The world has felt like a depressing place recently with conflict and slaughter on our screens from Iraq, Syria and Gaza. Yet standing among 400,000 people in New York on Sunday I was filled with a sense of profound hope for the world. I was in the Big Apple to take part in the largest ever mass mobilisation to raise awareness about climate change and attend the UN Climate Summit of world leaders taking place today.

Standing among the throng of people in New York, as well as the hundreds of thousands of others around the world (including 40,000 in London), I was struck by what a unifying power a common enemy can be. On Sunday, not only were there groups of Methodists and Baptists rubbing shoulders with Catholics and Presbyterians, there were Christians marching with Muslims, Jews, Pagans, Atheists and Baha'i. Anti-capitalist protesters stood alongside 'Concerned Moms for the Climate', doctors, firemen and vegans held banners next to indigenous people and victims of Hurricane Katrina. Almost every conceivable strand of society was represented in the huge column of humanity which snaked its way through the high rise blocks of Manhattan. There was even the odd celebrity thrown into the mix, with Leonardo DiCaprio taking to the streets in New York and Emma Thompson in London.

There aren't many things that show the same indiscriminate indifference to race, religion, age, profession or beliefs as climate change. And there's not much which needs such a unified, international response from people around the world. As Prince Charles said this week "even in a world full of daunting perils and crises, it is hard to imagine anything that poses a greater challenge and opportunity for humanity."

And that's the crucial word. Opportunity. Rather than feeling depressed by the scale of the challenge we face, this week I've felt inspired and encouraged that tackling the scourge of global warming could be one of humanity's greatest achievements. Seeing nearly half a million people from across America join with more than 2,000 other events in 160 countries shows the kind of public will needed to fix this problem. There was even a small gathering in Syria's war torn city of Aleppo.

It's also been encouraging to see faith communities respond to the challenge. Not only were religious groups marching together in New York, a special interfaith summit of 30 religious leaders, including US pastor Jim Wallis and Rev Suzanne Matale of the Zambian Council of Churches met yesterday to discuss the vital role faith groups have to play.

A statement signed by all 30 religious leaders said: "We recognize that climate change stands today as a major obstacle to the eradication of poverty. Severe weather events exacerbate hunger, cause economic insecurity, force displacement and prevent sustainable development. The climate crisis is about the survival of humanity on planet earth, and action must reflect these facts with urgency."

Thankfully this public call and urging of religious groups seems to have been heard by our political leaders. Today at the summit, heads of state are making concrete, national commitments to cut emissions and provide finance for the world's poor which will make a difference in tackling climate change.

The fight back against global warming is underway. It's on the top of the political agenda and it's on the front page of the New York Times. The world can be a depressing and divided place but our global response to climate change is already starting to bring people together.

Joe Ware is Christian Aid's Church & Campaigns Journalist