Climate change threatens to tip autumn into chaotic, wintry instability

My shadow stretches longer ahead of me whenever the clouds allow. So the slant of the sun, at least, is following its customary course. But autumn, long my favourite season, will unfold unknowably in the grip of climate change.

The fall of sunlight remains magical. As I write, it reached past the mountain to gild the breakers, the dunes, then swept up the hillside to sculpt the trees of our acre. There’s not a sign of leaf-fall in their fluid range of greens, this despite all the dry days of late summer, marked with dashes on the weather cards I send up to Met Éireann.

Unlike the earlier budbursts of spring, so crucial to the matching of birds and their insect prey, the fall of leaves in autumn has had to wait for the same kind of study. There was a wide expectation that warmer autumns would push leaf-fall back a couple of weeks. But a 2020 Swiss study, backed by experiments and some 60 years of records of European trees, suggests rather the reverse.

Higher levels of warmth, light and CO2, the research concluded, are driving leaves to be more vigorous in spring, taxing the trees’ internal limits to productivity and hastening an exhausted leaf fall by several days in autumn.

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An earlier leaf-fall, even of days, could reduce the amount of carbon stored each year by temperate forests. The research team at ETH Zurich estimated the lost carbon at about one gigatonne, or about a 10th of what humanity emits annually.

Few of our Thallabawn trees, as it happens, get the chance to show off much autumn colour, even in leaves left to linger by the wind. Only the beech, in the right weather, can sustain a coppery glow.

After the drizzles and hazes of summer, the crisper air and raking light of autumn sharpens edges and intensifies new hues. Mountains come nearer, bog sedges turn pink, dying moor grass swirls into golden lion’s manes.

For more theatrical, often spectacular, colour, I can look to autumn sunsets over the sea. What an off-season attraction for the Wild Atlantic Way this would make, followed perhaps after dinner by some star-gazing at the west’s dark skies. There’s salutary science in sunsets too – if, that is, you thought their spectacular colours owed something to man-made dust or pollution. The answer seems quite the opposite: clean air in the lower atmosphere prompts the brightest colours.

A good paper on the subject is by meteorologist Stephen F Corfidi, who works at the US's storm prediction centre. His explanations have to do with the scattering of air molecules in the beam of the sun, with wavelengths that vary the human perception of colour. It is our sensitivity to blue light, for example, that fixes the colour of a clear sky. At sunrise or sunset, the light from the sun takes a longer path through the atmosphere. This scatters much more violet and blue light, leaving more red for the human eye. Add in some Atlantic clouds and the richness of the sunset intensifies, with scarlet, orange and red.

The best clouds are cirrus and altocumulus, high enough to reflect the purest sunlight, but those lower can also glow brightly, given clean, transparent Atlantic air. The ocean, as I write, is mute and calm, as it has been most of the summer. A search of Atlantic hurricane maps finds nothing spinning off west Africa that’s imminently likely to bounce our way.

But the latest Atlantic study, from scientists in Ireland, the UK and Germany, finds the Gulf Stream system at its weakest in a millennium, a trend also measurable even in the growth rings of trees. This trend in the Atlantic threatens changes in the future winter storm track and even stronger hurricanes.

The Gulf Stream relies on AMOC, the Atlantic Meridional Overturning Circulation, one of Earth’s great Gaian machines. This moves nearly 20 million cubic metres of water a second, drawing warm surface water north from the tropics and sinking it deep in the Arctic to send it south and back round the planet.

Fresh water from melting Arctic ice, reducing water density, is helping to slow this crucial overturning. The modelled loss of 45 per cent of its flow by the century’s end threatens to tip autumn into chaotic, wintry instability.