Alex Renton: Our ship is sinking: we must act now

The cause of our weather shifts does not matter. The millions who will be affected are the priority

What is climate change doing to humans now? Apart that is, from causing them to mass in Cath Kidston tents on Blackheath? I've spent much of this year looking at the impacts on humans of climate change, eerily aware that for most of the rich world this is a phantom, a monster you may hear howling in the distance, if you want to – but as yet nothing to be seen or touched. If you believe – as Oxfam, who commissioned my report, does – that climate change along with poverty are the greatest threats to humanity this century, then the monster must be dragged out of the shadows and shown to the people who can get the world to act.

That's not so easy. Is the looming famine in Ethiopia and the Horn of Africa caused by a changing climate? You cannot with scientific certainty say any more than that the shifts in weather patterns and the seasons have made it harder to farm effectively. Floods in Bangladesh and the Philippines? It is certain that the sea level is rising, encroaching on liminal lands across the world – we will lose at least one nation, Tuvalu, during this century. Typhoons and hurricanes? Seen over 20 years there are far more "extreme weather events" – so many that, in the words of one of the UN's most senior emergency officials, "Disaster is the new norm". But is it climate change?

Professor Anthony Giddens, who has just published The Politics of Climate Change, estimates that all but 3 to 4 per cent of scientists are convinced that the core changes in the planet are serious and are principally caused by human interference in the climatic cycle. It's not an issue worth debating any more, in his view – "You cannot base decisions about the future of the human race on a 5 per cent chance [that the scientific consensus is wrong]", he told an Edinburgh book festival audience. I'm sure that is right.

The debate is sterile and absurdly over-extended: we've been wrangling at international level over the causes and the solutions to climate change for more than 20 years. Unforgivably, this has eclipsed a more immediate issue – how to help the poor world deal with the impacts of climate change now and in the future. This should not be complex, because – in terms of a Bengali rice farmer with paddy now too salty to grow anything, or a flood-stricken family in Orissa – it doesn't much matter why sea levels are rising, or the glaciers of the Himalayas and the Andes melting. What is important is how we deal with the reality – including the 26 million who, according to the consensus among the humanitarian agencies, are already displaced by climatic disaster.

Jane Cocking, who is in charge of Oxfam's emergency humanitarian operations, told me: "I've read endless research. But there's nothing that says how many wat-san [water and sanitation] engineers I need to station in Bangladesh in 2020."

I found that pragmatic need inspiring. Did I find the answer? No. But I can now say that the consensus of current science is that Bangladesh will, during this century, lose at least 17 per cent of its land area due to rising sea levels and related problems. That will deprive 25 million people of their land and livelihoods. And it will take more wat-san engineers to tackle than have ever been born.

I tried to make sure I kept up my sceptical guard as I set off into a clamourous world: the academic climate

change conference. It was necessary. Too often we journalists lazily quote science, and aid agencies, as gospel: "scientists agree" trips on to the page easily, as though that formulation sealed the argument. But scientists rarely agree – especially when they are in the business of prediction, which is what most climate change impact science is about. In fact they fight like cats, squabble over findings and funding, and behave like jealous artists everywhere.

The vast Copenhagen summit in March, attended by 2,500 academics, reminded me of the attention-hungry circus that is the Edinburgh Fringe. At that summit, in an open session addressing the probable collapse of maize as a viable crop across large swathes of Africa, an eminent agronomist announced that what we needed to consider was "when we need to move the population of East Africa to Siberia". You could almost hear his peers saying, "Damn him! Where's he going with that? And who's he got to fund the feasibility study?"

The message from the scientists has been blurred, dangerously so, because it has provided prevaricating politicians with more excuses. But consensus is emerging. No one argues now with the fact that rice, soya and maize production is shifting rapidly across the hot regions of the world; that sea levels will rise twice as fast as we thought only three years ago; or that diseases are migrating geographically, often to populations with no knowledge of them, and little defence.

Another interesting indicator is the rate at which the scientists themselves are getting bleaker: 90 per cent of them, according to two recent polls, don't believe the world can attain the emissions targets that will keep warming to an "acceptable" two degrees this century.

Lord Stern, whose hugely influential review for the British government of the economics of climate change was published in 2007, said in April this year: "I probably under-did it." He now believes that a devastating five degrees of warming this century is "not a small probability of a rather unattractive outcome... [but] a big probability of a very bad outcome". So what's a cash-strapped, over-advised rich world government to do?

I think the answer is quite simple. There are two issues here: one is the causes of climate change. The other is the problem of adapting the planet, and humanity, to what is already proven to be a rapidly changing climate. The Dutch are spending $1.9bn (£1.1bn) every year now on improving their sea defences – that is because there is an immediate need; it is equally so across the world.

We can continue to debate why our ship is sinking, and how and when we must cut carbon emissions: but we must start getting the lifeboats ready. That will be expensive. Most agencies have put the costs of at $100-$150bn a year. One of Britain's most eminent researchers on climate change, Professor Martin Parry, on Friday released a report estimating the price at three times that. It's a lot, but not so much seen in terms of bank bailouts. And, as Lord Stern has said, money spent well in the parts of the world where climate change will hit hardest, will be money that also helps those countries develop.

Every dollar spent now helping people adapt and prepare will be measured in lives saved, in agony mitigated. It will also act to prevent the great migrations of which world governments are so frightened. There is a disaster coming: even my sceptical heart has had to admit that. But I'm confident we can still limit the extent of it.

Alex Renton is the author of Suffering the Science: Climate Change, People and Poverty. It can be downloaded at www.oxfam.org.uk

Alan Watkins is away