The world is on track to become up to five degrees hotter, and sea levels could rise more than 80 centimetres this century, according to a leaked draft of a landmark climate change report prepared for the UN.

There is now a 95 per cent likelihood human greenhouse gas emissions are driving changes being observed globally, which in recent weeks have included extraordinary heatwaves in Asia and Alaska.

Rising tide: waves crash into sandbags where Victoria's Portsea Beach used to be. Credit:Ken Irwin

That degree of certainty has been revised up from 90 per cent in the last report in 2007, 66 per cent in 2001, and just over 50 in 1995. A sea level rise of up to 82 centimetres, which would have serious impacts on coastal cities everywhere, is now ''unequivocal'', Reuters reported.