What are natural hazards

Although the nomenclature is sometimes ambiguous, natural hazards are usually defined as extreme natural events that pose a threat to people, their property and their possessions. Natural hazards become natural disasters if and when this threat is realised. Rapid-onset natural hazards, which form the focus of this book, can be distinguished from the often disastrous consequences of environmental degradation, such as desertification and drought, not only by their sudden occurrence but also by their relatively short duration. Natural hazards that are geophysical in nature, rather than biological, such as insect infestations or epidemics, arise from the normal physical processes operating in the Earth's interior, at its surface, or within its enclosing atmospheric envelope. Most geophysical hazards can be conveniently allocated to one or other of three categories: geological (earthquakes, volcanoes and landslides), atmospheric (windstorms, severe precipitation, temperature extremes, and lightning) and hydrological (floods and debris flows). Others, however, are less easy to pigeonhole. Tsunami, for example, can be regarded as hydrological hazards in the sense that they form and are transported within the hydrosphere. Their origin, however, is almost invariably geological, and usually the result of a large submarine earthquake. Similarly, wildfires and snow avalanches are not so easily compartmentalised. In the context of environmental change, extraterrestrial hazards must also be considered, with a large and growing body of evidence pointing to asteroid and comet impactors as major initiators of environmental change and associated extinctions throughout the geological record.

Increasingly, during the last half of the twentieth century, the impact of natural hazards on communities was exacerbated by human action, particularly urbanisation, changes in land use and agricultural practice, and deforestation. In 1998, for example, large-scale logging was a major contributory factor to the devastating flooding in the Yangtze basin of China. During the previous year, huge forest fires raged out of control across Sumatra and Borneo, when the seasonal rains that put them out in other years failed to materialise, generating an enormous pall of smoke over South-East Asia that lasted for many weeks. The term na-tech (for natural-technological) is increasingly being used to describe these complex disasters for which there is a recognisable and significant human contribution.

In this book we focus on current knowledge of the relationships between natural hazards and environmental change, gleaned both from the geological record and from studies of contemporary geophysical phenomena that are linked to significant short-term environmental changes, such as ENSO (El Nino-Southern Oscillation). We restrict ourselves to rapid-onset geophysical phenomena (Table 1.1), rather than epidemics or environmental degradation, although drought and desertification are briefly touched upon. Furthermore, although social and economic issues, particularly where they relate to increasing vulnerability, are addressed, we concentrate primarily on the science of natural hazards and environmental change. The idea that natural hazards can also trigger environmental change as well as occurring in response to it is particularly emphasised, and in this context low-frequency but globally significant hazards (McGuire, 1999) such as volcanic 'super-eruptions' and asteroid and comet impacts are included. Inevitably, the human dimension is also explored, both through the exacerbation of natural hazards as a result of human activities and in terms of increasing vulnerability due to multiplying populations and increasing urbanisation.

Continue reading here: Natural hazards and environmental change

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