Implementation Plan for WCRP Grand Challenge on Understanding and Predicting Weather and Climate Extremes

Weather and climate extremes are an inherent part of climate. There is overwhelming evidence that the climate and its extremes are changing. As extremes affect every aspect of our society, decisionand policy makers, and stakeholders are increasingly asking for reliable predictions of extremes on time scales from days to seasons and centuries. To meet this societal need, the world climate research community is challenged by underlying science questions and the quality and coverage of the observational data that are used to monitor and understand extremes. Both the science questions and the data need urgent attention in order to better identify the factors and mechanisms that determine the location, intensity, and frequency of various climate extremes including droughts, floods, heavy precipitation events, heat waves, cold spells, tropical and extratropical storms, coastal sea level surges and extreme ocean waves. This information is needed in the near-term (from a season to a year) to mitigate risks to society and ecosystems, and in the longer term (from a decade to centuries) for effective adaptation planning. Despite the importance of the topic, progress has been fairly slow. However, recent developments suggest that the prospects for more rapid advancement of this WCRP Grand Challenge are excellent. These developments include: • recent substantial advances in modelling (including but not limited to model resolution) • advances in the understanding of the physical mechanisms leading to extremes • increased efforts to extend the historical observational record, including planned climate quality reanalyses over longer historical periods • expected improvements in remote sensing products, which now extend long enough to document trends and sample extremes • recent world wide coordinated efforts to attribute the causes of individual extremes, and other research activities already underway and planned. This white paper explains the challenges and opportunities, suggesting activities that can help move the community forward more rapidly, making use of above advances.