In November and December of 2005, I was in Montreal to attend the United Nations Climate Change Conference – the annual meeting of countries that are party to the Kyoto Protocol, which is aimed at reducing the emission of greenhouse gases. Each year, this large gathering of ministers, scientists, non-governmental organisations and concerned citizens is held in a different global city: more than 20,000 people were in attendance at Montreal. Although I came with full credentials from a leading US newspaper, my agenda was my own: I wanted to know if climate change was real.

For years, I had followed the testimony before Congress of Dr James Hansen, the world's pre-eminent climate-change scientist. However, like most Americans, I was confused. I didn't realise it at the time, but a systematic campaign of corporate-sponsored disinformation was at work sowing doubt in the collective American mind. These were the days before Al Gore's revelatory film, An Inconvenient Truth.

Naalak Nappaaluk, Inuk elder from Kangirsujuaq, Nunavik, Canada. From When It Changed by Joel Sternberg

What I heard and saw at this confence shocked me like nothing I've ever experienced. I live about five blocks from the former World Trade Centre and I was at my window as the first plane struck – but this was worse. In the opinion of nearly all the participants, not only was climate change occurring, it was also about to reach a tipping point and become irreversible.

This would mean that we would never be able to stop the ravages of climate change – or at least not without geoengineering solutions on a planetary scale. For example, Dr Paul Crutzen, who won the 1995 Nobel chemistry prize for his research into the hole in the ozone layer, has proposed a fleet of high-altitude balloons dispersing sulphur into the upper atmosphere. He believes this will reflect sunlight back into space.

Bill Clinton, from When It Changed by Joel Sternfeld

In Montreal, I tried to take photographs of delegates at the moment when the horror of what they were hearing was visible on their faces. At stake, after all, is the continuation of Earth as a planet fit for us to live on. No one can say how long the process of human extinction might take but, as it proceeds, the same global order will prevail that always prevails: rich nations will find ways to protect themselves and make themselves comfortable, while the poor nations and the poor people of the planet will suffer.

When I returned to New York, I spent two years researching and writing the text that sits alongside these pictures in When It Changed, my 2007 book inspired by the conference. At first, I pored over interviews we did in Montreal. Memorable among these were the words of Mohammad Reazuddin, minister of the environment for Bangladesh: "My voice may be small because I am from a small country. But those who will be washed away, their voices must be heard."

Discarded headphones at the conference, from When It Changed by Joel Sternfeld

Eventually, I concluded that a fragmented text might be the best way to convey the urgency of the matter, simulating the transmission of news stories on old teletype machines. The content spans 50 years, taking in reports of alarming scientific discoveries, the actions and inactions of governments and corporations, and Earth's ever more disastrous weather. Later, I converted the book into a DVD featuring my photographs of the delegates, a relentlessly scrolling version of the text, and – most importantly – a specially commissioned score by the experimental US musician Chad Matheny, who performs as Emperor X. He writes songs about all sorts of things, from broken air conditioners to plate tectonics. The music he composed for When It Changed is lyrical, haunting and unforgettable.

Is this "my best shoot"? That's hard to say. But I do know that nothing I've ever done has addressed a more desperate matter.