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International Strategy for Disaster Reduction
Platform for the Promotion of Early Warning

What's early warning
Basics of early warning - How early warnings are made - Milestones and first steps of early warning

How early warnings are made

Disasters and their warning signs
Often there are warning signs well ahead of a hazard event. Weather forecasters observe the start of hurricanes and storms and can calculate their likely future strength and tracks, vulcanologists can interpret telltale rumblings below the earth, ecologists keep watch on conditions that favour locust breeding, drought experts estimate the chances of refreshing rains, oceanographers plumb the depths of the Pacific Ocean for signs of the El Niño and plot the travel of ocean-crossing tsunamis caused by earthquakes.

Good early warning systems also need to consider community vulnerabilities as well as the hazards. What are the early warning signs for vulnerability? Key signs are growing poverty, environmental degradation, populations crowded in risky locations, civil strife, and lack of
knowledge and preparedness.

Prediction models
All warning systems, however simple they may be, are based on some idea of how the phenomenon behaves, whether it is a storm or a locust swarm, a landslide or epidemic, a migration of people or the slow destruction of a forest. Scientists call this idea the “model”, and they use the model to say what is likely to happen next. In the simplest cases, the model may amount to no more than common sense – for example the recognition that poor people who have settled in a river valley will lose their dwellings and all their belongings in even a small flood. At the other extreme, models of the physics of the global weather system are immensely complex and require large computers to do all their calculations and produce detailed forecasts for the whole globe.

Warnings are never perfect
Predictions are never perfect or precise. There is always some uncertainty. A hurricane may change course overnight or weaken; a tsunami may be higher than expected; an earthquake may be likely in a broad region but the location and time cannot be pinpointed. Often the warning may be expressed statistically – for example, as a 60% chance of an El Niño developing next year.

In addition, there is always a great deal of uncertainty about the social components of an early warning, and about the specific human impacts that might occur. This can make it difficult for decision makers to act. Decision makers and those at risk must weigh up the chances and consider the implications for their particular situation. Which option to follow – to close down a subway and disrupt a city or let the city keep running and risk chaos? To follow advice to evacuate or stay put and try to protect your home? To plant drought-resistant seed knowing it will not produce as much as the usual seed? Each person or community must make its own judgment as appropriate to their own circumstances. In each case there are costs of acting as well as costs of not acting.