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35,000 people died across Europe in the heatwave of 2003. By 2020, high temperatures we now experience once a century look set to occur on average every four years.

City folk face the main health risk because of so-called urban heat islands. Buildings absorb more of the sun's energy than open spaces do. Add to that the simple concentration of people, vehicles and air conditioning systems and a city the size of London can be 9?C hotter than its surrounding countryside.

London Underground is keenly aware of the dangers high temperatures already cause in crowded Tube trains and stations. At Victoria Station, the groundwater that gets pumped out of the tunnels to stop them flooding is piped around station concourses to cool them. Investment on a massive scale is necessary to keep Tube travel safe in the sweltering summers to come. Above ground, the cost of adapting transport networks for temperatures that buckle rails and melt tarmac will run into further billions of pounds.

Guaranteeing water supplies is more than just an engineering headache where large populations live far from upland rain catchment areas - in other words much of southeast England. Reducing losses from leaking pipes can only ever help. Now though, water companies are consulting on controversially flooding farmland to build the first major UK reservoirs in 25 years.

There is little prospect of Britain running out of water, but the need to manage supply and demand will see costs go up and people-s differing needs thrown more starkly into conflict.

The Experiment concludes that Britain should prepare for more frequent heavy rainfall events.

The residents of Bewdley, north Worcestershire, have got used to water threatening their homes whenever the Severn breaches its banks. Their stoicism is helped by an -11m flood barrier, installed after sustained heavy rain in Autumn 2000 saw 150 houses damaged by high river levels.

Of course, rivers flooding nearby areas is to be expected every so often and - arguably - if you choose to live by a watercourse, it's a chance you take. So why do a quarter of all the house floods caused by rainfall have nothing to do with river levels?

Many of Britain's sewers are no longer up to the task of handling rainfall run-off. In 2002, an intense summer deluge brought misery to Glasgow's east end as 500 homes found filthy water lapping their walls.

The problem is exacerbated when open areas are built on or paved over. In London alone, 30km <= of front gardens have been turned into parking spaces in recent decades. Where soil helps moderate the effect of sudden downpours, hard surfaces channel rain straight into drains for the overwhelmed sewers to cope with.

The solution of course is to spend massive amounts of money improving city sewers. Scottish Water alone faces bills reaching hundreds of millions of pounds. So stand by to pay more for your water services, pretty much wherever you live.

In the Scottish Highlands, the white winter plumage of the ptarmigan makes it an elusive bird, as long as there's snow. Winter snow cover protects both ptarmigans and the plants they eat. The Experiment concludes that Scottish winters by 2050 will be warmer and wetter, giving the ptarmigan a problem it may struggle to adjust to in time.

Alan Stewart can already see some evidence of climate change. The husky dogs he races in the Cairngorms now need showers before and after their outings, just to stay comfortably cool.

Scotland's world renowned salmon rivers face threats too, from the predicted increase in flash flood events. Scientists already observe that local salmon populations decline after extreme rainfall. That's due to fish eggs being washed out of river bed gravel.

In southern England, farmer Mark Diacono has spotted a business opportunity that he's confident climate change will make viable. He's produced ripe olives from the UK's first grove, near Honiton in Devon. His expansion plans include almond trees, lemons and kiwi fruit.

The Government estimate for sea level rise around the shores of SE England by 2080 is 0.75m (2?ft). Some experts think that's optimistic. Bill McGuire from the Benfield Hazard Research Centre puts the range at 1-2m. If that happens, he adds, "we're in real trouble."

Adjusting to warmer average temperatures and trying to reduce the rate of further climate change is likely to prompt lifestyle changes by the end of this century.

Architects and builders face the challenge of improving homes - potentially 22 million of them - to boost their energy and water efficiency.

The existing BedZED development in London could be a model. Thick walls, south-facing windows and natural ventilation keep temperatures inside comfortable without any direct heating or cooling. By collecting rainwater and generating some of their own electricity (from solar cells and burning locally sourced waste wood) the residents' environmental footprint is far below typical UK levels.

Energy sources by 2080 could well reflect a move away from fossil fuels, due to either their availability, cost or associated carbon emissions. Compared to fuels from crude oil, biofuels such as ethanol (made in the UK from sugarbeet) produce less carbon dioxide overall when burnt. They're no easy fix though. Growing enough crops to run all the UK's cars - let alone its aeroplanes - would require vast expanses of farmland.

And where will Brits want to drive or fly? Not Spain, says David Viner from the University of East Anglia. He's taken climate predictions on board to produce a tourism comfort index for 2080. The Mediterranean will be way too hot. The question instead is whether the UK can cope with millions of Spaniards heading for a new European riviera: Blackpool.

There are parts of the British Isles where the difference between low and high tide is as much as 11m. By comparison, 0.75m sounds manageable and much of the year it is. But whenever low atmospheric pressure and a high tide combine with winds in an unhelpful direction, the 'storm surge' that results is a menace - and 0.75m could make all the difference.

One such surge in 1953 killed hundreds of people from Lincolnshire to Kent. It would have badly affected London, if the high sea levels that destroyed Canvey Island and other parts of Essex had continued further up the Thames Estuary.

That disaster led to massively improved flood defences on the east coast, including the iconic Thames Barrier. They have already kept out floods on a par with January 1953. But as sea levels rise, in as little as 25 years' time, the current Barrier risks being overtopped by the very highest storm surges.

Money will no doubt get spent to protect London from the North Sea. As much as -20bn might be required for an additional outer barrier, stretching 10km (6 miles) from Southend to Sheerness. But the Association of British Insurers estimates doing nothing will cost -20bn every year come 2080 - and there are plans to build hundreds of thousands more houses in the Thames Gateway floodplain.

London's status will ensure flood protection there. Other parts of the country may not be so lucky. The Environment Agency admits maintaining defences around the entire vulnerable coastline will become increasingly costly. Instead, they intend to manage the risk. There are going to be land areas for which a sufficiently good economic case to defend them from the sea cannot be made.

If the damage has been done that makes 21st century climate change a reality, the same is not true of the years beyond.

The bleakest predictions assume humanity does nothing to reduce carbon emissions and feature not just an inexorable rise in global average temperatures but also a number of sudden 'trigger' events.

Polar ice sheets are already melting at rates enough to alarm some scientists. As the Earth's snow cover reduces, the amount of solar energy reflected off to space (the planet's albedo) falls.

Albedo is one example of a positive feedback mechanism, where warming effects promote further climate change.

Others include:

• Wildfires, if wet areas like the Amazon Rainforest dry out
• Methane releases from deep sea deposits or due to the melting of currently frozen peat bogs (such as the vast Siberian tundra)

Should the whole Greenland ice sheet melt, a 7m (23ft) rise in sea level would result. In the UK, that puts at risk the whole of the Fens - currently home to 385,000 people and almost half of England's prime agricultural land. Millions more residents of coastal towns and villages face upheaval, as do major centres of industry.

The greatest climate change impact on the UK will almost certainly be indirect. The people most at risk, according to Prof Peter Cox from the Met Office, have the least capacity to adapt, as they live in developing nations. Whether or not they are hit by catastrophic events, the potential exists for climate refugees, seeking productive land, to become an additional burden on development goals. Britain's reliance on trade (especially food imports) means that problems around the world are problems here too.

In David Attenborough's view it would be reckless in the extreme to do nothing now and merely hope such outcomes don't happen.