Four of the candidates competing to be London mayor have been promoting their environmental credentials at the first public debate in the contest.
The hustings was organised by the Green Alliance
Labour mayor Ken Livingstone said the main objective is to cut carbon dioxide (CO2) emissions.
Conservative Boris Johnson called for more renewable energy and Lib Dem Brian Paddick called for more tram use.
Green Party hopeful Sian Berry said the only way Londoners can tackle climate change is with a Green Party mayor.
All four were united in their opposition to the plans for a sixth terminal and a third runway at Heathrow.
Mr Livingstone said CO2 needed to be driven down even if it meant taking "fairly unpopular" or rigorous measures to do so.
Mr Livingstone defended his policy to introduce a £25 congestion charge on "gas guzzling" cars, saying the money raised would go towards promoting cycling and walking.
He cited the trials with hybrid buses, a programme of adapting public buildings to make them greener and the introduction of the Low Emission Zone (LEZ) as evidence of his green credentials.
But he came under criticism from Mr Johnson for allowing "garden grabbing" in the capital which he claimed had seen the loss of the equivalent of 22 Hyde Park's worth of green space in the last decade.
Ken Livingstone and Boris Johnson clashed at the debate
He called for reductions in energy consumption and more renewables such as combined heat and power and ground source heat pumps.
He also announced he would introduce a mayoral prize for low-carbon innovation.
Mr Paddick said he would develop local high streets to encourage people to shop locally.
He also unveiled plans to remove the congestion charge exemption from the greenest cars to discourage driving and to introduce a £10 charge for people who drove from outside the capital into Greater London.
Airport to close
He called for more trams and more high speed rail links for the UK and the continent.
Ms Berry said she would cut public transport prices by 20p a journey, triple the budget for cycling measures and use "every measure in the book" to help London generate more of its own energy.
She also said she would not build the proposed Thames Gateway bridge and would close City Airport.
The hustings on Thursday was organised by the Green Alliance.
Londoners will vote on who becomes the new mayor on 1 May.