PEOPLE in more than a thousand places around the world took to the streets on Sunday, motivated by a single issue: climate change.

I was proud to join in the biggest ever global event of its kind, the People's Climate March, with the thousands of others in London.

London's message was simple: climate change presents an ever-increasing threat to people around the world, and to nature, on which we all depend. We need to act now.

Sunday was also the International Day of Peace. There could have been no more appropriate day on which to reflect on the conflicts that climate change could instigate in the future, and on the choice that lies before humanity: whether to allow greenhouse-gas emissions to rise unchecked, or to bring them down to levels that scientists tell us are compatible with avoiding the worst impacts of climate change.

The recent Intergovernmental Panel on Climate Change (IPCC) report said warming in the climate system was "unequivocal" and that human influence on the climate is clear. It concluded that climate change poses a severe threat to people and the planet, including to food stocks and global security.

But the IPCC also said that it "doesn't cost the world to save the planet". This was confirmed in another report published last week, which found that tackling climate change was not only compatible with economic growth: it could even lead to better growth.

The Global Commission on the Economy and Climate said that ambitious action on climate change would also cut air pollution and thereby improve health, and improve agricultural yields in developing countries.

In short, the obstacles to action on climate change - scientific, economic, and political - are disappearing.

In the Church of England, the Shrinking the Footprint campaign seeks to cut our energy use and carbon footprint by 80 per cent by 2050. Among the progress made in London, the Chapter House of St Paul's Cathedral is now warmed by a ground-source heat pump, and St James's, Piccadilly, is one of several parishes to embrace solar power.

The feeling among the crowds at the weekend was that our neighbours are not only the people in the same street, area, or city as us: they are also the people in Bangladesh and other places vulnerable to rising seas, the spread of preventable disease, and increasingly unpredictable harvests.

The reality of our interconnected world is that we are all afloat in a great ark. Our first-class accommodation will not long remain immune from the effects of the leaks in the third-class cabins. For the sake of future generations, together we must make a difference today. Otherwise it is not just climate change but climate chaos that will prevail.

The Rt Revd Richard Chartres is Bishop of London.