Among the Indian novelists who arrived on the world stage after the publication of Midnight’s Children in 1981, Amitav Ghosh has long stood out for the range and consistency of his work. Indian novelists writing in English for the most part publish little. Gun Island is Ghosh’s ninth novel – more than Rohinton Mistry, Vikram Seth and Arundhati Roy combined. Ghosh has also expanded the horizons of Indian fiction intellectually – by infusing his novels with anthropology, history and science – and geographically.

Previous novels have been set in Burma, New York, southern China and Kolkata. Gun Island opens in Kolkata and ends on a ship in the Adriatic, stopping along the way in New York, Los Angeles, Venice and the Sundarbans, the mangrove forest in the delta of the Ganges in the Bay of Bengal. Its narrative fuses subjects as diverse as Bengali myth, the Little Ice Age of the 17th century and antiquarian manuscripts with the urgent contemporary matters of refugees and human trafficking. It contains all the strengths of Ghosh’s best work and takes in his most enduring interests: globalisation, the sea and, above all, climate change.

In The Great Derangement (2016), which started out as a series of lectures, Ghosh asked why so little contemporary fiction recognises, much less reckons with, the climate emergency we are facing. Gun Island is not the first novel in which Ghosh has answered his own call to arms. It is a sequel of sorts to The Hungry Tide (2004), which examined the impact of climate change on the Sundarbans. Many of the characters in that novel reappear here, most notably the Bengali-American marine biologist Piya Roy.

Gun Island’s narrator-protagonist, Dinanath ‘Deen’ Datta, is a dealer of rare books, originally from Kolkata but long based in Brooklyn. On his annual winter visit home, Deen meets an elderly aunt who tells him of an obscure temple in the Sundarbans that is dedicated to Manasa Devi, the goddess of snakes, and – so the legend goes – was built by Bonduki Sadagar, the ‘Gun Merchant’, a trader who travelled the world in an attempt to escape Manasa Devi, but was eventually captured and forced to become her devotee.

Deen visits the shrine and what he finds there so unsettles him that, back in Brooklyn, he is unable to return to his old life. An old mentor, a Venetian scholar called Cinta Schiavon, is fascinated by his description of the shrine and its legend. It is her encouragement that sets Deen off on the journey that leads from the Sundarbans to Venice, from one gun island to another.

‘The so-called realistic novelist’, says Javier Marías, ‘has confused his role with that of the historian or journalist or documentary-maker.’ Ghosh has never been a conventional realist: the plot of Gun Island is driven by a series of coincidences and spectacular events that might lazily be called ‘magical realism’. Piya tries effortfully to explain each new occurrence as the consequence of climate change. Cinta, who combines traditional erudition with a kind of extrasensory perception, thinks otherwise.

But the book’s form does more than rubbish the dichotomy between magic and the real. It demonstrates how the particular genius of the novel can be deployed to examine the subject of climate change in ways beyond the scope of journalism, history or documentary. Gun Island shows us how the psychological compulsions for order and security that drive our unwillingness to confront climate change are about to be first unsettled and then destroyed.

One aspect of Gun Island is mystifying. Why did Ghosh abandon the third-person narration of The Hungry Tide and the Ibis trilogy in favour of a first-person approach? Deen is a fussy and pedantic narrator, forever offering parenthetical qualifications, and has the genteel, cliché-filled, ‘convent-school’ diction of an Indian elite long past: ‘they were a motley lot’; ‘I was a bookish young fellow with my head in the clouds’. Three decades in the United States have left no imprint. And he is no storyteller.

The voice is true to the character but it impairs the narrative with the Pooterish banality of its exposition. As the characters witness a literal miracle, Deen tells us, ‘We were transfixed by this miraculous spectacle.’ But this is the rare novel that succeeds despite the shortcomings of its narrator. It is in Venice that the novel really starts to soar, when voices other than Deen’s take over, above all those of the rifugiati – migrants, many of them Bangladeshi, who, having survived the journey over land and sea to Venice, are now attempting to make a life in the shrinking space between the traffickers and the Italian state.

Gun Island has a kindred soul, a literary sibling, in Jenny Erpenbeck’s Go, Went, Gone (2015), in which a retired German classicist, as lonely and as cloistered as Deen, becomes involved in the lives of young African migrants to Berlin. There are important differences: Erpenbeck’s novel is quieter, perhaps less ambitious, and is told in a slightly distant third-person voice. But both she and Ghosh reject easy narratives of despair or redemption. And they remind us of the possibilities of fiction – of how to write a humane and rigorous novel of right now.