Muddy water
Flooding is in our future (8 minute read)
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Mary, grab the baby, the river’s rising
Muddy water’s taking back the land...
Whenever I hear about flooding, my mind plays a soundtrack for me. It’s a mournful song by bluegrass guitarist Phil Rosenthal called Muddy Water, covered by Nick Cave and the Bad Seeds in 1986. The song hauntingly captures a sense of hopelessness at the inexorable rise of the water, and it’s not easy to listen to. But it’s stayed with me since I first heard it in the 1990s.
The song has been on my mind lately. Partly, it has been the terrible flooding in South Africa, which has killed more than 400 people. But mostly, it has been because learning about climate change keeps leading me to the conclusion that the world is facing a future of floods, resulting from rising seas and more intense storms.
Two weeks ago, I wrote about the rising sea level, and how averages could be misleading, because different areas will face different amounts of sea level rise. But even for particular areas, it’s not the average that counts. What’s more important is what happens during periods when water levels are particularly high – during spring tides or storms.
Spring tides – which are nothing to do with the season of spring – are higher than average tides, and occur twice a month. They are caused by the position of the moon in relation to the sun. When the sun and the moon line up, the gravity of the moon is added to the gravity of the sun, causing the water on earth to bulge a little more. At certain times of year, the moon is also slightly closer to earth, and when it is, the gravitational pull is increased further, also increasing the height of the tides. When these factors all line up, coastal flooding can result in low-lying areas.
Coastal flooding is most often an inconvenience, but storm surges are another matter. They can be deadly and destructive. Storm surges interact with waves to batter coastal areas, eroding beaches and roads, and undermining the foundations of buildings.
A storm surge occurs when a combination of high winds and low atmospheric pressure raises the water level along a coastal area. Most of the storm surge results from high winds blowing more water towards the shore and raising the local sea level. But a small proportion of that rise in sea level – about 5% – is caused by low atmospheric pressure. The low atmospheric pressure at the centre of a storm allows the ocean’s water to expand. A storm tide is the result of a storm surge and high tide, and raises the sea even further.
Storm surges and storm tides are complicated. Their height depends not just on the size of the storm and the size of the tide, but the shape of the local coastline and the slope of the land which is under the water. A shallow slope causes the water to pile up higher while a steeper slope reduces the size of the surge. In the United States, this is bad news for somewhere like Louisiana, where the water is shallow off the coast, but gives some reassurance to Florida, where the water gets deep very quickly (you can see an animation showing that effect here).
Even the most conservative estimates suggest that tens of millions of people live in areas which are at risk of coastal flooding and storm surges. Some estimates put that figure as high as 250 million. By 2100, there are likely to be at least another hundred million people affected. While it is possible to manage the risk of coastal flooding, as demonstrated in The Netherlands, sea level rise will make that increasingly difficult.
Coastal flooding isn’t the only type of flood potentially affected by climate change. Away from the coast, flooding of rivers and streams results from heavy rainfall or snow melt. How will this type of flood be affected by climate change?
To answer that question, we need to look at the impact of climate change on storms. Like me, you’ve probably heard many times the statement that climate change is causing more frequent and more intense storms. But I’ve never understood precisely why, so I took a closer look.
The reason why storms will be worse as temperatures rise is actually relatively simple – warmer air holds more moisture. At higher temperatures, more water moves from its liquid phase to its gas phase – water vapour. At cooler temperatures we see the opposite effect, such as when a cold drink is poured into a glass on a hot day, and the water vapour condenses as drops on the outside of the glass. So, with warmer temperatures, more water evaporates from the oceans. As a result, there’s more rain to fall. There’s another effect too, because the rise of water vapour from the ocean fuels the wind, and the more water vapour that rises, the more wind there is. (The process is explained well in this video, even though the tone of the video is a little patronising.)
So warmer temperatures mean both higher winds and higher rainfall during storms. And with that higher rainfall, we are likely to suffer more intense floods. This is true for all the types of flooding associated with heavy rain – from river floods, which often result from many days of rain, to flash floods which result from intense rainfall over a few hours. We’re also likely to face more frequent landslides, which we don’t usually think of as floods, but which are closely linked to flooding in steep areas.
But while we mostly think about flooding as a disaster, it’s worth remembering that some floods have a positive effect as well. When rivers flood, they deposit silt over the flooded land, creating highly fertile soils when the waters retreat. Perhaps the most famous example of this is the fertility that resulted from the annual flooding of the Nile in Egypt (until the Aswan dam was built). The Nile’s floods were crucial for the development of Egyptian civilisation, and if the flood failed the country was plunged into famine. Fertile floodplains gave rise to other ancient civilisations, including Mesopotamia, on the Tigris and Euphrates rivers in modern day Iraq, and the Indus, in modern day Afghanistan, Pakistan and India.
So humans have a complicated relationship with floods – which can bring life and wealth as well as destruction. To try and shift this balance, we have been trying to manage rivers for centuries, building stop banks (or levees in the USA) and altering the channels of rivers to take water away more quickly. The result has not been a universal success. On the mighty Mississippi River in the United States, there is evidence that flood control engineering, while effective at containing small floods, makes large floods worse. Stop banks help as long as they hold, but if they collapse, as they did in New Orleans during Hurricane Katrina, the result can be catastrophic. Altering the channel of a river so that water flows more swiftly also makes floods more dangerous, as the faster-flowing water is more destructive.
There are other ways that we’ve been making floods worse too. We’ve covered urban areas with impervious surfaces, like concrete, so water cannot soak into the ground. As a result, cities are particularly prone to flooding. We have also removed the forest from large areas of land, and there’s some evidence that deforestation makes flooding worse, although that conclusion is debated.
With all the impacts of river engineering, deforestation and urbanisation on floods, it’s hard to get a sense of what climate change is doing to floods caused by excess rainfall. For example, were the disastrous floods in South Africa made worse by climate change, or were they simply the kind of bad flood that we’ve always faced? What about Australia’s recent floods?
Although it’s relatively easy to explain how climate change will result in more extreme weather, working out whether a particular weather event was linked to climate change is a lot more difficult. To do that, scientists need to know what the weather would have been like without climate change, and then compare that with what actually happened. This process involves looking at historical records for extreme weather, to work out how often an event of that size has occurred in the past. Scientists then use computer models to make two sets of predictions about weather events in the future – one with climate change and one without. Only then can they tell how much of a particular weather event resulted from climate change.
The process sounds relatively straightforward, but unfortunately it’s not. Rainfall varies a lot from year to year, and often on a very local scale, much more so than temperature. It is difficult to untangle that variation from the effects of climate change. So, while we are getting better at working out whether heatwaves have been influenced by climate change, we are much less certain about floods.
But even though it’s difficult to know whether a particular flood is the result of climate change, the general trend is clear. Whether we live along the coast, near a river or on steep land subject to landslides, flooding is in our future.
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Melanie, you always explain things so carefully and clearly - thank you!
I know that in the 13 years we have lived at our current location in Georgia, USA we have seen an increasing number of flash floods from massive downpours. Many of them in the past few years have been from hurricanes, which never used to make their way this far inland. We lost 2 bridges on our driveway a year and a half ago from a flood and were stranded at home. The area in North Carolina where I grew up is seeing regular landslides - they shut down a major highway after heavy rains to prevent trapping people between slides. The effects of climate change are real!