The World heritage site Sundarbans nature reserve in Bangladesh's south-west is one of the last untouched places on Earth - and home to the largest population of tigers left in the wild.

But the trees in the Sundarbans have suddenly started dying. And not just that: they have started dying in a way nobody has seen before, from the top down.

Nobody is sure what the cause is, but the country's leading scientists think the trees are dying because, in recent years, the water has turned from fresh to salty. The Sundarbans is a massive mangrove swamp, and the sea has begun encroaching. What we are seeing may be one of the first casualties of rising sea levels caused by global warming. "Nobody can say for sure whether it is climate change because there haven't been proper in-depth studies," says Professor Ainun Nishat, one of the country's leading environmentalists, and one of those involved in the UN's climate change report. "But this is the sort of effect rising sea levels will have on Bangladesh. We are fighting climate change on the front line. But the battle has to be integrated across all countries."