Whether or not you are convinced that global warming exists, one city is battening down the hatches. London already has a barrier in place to seal the River Thames [see Figure 1] from the sea in time of need, and it is now launching a multibillion-pound project to augment the barrier against the higher tides that the UK government believes warmer weather will hurl against the city.

"It's not a question of if, but when," says Baroness Barbara Young, head of Britain's Environment Agency.

Her agency's flood plans grew out of a June 2003 government report on how climate change was affecting the country. The report noted that between 1900 and 2002, the temperature in England rose by a countrywide average of 0.67 °C, a change more significant than it may seem because of the outsize effect at temperature extremes. Notably, the two warmest years in the last 300 fell in the past decade, in 1990 and 1999.

PHOTO: ENVIRONMENT AGENCY (UK)

The change has been nipping at Londoners' heels for years. The average sea level in the east of England has been increasing by more than 2 millimeters per year since the 1960s, in part as a result of the thermal expansion of water as it warms and the increase in meltwater from the polar caps. The sea has risen higher in the east, because the geologic plate on which that part of the country rests is sinking.

Again, the problem is worse at the extremes. Since record-keeping began, in 1780, the level of the highest tides has risen by 1.5 meters. About 40 percent of the increase comes from the land's sinking, the rest from the sea level's rising.

In the 1970s, London commissioned the Thames Barrier, an underwater gate that stretches 520 meters across the River Thames, in east London. Its 10 panels, each 20 meters high and weighing 3700 tons, normally nestle flat in concrete housings on the riverbed. When rotated to the vertical plane, they rise above the waterline to form a seamless wall strong enough to resist tidal surges, even those aided by an onshore wind, from pushing seawater upstream. In recent years, the barrier has also occasionally been raised to prevent flooding downstream following heavy rains.

The Barrier opened in 1982 at a cost of £530 million (US $954 million), and in its first decade it was closed on average twice per year. In the past decade, closures have been far more common—in 2003, the barrier was closed for nine successive tides.

Today, there is a renewed threat of a flood that could bring London to its knees. In December, Baroness Young told a meeting of meteorologists, town planners, and government officials that flooding threatens 1.25 million people, property worth £80 billion, 400 schools, 16 hospitals, and 100 railway stations.

Tim Reeder, project scientist for the Environment Agency's Thames Estuary 2100 Project, said that it is testing a number of proposals to hold back the tides, such as the creation of downstream flood plains to drain off much of the flow coming upriver before it hits the city. "But eventually, London will need a new barrier," he says. He is already studying four potential sites for a new barrier downstream from the current one. Another option is to make the present barrier taller and stronger.

Equally serious is the threat of heavy rains. As temperatures increase, great storms formerly expected perhaps once in a century will now strike more often, as might killer storms worse than any seen in London's history. In the summer of 2003, heavy rains filled the city's creaking sewer system to overflowing, sending raw sewage into the Thames and killing 100 000 fish in a matter of hours. The incident raised fears of a major health risk to river workers and to bathers and other people who use the river for recreation.

London's sewer system was built in the Victorian era, and it now cannot easily cope with even normal demands, let alone the additional challenges posed by climate change. The Environment Agency considers sewage spills so serious that it is contemplating construction of a £1 billion tunnel underneath the Thames to cope with future overflow. The tunnel, almost 35 kilometers long and 85 meters deep, would normally be kept dry; after a flood, water would be pumped out to a sewage treatment center, a task that would take 48 hours.

London has other tunnels that are also threatened by a worldwide warm-up. "Central London is a heat island that is currently 2 °C hotter than the surrounding area. The underground [railway] system is 5 °C hotter than that," says Chris West, director of the UK Climate Impacts Programme at the University of Oxford. The problem of heat exhaustion among commuters is so serious that the Mayor of London has offered a £100 000 reward to anyone who can come up with a reasonably cheap way to air-condition the network.

These problems began to hit home with the public in 2003, when a heat wave attributable in part to global warming killed 10 000 people in France. The question now is: how long will it be before a similar tragedy hits Britain?