Get used to flooding, climate change will bring more of it

More of this to come. Steve Parsons/PA Archive

Flooding continues to afflict many parts of southern Britain. Areas of the Somerset Levels have been submerged for weeks, large parts of the Thames Valley are under water and the River Severn is bursting to the brink. The coasts of Cornwall and Wales have been battered by storms, and farmland and buildings have been flooded from the sea. This barrage of wet weather begs the question: is climate change to blame and what might we expect in the future?

The evidence that human activity is leading to an increase in global average temperature is extremely strong, and was presented most recently in the IPCC’s report published in September last year. However, it is not really fair to ask whether one individual event (or indeed season) was caused by climate change. A more appropriate question is whether the increasing concentration of greenhouse gases in the atmosphere has changed the risk of experiencing extreme events.

A gambling analogy illustrates this: if we throw a pair of sixes, we put this down to luck. If we then throw several pairs of sixes, we begin to believe that the dice are loaded. Climate change is loading the dice.

Rising water

Global average sea level rose approximately 19cm between 1901 and 2010, and around 5cm since 1993. It therefore takes less of a storm surge for coastal flooding to occur now than it did a few decades ago. Warmer air can hold more water, and across many parts of the world we have seen more heavy rain over the last few decades. The UK has had several bouts of serious flooding in the last few years – the floods of 2007 and 2012 come readily to mind – but the flooding this year appears to be more extensive, more extreme and to have lasted for longer than we’ve recently experienced.

What is more uncertain, however, is the extent to which increasing greenhouse gas concentrations are leading to changes in the occurrence and intensity of storms. Most large scale flooding across the UK is primarily driven by prolonged and persistent rain coming – usually from the Atlantic – in a sequence of rain-bearing weather systems called depressions. We now know that such prolonged wet and stormy weather across the UK is strongly linked to the position and strength of the North Atlantic jet stream.

We in the UK are at the downstream wet end of the jet stream, and therefore very sensitive to how it varies from year to year. The North Atlantic Jet Stream has been very strong this year, which has been linked with unusually heavy rainfall over Indonesia, and the tropical western Pacific Ocean. But many different factors may also influence the jet stream. Weather in one part of the world can be strongly connected with what is happening elsewhere.

Forecasting future weather

Looking ahead, we expect sea level to carry on rising and heavy rainfalls to become even more frequent. Most projections of future climate across the UK suggest that we will experience wetter conditions in winter, and therefore that flood risk will continue. Because of our sensitivity to the position of the jet stream and the large variability from year to year, it is currently difficult to predict exactly how flood risk will change. However, we can be confident that relying on past experience and the historical record to estimate flood risk will probably lead us to underestimate our future exposure to flooding.

When the waters recede and we start to think about how to cope with future floods – as we clearly need to do – two things are necessary. First, we need to work out how to estimate what climate change might do to flood risk. This means we need to understand how different parts of the global weather system fit together – this is the subject of much new research. Second, we need to accept that we probably will not be able to estimate precisely how flood risk may change in the near future. The climate system is complex and requires that we develop ways of coping with risk that are flexible and adaptable – and which leave options open in the future.