Natural Hazards And Risk Analysis

Human beings live on a restless, dynamic planet. Our settlements and livelihoods depend on Earth's variations and variability, past and present, in the form of geology, topography, climate, and the distribution of vegetation and freshwater. At the same time, these variations and variability pose potential threats, which are called natural hazards. Extreme movements in Earth's crust release energy experienced as earthquakes, volcanoes, and tsunamis on Earth. Climate extremes such as hurricanes release gigantic amounts of energy. Heat waves, blizzards, and ice storms are other climate extremes. Floods and mass movements such as landslides, rock fall, and snow avalanches are more localized but can be very destructive and deadly, as are tornadoes and lightning strikes. Drought is a slow-onset hazard but nevertheless associated at times with great economic cost and displacement of people.

Hazards, however, are not in themselves a problem for humanity. Every day, many thunderstorms flash and rumble around the world in uninhabited areas and over the large surface of the planet covered by oceans. Ice, rock, and debris slides are a normal part of the erosion and deposition cycle, and many take place in uninhabited regions of Earth's mountains. To mention an extreme case, a sizable object (perhaps a meteor or comet) entered the atmosphere and exploded over Tunguska, Siberia, in 1908, flattening 80 million trees over an area of 2,000 square kilometers. But there were no human settlements. Had the event occurred over London or Paris, the outcome for humanity would have been quite different.

At the other end of a continuum of magnitude, even normal (statistically mean) values for some natural processes may endanger the health, livelihoods, and lives of people so poor or so marginalized by society as to live with little protection in highly exposed locations such as very steep slopes, in gullies, on temporary silt islands, or very near a river flood plain. In such cases, “normal” or “small” landslides, seasonal flooding, or the occasional “expected” rock fall may have catastrophic consequences for one or a few households while never being registered as a “disaster” for society.