Tony Juniper: This new political battleground is welcome

A potentially baffling array of "green speak" will emanate from Westminster this week. A Brown vs Cameron clash of environment speeches today may offer some clues as to how the green battle will go once Tony Blair stands down. Then the Government's Climate Change Bill will be published tomorrow. Later in the week, George Osborne will discuss plans for new aviation taxes.

This is hugely welcome - for years, politicians have said climate change is the most serious challenge we face, but relegated it to occasional speeches and too little action. So are things changing?

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The Climate Change Bill offers the best test. The principle is radical - it will make the UK the first nation to have a legal framework for managing carbon emissions. Ministers have proposed targets for reducing them through to 2050, with annual reports to Parliament on whether we are getting there. An independent body will oversee progress.

The questions that remain are the ones we must ask of every party's policies. First, are they ambitious enough? So far all parties have promised a 60 per cent cut by 2050 - yet the latest science shows this isn't enough if the UK is to play its part in keeping down global temperatures. This is particularly the case if the Bill fails to take into account emissions from international shipping and aviation.

And if the Bill is ambitious, can we be sure its targets will be met? This Government has a great record of setting targets, but a terrible one for meeting them. Despite three manifesto promises for a 20 per cent cut in CO2 by 2010, emissions have risen since 1997. We must know what ministers are planning to deliver every year - and have them report to Parliament on whether they have achieved it. The independent body must have real teeth, and ministers under legal duties to change policies if they are not working.

The real test will be how the parties vote when the Bill comes before Parliament. It is not easy to weigh up the precise effects of numerous policies coming forward - and no party yet offers a complete policy suite that can be added up to a particular percentage cut in CO2 emissions. But the parties who are serious about developing one will have no problem in backing a Climate Bill with muscle.

The Big Ask, Friends of the Earth's campaign for a strong climate change law, has received a huge amount of support. There will be awkward questions put to anyone promising to stop climate change, but not supporting the necessary targets and scrutiny. This week's scramble for green votes offers hope that no party wants to be left answering them.