Volcanic eruptions can affect global warming—or cooling. And one of the ways we measure their impact is by looking at a lunar eclipse.

To understand this strange connection, we need to know a bit about Earth’s atmosphere.

The lowest part, the troposphere, extends from the surface up 4 to 12 miles. It’s where Earth’s weather happens.

The stratosphere, above that, is dry and less active.

Dust and ash particles carried into the troposphere by volcanoes, fires, or other sources will quickly fall or be rained out.

But when a large volcano erupts, it blasts ash and gas through the troposphere into the stratosphere.

There, sulfur gas forms microscopic droplets of sulfuric acid. These “sulfate aerosols” can remain suspended for years.

One way scientists measure them is by looking at lunar eclipses. If they’re murky and dim, the stratosphere is clouded with aerosols. If they’re bright and defined, the stratosphere is clear.

Large volcanoes in the past have put up enough aerosols to reflect the sun’s heat and cool the surface of Earth by as much as 1°F.

But in the last two decades, the stratosphere has been remarkably clear, which has allowed more solar energy to reach Earth.

Some scientists estimate this has caused half the global temperature increase seen during this period.

Another example of how Earth’s processes are linked in surprising and complex ways.