Margaret Atwood, her assistant informs me down the line from Toronto, will be with me shortly.

The minutes tick expensively by, giving me plenty of time for my imagination to run amok. I imagine her directing operations like one of those super-efficient, ultra-confident, stiletto-heeled characters, invariably played by Glenn Close or a clone, who scares the living daylights out of her wilting employees. In Atwoodworld, there's always something on the go, a book to write or promote, a cause to champion, a keynote speech to deliver. Like Doctor Who, she seems forever to be fighting forces bent on our annihilation, be they governments or corporations or the local university, to which she recently took a cane for replacing a grass-covered sports field with artificial turf.

The last time I spoke to her she had been reading a book about how quickly cities could be restored to nature. It wouldn't take very long at all, she said, sounding excited at the prospect, as if in her lifetime Manhattan could be reforested. Her voice, when she does eventually come on the phone, is that of someone who demands to have her interest whetted. Small talk is not her lingua franca. Bores, moreover, must beware and fools can expect the Atwoodian equivalent of the hairdryer treatment. At 73, she is as feisty and fecund as ever, words pouring out of her like coins from a fruit machine.

Her latest novel, Maddaddam, is the final part of a trilogy which opened with Oryx And Crake, after which came The Year Of The Flood. Thus it is one her "speculative fictions", in which everything that is described in it has already happened or could happen. Indeed, she says, some of the things that were deemed possible in Oryx And Crake, which appeared a decade ago, have now come to pass.

To get new readers up to speed, Atwood offers a summary in Maddaddam of the first two books, though how helpful that will prove is debatable. Suffice it to say that the Earth is still in the grip of the man-made plague which has wiped out human beings but has brought to the fore the bio-engineered Crakers, including Toby, a former member of the God's Gardeners, whose aim is to preserve all animal and plant life, and Zeb, "who has an interesting past".

Other principals include the prophet Snowman-the-Jimmy and Amanda, who is recovering from a battering at the hands of the Painballers, who remain a force to be reckoned with. If none of this makes any sense, worry not. Taken out of context, this trilogy, routinely described as "dystopian", is as mind-warping as the politics of Middle Earth.

Atwood did not have a trilogy in mind when she wrote Oryx And Crake. Thereafter, she says, it was simply matter of wanting to know what happened next. Of course, she, being the author, knew what was going to happen next but putting it down on paper took longer than expected. She wrote three novels -The Penelopiad, The Tent and Moral Disorder - between Oryx And Crake and The Year Of The Flood.

She also, I note, wrote a non-fiction book, Payback: Debt And The Shadow Side Of Wealth, during those years. In it, she explored the idea of debt philosophically, viewing debt as "an imaginative construct, and how this construct mirrors and magnifies both voracious human desire and ferocious human fear". The debt Atwood writes about in Payback is that which we owe to the planet and to other people, the debt which we should find a way of repaying before we ourselves shuffle off.

It is a notion which, at least to this reader, seems to inform much of her work, whether fiction or non-fiction. What is remarkable about Atwood, however, is her ability to adapt her style of writing to the task in hand. For, as she suggests, who wants every book to read the same? In Maddaddam her style is largely shaped by her male narrators, which is why there is a lot of swearing in the novel, and which, she adds, is a trait of modern life. Even Canadians swear, she says. Has she ever asked anyone to refrain?

"No, I haven't actually. I think it would be very tricky to do because to them it's just ordinary conversation. They would not really understand it at all. It has now become a punctuation whereas once upon a time it was considered quite daring to do it in writing. I think people did swear a lot but you couldn't write it?"

She reckons that a younger generation will come to see swearing as "uncool", which ought to lead to its disappearance. She expects tattoos to go the same way, which is a wonderful thought but which strikes me as overly optimistic. "Are we going to do 'ain't it awful?', 'what's the world coming to?'" she asks, alarmed that our conversation is beginning to sound like that of two grumpy gits.

Is she an optimist?

"What, about tattoos?"

No, the future of the planet.

"It's not a question of whether the planet will survive; it's more a question of whether the human race will survive. Because I think the planet will keep spinning around. It will survive in some kind of form but it has always survived in some kind of form."

In a sense, her anxiety is bred in the bone. Her father was an entomologist and, early on, informed his curious daughter of the ecological importance of insects. Thus science is second nature to Atwood, unlike so many of her literary peers who couldn't tell one end of a microscope from another. Once, she recalled, the main topic around the table at Christmas was likely to be "intestinal parasites or sex hormones in mice", sure killers of appetite in any normal household. Now, for fun, she reads the likes of Stephen Jay Gould or Scientific American and New Scientist, from which she takes cuttings to germinate her imagination and to which the origins of Oryx And Crake may be traced. "Every novel," she once wrote, "begins with a what if, and then sets forth its axioms. The what if of Oryx And Crake is simply, What if we continue down the road we're already on? How slippery is the slope? What are our saving graces? Who's got the will to stop us?"

These are similar questions asked by writers such as Philip K Dick and HG Wells, both of whom Atwood admires. Indeed, as she reminds me, she once wrote an introduction to Wells's 1896 novel The Island Of Doctor Moreau. Like her, Wells veered between utopianism and pessimism. The Island Of Doctor Moreau falls into his latter phase. It has obvious points of similarity with Atwood's trilogy, for where Wells imagined human beings created from animals through vivisection, Atwood replaces us with the bio-engineered Crakers. Like Wells, too, she writes about what worries her, which is the world described in her trilogy, a world in which people make drugs like Blysspluss, which offers its users unlimited sexual energy while at the same time acting as a prophylactic, thus preventing the population from growing.

It is the kind of pill, argues Atwood, like so many nowadays, which while seeming to do us good actually does the opposite. "Any ingested medication can have negative effects," she says going into activist mode. "Blysspluss carries it a bit farther, in that it's guaranteed to kill you. But of course, the drug company in the book, the God's Gardeners, are giving people pills they know are going to make them sick. That goes on a lot."

She may be deadly serious but she is also seriously funny. The apocalypse may be possible, even probable, but it is unlikely to be upon us before she arrives in Edinburgh today for the Book Festival. It is her second home, where in the early 1980s she read in a bookshop to an audience of three.

This time around it will be standing room only, ironic proof that there's nothing more popular than the prospect of no future.