The life of the writer RB Cunninghame Graham is now much more interesting than anything he wrote – a summary such as "Scottish laird, old Harrovian adventurer, Argentinian gaucho, Spanish gold prospector, Britain's first socialist MP" touches only half of it. But one of his short stories still finds a place in literary anthologies. This is Beattock for Moffat. It's a grim little story that mixes sentiment with a brutal matter-of-factness. An exiled Scotsman dying of bad lungs takes the night train north from London, determined to last long enough to see his birthplace, Moffat, just across the border. He gets as far as Beattock, the junction for the branch line, where he dies on a station bench with "a faint bloody foam [on] his pallid lips" and a "fine rain beating on the platform".

I think of it whenever I make the same journey from London by train and see the old terrace of railwaymen's houses at Beattock (the station itself closed long ago), or the turn-off signposted to Moffat if we're driving up the M74. The business of crossing borders – England/Scotland, life/death – is you might say at the heart of the story, but those aren't the borders I think about now. "Do ye think it will be rainin' aboot Ecclefechan?" asks the sick man of his brother when their train is still in Euston, and then decides for himself that it's "sure to be rainin" by Lockerbie. And usually it is. London will be in sun, the clouds will come somewhere between Stafford and Preston, the rain will be hitting the window or the windscreen long before Penrith. The border to consider (and regret) in these conditions is not so much cultural or political as meteorological: between wet north-west Britain and the drier country to the south and east.

Of course, the division isn't new. Cunninghame Graham wrote his story 100 years ago. Moist air blown in from the Atlantic has been precipitating over Britain's western hills and mountains for as long, probably, as they have existed. Nor, obviously, is the wetness and dryness absolute. When Luke Howard, the scientist who devised the modern classification of clouds, wrote that "habit reconciles the Englishman to a sky … which drips, more or less on half the days of the year" he gave no exemption to Londoners or the farmers of Kent. There is, however, no denying that the west is getting wetter. The floods that last week ruined Cockermouth and cut Workington in two had their origins upstream in the highest rainfall ever recorded in the UK over a 24-hour period: 314.4mm, about a foot of rain, was recorded at Seathwaite in Borrowdale, which is 35mm higher than the previous record (at Martinstown, Dorset, in 1955). Across the border at Eskdalemuir the weather station has already set records for November.

These are the peaks in a long-term pattern. The Met Office averages figures over 30-year periods to iron out the random excesses of climate variability. In western Scotland the average monthly rainfall for 1961-1990 was 473mm, which rose to 533mm in the period 1971-2000 – roughly an increase of 12% – and shows no signs of levelling off ("The only direction is up," the Met Office said this week). Bute, where I spend every family summer, had the wettest August since the island began to record its climate in 1800. Almost every day and night the rain would patter on the roof and gurgle down the gutter. The mere state of not-raining became the cause for celebration. During these August evenings we'd sit before a coal fire and watch the weather forecast: another Atlantic low would be heading our way even before the present one had left, while on the weather map London glowed irritatingly in sunshine. The south and the east (even Edinburgh) were foreign countries; they did things differently there.

Until recently, the people of the west accepted their climate as an almost frivolous disadvantage. People sighed – ah, but you never got good weather for the Glasgow holiday fortnight – or converted sullen days of persistent drizzle into funny stories of drowned putting greens and seaside landladies. There was also, then, the much likelier possibility that a brilliant blue day or two would bring out the cliche known everywhere from Snowdonia to Stornoway – if you get the weather, there's nowhere in the world to beat it. But these stoic attitudes date from the time when the Atlantic's diagonal track of depressions passed farther north, almost at Iceland, and northern Britain got only their eddies, which, to quote a meteorological study of 1928, "seldom bring really heavy rains".

The storm track has now slipped south. Science can't be certain why – changing sea surface temperatures may be the cause but as part of a process not yet sufficiently understood. The consequences are no longer containable by wry jokes. Unprecedented rainfall has spectacular and noticeably tragic effects – see Cumbria this week – but the general trend to a more watery climate could, slowly and less dramatically, eventually undo an economy and a way of life. Crops can't be planted or rot at their roots; tourists and their money go elsewhere; basements and lofts that have been dry for centuries spring leaks; gardens never stop squelching; doors squeak and wood crumbles; sewers overflow; buried cabling sparks out. In 2004, the A83 from Glasgow was closed for days by a landslip, marooning the people of south Argyll. The same thing happened last year and this. Peat, soil and stone that had been secure on the hillside for thousands of years suddenly romped down the slope. Nobody could remember such a phenomenon happening in summer before.

A certain amount of solace can be found in history, especially for anyone who doubts or denies that climate change is man-made and attributes global warming to the planet's natural cycles. Could the future, after all, be any worse than the little ice age of the 16th and 17th centuries? In north Britain, especially Scotland, crops failed, bread was baked from tree bark, and peasants maddened by starvation fought each other to feed from the choicest nettles. Snow on the highest mountaintops survived the summer, great winds obliterated coastal villages, and the upper limits of cultivation on frosty Lothian hillsides fell by 200 metres. Eskimos reached the Orkneys by ice floes and kayak, and one of them paddled as far south as Aberdeen. It marked the beginnings of Scotland as an emigrant country – a good place to see the back of, unless you were an Eskimo – and yet, slowly and erratically, warmth and a more prosperous civilisation returned.

But let's be even more Pollyanna-ish. After Peak Oil comes Peak Water (the title of a new book) and an era of Mediterranean thirst that north-west Britain, Wet Britain, is well placed to slake. I see water tankers sailing out of forgotten ports along the Cumbrian and Scottish coasts, bound for Naples and Cadiz. In Ecclefechan, where it will certainly still be raining, people relieve their depression by thinking of themselves as the Saudis of H₂O.

www.guardian.co.uk