Would volcanic ash increase or decrease temperature in the earth's atmosphere?

Answer 1

Temperatures tend to fall as atmospheric particulate matter increases. This has been known from massive volcanic explosions, and a small handful of scientists some years back proposed a global thermonuclear war could potentially plunge earth into a "nuclear winter," or new ice age. We now know particulate settles out relatively quickly, so that the nuclear winter would only last 2-5 years.

Both the particulate and the hydrogen sulfide gas would have an immediate cooling effect. Volcanoes also emit CO2, which lingers in the atmosphere thousands of times longer than the particulate. So although the initial effect would be cooling, the long term effect would be a slight warming. A LOT of volcanoes could build up CO2 levels significantly over thousands of years.

At the present time humans release about 100 times the CO2 of all earth's volcanoes combined, according to the USGS.

Answer 2

Volcanic ash can have different effects on temperature depending on its altitude in the atmosphere. Lower altitude volcanic ash tends to block incoming sunlight and can decrease temperatures below it, while higher altitude ash reflects sunlight back into space, potentially leading to lower global temperatures.