As we know, human greenhouse gas (GHG) emissions warm the planet by increasing the abundance of greenhouse gases in the atmosphere, thus increasing the greenhouse effect.
Solar activity also warms or cools the planet by increasing or decreasing the amount of radiation reaching the Earth’s atmosphere and surface.
Volcanic activity generally cools the planet over short timeframes by releasing sulfate aerosols into the atmosphere, which block sunlight and reduce the amout of solar radiation reaching the surface. However, unlike many greenhouse gases, aerosols are washed out of the atmosphere quickly, mostly after just 1-2 years. Thus the main volcanic impact on long-term temperature changes occur when there is an extended period of particularly high or low volcanic activity.