Environment: Johnson unveils secret weapon in war on climate change - the roof garden

· London mayor announces plan to soak up rainwater
· Strategy outlined to deal with heatwaves and floods

To some they are a rural escape in the centre of the city, to others they are a chance to test their green fingers and design skills. Now London mayor Boris Johnson has found a new use for urban roof gardens - as a key weapon on the front line against global warming.

An increase in the number of rooftop gardens to soak up rainwater across the capital is among a series of measures suggested by Johnson yesterday, as part of efforts to prepare London for the effects of climate change.

The mayor's adaptation strategy, billed as a world first, aims to address the challenges of flooding, extreme temperatures and drought. It calls for compulsory water metering, greater awareness of flood risks and more tree planting, alongside stronger efforts to resist attempts by local authorities and insurance companies to fell existing urban trees.

The mayor's team said they were also looking to copy a heatwave emergency plan used in US cities, including Philadelphia, where old and vulnerable people are collected in air-conditioned buses and taken to cool public buildings, such as libraries, shopping centres, churches and offices.

Despite previously attacking the Kyoto Protocol - which regulates international carbon emissions - as "pointless" and saying that anxiety over climate change was "partly a religious phenomenon" Johnson now admits that the 2006 Stern review on the issue had convinced him of the need to act. "When the facts change, you change your mind," he said.

Launching the strategy at the Thames Barrier, in east London, he said: "We need to concentrate efforts to slash carbon emissions and become more energy efficient in order to prevent dangerous climate change. But we also need to prepare for how our climate is expected to change in the future. The strategy outlines in detail the range of weather conditions facing London, which could both seriously threaten our quality of life, particularly that of the most vulnerable people, and endanger our pre-eminence as one of the world's leading cities."

He said the strategy, which is a legal requirement under the Greater London Authority Act, put London in a strong position.

"London is not unique. All major cities, such as New York and Tokyo, are at risk from climate change," he said.

The strategy does not address so-called mitigation of climate change - measures to reduce emissions - but Johnson said he supported a target set by the previous mayor, Ken Livingstone, to slash carbon pollution 60% by 2025. He said measures to meet the target would include incentives for Londoners to better insulate their homes and switch to more efficient condensing boilers. His team is assessing whether households could be given council tax rebates for adopting such energy-saving measures.

Global warming is expected to give London and its surrounding area longer, hotter summers as well as warmer, wetter winters with the added problems of more frequent heatwaves, droughts and flash floods from rising sea levels and downpours. About 600 Londoners died as a result of the 2003 heatwave that killed about 15,000 in France, while low rainfall during 2004 and 2005 led to water shortages in the capital.

Fifteen per cent of London is at high risk from flooding due to global warming - an area including 1.25 million people, almost half a million properties, more than 400 schools, 75 underground and railway stations, 10 hospitals and London City airport. At stake is an estimated £160bn worth of assets, not just in London, but along the Thames estuary, where large housing developments are planned.

Johnson said work was under way to address stifling summertime temperatures on the underground network, with air cooling on subsurface tube lines to begin in 2010. By 2015, he said all trains on subsurface lines, around 35-40% of the network, would be air-cooled.

The draft adaptation strategy, which will be finalised next year, calls for a citywide "urban greening" programme, using green spaces and trees to absorb and retain rainwater, and pledges to map London's drainage network to reduce surface water flood risk. It also recommends greater use of rainwater harvesting and "grey water" recycling in new buildings, as well as London-specific guidance for designers and architects to reduce the risk of buildings overheating in summer.

Jenny Bates, of Friends of the Earth, said: "It is essential that the capital prepares for the impacts of climate change, which is already affecting Londoners through increased flood risk, heavier and more frequent downpours and extreme heat. But Boris Johnson is also committed to cutting London's carbon dioxide emissions by 60% by 2025 in order to prevent dangerous climate change, and has so far failed to explain how he will achieve this.

"The mayor must provide a comprehensive action plan for reducing emissions that includes ways to make it cheaper and easier for Londoners to go green."

Key points

On flooding

Citywide urban greening programme based on green spaces and trees to soak up water

Campaign to raise public awareness of flood risk

Identify and protect critical infrastructure and vulnerable communities

On drought

Reduce leakage from water mains

Compulsory water metering where feasible

Rainwater harvesting and 'grey water' use

On heatwaves

Enhanced access to air-cooled buildings for London's vulnerable people

Specific guidance to architects

Requirement for new developments to help cool the city

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