Nelo said, ‘Forgive me, Uncle, but why don’t you just tell us what it is you have learned? We can at least try to understand. And then, and then—’

‘And then if you have to turf out my stiffening corpse into the snow before Etxelur the message has a chance of getting through? Is that what you think?’ He sounded fretful.

Avatak said gently, ‘Perhaps they will understand a little of it. And it might make your mind easier.’

Pyxeas sat up awkwardly and regarded him. ‘You are wise, Avatak, wiser than I ever was. Very well.’ He beckoned them closer, and began to whisper. ‘It is a great truth that I, Pyxeas, have discovered. But not a complex one. I will tell you the essence of it – you may check my facts and conclusions from the material in the trunk, the presentations I made in Carthage . . .

‘It is simply this: fixed air. That is the secret of the weather.

‘I told you, Avatak, that the weather is controlled by the dance of the world around the sun, its nodding axis, its wobbling circuits. So it is – but not by that alone. There is a second factor – well, probably many more we have yet to discover. But the second most important factor, as the results of Bolghai clearly show, is the fraction of fixed air in the atmosphere that we breathe. For if the sun delivers heat to the world, fixed air, you see, traps that heat. The more of it there is, the more the heat is trapped.

‘Bolghai proved too that the living things on the surface of the world affect how much fixed air is present. For a tree, as it grows, will absorb a great deal of fixed air – much more than the scrap of land it stands on, if that land is farmed. And conversely when a tree is burned, or rots away, the fixed air that it consumed in the growing is released again.

‘And that, Avatak, is why the world’s descent into longwinter has been such a puzzle to me. Not the fact that it is happening, but that it is happening now. We should have been in its grip already – and we are not, because of human actions. It is an astonishing thing to say, but it is true. It is clearly proven by an inspection of history, and the detailed records of the weather kept at Northland and elsewhere.

‘Several thousand years ago the world began its slow descent into the next longwinter. But unlike all the previous longwinters before, now the farmers were at work, in Cathay as in the Continent, planting their crops. They worked their way across the Continent from the east, clearing a landscape that had been choked with forest. Do you see?’

‘Ah,’ said Avatak. ‘And all that fixed air in the trees was released.’