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Carbon Brief Staff

27.11.2012 | 3:00pm
ScienceFive things to know about flooding and climate change
SCIENCE | November 27. 2012. 15:00
Five things to know about flooding and climate change

Heavy rainfall continues to batter the UK this week, causing widespread disruption. The Environment Agency has issued hundreds of flood warnings and evacuated many from their homes. Is this unpredictable weather normal, or is there a link between flooding events like this and climate change?

Here’s five things you should know about linking climate change with flooding.

1. A warmer atmosphere holds more moisture         

According to the Intergovernmental Panel on Climate Change ( IPCC), the atmosphere is about 0.75 degrees warmer than it was at the start of the century, which means it can hold 5-6 per cent more moisture.

That doesn’t automatically mean more heavy rainfall for the UK because complex weather patterns govern the amount, timing and distribution of rainfall. But it does mean that with more water in the atmosphere, the volume of rainfall may increase when it does pour.

This isn’t the end of it though, because climate change may affect atmospheric circulations like the Jet Stream, which control the movement of weather systems over the UK. Research on this is still fairly new, however, and the link remains less than certain for now. But it’s another reason why the climate change link to flooding is complicated.

2. Evidence of heavier rainfall in the past is limited, but growing

Extreme events are rare, and detecting trends outside natural variability requires decades of continuous observations. So even though there’s a theoretical link between climate change and rainfall, it’s hard to find clear evidence yet because the record of measurements is short and doesn’t cover all parts of the world.

Globally, that makes it hard to distinguish any trend in the intensity or frequency of flooding due to climate change, as the IPCC concluded in its Special Report on Extreme Events. But in Europe, where more data on rainfall exists, there’s some evidence for a trend towards heavier rainfall.

A recent study finds greenhouse gas emissions contributed to observations of more intense precipitation over two thirds of the northern hemisphere between 1950-2000. Finding trends like this is difficult with a small data set on rainfall, but should become clearer as more extreme events occur over time.

3. Attributing specific events to climate change is tricky, and flooding is no exception

In general, scientists are wary of attributing specific extreme events to climate change because it’s impossible to say whether an event would have happened if global temperatures weren’t increasing. It’s more helpful to consider how climate change may have contributed to the frequency or severity of such events.

A recent study found although the precise human contribution to widespread flooding in England and Wales in 2000 is difficult to pinpoint, global greenhouse gas emissions increased the risk of flood by up to 90 per cent. Such careful analysis of the data takes time and linking the current flooding in the UK to climate change is not possible yet.

4. Scientists predict that heavy rainfall will increase in the future

Flooding occurs in a number of ways, and each may be affected by climate change. Surface water flooding occurs where heavy rainfall can’t absorb into the ground or drain away. River flooding, closely linked to surface flooding, occurs when streams burst their banks. Coastal flooding results from high tides, storm surges and sea level rise.

Rising sea levels present a clear threat to flood coastal areas. But there’s less certainty in projections of surface water and river flooding. That’s mainly down to uncertainty in modelling, and difficulty simulating how weather patterns may change.

Based on physical reasoning, however, many studies – including a government study on climate change risks – predict the UK will experience more intense rainfall and more frequent high river flows in the future, particularly during the winter. A recent report from the European Environment Agency (EEA) echoes this, predicting increases in both river and coastal flooding in the UK.

5. Flooding isn’t just about rainfall; other human factors contribute too

Flooding and extreme precipitation go hand in hand, but they aren’t the same thing. While climate change may directly alter precipitation in the UK, flooding is a consequence of heavy rainfall which also has a human component.

The rising cost of damages associated with flooding is a perfect example of this. The rise could be down to a number of factors, not simply the amount of rain that falls. People are getting better at reporting damages, but human activities have a major impact too.

Changes in land use, such as building houses on flood plains and paving over natural surfaces, are making people more vulnerable to flooding. According to the EEA report, climate change could contribute more in the future, but for now, land use changes are the reason behind rising damage costs in the UK.

So in summary, it’s pretty difficult to assess the effect climate change has on flooding. While in theory warmer temperatures may lead to more rain, the climate system is pretty complicated in practice. It looks likely that the UK will experience more surface, river and coastal flooding in the future, but it’s hard to be more specific than that

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