Volcanic winter

Large explosive eruptions can loft sulfur dioxide and fine particles into the stratosphere. Sulfur gases undergo chemical reactions that form sulfate aerosols. These small particles scatter incoming sunlight, reducing the solar energy reaching the lower atmosphere and surface, while also absorbing radiation and warming parts of the stratosphere. Unlike much heavier ash, which tends to settle relatively quickly near the volcano, stratospheric aerosols can spread over large areas and persist for months to a few years. Their influence changes as particles form, circulate and gradually leave the atmosphere.

The most familiar effect is temporary cooling of the surface and lower atmosphere, but the response varies by location and season. Aerosol heating in the stratosphere can affect winds and atmospheric circulation, sometimes producing regional winter warming even while the global troposphere cools. Climate models and observations also associate large eruptions with changes to rainfall and monsoon circulation. The outcome depends on how much sulfur reaches the stratosphere, the altitude and latitude of injection, particle evolution, and the climate system's internal variability. Therefore, an eruption does not produce the same temperature change everywhere.

Mount Tambora, on the Indonesian island of Sumbawa, underwent a major eruption in April 1815. The eruption injected material into the atmosphere and was followed by unusual climate conditions across parts of the Northern Hemisphere. The summer of 1816 became known as the Year Without a Summer in Europe and North America, where cold, frost and poor growing conditions damaged crops and contributed to food shortages. Historical accounts and later research connect Tambora's aerosol-driven climate impact to these conditions, while recognizing that local weather and societal effects varied. Scientific reviews combine instrumental records, climate proxies, historical evidence and simulations to assess the event; uncertainties remain in reconstructing aspects such as aerosol distribution.

“Volcanic winter” describes a cooling episode associated with volcanic forcing; it does not mean the planet enters a literal ice age. Major eruptions can temporarily perturb climate for a limited period, whereas the present long-term warming trend is driven primarily by human greenhouse-gas emissions. Studying eruptions helps scientists understand how sunlight, atmospheric chemistry and circulation interact, but a single eruption is not a direct analogue for ongoing anthropogenic climate change.