Termination Shock

Pinatubo was the name of a volcano in the Philippines that had exploded in 1991. It had blasted fifteen million tons of sulfur dioxide into the stratosphere. The result had been a couple of years’ beautiful sunsets and reduced global temperatures. The two phenomena were directly related. The sulfur from the volcano had eventually spread out into a veil of tiny droplets of H2SO4. Light from the sun hit those little spheres and bounced. Some of it bounced directly back into space—which accounted for the planet-wide cooling, as energy that never entered the troposphere in the first place couldn’t contribute to the greenhouse effect. Other light caromed off those droplets, billiard-ball style, and came into the troposphere at various oblique angles. Since that was where humans lived, those who lifted their gaze saw that light as a general brightness of the sky. This was hard to notice in the daytime but quite obvious when the sun was near the horizon, the sky was generally dark, and the light was red.