"Heat?"
"Heat is produced as a waste product of civilization."
"I fail to understand," said Speaker-To-Animals.
Louis, who as a flatlander understood perfectly, forebore to comment. (Earth was far more
crowded than Kzin.)
"An example. You would wish a light source at night, would you not, Speaker? Without a light
source you must sleep, whether or not you have better things to do."
"This is elementary."
"Assume that your light source is perfect, that is, it gives off radiation only in the spectra
visible to kzinti. Nonetheless, all light which does not escape through the window will be
absorbed by walls and furniture. It will become randomized heat.
"Another example. Earth produces too little natural fresh water for its eighteen billions. Salt
water must be distilled through fusion. This produces heat. But our world, so much more
crowded, would die in a day without the distilling plants. "A third example. Transportation involving changes in velocity always produces heat.
Spacecraft filled with grain from the agricultural worlds produce heat on reentry and distribute
it through our atmosphere. They produce more heat on takeoff."
"But cooling systems --"
"Most kinds of cooling systems only pump heat around, and produce more heat for power."
"U-u-urr. I begin to understand. The more puppeteers, the more heat is produced."
"Do you understand, then, that the heat of our civilization was making our world
uninhabitable?"
-- From Larry Niven, ringworld, 1970