We cannot live without optimism, even when pessimism is far more logical. A Christmas thought: you can genuinely feel a sense of peace on earth and goodwill to all men knowing all the time that the following day’s paper will contain as many atrocities as today’s. It is the contradiction that makes life possible. It is most certainly the contradiction that makes life possible for anyone who believes that looking after our planet is a fairly good idea.

The polar bears got to me. The global warming and the shrinking of the Arctic ice seem likely to bring about the first mega-extinction: the first of the iconic must-save species to go. And on Boxing Day spring arrived: and sent a chill up my spine. The great tits were singing teacher-teacher from the little copse, the coal tits were doing their stuff from the top of the pine trees, and then, impossibly, I heard the year’s first song thrush, singing each song twice over, to recapture that first fine careless rapture.

It seemed a cheerful and beautiful way to introduce the beginning of the end of the world. I tell myself that the greatest crime in conservation is giving up: that optimism is necessary for personal and perhaps planetary survival. And I found a smidgen of cheer by going to London.

Millions of words have been written in thousands of books about a return to the beloved haunts of childhood, only to find them changed beyond salvation. Well, when I go to the Tideway on the River Thames I find the exact opposite. A stinking horrible place has been changed beyond damnation: loveliness prevails where the river once sweated oil and tar.

I coxed eights on the Thames, frequently striking bridges and other craft, when I was at school, and even more frequently gagging with the stench of the place. This was boating on a river of death, a winding sewer that rose and fell with the turning of the tides. These days it is lovely. I walked up to the junction of the Grand Union Canal to observe the heronry. You never saw herons when I was reshaping London’s bridges. There was nothing in the river for them to eat.

There are 13 nests in the metropolitan centre of the heronry, and half a dozen farther out, in the boondocks of Metroland. Many bore a one-legged sentinel: not breeding, not yet, but roosting. The big, haphazard, big-sticked nests look solid enough to raise plenty of herons in the coming year: in a few weeks the place will be a hive of activity. Bloody big bees, though.

Herons were fabulous exoticisms when I was a boy in London — occasionally seen flying in that slow, purposeful way on broad, big-arched wings, or doing the motionless laser-glare from the water’s edge — and it would be a day of special excitement. But now they are a fixture on the fishable portions of the Thames, and on the last chunk of Tideway between Hammersmith and Teddington you see them all the time. The herons have reclaimed their river.

And they have done so because humans, after centuries of getting things wrong, have started getting things right. The Thames has been, to a large extent, undestroyed, rewilded. A raft of birds in the main stream: black-headed gulls, mallards, a tufty or tufted duck, a couple of cormorants deep-diving for fish. If humans can reclaim that stinking river for life, there is plenty of scope for putting more things aright.

So I told myself, anyway, optimism being not only necessary but actually sustainable along the living river. Another heron, perched at the top of a weeping willow, savage reminder of what the future needs to be. Sweet Thames, run softly till I end my song.

• Laughter is also good for optimism. The pied wagtail is a lovely bird, familiar to many for doing things like setting off the electronic line-judge at Centre Court at Wimbledon. They are dainty birds, masters of tarmac surfaces and flat roofs, where they dance for insects: Italians call them ballerinas, and they pas de chat and pas de poisson for their suppers. They have an extravagant bounding flight, proceeding in steep and exaggerated undulations, and when they do so, they always shout at the tops of their voices.

  “Chiswick!” they say, and again, “Chiswick!”. A moment of delight, then, when I saw a pied wagtail bounding along the middle of the Thames, not only over-flying a bridge but actually announcing the bridge’s name in strident wagtail-voice. A necessary touch of optimism. Bloody dreadful about the polar bears, though.

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