It looked up at me, at the only biped on the planet. Its eyes were glazed, incurious. Then it shambled toward the river, seeking water.

The little robot was still at my feet, her glass eyes blank.

“So what’s happened?”

“There have been major eruptions,” the robot said. “Far from here, in Siberia, too far away to hear or see. But they have had a global influence.”

They had not been volcanoes, I learned. Magma, “flood basalts,” had come seeping from fissures in the ground, and had covered vast areas.

“When the eruptions began the injection of dust into the air caused a snap freeze, a couple of cold summers. But since then the huge flows of basalt have been emitting carbon dioxide and sulphur dioxide.”

“Greenhouse gases.”

“Yes. The result is a rise in air temperatures, all around the planet. Life here was always precarious. Even a one-degree rise in temperature has been enough to kill off many plants, and the herbivores that fed off them—”

“And the carnivores that fed off them. ”

“Yes. There have been few actual extinctions yet, but biotas have been disrupted, and populations of plant and animal species have crashed. The creature by the river is a dicynodont. Its habit of burrowing in the ground to estivate has enabled it to ride out the worst of the heat where others have succumbed.”

“Let me guess,” I said. “Worse is to come.”

“Yes.”

Another snap transition; I staggered, shocked.

That baking sun had disappeared. Suddenly rain poured out of a cloud-choked sky, hammering on my head and shoulders hard enough to hurt — but then I realized that the water was actually stinging me, like a mild acid. Hastily I dragged my jacket over my head. But there was no relief from the sweltering heat; the air was so humid it was actually difficult to breathe.

The robot stood patiently at my feet, rain splashing from its paintwork.

“Shit,” I shouted. “You might have warned me.” I could barely hear my own voice above the roar of the rain.

“The dicynodonts had no warning.”

“More eruptions?”

“Yes. The basalt traps are pumping out chlorine, fluorine, sulphur dioxide, as well as carbon dioxide. These gases are combining with the high air to form a cocktail of acids, hydrochloric, sulphuric, carbonic—”

“Acid rain.”

“Yes. The last of the trees and the larger plants are now being burned off. Animals are being flayed alive. Those dicynodonts still shelter in their holes, but there is nothing to eat but a few ferns and moss and lichen in crevices by the river.”

“And it’s going to get worse, isn’t it?”

The robot seemed to hesitate. “Would you prefer a warning this time?”

“Just do it.”

Another bewildering change.

The rain had cleared, the sky stripped of clouds to reveal the sun once more. But the sky was tinged a washed-out orange-brown, with barely a scrap of blue. I still stood beside the river valley, but it seemed much broader than before, its banks roughly scoured away. Its floor was littered with boulders and banks of gravel, and further downstream it was incised by channels that cut across each other like braided hair.

The land was bare rock. I couldn’t see a scrap of green anywhere: there wasn’t even soil cover. It was as if a great knife had scraped over the ground, cutting away all the soil and plants and trees, all the animals and insects.

That robot still stood at my feet, unchanged, patient. I felt irrationally angry at her, as if she had caused all this devastation. I took a deep breath — and my lungs ached. “Jesus. I can barely breathe.”

“Actually,” the robot said, “I have used some artistic licence. If you tried to breathe the authentic air of this period you would shortly die of anoxia.”

“What happened, Gea?”

“Gas hydrates,” she said simply.

The carbon dioxide injected into the air by the Siberian volcanic event had caused global temperatures to rise by several degrees. Eventually, at the poles, the ocean margins and tundra began to thaw out. Just as Tom had experienced in 2047, there had been an immense release of the methane and carbon dioxide trapped in the ice — a hundred-fold expansion in volume when that great lid of cold was released. All over the polar regions there had been vast bubbles, water spouts, the very ground crumbling and cracking open as geysers of the stuff spewed into the air. It must have been a hell of a sight.

The effects were disastrous.

Gea said, “The injection of the first of these deposits into the air fed back into the greenhouse cocktail already working in the atmosphere, and warming increased further—”

“Which thawed out more hydrates, which released more gases, which increased the warming effect.” Any engineer would recognize a positive feedback cycle. Around and around it went, getting hotter and hotter, the air filling up with the noxious gas.

When all the hydrate banks had emptied, a violent warming pulse followed. Not only that, there was now so much carbon dioxide in the air that as it seeped around the planet the levels of oxygen were reduced to far below normal.

“The last of the dicynodonts probably suffocated rather than starved,” the robot said. “At least it was quick. The ferns and cycads and other opportunistic species that took the chance to propagate during the warming pulse — even they have gone now. A few plants survive, just weeds, clinging on in pockets of soil.”

I looked around at the barren landscape. “It’s like being on Mars. Nothing but rock and dust.”

“That isn’t a bad comparison,” the robot said. “Look at the river. Can you see the heavy debris, the braiding effect in the channels? There is no life here, nothing to bind together the soil. When the rains came all the remaining soil was quickly washed away, and heavy debris scoured the channels. You can find such braiding in rivers on Mars, which likewise formed their courses in the absence of life.”

The oceans had suffered, too. When the rains came all the dead and stinking vegetation was washed away into the rivers and swept downstream to the sea. Around the mouths of the rivers the seabeds were covered in a carpet of organic matter, the rotting corpses of animals, dead vegetation, all mulching down to a thick black slime that choked the life of the seafloor, the molluscs, shrimps, worms, arthropods. As it decayed this foul stuff drew down yet more oxygen from the air and emitted yet more carbon dioxide and other foul gases. Meanwhile the excess carbon dioxide poisoned the plankton, the tiny organisms that were the basis of all marine food chains. Thus oceanic populations imploded just as those on land had.

Gea said, “Biodiversity was reduced to about a tenth of what it had been before. Ninety percent of all marine species went extinct, seventy percent of land vertebrates. The numbers are approximate, of course; we will never know it all. It was far worse, by an order of magnitude, even than the dinosaur-extinction event. This disaster must have come close to ending the story of multicelled life altogether.”

“But life recovered,” I said. “Didn’t it?”

“Oh, yes. Eventually.”

After this end-Permian extinction event the world was a desolate place, its complicated ecosystems imploded — and its ruins became dominated by a single animal. The lystrosaur, a kind of dicynodont that looked something like a pig, was a chance survivor that took its extraordinary opportunity; soon ninety-five percent of all the animal flesh in the world was lystrosaur meat.

Recovery came as the descendants of such survivors, shaped by time and evolution’s scalpel, diversified to fill all those empty niches. A new world of dinosaurs and pine trees would emerge from the rubble of the old, and at last, after further extinction events, would follow the flowering plants and the grasses and the true mammals, and mankind.