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close this bookNatural Disasters - Be Prepared! (UNESCO, 1997, 50 p.)
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Sounding the alarm

From tolling bells to remote sensing by satellite, disaster warning systems have come a long way

BY FABRIZIO FERRUCCI


Hurricane Fran (September 1996) as seen from a weather satellite. [Photo © NASA/Sygma, Paris]

The tolling of bells was once the usual way to warn people of impending disasters like floods, fires or approaching armies.

Warnings based on eye-witness reports were passed on from town to town. The further away the town was from the disaster, the longer it had to prepare its defences. These days, the approach is much the same even though the technology is very different. Warnings are still issued at one point and propagated across a network of users who deploy skills to mitigate the impact of the event.

Today’s warning systems rely on measurement techniques, mostly from physics and geophysics, and on communications technology. The science of risk prediction is young, and the systematic observation of natural processes is not much older. Quantitative meteorology dates back to the 1940s, quantitative seismology to the 1960s, and quantitative vulcanology is only about twenty years old. Remote-sensing from space only began in the late 1970s. Meanwhile, advances in communications technology have made it possible to speak to people anywhere on earth from a disaster site.

Technological risks and natural risks

Technological, or human-made risks are usually limited in time and place, even if they cause dramatic and long-term effects. Warning systems are built into industrial plants and are based on all possibilities of failure known to the designers. Natural risks are much more dangerous because they involve the biggest energies on earth, and because we have less access to risk sources that were not designed by human beings.


Houses in flames as fire spreads through a city block following the January 1995 earthquake in Kobe (Japan). [© John Pryke/Reuters/MAXPPP, Paris]


A mother and daughter look on helplessly as their house burns down. [© Peter Fownes/ Ask Images, Paris]

The concept of warning implies forecasting in space, time, or both. In some cases warnings can be issued if we can foresee the short, medium- and long-term evolution of an impending event, and its consequences. In others they are possible if we can identify minor events that are precursors of a much bigger one. Good examples of early warning are the forecasting of weather conditions twelve, twenty-four and forty-eight hours ahead, and of extreme meteorological events such as hurricanes, their path and timing. A bad example is earthquake prediction, which is seldom attempted and when it is all too often fails. The Table on this page shows today’s capacity to forecast the occurrence of some major natural events.

As a general rule, the capacity to issue reliable early warnings requires: good approximate knowledge of the specific source of the hazard in question; availability of one or more models allowing description of the event in time and place; appropriate surveillance systems being in operation; transmission of information ahead of the development of the hazard.

Sometimes one type of risk is linked to another. Extreme meteorological events may lead to flash floods, plain floods and landslides. Industrial fires may lead to explosions, severe chemical pollution of air, ground and water. If there is full control of the source of primary risk, warnings about secondary effects are possible.

How do we stand in practice with regard to warning techniques of some natural phenomena today?

Weather forecasting

Weather forecasts are generally reliable because meteorologists can model in detail the behaviour of large masses of air and because a very complete set of worldwide observations at all scales is available. Weather satellites provide a flow of images of cloud distribution over continents and oceans, while a dense network of ground stations reports continuously updated information about temperature, humidity and wind direction and speed at ground level and in the troposphere (the lowest level of the atmosphere).

As the Table below shows, we must answer a categorical “no” about the reliability of many types of forecast in time. This does not exclude the possibility that, locally and sometimes, correct forecasts are issued. However, these cannot be “generalized” and extended successfully to other places and other scenarios. F. F.

IS IT POSSIBLE TO FORECAST THE OCCURRENCE OF DISASTROUS NATURAL


IN SPACE

IN TIME

Long term (a year or more)

Medium term (month to weeks)

Short term (days to hours)

Earthquakes

Yes

Yes, almost

No

No

Extreme meteorological events

Yes

No

No

Yes

Flash floods

Yes

Yes

No

Yes/almost

Forest fires

Yes

No

No

No/almost

Landslides

Yes

Yes

Yes

Yes/almost

Plain floods

Yes

No

No

Yes

Volcanic eruptions

Yes

Yes

Yes

No/almost

Earth observation from space and efficient telecommunications on land and with space make it possible to combine national forecasts into a global system which can predict extreme events in time for precautions to be taken.

Weather forecasts can warn of heavy rainfall which may lead to flooding because of its concentration in space and the structure of river basins. Ground-based weather radar plays an important part in short-term flood warning by making it possible to estimate the location and amount of rainfall, whose effect on a river basin can be monitored by sensors which make it possible to issue short-term warnings and take steps to avoid or limit overflows. A good topography of the river basin and the river bed are needed to predict the behaviour of flood waters on the basis of rainfall density and duration and soil type.

Although flood warnings are not always timely, many recent instances show that lack of co-ordination rather than lack of warning is the weak point in flood risk mitigation.


Decontamining a building at Cernobyl (Ukraine) in 1986. [© R.I.A./Gamma, Paris]

Volcanic eruptions

It is relatively feasible to predict volcanic eruptions because the volcanic process is conditioned by many physical and chemical factors that can be individually monitored. Eruptions are always preceded by intense seismic activity and ground inflation, and the reawakening of a dormant volcano can easily be detected by just a few seismic instruments, making it possible to issue a long-term warning.

As the magma makes its way to the surface, ground inflation will begin and new gas vents will appear, along with changes in the gas composition and local disturbances in the earth’s gravity and magnetic fields. The appearance and growing intensity of these phenomena make it possible to issue a medium-term warning based on data provided by a complex blend of instruments including a seismic network, networks for monitoring the horizontal and vertical components of the ground deformation, a network of magnometers, a network for measuring very small changes in the local gravity field, and instruments for analysing volcanic gases.

At this point forecasting becomes more complex. Firstly, as the magma gets close to the surface, its effects are concentrated in an ever-smaller area, so many more and closer spaced instruments are needed to pinpoint the highest area of risk.

Secondly, the shallow part of the earth’s crust (2-3 km) is much weaker than the intermediate crust where the magma was earlier and so eruption is easier. The moment of eruption is preceded by much more rapid physical and chemical changes. The closer we are to eruption time, the greater the forecasting uncertainties.

Short-term warnings of eruptions are therefore still rare and mostly unreliable, however, because the complete range of measurement networks is only available for one per cent of the world’s active volcanoes.

In the case of explosive volcanoes, which pose a major threat to people living nearby or to aircraft flying into the dense volcanic ash thrown up - a medium-term warning should lead to the establishment of no-go zones but this is difficult to enforce for understandable social and economic reasons, especially in densely populated areas.

Earthquakes

The accuracy of earthquake prediction is poor, the main reason for the failure of short-term forecasting being the abundance rather than the lack of events that precede an earthquake. Many earthquakes are preceded by tremors, sizeable ground deformation, local changes in the earth’s electrical or magnetic fields, changes in the level of water wells, and emissions of radon, carbon dioxide and other gases along fault lines. But these events also occur independently of earthquakes, and in the case of big quakes have never all occurred together or been observed on the right scale by use of a single instrument network.

Although we know a lot about the source of earthquakes these days, we do not know much in detail about the earthquake preparation process nor about the exact meaning, at any given moment, of a hundred or so potential earthquake precursors. So it is easy to understand why governments and scientists are reluctant to risk their credibility by issuing warnings. The most reliable warning that can be given for earthquakes today is in fact simply to note where and when they have occurred and act accordingly.

Regional networks of seismographs can be set up in these earthquake-prone areas. Data can be processed in real time or near-real time through automated systems which make it possible to pinpoint the time, magnitude and epicentre of a quake a few minutes after the event. In this way rescue services can be alerted and directed to the places where they are most needed.

Today early warning of natural disasters is either possible or else we know why it is not. There is every reason to believe that through technology, our disaster-warning capacity will improve and that soon it will be possible to organize a timely response to all natural risks.

Until then, keep an ear cocked for the tolling bells. It may be just a wedding, but you never know.


A demonstration provoked by the escape of toxic fumes (dioxin) from a chemical plant in Seveso, near Milan (Italy), in 1976. [© E. Fornaciari/Gamma, Paris]