Critical Difference by Murray Leinster — Read

During the last night of preparation, Massy sat by a thermometer registering the outside temperature. He hovered over it as one might over a sick child. He watched it and sweated, though the inside temperature of the drone-hull was lowered to save power. There was nothing he could actually do. At midnight the thermometer said it was seventy degrees below zero Fahrenheit. At halfway to dawn it was eighty degrees below zero Fahrenheit. The hour before dawn it was eighty-five degrees below zero. Then he sweated profusely. The meaning of the slowed descent was that carbon dioxide was being frozen out of the upper layers of the atmosphere. The frozen particles were drifting slowly downward, and as they reached lower and faintly warmer levels they returned to the state of gas. But there was a level, above the CO_{2}, where the temperature was plummeting.

The height to which carbon dioxide existed was dropping -- slowly, but inexorably. And above the carbon-dioxide level there was no bottom limit to the temperature. The greenhouse effect was due to CO_{2}. Where it wasn't, the cold of space moved down. If at ground-level the thermometer read ever so slightly lower than one hundred and nine below zero -- why -- everything was finished. Without the greenhouse effect, the night-side of the planet would lose its remaining heat with a rush. Even the day-side, once cold enough, would lose heat to emptiness as fast as it came from the sun. Minus one hundred and nine point three was the critical reading. If it went down to that, it would plunge to a hundred and fifty -- two hundred degrees below zero! And it would never come up again.