Sailing up the Clyde was once how most travellers from abroad reached Glasgow, as well as those from Ireland, the Hebrides and the peninsulas of Argyll. Passenger liners came right up the river to within a mile of the city centre from New York and Boston and imperial cities where Scottish influence was particularly marked: Calcutta, Rangoon, Halifax, Montreal. As late as the 1960s, a traveller could step ashore in Glasgow having embarked in Dublin, Belfast or Bombay, though by that time the bigger transatlantic ships were ferrying their passengers ashore in the deeper water off Greenock, 25 miles downstream.

This trade contracted slowly in the last 50 years of the last century, and then suddenly it vanished. Other port cities have been similarly deserted, but perhaps in none of them, even New York, did the loss feel so confounding. Glasgow, after all, had seen a steamship long before it saw a steam locomotive; the first commercially successful steamboat in Europe began operating on the Clyde in 1812, beaten only to a global first (so Glasgow patriots reckoned) by the sharp practice of an American who had stolen the idea and put it to work on the Hudson. Today, the last vessel to perpetuate this two-centuries-old tradition is the elegant paddle steamer Waverley, built in 1947, which sails to and from Glasgow for two or three months every summer and whose near-miraculous survival is a testament to the love that Britain affords old machines.

A voyage upriver on a fine evening this week revealed that deindustrialisation has its compensations. After we left the beautiful firth behind – in the days of international sea travel it was considered the most majestic entrance to Europe – a pod of dolphins played around the ship at Greenock, while a few miles farther on hundreds and hundreds of swans gathered on the sandbanks near Dumbarton. The shipping channel was narrow by now, the result of an ingenious but simple piece of 18th-century engineering which deepened the river by forcing its flow through a tighter space, scouring the bottom to allow ocean-going ships as far inland as the Glasgow warehouses. “The Clyde made Glasgow and Glasgow made the Clyde” was a saying that every Glaswegian knew once, because the deepening also enabled a flourishing shipbuilding industry, which grew through the 19th century until by the Edwardian era the Clyde was making a fifth of the world’s tonnage.

On deck on our vessel, elderly men (each year, a shrinking number) pointed at pieces of wasteland or new buildings in the yellow/orange brick that bargain architecture seems to favour, and announced a name.

“That was John Brown’s.”

“There’s where Connell’s yard used to be.”

“Barclay Curle next. The Nevasa came out of there.”

Sometimes there was a disused slipway, sometimes a wharf of rotting timber, sometimes a field of cows. When blocks of new flats rose on either bank, the beat of our paddles echoed from them. Families came out on their balconies to wave. We passed a coaster loading scrap, the only ship in the river to be seen since Greenock, and then, close to our destination, the unlikely sight of a half-built frigate resting on the stocks.

The daylight was fading when we disembarked, not far from a hammerhead crane, built to load ships with cargoes of railway engines, which sits as a kind of memento mori among the conference centres, media headquarters and upmarket hotels that have replaced the docks. The river, which no longer bubbles and stinks, is crossed by new bridges and lined with little trees. The lighting is cosmetic. It was hard to reconcile this clean, self-conscious landscape with a 1960s memory of a quayside pub filled with unsteady seamen and dockers, or of departing from a nearby wharf on the overnight boat to Belfast. (The bar for those travelling first class was warmed by a coal fire.)

Cop26, the UN’s 26th annual conference on climate change, will convene on this piece of old dockland in November for what it describes as “the world’s last best chance to get runaway climate change under control”. As many as 25,000 government representatives, media people and environmental climate campaigners are expected; Covid regulations for overseas visitors will be relaxed. There is a world to be saved – and a civic reputation to be reinvented and burnished. The UK government, this year’s Cop president, chose Glasgow as the conference venue, and Glasgow is determined to prove that better reasons lay behind that decision than a surplus of hotel rooms. According to the promotional video the city council made for Cop, “Glasgow is a city transformed … a city that continues to embrace change” by using its traditions of innovation and of social justice to overcome the legacy of its past and give its people a cleaner, greener, fairer future. “Glasgow can show the world we are becoming the city of our times on [sic] the issues of our times.”

Promotional videos never speak on oath; still, the language is ridiculous. Away from its new riverside, Glasgow is often ruinous. Shopping streets – Sauchiehall Street most notably – look shoddy and abandoned; shrubs sprout – an unplanned greening – from the roofs of Victorian offices. The legacy of its finest architects, Alexander “Greek” Thomson and Charles Rennie Mackintosh, faces an uncertain future. Thomson’s Egyptian Halls sit, as they have sat for years, rotting behind cloth screens and scaffolding in Union Street. Nobody can say whether Mackintosh’s Glasgow School of Art, badly damaged by fire in 2014 and destroyed in another fire four years later, will be rebuilt or in what way. Earlier this year, a confusing reorganisation and reduction of refuse collections left rubbish piled high on the pavements. Loss of revenue at Glasgow Life – the so-called arm’s-length executive organisation that runs the city’s culture and sport department – led to the closure of 80 out of 171 venues, many of them galleries, museums and libraries. Some have since reopened, many others not. Around 500 jobs will be lost.

Naturally, the causes of Glasgow’s degeneration are complicated and have long histories. Blame for the most recent failure, however, tends to go to the present state of Scottish politics and the fierce centralising instincts of the Scottish National party under the leadership of the first minister, Nicola Sturgeon. The SNP won control of Glasgow city council in 2017 after generations of Labour in power, but its city leaders have been found to be ineffective local champions. In the words of the journalist and commentator Gerry Hassan: “Glasgow councillors are more like agents for the national SNP administration in Edinburgh than independent advocates for the city that elected them. And yet their city is decaying. There’s a real, patent anger about the place.”

Between 2013 and last year, Glasgow, which remains Scotland’s largest city, lost GBP270 per head per year in funding from the Scottish government; only one or two local authorities in Scotland did worse. The SNP this week formed a useful alliance with the Greens, which may invigorate the party and enhance Scottish credibility at Cop26. And so, come November, the new riverside may shine with the kind of optimism not seen since John Brown’s shipyard launched the Queen Mary. The old city, understandably, reserves the right to feel left out.

• Ian Jack is a Guardian columnist