And it was the amount of energy a single human could produce that dictated military potential, standard of living, happiness, and all besides. - Isaac Asimov

The City was the acme of efficiency, but it made demands of its inhabitants. It asked them to live in a tight routine and order their lives under a strict and scientific control.

It had been a pretty good world, he thought. Not perfect, far from it, but good enough. Most people had been reasonably happy, most were prosperous, there was progress being made on all fronts — toward deeper scientific understanding, toward greater economic expansion, toward stronger global cooperation. The concept of war had come to seem quaintly medieval and the age-old religious bigotries were mostly obsolete, or so it had seemed to him. And now it was all gone, in one short span of hours, in a single burst of horrifying Darkness.

What, then, of human activities? Is humankind itself hastening its own end? Man has, for instance, been burning carbon-containing fuel — wood, coal, oil, gas — at a steadily accelerating rate. All these fuels form carbon dioxide. Some is absorbed by plants and the oceans but not as fast as it is produced. This means the carbon dioxide content of the air is going up — slightly but nevertheless up. Carbon dioxide retains heat, and even a small rise means a warming of the Earth's atmosphere. This may result in the melting of the polar ice caps with unusual speed, flooding the world before we have learned climate control. In reverse, our industrial civilization is making our atmosphere dustier so that it reflects more sunlight away and cools the Earth slightly — thus making possible a glacial advance in a few centuries, also before we have learned climate control.