Apart from crags and pockets of ancient woodland, the British uplands are manmade. Three thousand years before Christ, neolithic farmers felled the trees and gave us a landscape stripped to grassland by grazing sheep we take as “natural” today. Two thousand years after Christ, new forces are moulding the British uplands. They will bring back at least a part of what stone age men destroyed.

It’s hard to believe in an unequal country, where wealth and land are so unevenly distributed, but the ecology of the hills depends on popular approval. When public opinion moves, the hills move with it. However solid their drystone walls are, they will not be strong enough to hold back political change, climate change and changes in fashion, which affect the countryside as surely as they affect clothes and music.

The sale of Thorneythwaite, a sheep farm near the head of Borrowdale, encapsulates the coming revolution. If you don’t know Borrowdale, you should. To my mind, it is the most beautiful valley in England. Thorneythwaite is high up the river Derwent, almost at the end of the metalled road, and overlooks the route to Scafell Pike. The National Trust bought it last week with an astonishingly high bid of £950,000, £200,000 above the guide price and £450,000 above what farmers could match

Local opinion was scandalised. The great Cumbrian Melvyn Bragg said that had a billionaire bullied his way to “this disgraceful purchase”, as the trust had done there, would have been “a deserved outcry”. James Rebanks, the Lakeland author of the wonderful The Shepherd’s Life, said the trust should be protecting hill farmers, not gazumping them.

I spent my childhood holidays on a Cumbrian sheep farm and would once have defended Bragg and Rebanks with all my heart. But although I feel like a traitor, I can see good reasons for saying not just that their opponents will win, but that they should win.

The inability of farmers to meet the guide price for Thorneythwaite will only grow. Of all the consequences of Brexit, the cutting of agricultural subsidies is the easiest to predict. Farmers who voted leave – 58% according to one survey – astonished friends of mine who campaigned for remain in rural Britain. What turnip truck had these yokels fallen off? When the promise to maintain common agricultural policy subsidies runs out in 2020, parliament will not continue to pump public money to farmers. Free-market conservatives do not believe in state subsidies. As for centre-left politicians, can you imagine them putting farmers before the NHS?

Before the vote to leave the EU, farm subsidies were almost beyond political debate. Now we are heading for the exit, they will be open to negotiation. British farmers receive up to 60% of their income from EU subsidies. Hill farmers are the most dependent of all. If they are to continue to take public money, they will have to do what the public wants and raising sheep will not be enough.

The most telling point about the trust’s intervention is that it did not buy the Thorneythwaite farmhouse. Rather, it wanted to transform the hills sheep have stripped of all except a few pockets of woodland. The trust will replace the monoculture with “healthy soil, natural water management and thriving natural habitats”. Following a succession of floods, the trust will also “explore” whether tree planning can “slow the flow of the upper river Derwent” and help prevent flooding downstream in Keswick and Cockermouth.

The aftermath of flooding in Keswick in 2015. Photograph: Paul Ellis/AFP/Getty Images

A writer in the Spectator, who accused the trust of wanting to turn the Lake District into Disneyland, could not have been more wrong. Keswick and Cockermouth have been flooded repeatedly. Flood defences are unable to spare them as global warming intensifies. The same is true for Scottish, Lancashire and Yorkshire towns downstream of the Highlands and Pennines. A part of the answer is to expel sheep and plant trees that might hold up the water on their old grazing lands. You don’t need to cloak the landscape, just to forest about 20% of the hills to slow run off and create natural dams in mountain streams. It’s not that sheep farming cannot coexist with flood management, but it will have to be restricted. The monoculture of the uplands must change, in other words, and the public, whose representatives will soon have exclusive power to decide agricultural policy and whose taxes will keep the farmer in business, will want it to change.

Before the Romantic movement, most saw the Highlands as wastelands. Our love for them is a result of the romantic reaction against the Industrial Revolution, which in turn produced its own revolution in sensibility. Another revolution is upon us. It is easy to mock the rewilding movement just as it was easy to mock the Romantics. But I would keep the “Disneyland” jeers to a minimum if I wanted to get a hearing.

Reintroducing beavers in Scotland has been popular with the public. Photograph: Alamy

Rewilding the fells is not just townies forcing their naive fantasies on the countryside. It is a hard-headed policy: in a tiny way, it will help offset global warming; more tangibly, it will slow the floodwaters climate change is bringing. It will also be popular. If you doubt me, look at how many go to see the new beaver colonies in Scotland or the wetlands in East Anglia and Somerset. Or listen to the sympathetic hearings plans to reintroduce lynx to the Kielder Forest receive. Look even at the seeds on sale in supermarkets and notice how popular the wildflowers we once dismissed as weeds have become.

“Taxpayers should only pay public subsidy to farmers in return for things that the market won’t pay for, but are valued and needed by the public,” said the National Trust’s director general, Helen Ghosh, after the Brexit vote. Her shopping list included wildflowers, bees and butterflies, farmland birds, water meadows and meandering rivers, which themselves slow flood water.

You can mock her if you want, but your mockery won’t stop her. Romanticism was a reaction against industrialisation and rewilding is a reaction against global warming and the mass extinction of species. It is likely to be as uncontainable.

In any case, I don’t want to mock. I have walked the Lake District since I was a child. As I tramped the hills above Borrowdale and Buttermere this spring, I noticed the absence of trees for the first time and found it disconcerting, almost shocking. All my life I thought I’d want to see the landscape the Romantic poets saw stay the way it was. All of a sudden, I wanted something new – and better.