Nine years ago in the year 2000 after most of the UK was swamped with what was pronounced a "1 in 100- year" or even a "1 in 500-year" flood, some people thought they would never again see such a disaster in their lifetime. Some even reduced their flood insurance. Big mistake!

Now, with some regions struck in 2007/08 plus the floods of 2003 on the Thames and those in Boscastle and Carlisle, by a supposedly once-in-a-lifetime flood for the second time since 2000, some scientists say the use of terms like "100-year flood" should be re-evaluated because they are often misunderstood and can give the public a false sense of security.

The United States Geological Survey, say they need to stop using the term 100-year flood and it could happen twice a year, if you're unlucky." Or 200 years could go by without a 100-year flood. US university professor Robert Traver, who specialises in storm water management, was more succinct: "Whoever invented that term should be shot." Some Government scientists both in the US and the UK say they have tried to move away from using the terms, yet they also say they routinely fall back on the labels as shorthand for measuring a flood's severity.

The terms have practical consequences; they are used for such things as classifying a flood bank’s protection level and setting insurance requirements for people who live in floodprone areas. Many people seem to believe that a 100-year flood should happen once every 100 years, or that a 1000-year flood should happen every 1000 years. But that's not how it works. A 100-year flood is defined as a flood so big that it has a 1 percent chance of happening in any given year.

The construction of flood defences and the building of flood relief channels are also contributing to the risks; it is possible climate change will increase flooding in the UK. For a 1 in a 100-year flood, "we should be talking about a 1 percent annual chance flood," said Laurence Waterhouse a trustee of the National Flood Forum. Will it get worse? The answer is YES! Global climate change caused by the accumulation of greenhouse gases in the lower atmosphere will change the frequency of flooding. The global concentration of these gases is increasing, mainly due to human activities such as the combustion of fossil fuels (which release carbon dioxide) and deforestation (because forests remove carbon from the atmosphere).

The atmospheric concentration of carbon dioxide, the main greenhouse gas, has increased by 30 percent since preindustrial times. Projections of future climate change are derived from global climate model or general circulation model (GCM) experiments. Climatologists review the results of these experiments for global and regional assessments. It is estimated that global mean surface temperature will rise by 1.5° to 3.5°C by 2100. This rate of warming is significant. Large changes in precipitation, both increases and decreases, are forecast, largely in the tropics. Climate change is very likely to affect the frequency and intensity of weather events, such as storms and floods around the world. Climate change will also cause sea level rise due to the thermal expansion of the oceans and the melting of the mountain glaciers.

Global mean sea level is anticipated to rise by 15 to 95 centimetres by 2100. Sea level rise will increase vulnerability to coastal flooding and storm surges. The faster the climate change, the greater will be the risk of damage to the environment. Climatic zones (and thus ecosystems and agricultural zones) could shift toward the poles by 150 to 550 kilometres by 2100. Many ecosystems may decline or fragment, and individual species may become extinct. This is all a chilling picture. So when you have had your last 1 in 100 year flood and are thinking about the fact that it is with luck that there will not be another one for 99 years, think about the changes in climate and the fact we would be having that 1 in 100 flood every year…

Laurence Waterhouse Jan 2009