Dhaka, Bangladesh

Shortly after dawn, a small seaplane hovers over the eroded chars (islands) of Bangladesh, and then touches down on the Jamuna River near one of a string of villages where some of the world's poorest people live. This is the front line in the battle against climate change, a place that should defy the fiercest sceptics. We emerge from the seven-seater, led by Ed Miliband and Douglas Alexander, the Secretaries of State for Climate Change and International Development, respectively. The two cabinet ministers who are working so closely together - they are in matching chinos and blue shirts as if to prove the point - are visiting local people who are direct beneficiaries of aid from the UK. The UK is the single biggest bilateral donor to Bangladesh, with £125m given this year and £150m set aside for 2010.

The amount is largely welcomed by Bangladeshi officials. But some development NGOs have called for more, as £150m translates roughly into a pound per person per year. “Why not £250m?" they say. Critics also ask why it is that countries such as Pakistan are considered fit for direct grants, able to choose how they spend the money, while others, such as Bangladesh, are not.

Here, the DfID-funded Chars Livelihoods programme provides a one-off asset worth up to £150 (plus a small monthly stipend for 18 months) to households whose huts stand on plinths, raised above flooding danger levels. In the near-tropical heat - up to 40° by day and over 30° at midnight - Tasleema, a farmer's wife, thanks the ministers. "Before, it was a very difficult time," she says. "But after I got [six] cows, my life is better than before. I have been able to increase my livestock." In another hut, Rehena asks Miliband - through a translator - if he has children. I turns out they have babies born within a month of one another. The discovery is poignant, because so much of the British government's message, with a hundred days to go before the make-or-break climate-change summit in Copenhagen, is about the interconnected nature of this crisis.

For if, in the UK, the impact of climate change appears remote, it is ever present here (the highest ever flood was in 2007). Bangladesh is no bigger than England and Wales, but has more than 143 million people, most of whom live in lethal conditions of overcrowding and poverty: 119 million subsist on less than $2 a day.

The high-level British delegation will be moving on to meetings in New Delhi aimed at bridging the divide between a developing India, which has resisted greenhouse-gas reduction, and the developed west, which NGOs say must cut emissions by at least 40 per cent by 2020 to prevent a global temperature rise of 2° or more. The independent UK Committee on Climate Change argued in December 2008 that this pledge should be strengthened to 42 per cent for a 50-50 chance of preventing the rise, and the UK under Miliband has led the way with the promise of a 34 per cent reduction. According to a diplomatic source, when Alexander and Miliband met privately with the Bangladeshi prime minister, Sheikh Hasina, they urged her to make her voice heard on this issue at the World Climate Conference in Geneva, where she flew for preparatory talks on 1 September.

Alexander repeatedly stresses that the ministers will be going to India "not to lecture but to listen". But in an interview during the return flight to Bangladesh's capital, Dhaka, both men (who first met in Miliband's kitchen in 1990, and are now on their first joint ministerial trip) spoke of the need for a deal in Copenhagen. "The situation in Bangladesh emphasises the urgency of the situation," says Miliband. "Climate change is not a theoretical prospect in the future but is actually happening at the moment. I was struck at the char over the difference the UK is making to tens of thousands of people." Alexander says he was moved by the "resilience, dignity and optimism" of the villagers he met on this, his third visit to Bangladesh.

Miliband accepts the "historic responsibility" of developed countries towards developing nations, which blame climate change on the west. He points out that between 1830 and 2000, 30 per cent of carbon emissions were from the US, 30 per cent from Europe and 6 per cent from China. But, he says, countries such as India have to accept the need for reductions, for their own sake. "Developing countries must be part of the solution . . . There is the moral case and the self-interested case - the 'better life' argument." For example, "Bangladesh is on the front line of climatechange, but India itself is affected by that change." Miliband, "an optimist by nature", says he is encouraged by the change of direction taken by the US under President Obama, and his pledge to cut emissions by 14 per cent by 2020 - a far cry from the Bush era.

Both ministers believe there is a "real dividing line" here - and it is true the programmes I have witnessed are among the government's unsung achievements. Bangladesh is a long way from the partisan Westminster hothouse, but it is worth remembering that while the Tories have ring-fenced international development spending from cuts, they have refused to match the government's promise of a cap of 10 per cent of aid funding to be spent on climate financing.

In addition, according to aid workers I met in Dhaka, the Tories have tried, in private discussions, to downplay the "historic responsibility" of the west for climate change.

For Miliband, Copenhagen is among "the most important things I'll ever to do in my life". And Alexander explains: "I came into politics to change things . . . For our generation of progressive politicians, climate change is the defining test."

The island on which Tasleema and Rehena live is likely to disappear within their lifetimes. In that sense, their plight is a microcosm of climate change across the world. And while the DfID programme is stemming the tide here, time is swiftly running out.

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During the same chat, wedged inside the plane, Ed Miliband clears up a few domestic matters. First, a recent report suggesting he had considered quitting politics was "rubbish". Second, he won't discuss growing talk of him being a strong contender for the Labour leadership: "I'm getting on with what I'm doing and it's a huge privilege." He genuinely hates the subject, but he won't be able to avoid it for ever - and I watched him work the room in Bangladesh like a leader-in-waiting. Finally, on our very own Peter Wilby's proposal that he should enhance his own position by resigning on a matter of principle, he chuckles: "With friends like Peter Wilby . . ."