Cop26 has been billed as the last best chance to keep global temperature rises to no more than 1.5C, with Sir David critical of those who deny the climate crisis.
In conversation with BBC science editor David Shukman, the naturalist and broadcaster said: “Every month that passes, it becomes more and more incontrovertible, the changes to the planet that we are responsible for that are having these devastating effects.”
He added: “If we don’t act now, it will be too late. We have to do it now.”
Wealthier western countries like the UK have a “moral responsibility” to help refugees displaced by climate change, the 95-year-old said.
“We caused it – our kind of industrialisation is one of the major factors in producing this change in climate. So we have a moral responsibility,” he told the BBC.
“Even if we didn’t cause it, we would have a moral responsibility to do something about thousands of men, women and children who’ve lost everything, everything. Can we just say goodbye and say this is no business of ours?”
Earlier in October, Sir David, who will be present at the summit, said the possibilities of Cop26 gave him “some hope”.
“For the first time people around the world will hear the arguments as to what we should do, the analyses as to what the problems are and what the solutions are,” he said.
“Those two things bring me some hope.”
Boris Johnson will welcome world leaders to Glasgow for the summit which begins on Sunday after saying it was “touch and go” whether key goals would be met as the the heads of some major polluting nations are set to skip the event.
Earlier this month, the Queen acknowledged she would attend Cop26 and appeared to suggest she was irritated by a lack of action in tackling the climate crisis.
It came days after her grandson, the Duke of Cambridge, warned world leaders in Glasgow against “clever speak, clever words but not enough action”.
Recent analysis suggests that even with the latest pledges and targets, the earth is heading for around 2.4C of warming in the long-term.