Volcanoes Cool and Warm, without Doubletalk

Right after World War II, when industry powered up in peacetime and started cranking out consumer goods, emissions increased rapidly for both CO₂ and particles from smokestacks. If emissions are suddenly ramped up like that, and then held constant, the number of sun-blocking particles in the air increases for a week or so and then stabilizes, because particles are falling out of the air as rapidly as they are added. But, for a given rate of emission, the CO₂ concentration of the air will rise for a few hundred thousand years, until the rate of rock weathering balances the new, raised rate of emission. (Human emissions did not remain constant, but this may help you think about things.) For industry after the war, the particles emitted in the first week had a cooling effect that was much larger than the warming effect of the CO₂ emitted during that week. But, as years became decades, the particles fell down, much of the CO₂ stayed up, and the warming grew to outweigh the cooling.

Volcanoes emit CO2, but they also put up many sun-blocking aerosols, including ash such as seen in this USGS photo of the 1980 eruption of Mt. St. Helens.

Source: USGS Cascades Volcano Observatory

Volcanic eruptions have essentially the same story. Over short times, the sun-blocking cooling from particles exceeds the warming of the CO₂. The volcanic particles typically get thrown into the stratosphere, above most rainfall, and so stay up a year or two rather than a week or so, but then fall out. So if extra volcanism continues long enough, the particles fall down, the CO₂ builds up, and warming results. Exactly how long you have to wait for “short time” to become “long time” depends on the types of volcanoes and many other issues. In general, an increase in volcanic activity (typically involving many volcanoes, or huge volcanic provinces) will cause cooling over times of years to centuries that most economists worry about, but with warming over longer, geologic time.