Events over the past few decades have brought extreme weather and climate events to the fore of societal concerns. Ordinary citizens, individuals in the private sector, and people at the highest levels of government worry about the apparent increase in the frequency of weather and climate events causing extreme, and in some instances catastrophic, impacts. We differentiate between weather events – relatively short-term phenomena associated with, for instance, tropical cyclones (hurricanes and typhoons, for example), severe floods, and the like – and climate events – longer-lived and/or serial phenomena such as drought, season-long heat waves, record wildfire seasons, multiple occurrences of severe storms in a single season or year, etc. The differentiation is related to the distinction between weather, which can be forecast on short timescales of less than 1–2 weeks, and climate, which can be forecast on monthly, seasonal, and annual timescales. The adage “Climate is what you expect and weather is what you get” probably originates from the fact that climate is the statistical average of the weather over a specified time period. Regardless of whether an extreme event is weather- or climate-related, it could have significant and numerous implications for society.

This book summarizes our knowledge of different aspects of weather and climate extremes and then focuses on their recent and potential future consequences for different socioeconomic sectors.

Extreme events are critical determinants in the evolution and character of many natural and human-influenced systems. From such a perspective, extreme climatic events, in particular, present society with significant challenges in the context of a rapidly warming world. The societal impacts of recent extreme climatic events around the world motivated us to bring together in one book a scientific exploration of the nature of climatic extremes – past, present, and future – and examples of efforts aimed at making these events more comprehensible and manageable.

Extreme climatic events can affect both natural systems (e.g., coastal and riparian ecosystems) and human systems (e.g., the city of New Orleans). Despite having one of the most effective emergency response systems in the world, the United States has experienced months, and will likely continue to experience years, of difficulties in coping with the aftermath of Hurricane Katrina. Furthermore, while Hurricane Katrina may not be classified as an “extreme” hurricane in terms of its wind intensity at landfall, or a rare event in terms of the wind speed return period, the consequences of its landfall along the northern Gulf Coast would likely qualify as an extreme and, one hopes, rare event.

The capacity of society to respond optimally to climatic events such as active hurricane periods or long droughts depends on its ability to understand, anticipate, prepare for, and respond to extremes.

Abstract

A broad range of paleoclimate indicators is available to study the El Niño/Southern Oscillation (ENSO) phenomenon. These climate proxy records, each of which is sensitive to somewhat different aspects of climatic variations, can be used for studying ENSO – to answer specific questions, such as, How long has ENSO been operating in its present form? In this chapter, we evaluate the information provided by the various proxy records, highlighting their possible strengths and weaknesses, and pointing out areas where outstanding research questions into the nature of ENSO variability still remain.

At present, the preponderance of evidence suggests that during glacial times, the ENSO phenomenon (if it was operating as an alternating east-west source of heat and moisture to the atmosphere) did not leave the same spatial or temporal expression in the paleoclimate record that is evident more recently. Conditions in the early Holocene, prior to ~6,000 years before present (BP), are indicative of changed atmospheric and oceanic patterns, substantially different from those of today. Only after about 6,000 years ago did the climatic associations related to the changes in sea surface temperature and atmospheric circulation patterns related to ENSO that are seen today begin to be systematically recorded in the paleoclimate record.

Introduction

One of the major expressions of global interannual climate variability is related to the coupled ocean-atmosphere phenomenon of El Niño/Southern Oscillation (ENSO). Because of the far-reaching effects of ENSO on societies, concerns about its future behavior have promoted research into the ENSO phenomenon's long-term history, especially of those intervals when climate boundary conditions (orbitally determined seasonal insolation, global ice cover, sea level, aerosols, etc.) were markedly different from those of today (Diaz and Markgraf 1992).