Extreme weather

Is climate change increasing the frequency and severity of drought in the UK? Is climate change increasing the frequency and severity of extreme storm events? Is climate change increasing the volume and intensity of rainfall and flooding in the UK? Are warmer air temperatures causing glaciers to melt?

Drought

Drought is generally defined as an extended period of low or absent rainfall relative to the expected average for a region. Meteorological drought describes a prolonged period with less than average precipitation while a hydrologic drought occurs when available water reserves (in lakes, reservoirs and groundwater stores known as aquifers) fall below acceptable levels. The latter condition can arise even when there have been recent rains.

In many tropical countries, extremely arid conditions are an everyday state of affairs. Global belts of hot, dry sinking air girdle the tropics of Cancer and Capricorn. This is a result of a circulatory movement of air that initially rises close to the Equator along the Inter-Tropical Convergence Zone (ITCZ). Shedding its moisture there as convectional rain, the air moves pole-wards at high altitude before being forced to descend at the tropics as a result of jet stream activity. Now hot and dry, this descending air is responsible for high pressure and arid desert conditions throughout much of the tropics. The entire circulatory system, comprising of low-pressure along the ITCZ and high pressure at the tropics, is known as the Hadley Cell.

Some of the tropical air that subsides over the tropics in the northern hemisphere will often travel further northwards into Europe instead of returning to the ITCZ. If it passes mainly over land then it is termed a tropical continental air mass and is the usual cause of extended drought in the UK. During our very hottest summers, such as 1976, 2004 and 2006, clear blue skies – and lack of rain - are experienced as a constant. This indicates that hot dry Saharan air has advanced over mainland Europe into southern parts of the UK to form a ridge of high pressure. Meteorologists call this a blocking anticyclone. It can be recognised on weather charts as a circular or pear-shaped pattern of widely-spaced isobars with a 1026-1040 mb high pressure centre and light winds which diverge clockwise from the centre in the Northern hemisphere.

Is climate change increasing the frequency and severity of drought in the UK?

Warmer temperatures resulting from climate change will not simply be experienced around the world as a uniform and linear rise. With greater inputs of energy, complex systems such as the Hadley Cell will begin to function in modified ways. As a result, periods of drought in the tropics might last longer or become less predictable. For instance, scientists know that cycles of periodic drought currently operate in tropical regions such as sub-Saharan Africa and the Middle East. However, past observed trends will become a less reliable guide for future weather prediction as global temperatures rise.

The UK may also be facing a less predictable future where drought is concerned. The apparently increased frequency of extreme drought may be a result of changes in the behaviour of tropical air masses. For instance, back in 1976 an unusually high surface pressure persisted over southern England for much of the year (part of a ridge that extended northwards all the way from the Azores High). At the time, this was regarded as a very unusual situation. It was the driest period that had been recorded since 1727 and scientists calculated that there was only a 1 in 200 chance of its re-occurrence in any subsequent year.

However, some scientists now believe that the conditions experienced during the 1976 drought may return more frequently in future, perhaps once every fifty (or even thirty) years. The drought conditions experienced in south-east England during 2006 – a mere 30 years on from 1976 – may be used as evidence to support this hypothesis (especially when one recalls that record summer temperatures were also reached in the UK in 2004, while 20,000 deaths were attributed to a heat-wave in mainland Europe during 2003).

Looking towards the future, there is a strong likelihood that the UK is set to experience such dry conditions with ever-increasing frequency. The recent findings of the Intergovernmental Panel on Climate Change (IPCC) suggest that unless action is taken to keep the global temperature rise below 2C then summers such as those experienced in 1976 and 2006 could become the norm. London summer temperatures could reach 40C and above by 2050 according to climate models.

Student Practice Question:

Examine the causes and consequences of anticyclonic weather conditions in the UK

As well as referencing key ideas such as the origins of tropical air masses (and the importance of convergent flow in the upper troposphere, perhaps referring to Rossby Waves), a good response to this essay might ask whether climate change could be a cause of anticyclonic conditions dominating for longer periods of the summer, especially in southern England.

Storms

Hurricanes are large rotating storms centred around low pressure areas. They usually develop between 5 and 30 degrees of latitude, starting life as a body of warm moist air over a tropical ocean that has reached the critical temperature of 26°C. Driven by the rotating earth’s Coriolis Force, a flow of air develops around a central eye whenever prevailing winds near the ocean surface are able to form inward spirals of air.

The ocean’s heat drives this process, causing evaporation and sending moisture-laden clouds high up into the atmosphere. Hurricanes play an important role in transferring heat and energy between the equator and the poles. There are five levels of hurricane strength, according to the Saffir-Simpson scale (below which lie an additional two weaker levels, labeled “tropical storms”).

Storms are not confined to the tropics of course, and the UK experiences them too – less extreme manifestations known as depressions. Depressions are cousins of hurricanes – cyclonic systems with a low pressure centre, formed when tropical and polar air masses converge at the polar front (between 40N and 60N). Under conditions of divergent flow in the jet stream (a meandering belt of air moving 12 km above ground level), cyclogenesis (depression formation) takes place and a wave form is produced with a warm front leading a cold front.

Is climate change increasing the frequency and severity of intense storm events?

In the film "An Inconvenient Truth", Al Gore suggests that the US is facing increased risk of hurricane damage. The 2004 hurricane season certainly brought unusually high costs to the southern states of the US as well as to the Caribbean. Four major hurricanes – Charley, Frances, Ivan and Jeanne - brought around two thousand deaths to the region, as well as property damage amounting to many billions of dollars.

Then the 2005 season brought Hurricane Katrina to New Orleans, devastating an entire city.

Can these events be attributed to climate change? Hurricanes are fed by water vapour over oceans and the evidence does suggest that unusually warm waters over the Gulf of Mexico helped elevate Katrina from a Category 1 hurricane to Category 5. It was the third strongest hurricane ever to make landfall in the US.

Meanwhile, the UK has been reported as becoming twice as stormy in the past 50 years. According to new Hadley Centre research, climate change has modified the North Atlantic oscillation (NAO) and forced deep depressions that used to hit Ireland further south, causing more of them to pass over the UK. Once again, this shows that climate change is not simply bringing about simple uniform warming of the atmosphere. It is apparently modifying the structure and functioning of circulatory systems, further compounding the challenges ahead.

Student Practice Question:

Explain why global patterns of pressure and wind vary over time

Part of your answer may explain why low-pressure hurricanes and storms occur in certain regions and at certain times of the year. You might also consider whether long-term changes are now taking place in the frequency and severity of extremely low-pressure cyclonic events as a result of climate change.

Floods

A flood occurs when the discharge of a river exceeds that capacity of the river channel to contain it. Once bankfull discharge is exceeded, inundation of the river valley flood plain must follow. An extremely flashy storm hydrograph can indicate that a flood is imminent. Flood conditions are often associated with precipitation moving through a drainage basin via the process of overland flow as part of the hydrological cycle. Water flows directly over the land surface as result of one of two rainfall scenarios:

1. soils are saturated, perhaps due to rainfall duration, with a large volume having fallen previously

2. extremely high intensity rainfall outstrips the infiltration capacity of even dry soils

Both conditions result in overland flow, although the latter is more likely to produce the most dramatic effects. Overland flow – and the potential for flooding – is also exacerbated by steep slopes, thin soils, impermeable bedrock (or concrete) and a lack of vegetation which would otherwise intercept some of the rainfall (this can reduce or slow the transmission of water through the system, thereby alleviating flood risk).

Unpredictable flash flooding (associated with the sudden onset of high-intensity rainfall) is less common in the UK than gradual-onset flooding associated with a longer period of steady rainfall, often over several days or weeks.

It has been suggested that the flash flood in Boscastle (Devon) in 2004 may be a harbinger of climate change – and that such extreme flood events may become more frequent in the future.

Is climate change increasing the volume and intensity of rainfall and flooding in the UK?

Flash floods in Cornwall devastated the tourist village of Boscastle during August 2004. The village experienced the heaviest rains in living memory, with 185 mm falling in just five hours. Given that the total annual rainfall for much of southern England is only around 1,000mm, this was an exceptional amount for a single rainfall event. Attention must be paid not just to the total volume of rain but also the intensity with which it fell. the majority falling in the first two hours. Under such conditions, infiltration-excess overland flow was inevitable, with the rate of input of rainwater greatly exceeding the infiltration capacity of the soil. In total, an input of 3 million tonnes of water was added to a tiny drainage basin whose size is just 40 square kilometres.

Viewed in isolation, one very heavy storm is not necessarily a symptom of climate change. However, the Boscastle flood is part of a "bigger picture" that suggests Britain's weather is changing. The summer of 2004 as a whole was the wettest in the UK since 1956. In fact, more than double the normal amount of rain fell in England and Wales during June, July and August 2004. It is also notable that it followed on more or less directly from the widespread European floods of 2002 (when 600,000 people experienced economic losses topping £10 billion and over 100 died).

Could northern Europe be moving towards a more unpredictable and stormy climatic regime, as some meteorologist currently suggest? Warming temperatures associated with climate change certainly have the potential to bring more extreme flood damage. Higher temperatures result in raised evaporation rates over water surfaces, meaning that there is greater moisture available to create rain; and if air masses are warming in response to climate change, then their potential to hold ever-greater levels of moisture is increasing (water vapour content is determined by the amount of heat energy available). The possibility therefore exists that the volume and intensity of frontal rainfall over parts of the UK is set to increase. In addition to getting warmer, we may also get wetter - due to an increase in high-intensity rainfall events and flash flooding.

Student Practice Question:

"Heavy rainfall causes floods" - discuss.

A discussion of this statement should begin by recognising that the word "heavy" has two interpretations: long-duration and high-intensity. The role of climate change as a factor that might be driving an increase in high-intensity events is definitely worth flagging up. However, other factors must also be brought into the discussion - land use changes and physical catchment characteristics such as vegetation and slopes help determine how quickly water moves through the hydrological system and whether heavy rainfall actually prompts a flashy response from a river.

Glacial melt

The size and movement of a glacier is controlled by rates of ablation (melting) and accumulation (of snow and ice). During periods of global cooling (such as the onset of the Pleistocene ice-age) rates of ablation will fall and the result is a world-wide advance of glacial ice. However, during periods of warming the opposite happens. Ablation rates rise and the snout of a glacier recedes up-valley, exposing landforms such as moraine and fluvial deposits.

The production of vast quantities of melt-water and the growth of pro-glacial lakes was one of the most significant impacts of global warming at the end of the last glacial period - both in terms of landform development and for local climates. Notably, the sudden release of vast amounts of cold melt-water from melting glaciers and over-flowing melt-water lakes in North America caused sudden changes to ocean circulation in the north Atlantic. Interfering with the flow of warmer waters into the region, the cold melt-water served to cool Europe's air to such an extent that it delayed the return of warmer temperatures to the continent for a further 1,500 years. 

The huge volume of ice-cold water had disrupted the flow of the Gulf Stream - the ocean current that brings warm waters to areas of the North Atlantic such as the UK. Also known to oceanographers as the Atlantic thermohaline circulation, the Gulf Stream carries one billion watts of heat annually from the tropics to the Arctic via Scotland.

Could another interruption of the Gulf Stream happen again as a result of climate change? Could the UK really get colder on account of climate change?

How might melting ice impact on ocean currents and surface air temperatures?

Will the Gulf Stream really get diverted as result of glacial melting caused by climate change? Scientists are certainly concerned about the possibility of the Gulf Stream being diverted, leading to the cooling of the UK. One recent conference on climate change reported that the chances of this occurring are now almost fifty-fifty. The current has recently been weakened by 10% and scientists warn that a 3C rise in global temperatures – which is well within current predictions – would bring a 45% chance of complete shutdown.

This is because patterns of ocean circulation will be modified by huge inputs of cold glacial melt-water from Greenland. The resulting climate in Europe could be even colder than that experienced in the 1600s, during the so-called Little Ice Age when the Thames regularly froze over in winter. Without the Gulf Stream, scientists warn that the UK's air temperatures could actually fall by 10C!

Melting of Antarctic ice is also a cause of great concern. Temperatures on the Antarctic Peninsula (an 800-mile long westward extension of the continent) have risen by 4C over the last 50 years and the winter temperature of the surrounding waters is thought to have risen by 10C. Physical changes are also evident. In March 2000, the world’s biggest iceberg, B15, broke away from the Ross Ice Shelf in Antarctica. 200 feet high and the size of Jamaica, it split in half in October 2003 and is now beginning to waste further.

Scientists believe this is a result of global warming and is linked to recent reports that glaciers in neighbouring Chile and Argentina are melting at twice the rate observed in the 1970s. Given that 70% of the earth’s fresh water is stored as ice in Antarctica, there is a clear threat of global eustatic sea level rises should Antarctica’s ice mass begin to decline further.

Student Practice Question:

Discuss the causes and consequences of increased rates of glacial ablation

As well as referencing important year-to-year ideas about seasonality, sun-spot activity and avalanching, a good answer will recognise that most glaciers are now experiencing a long-term retreat and negative mass balance as a result of ablation that most scientists believe is the result of climate change. Local consequences (creation of depositional landforms and flooding of low-lying areas) can be contrasted with possible long-term regional impacts (interference with Gulf Stream).

Fieldwork and practical investigations

A-level students may want to undertake a personal investigation examining possible meteorological signs of climate change, as experienced in the UK or elsewhere - especially those who will be following the new (2008) Edexcel Specification that includes "Extreme Weather" as an optional AS Investigations (Paper 2) study topic.

Fieldwork opportunities include a weather log, flood impacts survey, flood/drought risk assessments or flood management assessments. Practical work/GIS could scrutinize weather records, satellite images, hurricane data, and statistics for flood/drought events; or might consist of an evaluation of various management strategies.

Two engaging themes for study might be:

• Assessing evidence that flood impacts are apparently becoming more frequent or severe - and whether this is due to climate change or to changing demographics (population growth along rivers and coastlines) and better media coverage (global information links make it appear the situation is worsening).

• Assessing ways to manage drought through physical, social, economic and political responses in contrasting areas like SE England or the Sahel (e.g. water management schemes such as hosepipe bans or adapting farming techniques to drier conditions).