Which natural event would most likely contribute to cooling the Earth's atmosphere?

The natural event that most likely contributes to cooling the Earth's atmosphere is volcanic eruptions. When a volcano erupts, it can release a large amount of ash and sulfur dioxide into the atmosphere. These particles can reflect sunlight back into space, leading to a temporary, global cooling effect known as "volcanic winter." This phenomenon can significantly lower the Earth's surface temperature for an extended period following a substantial eruption.

In contrast, forest fires are primarily associated with the release of carbon dioxide and particulates, which can contribute to warming rather than cooling. Earthquakes, while they can lead to surface changes and potentially trigger other events like tsunamis, do not directly cause significant atmospheric effects related to temperature regulation. Hurricanes are associated with the release of heat through the warm waters they interact with, leading to localized warming instead of cooling the atmosphere.

Thus, volcanic eruptions are well-documented as a significant natural contributor to short-term cooling of the earth's atmosphere, achieving this through the mechanism of particulate and gas emissions that alter solar radiation dynamics.