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Climate change and rivers: a brief introduction

Last updated: May 17, 2012.

What's the biggest thing you can imagine? How about the climate of our planet changing so much that it becomes impossible for us to live on?

Something so immense and so drastic is almost beyond the realm of anything we could imagine—and yet, if the predictions made by the world's scientific community are true, a climate catastrophe of this magnitude may be a matter of "when" and not "if". Even in the short term, in our own lifetimes, we can expect to see dramatic changes in the weather, rising sea levels, and huge disruption in everything from the survival of species to the production of food. There will be some benefits from climate change, but many losses too. As we'll consider in a moment, we can also expect to see significant changes to the UK's rivers—and this short article is designed to give a brief overview of what we know so far.

What is climate change?

First, let's find out a bit more about climate change: what it is, why it's happening, and what it could mean.

Climate change is often referred to as global warming: the basic idea is that Earth, as a whole, is gradually getting hotter, prompting changes in the climate (long-term weather patterns) around the world. Some places will get hotter, some will get cooler, some will be drier, some will be wetter—that's why many scientists prefer the term climate change to "global warming", which suggests, rather misleadingly, that everywhere gets hotter by the same amount, in the same way.

Global warming map of sea temperatures

Photo by courtesy of Great Images in NASA.

Climate change is very likely caused by humans, mostly by burning fossil fuels (such as oil, coal, and natural gas) to release energy in everything from power plants and home heating systems to car engines and waste incinerators [1]. Burning fuels releases carbon dioxide gas, which accumulates in the atmosphere and acts like a kind of heat blanket, trapping some of the Sun's radiation and causing the planet to warm up like a greenhouse. Other "greenhouse gases" (including methane, ozone, and water vapour) also contribute to global warming, though carbon dioxide is the one that gives most concern. Since the mid-19th century Industrial Revolution, the concentration of carbon dioxide in the atmosphere has risen from about 280 parts per million to over 380 parts per million today; annual emissions of carbon dioxide grew by about 80 percent in just three decades from 1974 to 2004 [1]. If these trends continue, we can soon expect to see the concentration of carbon dioxide doubling compared to the level it was at before the Industrial Revolution. Although there are differing opinions on what this will mean, most climate scientists believe it will cause Earth to warm up by several degrees by the year 2100 (three degrees is thought to be the most likely amount). If the concentration of carbon dioxide increases even further, the warming will be correspondingly greater—as much as 6.4 degrees by some estimates [1].

Why does this matter? A few degrees of warming doesn't sound like a bad thing, especially if you live in a (sometimes) chilly country like the northern United States, Canada, or the UK. But let's put things into perspective. When the last great Ice Age ended some 15,000 years ago, Earth warmed by about five degrees over a period of several thousand years. Now we're potentially talking about a comparable amount of warming taking place over a century—perhaps 10–50 times faster [2].

Who says the climate is changing?

While you may have read about climate sceptics and the very public, often very acrimonious debate over climate change, it's important to remember who is saying what—and why. The idea that climate change is real, happening now, very likely caused by humans, and will produce unprecedented changes to the world we live in is endorsed by the vast majority of the world's climate scientists (and other scientists), notably the Intergovernmental Panel on Climate Change (IPCC, which compiles regular, official scientific reports on climate change for the world's governments), the World Meteorological Organization (WMO), the American Geophysical Union (AGU), the Royal Society of London, and many of the world's other key scientific institutions. Scientists work for science (the process of finding and interpreting evidence to understand our world), which helps us develop technologies to improve our lives.

Climate sceptics, on the other hand, are generally amateurs and opinionated journalists with little or no scientific training; considering how few of them there are, their views attract a disproportionate amount of media attention. While some sceptics are genuine, well-meaning critics whose arguments have undoubtedly sharpened the scientific debate, some are known to have been funded by vested interests, such as oil, coal, and car companies, who fear the economic effects of government policies that seek to reduce carbon dioxide emissions [18].

What are the likely effects of climate change?

No-one knows for sure what Earth's climate will be like in 10, 50, 100, or 500 years. What we do know is what we're seeing already and what climate scientists predict will happen through computer models [3]. Broadly speaking, a warmed up Earth will have significantly less ice cover at the poles and more water in the sea (both because of the melted ice and because warm water takes up more space). Weather is expected to become more erratic and extreme, with more droughts, floods, and forest fires, though it's uncertain whether hurricanes will increase [4].

What about plants and animals? The millions of species that thrive on Earth are adapted to living in particular ecosystems (such as those based in coral reefs, tropical rainforests, mountains, or deserts) and they cannot evolve (or, in the case of planets, move) fast enough to cope with such relatively rapidly changes [2]. As a result, climate change is expected to lead to massive extinction of species in the coming decades. It's easy to brush such things aside if you're a city-dwelling stock-trader who never comes into contact with polar bears or lemurs. But it's important to remember that humans are animals who are also adapted to living on Earth. Major changes in the climate are expected to lead to significant changes in farming and agriculture; some parts of the world will become more arid and less productive, though there will be gains in other areas [19]. (Stock traders who run on espresso might like to note, for example, that climate change is already having a huge impact on coffee production [20].)

How will climate change affect rivers?

In a warming world, the general trend is going to be toward greater water scarcity and drier rivers [19]. A major study of 900 rivers published in the American Meteorological Society's (AMS) Journal of Climate in 2009 concluded that flows into the oceans have decreased significantly over the last 50 years, with that trend predicted to continue [5]. According to the IPCC's 2007 report, direct climate change impacts on rivers will occur through rising temperatures and carbon dioxide concentrations, while indirect effects will be caused by changing precipitation (rainfall, snow, fog, and so on) and melting glaciers/ice. Globally, the IPCC suggests that climate change impacts on rivers will include changes in water quality, impacts on wetland plants and animals, and biodiversity loss, concluding "Changes in climate and land use will place additional pressures on already-stressed riparian ecosystems along many rivers in the world" [6].

Water quantity

Climate scientists believe the world's weather is going to become more erratic and extreme. So in countries like the United States or the UK, we might get more droughts and more flash floods, with rain or snow being more concentrated and intense—falling harder and for longer—than we're used to. Why? In a warmer world, more water will tend to evaporate from the oceans (and freshwater sources) into the atmosphere, which will hold and carry much more water than it used to. When it does fall, it will produce more intense rain and snow: a greater precipitation intensity, as the weather forecasters say.

Periods of intense dry weather followed by periods of intense rainfall place rivers (and freshwater life) under huge stress; both can be bad, in different ways. On one hand, rivers obviously need a certain amount of water to support life. More selfishly, humans need a certain amount of rain to fill up aquifers (groundwater supplies) and reservoirs that supply everything from drinking water and irrigation to manufacturing industry and power generation. According to Trevor Bishop of the Environment Agency, quoted by BBC Panorama in September 2011: "Climate change is highly likely to mean a drier, warmer climate that pushes demands up and it will probably reduce the amount of water available in rivers. So there is going to be a mismatch between how much water is available and how much society and the economy needs" [7].

On the other hand, too much rain falling too quickly soon overwhelms floodplains and rivers, while the recent, unfortunate tendency to build on floodplains makes it harder for the ground to absorb water as effectively [8]. Rivers aren't just nice wet lines you walk along; they're the highways that carry water to the sea. Rising rivers don't simply carry more water, they carry it much more quickly. High river levels can produce dangerously fast-flowing rivers that brush bridges aside, causing major problems for roads, railways, and the communities who depend on them. Rising sea levels will increase coastal erosion, but they'll also pose problems in low-lying estuary towns and cities, where tidal rivers will be more likely to cause floods [9].

Climate change seems certain to lead to more (and worse) flooding, which will lead to greater pressure for perhaps inappropriate flood defence and heavy engineering schemes that may see some of our rivers increasingly turning into high-speed drains. We're certain to see renewed calls for a total ban on floodplain developments... but will our governments and planners actually heed them? [8]

Water quality

Floods don't just change the quantity of water flowing into rivers: they also change its quality. Toxic chemicals build up on warm roads during dry weather and flash floods can quickly wash them into rivers or other watercourses as "runoff", causing major pollution episodes (either chemical or thermal) sufficient to wipe-out entire fish stocks from stretches of river. Excessive stormwater also causes problems for sewage disposal; high water levels cause sewage treatment plants to overflow through what are known as combined sewer overflows (CSOs), causing raw sewage and other waste to discharge onto rivers and beaches [10]. Less obviously, faster-flowing rivers will behave in subtly different ways, with sediments traveling and being deposited in different ways, banks eroding at different speeds, and so on.

Interestingly, climate change will bring different seasonal patterns to our rivers. In 2008, a major study of climate change and UK rivers by scientists working for the Environment Agency modelled the likely impacts on five very different rivers throughout the UK [12]. Impacts differ from river to river and it's hard to generalize. Nevertheless, the study reached these broad conclusions (summarized and paraphrased here):

Surprises in store?

One of the interesting things about climate change is how it will affect existing environmental problems; we have no idea yet whether it will make things better or worse—and there are bound to be surprises in store. In 2008, for example, Cardiff University scientists suggested that climate change is hampering the recovery of Welsh rivers from acid rain. Wetter winters have led to greater acidification of upland streams, raising acid levels in the rivers they feed into and undermining efforts to reduce acid rain by cleaning up air pollution [13].

Another thing we don't know is how quickly the impacts of climate change will occur. According to Hayley Fowler of the University of Newcastle, changes in rainfall in the UK may be happening much earlier than expected: "This pattern of change is the same as that project by climate models under global warming for the end of the twenty-first century" [14].

Is it happening already?

Climate and weather are inherently changeable and it's impossible to say with certainty that any changes in the weather we experience are caused by climate change. Every time there's a major flood or drought, people ask whether climate is responsible. No-one knows. We should expect to see more intense and erratic weather and it shouldn't surprise us, but storms, floods, heatwaves, and droughts have always occurred in the past and we still can't say with certainty what causes any particular weather event.

We do already some anecdotal evidence about the impacts of climate change. For example, in 2010, a study by Cardiff University scientists found that salmon and trout populations in the River Wye and its tributaries fell significantly between 1985 and 2004, and suggested warmer water and lower river levels (correlated with changing climate) were to blame [15].

Spring 2012, in which the UK lurched from drought to flood in less than a month, may also be a sign of things to come. An Environment Agency drought bulletin from May of that year suggests this unseasonal weather might be another indication of a climate beginning to swing between extremes [16]:

"In the week ending 6 April, the Environment Agency's weekly water situation report showed:
  • Rainfall at between 29–68 per cent of average for March
  • River flows below normal or lower at every indicator site in England
  • Reservoir stocks below normal or notably low at 65 per cent of indicator sites in England
  • Groundwater levels below normal or lower at all but four indicator sites (84 per cent), with 12 sites rated exceptionally low in England.

After the persistent and heavy rain of recent weeks, the Environment Agency’s weekly water situation report for week ending 11 May showed:

  • The most rainfall in April in over 100 years
  • River flows notably or exceptionally high at 48 per cent of indicator sites across England
  • Many reservoirs are recovering
  • Groundwater levels still exceptionally low in 42 per cent of indicator sites in England."

Then again, it could just be a change in the weather; no-one knows for sure. Even freak flooding events are very hard to link conclusively to climate change—even if a link does indeed exist. A review of the major summer floods of 2007 by the Centre for Ecology and Hydrology (CEH) found no link with climate change. According to lead author, Terry Marsh, quoted by BBC News: "The river floods of summer 2007 were a very singular episode, which does not form part of any clear historical trend or show consistency with currently favoured climate change scenarios" [17].

Even when climate change is occuring, it's difficult to spot—and that makes it even harder to convince people that the threat is real.

Written by Chris Woodford for the UK Rivers Network. © Copyright Chris Woodford 2012.


  1. See the IPCC's Climate Change 2007 Synthesis Report: 2. Causes of Change. For a user-friendly summary, see Humans blamed for climate change by Richard Black, BBC News, 2 February 2007.
  2. The figures in this section are mostly based on those quoted in the excellent, concise Climate Change Fact Sheet written and regularly updated by Prof Stefan Rahmstorf of Potsdam Institute for Climate Impact Research.
  3. See Models 'key to climate forecasts' by Dr Vicky Pope of the UK's Hadley Centre.
  4. For a very good, balanced introduction to the likely effects of climate change, see The Rough Guide to Climate Change by Robert Henson.
  5. World's major rivers 'drying up' by Matt McGrath. BBC News, 21 April 2009.
  6. 4.4.8 Freshwater wetlands, lakes and rivers in IPCC Climate Change 2007: Working Group II: Impacts, Adaptation and Vulnerability.
  7. Quarter of England's rivers at risk. BBC Panorama, 18 September 2011.
  8. See March against flood-plain housing, BBC News, 18 August 2007. Also In Deep Water (BBC Panorama).
  9. For example, the Environment Agency's 2009 Limiting and adapting to climate change: position statement says: "Rises in sea level, increased rainfall and storm frequency mean that London and the Thames Estuary will be at greater risk from flooding in the future once the current barrier expires."
  10. See Combined Sewer Overflows (CSOs) by Surfers Against Sewage for an introduction.
  11. Eel populations in London's River Thames crash by 98%. BBC News, 22 January 2010. Dr Matthew Gollock is quoted: "It is difficult to say what is going on - it could be due to a number of potential factors including changes in oceanic currents due to climate change..."
  12. Potential impacts of climate change on river water quality by P. Whitehead, D. Butterfield, and A. Wade. Environment Agency Science Report – SC070043/SR1.
  13. River recovery 'dampened by rain'. BBC News, 3 December 2008.
  14. Quoted in the box "England and the Rains of Autumn" on p60 of The Rough Guide to Climate Change by Robert Henson.
  15. 'Climate change' danger to salmon and trout in Wye. BBC News, 17 May 2010.
  16. Heavy rain results in review of drought-affected areas. Environment Agency Weekly Drought Update, 11-May-2012.
  17. 2007 floods 'no link to climate' by Richard Black, BBC News, 10 March 2008.
  18. Andrew Rowell's Green Backlash is a lengthy, though now dated study. For more up-to-date rebuttals of climate sceptics, see George Monbiot's columns A Beardful of Buncum (December 9, 2008), Death Denial (November 2, 2009), Why Libertarians Must Deny Climate Change (January 6, 2012).
  19. Climate change-related water scarcity to affect global food production—UN. UN News Centre News Release, 9 June 2011.
  20. See for example Fairtrade coffee producers face challenge of climate change by Sarah Butler, The Guardian, 24 January 2011.

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