Overarching Theme: Document

Are existing observations sufficient to underpin the assessment of extremes?

This theme deals with documenting and assessing past changes in extremes. Observations are the key foundation for understanding long-term climate variability and change. However, observations are often not well-constrained, and critical gaps exist in the amount, quality, consistency and availability of observations, especially with respect to extremes.

Overarching Theme: Understand

What are the relative roles of large-scale, regional and local scale processes, as well as their interactions, for the formation of extremes?

This theme deals with the investigation and characterization of physical mechanisms leading to the occurrence of extreme events such as heat waves, droughts or floods. Recent investigations have shown that the interaction between large-scale phenomena (weather types, modes of variability) and regional-scale land-atmosphere feedbacks or forcings can be critical.

Overarching Theme: Simulate

Are models able to reliably simulate extremes and their changes, and how can this be evaluated and improved?

There is a lack of understanding in the types of events that current models can provide credible and robust simulations for, and in the identification of key processes for climate models to capture in order to produce credible simulations of weather and climate extreme events and thus improve prediction of those events. Furthermore, the ability of models to simulate particularly small-scale extremes depends on resolution and sometimes requires downscaling.

Overarching Theme: Attribute

What are the contributors to observed extreme events and to changes in the frequency and intensity of the observed extremes?

A key challenge for the community is to provide access to the latest information on how extremes have varied or are likely to vary under a changing climate and a range of greenhouse gas emissions scenarios. The extent to which humans are responsible for changes in extremes and particularly individual extreme weather is a challenging topic.