we just don't know how much. the scientists are warning that there is going to be some rise in sea level whatever we do, maybe half a metre by the end of the century if emissions are brought under control, or one metre if they are not and that would be really devastating for millions of people on coastlines around the world. but they can't rule out a far bigger rise approaching two metres if the polar ice sheets collapse. the great ice sheets are already adding to the level of the sea, but this process may suddenly accelerate. scientists aren't sure, but the implications would be disastrous. india is now in the grip of flooding. high seas would make it worse. the science has never been so clear that we can head off the worst of climate change, but not all of it, so we urgently need to get ready. david shukman, bbc news. david shukman, bbc news.