2124: 7.6 billion; 2125: 7.8 billion; 2126: 8.0 billion…

But suppose that, without future surprises or another great war, humanity over the span of centuries dwindled and faded and vanished? What did the modeler do then? If Kate were correct, and the model runs all went that way, Alex had better have some kind of an answer ready for the meeting with Mischa Glaub. Kate thought Glaub might have a couple of other people with him, members of the project review committee.

2134: 9.2 billion; 2135: 95 billion, 2136: 99 billion…

They had come to the year of the model run where Alex had left last night — the place where he had dragged himself away from the displays and headed off with his mother to meet Cyrus and Lucy-Maria Mobarak. At this point Kate had taken over. Now he had to pay extra attention to other variables, while continuing to monitor population growth.

2137: 10.0 billion. Running along smoothly, except that the rate of population growth was suddenly down. Now he recognized another complication. The model was set up to accept inputs, where appropriate, from other sources. Before the Seine came into operation those other sources were limited and well-defined. Now, suddenly, a million new data sources could feed the model. They included other predictive models whose outputs Alex did not trust.

“How did you limit exogenous variable inputs?” He snapped the question to Kate, without taking his eyes off the display.

“I cut them out.” She was standing very close to him, where she could see everything he saw. Her breath on his cheek was as warm as her voice was cold. “You left without telling me how to pick them, or what values to use. I didn’t take any that you hadn’t included.”

Alex nodded. New exogenous variables were a source of possible instability. Kate had made the conservative choice, by prohibiting new ones. All the macroscopic measures looked good to him. He could see no sign of the precursors of war, the ominous indicators that had popped up all over when he did simulations of System activity for forty years ago.

But something odd was happening. The model was now forty-five years out, and although the population count continued to creep up, two other variables had reversed their trend. The index for Outer System activity was down, with the cancellation of three development projects among the moons of Neptune. Just as disturbing, no new extra-solar probes had been launched for the past seven years in any of the model’s predicted futures.

“Can you see it?” Kate didn’t sound angry anymore. She was just tense.

“Yes.”

“Do you know what’s causing it?”

“No idea.”

“What are we going to tell Mischa Glaub and the review committee?”

A good question, but not one to concern Alex at the moment. He had too much else on his mind. Sixty-five years out, with no hint of coming war; but transportation cargo volume and inter-world passenger traffic were down. So were terra-forming activities, Oort Cloud exploration, free-space research stations, and what Alex thought of as “inverse terraforming” — the genetic modification of Earth’s plants and animals to match the geography and physiography of other worlds.

And now, in 2140, the population curve was totally flat and he thought the fitted curve showed the first hint of a downward slope. Why, when the solar system was peaceful and stable?

“Mineral deficiencies?” He hit the sequence to provide figures on population breakdown. “Maybe reduced fertility?”

“You think?—” Kate was crowding him, almost sitting in his seat.

But Alex had an answer before she could complete a question. The available minerals and trace elements needed for human existence were on the increase. The fertility indices were fine, general health was better than ever, longevity was increasing — and still the figures for total System population were declining. As the model moved forward, ticking ahead another five years, the gentle decrease was turning into a nosedive.

“What’s happening, Alex? What’s causing it?”

“I don’t know.” He wanted to say, this is impossible, it can’t be happening. Either you have a steady human expansion, or you have a war. Humans just don’t die out, with no reason. That had never happened before in any of the models he had seen — his own, or other people’s.

2152: 7.1 billion; 2153: 6.4 billion; 2154: 5.7 billion; 2155: 5.0 billion; 2156: 4.3 billion…

The population prediction wasn’t just decreasing, it was plummeting. Alex waited and watched, but in his mind he had already extrapolated the curve. They were losing seven hundred million people a year. Unless the curve flattened, in a little more than six years the projected human population of the solar system would be zero.

A plague, a major universal plague which left no survivors? That was the only thing he could think of. Such a disaster could certainly occur, as one of the surprises which any real future might contain. But from the point of view of the model, the plague would have to be fed in as a new exogenous variable. Neither he nor Kate had introduced any such event.

“Alex…” Kate said.

She didn’t need to say more. The year index read 2160. The population count was 1.5 billion. As they watched, the year advanced to 2161 and the count fell below a billion. 2162, 2163, 2164… The count slowed, steadied, hovered around the one hundred million mark. And then — 2165, 2166, 2167, 2168 — the number began a final and implacable downward run.

By 2170 it was over. In that year, and in every year beyond, the human population of the solar system was a steady, flat zero.

They stared at the display in silence. Finally, Kate said, “Well, it is only a model.”

Normally those would have been fighting words. To Alex’s mind, provided that you fed the model reasonable inputs and possessed enough computer power, the results you got back were a possible future. And more than possible; plausible. Not the only conceivable future, certainly, because of surprise factors that no model could include. A future, however, that was far more than an assembly of random predictions.

Now a hundred different runs, with a hundred different sets of initial conditions, pointed to the same melancholy conclusion: No humans by the year 2170. Alex was reluctant to believe the results, but he could see no basis for rejecting them.

Population zero; and two years before that, all transportation, development, and outward expansion had ended. He was still staring at the flat-lined results when Kate made the day’s discomfort complete.

“Come on,” she said. “We’re running out of time. In half an hour we have to brief Mischa Glaub. I hope you have something sensible to say. Because I sure as hell don’t.”

Kate had warned of a tough review session. Alex decided in the first thirty seconds that the real thing was going to be worse than her fears.

It began as soon as Kate led Alex into a small conference room flanked by modern display panels and old-fashioned pictures. The latter were of stern-faced individuals, presumably past members of review committees.

Seated at the table were four people — Kate had promised at most three — no less stem-faced than the images on the wall. Alex had met Mischa Glaub before. He was a short broad man with a shaved head, a sour expression, and a permanently angry disposition. Old hands in the Department of Planning made it a point to avoid meetings with Mischa in the morning. Food, it was said, softened his ire. Unfortunately this session was starting an hour before lunch, and it would run until the committee members were satisfied.

Alex had also met, at least briefly, two of the others. They were Glaub’s boss, Tomas de Mises, and Ole Pedersen, head of the Methods and Logistics Directorate which sat at the same organizational level as Mischa Glaub’s empire. It was no surprise to find them here, though Pedersen’s presence might be a problem. Kate had warned Alex that Ole Pedersen was wily and ambitious, always promoting his own group’s products and critical of anyone else’s. Tomas de Mises was less of a worry. He was older, close to retirement, and reluctant to say or do anything likely to cause trouble.