March 2014
Rainfall is a part of the natural hydrological cycle. It is one of the vital water flows that link atmospheric systems with ecosystems and human systems. Any long-term changes in the amount, timing or duration of rainfall could bring a range of serious geographical effects. Two recent reports examine the evidence for changing water flows. Firstly, the Living With Environmental Change (LWEC) 2007 report focuses on the UK. Secondly, the 2013 Intergovernmental Panel on Climate Change (IPCC) assessment examines evidence for change globally. Drawing on both, this article provides GCSE and AS-level students with an update on recent and predicted trends in the UK’s water cycle and weather. Could it be that climate change is to blame for recent flooding and extreme weather?
Water availability in the UK is naturally unreliable. Before considering how climate change could affect water supplies, it is important to recognise that hydrological flows are naturally variable (i) from place to place in the UK and (ii) over various timescales.
Natural variability in water supply complicates climate change predictions for the UK greatly. For instance:
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Regional variations occur between different parts of the UK. East and southeast England receive just 500-700mm annually. Rainfall is highest in the west and north, especially in mountainous areas. In some parts of the Highlands of Scotland, high levels of relief rainfall bring the annual total to 3,000mm
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Seasonal variations occur naturally in the UK. England and Wales have mild winters and warmer summers. There is no pronounced dry season, unlike Mediterranean or Tropical climate regions, although UK rainfall is lower in summer than it is in winter. London’s average December rainfall is around 80mm, whereas a lower figure of 50mm in more usual for July. In the wettest parts of the Scottish Highlands, both figures are typically double this
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Longer natural cycles also affect the UK’s rainfall. Clusters of drier or wetter years are a feature of historical climate records (1975 and 1976 were exceptionally dry years, for example with only 541mm of rainfall between March 1975 and August 1976). This variability is attributed to cyclical variations in the way that the ocean and atmosphere interact on a planetary scale. Of particular importance is the periodic movement of the jet stream, a narrow band of fast moving winds found high up (10 km) in the atmosphere. The jet stream moves in a westerly direction over a region of the north Atlantic where cold air from the Arctic meets warm air from the tropics, before reaching the UK. Rain-bearing depressions develop along the edge of this boundary, known as the polar front, like a series of whirlpools, before migrating westwards over the UK, following the jet stream. Periodic strengthening and weakening of the jet stream occurs, as well as changes in its position, which determines whether storms pass over England or Scotland to a greater extent. The causes of these fluctuations, which are still not fully understood, include the North Atlantic Oscillation (a periodic change in Atlantic Ocean atmospheric pressure) and the Quasi-Biennial Oscillation (a periodic change in equatorial air circulation with knock-on effects for the jet stream). The result is complex, natural variability in the occurrence of ‘flood-rich’ and ‘flood-poor’ years in the UK.
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All of this means that it is hard to predict with any great certainty what the effects of climate change may be for local UK regions! It is also the reason why many scientists are reluctant to attribute any particular event, such as the winter flooding of 2013-14, directly to climate change. Naturally-occurring extremes are always to be expected, given the complex and dynamic character of our climate.
Evaporation
Warmed by the sunlight, water returns to the atmosphere in gaseous (vapour) form.
Condensation
Tiny droplets of water form when rising water vapour cools, sometimes visibly as cloud.
Precipitation
As tiny cloud droplets collide and grow, they merge to form bigger droplets that fall as rain, hail or snow.
Overland flow
Rain that falls at high intensity, or onto saturated or impermeable surface, cannot soak in and travels quickly over the ground surface, downhill, towards rivers and lakes.
Throughflow
Rain that can soak into the soil (by a process called infiltration) flows slowly downslope, through the soil, until it reaches a river or lake.
Groundwater flow
On entering the soil, some rainwater seeps into the rock below, moving slowly through pores, or more rapidly along fractures and lines of weakness (joints and bedding planes).
A warmer world will be one where more evaporation takes place over the oceans – and what goes up must ultimately come down! Climate change scientists therefore believe that rainfall patterns are likely to change in the UK as the world’s oceans warm, and may in fact be doing so already (though the effects are hard to disentangle from the jet stream’s naturally erratic behaviour). Many scientists do believe, in general, that the UK may in future become faced with:
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longer, hotter summers, especially in London and the southeast
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warmer, wetter winters, with more rain-bearing storms, of greater intensity, for all parts of the UK, not just northern regions.
Some of the main climate risks for the UK are shown in the table below, along with recent examples. Could these hazards occur more frequently in the future?
River flash flooding
A warming climate and ocean could increase the potential for extremely high intensity rainfall events which outstrips the infiltration capacity of soils. Overland flow – and the potential for flooding – is also exacerbated by steep slopes, thin or compacted soils, impermeable bedrock (or concrete) and a lack of vegetation which would otherwise intercept some of the rainfall. This brings a flash river response that can onset suddenly and be hard to predict. This is the kind of flooding that overwhelmed Jedburgh In December 2013 (and Boscastle in 2004)
Slow-onset river flooding
Even if rainfall intensity is not always high, an increase in the frequency of rain-bearing storms will result in soils becoming saturated. Also, in southern and eastern England the chalk rock below the soil is permeable; water sinks even deeper into the ground. This groundwater emerges slowly into rivers, sometimes many months after the rain fell. When the River Thames burst its banks in January 2014, it was the result of prolonged rainfall, saturated soils and high groundwater levels
Surface water (or pluvial) flooding
When high intensity rainfall collects in hollows and depressions where homes are located, sudden local flooding can occur - and this can happen far from a river or coastline. 3 million UK homes are at some risk of pluvial flooding, many homeowners simply don’t realise it. Surface water flooding in June 2007 left the insurance industry with a £3 billion bill
Coastal flooding
This can happen when there is a high tide and a storm is blowing at the same time. Winds drive the high tide further inland than usual, also generating storm waves. The Thames Flood Barrier protects London from storm surges. However, many coastal regions of the UK have limited sea wall protection and were overwhelmed by coastal flooding in January 2014, when waves as high as 20m were reported. Large parts of Devon and Cornwall were badly affected and the sea wall at Dawlish collapsed, destroying a section of the Plymouth-London railway line
Living With Environmental Change (LWEC) is an ambitious and innovative partnership of UK government departments and agencies, including the Environment Agency and the Met Office. LWEC provides environmental reports for the public that aim to make the best use of existing knowledge for policy and practice. LWEC’s recent Water Climate Change Impacts report considers the effect of climate change on fresh water in the UK, including rainfall, floods and droughts. The report card is intended to help people understand the scale of possible change.
Based on a review of the available evidence, LWEC is certain of the following ‘headline messages’:
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UK annual average rainfall has not changed since the 18th century. However, in the last thirty years more winter rainfall has fallen in heavy event
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Since the 1980s, flooding has increased in both size and duration during winter
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UK temperatures have increased by about 1°C since 1980
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Sea level is rising by about 3 mm every year.
Although the effects on the UK are likely to vary as highlighted by the Parliamentary Office of Science and Technology: "the south of the UK is sinking at a rate of approximately 1mm per year in comparison to global mean sea levels, whilst the north is rising at a rate of 1mm per year". The POST note can be accessed here.
But can these trends be attributed to long-term climate change with any real degree of certainty? The infographic shows LWEC’s conclusions and the degree of certainty it attaches to them. If a large number of separate investigations have led to the same conclusion (i.e. that there is both an observable trend and that this trend is linked with a changing climate) then LWEC reports that trend with a high confidence level. Where that have been fewer investigations, or there is greater disagreement on whether a causal link can be established with climate change, then LWEC reports that trend with a lower confidence level.
What does the LWEC report say could happen?
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The risks to agriculture and forestry include the problem of drier soils, reducing crop and timber yields and leading to extra demand for water for irrigation. At the same time, increased flooding could reduce the productivity of high-quality agricultural land, which is often in the floodplain. Warmer and drier summers could lead to more wildfires
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For businesses the main climate risks include flooding, increased competition for water, and the disruption of transport networks and communication links, for example by floods. There are also indirect risks from changes in agriculture and the natural environment
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People’s health and wellbeing is particularly affected by flooding, not only through direct injuries and deaths but also because there can be mental health effects. Water-borne diseases may also become more of a problem with increasing temperatures
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Buildings and infrastructure may be affected both by extreme weather and long-term changes in climate. Energy infrastructure is at significant risk from flooding. Increased energy demand may increase the demand for water to cool power stations. Roads and railways are disrupted by flooding, and river bridges are at risk from erosion when river flows are high. Our water supply depends on rainfall, and increased rainfall variability may make provision of water supply more difficult. Buildings can be damaged by flooding
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The natural environment depends on water in many ways. Lower summer river flows may lead to poor water quality. Warmer rivers and lakes may suit some species, but others will not thrive. Flooding and erosion can damage important habitats
Source: LWEC
The Intergovernmental Panel on Climate Change (IPCC) recently published its fifth assessment report on climate change. According to the IPCC, each of the last three decades has been successively warmer, and all have been warmer than any preceding decade as far back as 1850. The period from 1983-2012 in the Northern Hemisphere was ‘likely’ the warmest 30-year period of the last 1,400 years. The following climate change observations and predictions made by the IPCC are highly relevant to climate risk assessment for the UK:
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The average land and ocean surface data show a temperature rise of 0.85C over the period 1880-2012
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The IPCC is now ‘virtually certain’ that the upper 700m of the Earth's oceans have warmed during the period from 1971 and 2010 (this could indicate a heightened potential for evaporation and thus the formation of rain-bearing storms over the North Atlantic
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However, Arctic sea-ice has continued to fall in extent (introducing more cold water into the North Atlantic)
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Thermal expansion of warmer oceans, and run-off from the Greenland and Antarctic ice sheets, means that global mean sea level rise for 2081−2100 is projected to be between 26cm and 82cm (this will heighten flood risk for low-lying coastal areas of the UK).
The UK has been no stranger to flooding in the last decade. In 2004 a flash flood hit the village of Boscastle. 2007 brought nation-wide surface water flooding at a cost of £3 billion. 2012 was also unusual: the whole calendar year was the second wettest UK-wide since 1910, and just a few millimetres short of the record set in 2000 (the wettest year since 1766). January 2014 was the wettest on record (particularly for England and Wales), as successive rainstorms brought saturated soils to Dorset, Wiltshire and Somerset, while the Thames bursts its banks at Henley. Reviewing this evidence, it is clear to see why the impact of climate change on recent weather is a subject of ongoing research.
Isn’t it obvious that the climate is changing, to judge by recent rainfall?
No, it’s not. The historical record shows unusually wet months of years, sometimes in clusters, are a natural feature of the UK’s climate. December 1914 was almost as wet as January 2014. Periodic movements in the jet stream have always brought rainfall variability over differing time scales.
So the recent flood have nothing to do with climate change?
Actually, they might do. Scientists are making progress with ‘event attribution’ – the confidence with which it is possible to say that a particular period of extreme weather could be attributed to climate change. Recently, scientists have begun to understand to a greater degree how the odds of events have changed, and whether the likelihood of having an extreme weather event has increased or decreased. Peter Stott of the UK Met Office has said: ‘It’s like loaded dice. The chances of throwing a six have gone up a lot.
So all of the recent floods are in line with climate change models, it just can’t be proved for sure.
Not all of them. In particular, the wet early summer weather in the UK in 2012 is actually at odds with IPCC climate change scenarios, in which the UK is projected to experience drier summers. Increased winter rainfall and floods is more in line with what the IPCC have been saying. However, the climate system is so complex that unexpected extreme weather events could also be generated in unanticipated ways.
Is there any way we can be sure of what will happen in the future?
That depends on what you mean by ‘sure’. There are many different systems and factors to consider and our understanding is improving all the time. But currently, there is a lot of uncertainty, which is the reason why the LWEC report card does not show ‘high certainty’ very often. Indeed climate is an average measure of temperature and precipitation taken over 30 years on a rolling basis, so we cannot – and should not - make judgements about individual events.
We do know, for instance, that a warmer Atlantic Ocean would produce more evaporation and heighten the potential for rain-bearing storms to be formed over the Atlantic, close to the UK. But we also know that the melting Arctic ice will introduce more cold water into the Atlantic, and it remains to be seen how this affects ocean currents and temperature patterns. What happens in other parts of the world has an influence too. One theory is that warming in the Pacific is modifying the jet stream by increasing the amplitude of its meanders. This could have the effect of increasing the length of dry and wet periods for parts of the UK (which could help explain the lengthy series of storms in 2013-14). Find out more here.
However you view it, this is bad news for Britain, right?
Not for everyone. During the recent floods, sales of dehumidifiers increased. Pest control firms had a bumper season, provided their own premises did not flood. Rising groundwater in southern England flooded the burrows or rats and mice, triggering a wave of rodent migration into people’s houses (Financial Times, 14 February 2014).
This report has been prepared for students sitting GCSE, A Level or Diploma examinations that contain a Rivers, Extreme Weather, or Climate Change component.
Author Dr Simon Oakes is International Baccalaureate Chief Examiner for Geography and previously worked at Middlesex University’s Flood Hazard Research Centre on a range of flooding-related projects.