasters and risk communication experts to deliver timely, scientifically grounded analyses.

The analytical methods used by WAC Japan have been developed by researchers participating in the center, and the details have been published in peer-reviewed scientific journals. This approach conducts event attribution by analyzing observational and reanalysis data, in addition to utilizing data from the large-ensemble climate simulations of d4PDF, which were produced under the climate change research program supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT).

Disentangling the Causes of Extreme Weather

“Is this extreme weather caused by global warming?”

Climate change is already having profound impacts across all regions of the world, and these effects are predicted to intensify in the future. In Japan, average temperatures are rising at a faster pace than the global average, with extreme weather events—including heatwaves and torrential rainfall—becoming increasingly prominent throughout the country.

Extreme weather events naturally occur as random occurrences within the inherent, natural variability in the climate system. Until recently, it was considered nearly impossible to scientifically quantify and demonstrate the influence of anthropogenic global warming on actual extreme weather events, as disentangling anthropogenic effects from natural variability posed significant challenges. However, the advent of a groundbreaking approach called “event attribution” has made such analyses possible.

Particularly in Japan, being surrounded by oceans requires evaluating the influence of surrounding marine conditions and distant phenomena such as El Niño. Accordingly, a Japan-specific event attribution methodology has been developed and continues to evolve, enabling researchers to separate and visualize the impacts of anthropogenic global warming from those of natural variability.

WAC Japan utilizes this Japan-specific event attribution system to assess and disseminate the extent to which global warming has contributed to extreme weather events. Through this work, WAC Japan aims to enhance the public accessibility of climate science and promote a deeper understanding of the impacts of climate change, grounded in scientific evidence.

Rapid and Reliable: The WAC Method

At WAC Japan, we employ an innovative event attribution methodology originally developed under the Ministry of Education, Culture, Sports, Science and Technology’s Program for Advanced Research on Climate Change Projection. This novel approach not only disentangles the impacts of anthropogenic global warming from those of natural variability, but also integrates statistical techniques to achieve substantially faster analyses.

Conventional event attribution studies have employed climate models that reproduce atmospheric and oceanic phenomena based on physical laws. These models allow researchers to simulate both the current Earth influenced by historical global warming and a hypothetical Earth without anthropogenic global warming. By conducting large ensembles of simulations under both scenarios and comparing them, researchers can assess the influence of global warming on extreme weather events. To date, approximately 7,000 years’ worth of climate simulation data have been accumulated, providing a valuable foundation for ongoing attribution research.

The method adopted by WAC Japan utilizes this accumulated climate simulation database in conjunction with statistical mathematics to estimate the probability of an extreme weather event. By evaluating how frequently phenomena similar to the observed event occur within the large simulation archive, the analysis can produce reliable attribution results within just a few days after the event occurs.

For example, in our analysis of the July 2024 heat event in Japan, we estimated that the probability of air temperatures over Japan exceeding the observed record was 21.3% under present-day climate conditions influenced by human-induced global warming. By contrast, under a hypothetical climate influenced only by natural variability, the probability was estimated to be nearly 0%. This demonstrates that without global warming, the extreme heat experienced in July 2024 would not have occurred.

WAC Japan Researchers

Yukiko Imada
Associate Professor, Atmosphere and Ocean Research Institute, The University of Tokyo

Dr. Yukiko Imada received her Ph.D. in Science from the Graduate School of Science, The University of Tokyo, in 2010. After conducting research at the University of Tokyo and Tokyo Institute of Technology, she joined the Meteorological Research Institute in 2014. Her expertise includes predictability studies using seasonal forecasting models, applications in the hydrological field, research on extreme weather, and studies related to climate change and global warming. She has received several awards, including honors from the Meteorological Society of Japan and the Nishida Prize from the Japan Geoscience Union. She serves on several international committees, including the World Climate Research Programme Light House Activity.

Tomonori Sato
Professor, Faculty of Environmental Earth Science, Hokkaido University

Dr. Tomonori Sato obtained his Ph.D. in Science from the University of Tsukuba in 2005. He previously served as a Research Fellow of the Japan Society for the Promotion of Science; Project Assistant Professor at the Center for Climate System Research, the University of Tokyo; and both Project Assistant Professor and Associate Professor at the Faculty of Environmental Earth Science, Hokkaido University, before assuming his current professorial position. His research focuses on atmosphere–land and atmosphere–ocean interactions, as well as long-term changes in regional meteorological processes that give rise to extreme weather events. Through this work, he investigates the complex and significant impacts of global warming on regional climate and society.

Chiharu Takahashi
Project Assistant Professor, Climate System Research Division, Atmosphere and Ocean Research Institute, The University of Tokyo

Dr. Chiharu Takahashi holds a Ph.D. in Science from the Graduate School of Environmental Studies, Nagoya University, and is a certified weather forecaster. She took up her current position after serving as a researcher at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and The University of Tokyo. Her research focuses on extreme weather and climate change.

Tetsuya Takemi, Dr.
Professor, Disaster Prevention Research Institute, Kyoto University

Dr. Tetsuya Takemi received his Ph.D. in Science from Graduate School of Science, Kyoto University in 1999. He previously served as Assistant Professor at Osaka University, Lecturer at Tokyo Institute of Technology, and Associate Professor at Disaster Prevention Research Institute, Kyoto University, before serving as Professor at the current affiliation. His area of expertise is studies on severe storms and extreme weather such as heavy rainfalls and tropical cyclones, climate change impacts, and urban meteorological hazards. He has received academic awards including the Award of the Meteorological Society of Japan. He has long been contributing to international journals as an editor and an Editor-in-Chief.

Nobuhito Mori
Professor, Disaster Prevention Research Institute, Kyoto University / Yokohama National University

Dr. Nobuhito Mori holds a Ph.D. in Engineering from the Graduate School of Engineering, Gifu University. He previously served as a Senior Researcher at the Central Research Institute of Electric Power Industry, Lecturer at Osaka City University, and Associate Professor at the Disaster Prevention Research Institute, Kyoto University. He currently holds professorships at both Kyoto University and Yokohama National University. Dr. Mori specializes in the assessment of extreme disasters. He has received numerous honors, including the Award of the Meteorological Society of Japan, the Coastal Engineering Journal Paper Award from the Japan Society of Civil Engineers, and the Commendation by the Minister of Education, Culture, Sports, Science and Technology. He also served as a reviewer for the Japanese government in the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report.

Masahiro Watanabe
Professor, Atmosphere and Ocean Research Institute, The University of Tokyo

Dr. Masahiro Watanabe received his Ph.D. in Science from the Graduate School of Science, The University of Tokyo, in 2000. He previously served as a researcher at the University of Hawai‘i and as Associate Professor at Hokkaido University before assuming his current position. His area of expertise is climate science. Dr. Watanabe has received numerous awards, including the Award of the Meteorological Society of Japan, the Nishida Prize from the Japan Geoscience Union, and the Commendation by the Minister of Education, Culture, Sports, Science and Technology. He was also a contributing author to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report.

Partners

WAC Japan is in cooperative relationships with the following programs and research institutions in research and development as well as in the dissemination of research results.