North American Weather and Climate Extremes: Progress in Monitoring and Research

This workshop assessed long term observations of weather and climate for North America in terms of trends in extreme weather and climate. Physical mechanisms and processes were discussed, exploring what could be expected in a warming climate and whether or not those evidence for those expectations can be found in the observational record. Impacts of extreme events on different sectors were also part of the workshop discussions as were the spatial and temporal scale limitations of models for resolving certain types of extremes.

keywords: weather, climate, climate change, extremes, observations

Description
There is preliminary evidence from observations and model simulations to suggest that in a gradually warming world there will be appreciable changes in the variability and long-term trend of extreme events, i.e., events that are at the high end of a probability percentile ranking. If this is the case, then monitoring extremes is an important additional indicator or index of climate change and a valuable tool for identifying how society might respond to changes. In other words, knowledge of extremes can strengthen the science of climate change detection and attribution, as well as prepare society for anticipated changes in weather patterns and near-term climate perturbations.

“Extreme events” span many weather and climate variables. Key variables include temperature-related parameters (severe freezes, heat waves), precipitation-related parameters (wet spells, heavy precipitation events, droughts), tropical and extra-tropical storm frequency and intensity including extreme waves and coastal erosion, ice and hail, snow cover and depth, etc. The impact of climate extremes can be severe and wide-ranging. Extremes affect all sectors of the economy – agriculture, utilities, transportation, water resources, insurance industry. Among its many uses, information on climate extremes allows for real-time evaluation of ongoing or recent events placed in an historical context.

It is timely to re-visit this topic given recent progress in this field (e.g., new observations and analyses, climate model simulations at regional scales, new analysis techniques). In 1997 an international workshop was convened to identify knowledge on changes in climate extremes and the impact on user communities (CLIVAR/GCOS/WMO Workshop on Indices and Indicators for Climate Extremes, National Climatic Data Center, NOAA, Asheville, NC, USA, June, 1997 — published in book form by Kluwer Academic Publishers in 1999). The workshop examined the following questions:

1. What needs to be done to improve data sets and analyses for extreme weather monitoring?
2. Can we establish priorities for specific data set development and improvement?
3. Can we establish indices and indicators of extreme weather and climate?
4. What are the impediments to improving the monitoring of climate extremes?

In addition to the NCDC meeting in 1997, in 1998 the Aspen Global Change Institute held a meeting also co-chaired by Tom Karl and Jerry Meehl titled “Climate Extremes: Changes, Impacts, and Projections.” This led to a report, Elements of Change 1998 published by AGCI in 1999 and a series of articles in BAMS and an article in Science in 2000 by a number of the session participants.

The WMO Commission for Climatology / CLIVAR Expert Team on Climate Change Detection, Monitoring and Indices has been conducting regional meetings to produce various climate indices from high-resolution regional data. The US, Canada, and Mexico have been working in a tri-lateral mode on a North American assessment of climate extremes. This work provides a cooperative foundation for a closer examination of the science of climate extremes for the North American continent. It is this geographical area that will be the primary focus of the workshop. However, the participation of scientists from outside North America and the presentation of other work recognize the connectivity between phenomena on different regions of the globe and will help to place the North American experience in a global context.

A workshop would respond to the current needs associated with the assessment activities of the US CCSP and be able to complement the global focus of IPCC with an emphasis on extremes in North America. The CCSP Strategic Plan states “A cause for concern for which there is considerable uncertainty is the potential for changes in extreme events and rapid, discontinuous changes in climate”. Such changes would test the ability of societies to adapt (e.g., extended and severe drought). One of the 21 Synthesis and Assessment Products to be produced under the auspices of the CCSP is Product 3.3 “Climate extremes including documentation of current extremes. Prospects for improving projections.” There are potential intersections between this product and several other CCSP products of related interest

The primary objective of the workshop is to assess the latest scientific findings related to monitoring and projections of extremes and to see collectively where we are in our efforts to deal with research and monitoring of climate extremes for the North American continent. A specific outcome will be an action plan to produce the required CCSP product, i.e., an assessment report on climate extremes.

Three general areas will be addressed: (1) develop a framework to define climate extremes of particular ecological or economic impact, (2) assess the state of the science in the historical and contemporary measurement of climate extremes; (3) examine and clarify the ability of climate models to simulate and project climate extremes including
changes on the frequency, intensity, and duration of extremes; and (4) define the measurements, analyses, and other actions required to reduce uncertainties about changes and trends in future climate extremes.

The workshop format will be about 70% plenary and 30% break-out, with international representation and with experts drawn from the observational, modeling, and analysis communities. There will also be representatives of the impacts community; their expertise will help shape the discussion about relevant physical variables and derived products. Days 1 – 3 of the workshop will be spent mainly in a plenary session in which recent developments and the state-of-the-science in the three general areas are reviewed.

Specific topics that will be presented/discussed include:

•  How can we best define “extremes”?
•  Which phenomena/physical variables should and can be covered?
•  The role of models
•  Appropriate indicators/indices of change
•  Relationship between natural modes of variability (ENSO, NAO?)
and changes in extremes
•  Mathematical/statistical characterization of extremes
•  Monitoring of extremes
•  How have extremes changed in the 20th century, and how may they change in the future?
•  What needs to be done in the future in research and monitoring?

Break-out groups will then meet late on Day 3 and all of Day 4 to start discussions on (1) short-term actions (i.e., those that can be initiated within a year or so and will contribute to satisfying requirements for information, including the CCSP synthesis document on climate extremes and the IPCC; and (2) longer-term needs, such as improved observations, analyses, data management, etc. (specific, workable steps given existing resources and the most urgent unfunded needs). The morning of Day 5 will have plenary presentations of the group results. The afternoon of Day 5 will address the CCSP Prospectus for producing the synthesis product. The discussion of the CCSP process will conclude on the morning of Day 6.