In Grosvenor Square today, the dividing line between the crowds and the US embassy will not just be the rows of concrete blocks and security personnel. It will be between those outside demanding international action to tackle climate change, and the hugely powerful forces inside who are the biggest obstacle.
There is a real possibility that within 20 years we will reach a point of no return, when the warming unlocks hitherto contained sources of greenhouse gases, such as the methane trapped under the polar ice caps.
A drastic situation calls for drastic measures. That's why domestically, the Lib Dems have called for a cross-party consensus on climate change, responded to positively by the Conservative spokesman Oliver Letwin, though not yet by Margaret Beckett for the Government.
There is a need to agree on annual targets for reductions in greenhouse gas emissions and to face up to the need to take radical action on a number of fronts, for example the damaging emissions from aircraft.
'US is burying head in sand' Nick Rau, Friends of the Earth
Thousands of people in 32 countries across the world will take part in mass rallies to demand international action on climate change.
They will call on the international community, meeting for UN climate change talks in Montreal, to take action to tackle global warming.
The first week has seen progress. Growing scientific evidence has installed urgency. But the US administration is burying its head in the sand, refusing to take any meaningful action, while accepting its emissions will rise by 30 per cent by 2012.
UK credibility is also hampered by its woeful domestic record. Despite promising to cut carbon dioxide emissions by 20 per cent of 1990 levels by 2010, emissions have actually risen under Labour.
Earlier this year Friends of the Earth launched the Big Ask campaign to get the UK Government to act at home and develop a low-carbon, nuclear-free economy.
As part of this we want to see a legal obligation on the Government to cut domestic emissions by 3 per cent year on year.
'Serious political will needed' Caroline Lucas, Green Party MEP
The logical conclusion that climate change is a weapon of mass destruction - and leaders who don't tackle it are guilty of crimes against humanity - is too infrequently stated.
The tens of thousands expected to take to the streets will make this point.
The targets most governments accept is cuts of 60 per cent in greenhouse gas emissions by 2050. This requires serious political will - and Tony Blair doesn't have it. CO2 emissions have risen under his premiership and he allowed business emissions to increase. He has backed a huge expansion in the UK's airport capacity and a major road-building programme.
We must adopt the Contraction and Convergence principle, which holds that richer countries who have done more to create the problem should reduce emissions faster than developing nations. This allows them both to catch up and trade a surplus emissions allowance.
Nationally, we must cap personal carbon emissions (domestic electricity and gas, travelling etc) - and allow surpluses to be traded.Reuse content