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Indicators for the Detection and Attribution of Concurrent and Compound Extremes in CMIP6 Climate Model Simulations

Recent studies have shown that climate extremes can result in compounding effects when
occurring simultaneously or in a cascading fashion, which can lead to more frequent and more
severe hazards than otherwise expected. While the impact of climate extremes on societal, natural,
and built systems is well-known, most of the current indicators of extreme events are univariate
and they do not provide information on compound extremes such as droughts and heatwaves.
Here, we propose frameworks to address the increasingly important issue of capturing and
quantifying compound and cascading extreme events. The primary objective of this proposal
is to develop, validate and apply indices for the detection and attribution of compound and
cascading extremes that can be integrated into the U.S. National Climate Assessment (NCA).
We propose to use a variety of methods, ranging in complexity from empirical methods to more
advanced copula theory, to create indicators of projected change during the 21st Century across
the United States. Using the CMIP6 model projections, our proposed indicators seek to provide a
more thorough measure of compound and cascading extreme events.

Our main objectives include: (a) Develop, test and implement a set of indicators designed for
assessing change in compound and cascading climate extremes; (b)Develop, test and implement a
set of indicators and multi-hazard scenarios for attribution analysis of compound and cascading
extreme events; (c)Develop a generalized toolbox (including source code and sample data) based
on items (a) and (b), and make it freely available to the public; (d) Use historical data and CMIP6
simulations to study changes in compound and cascading extremes in the past and future. Some of
the key questions are: How are compound events projected to change in a warming climate? How
is the interdependence between climate hazards expected to change under different representative
concentration pathways?; and (e) Support the National Climate Assessment (NCA) Indicator
Working Group. Collaborate with the team members and refine and adjust the proposed
methodologies to meet NCA needs (the PI is a co-author of the 4th NCA and is familiar with the

The resulting indicators from this project, including source codes and sample data, will be made
freely available to the research community. The proposed indicators will contribute to the NCA
by providing a more comprehensive multivariate assessment of changes in compound and
cascading extreme events based on CMIP6 projections. The indicators will shed light on the
interconnected nature of climate extremes. Throughout the project, our team will work closely
with the NCA Indicator Working Group, and will refine and adjust the proposed methodologies
and objectives to ensure their relevance to the NCA.

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