A
volcanic winter is a reduction in global temperatures caused by
volcanic ash and droplets of
sulfuric acid obscuring the
Sun and raising Earth's
albedo (increasing the reflection of solar radiation) after a large particularly explosive
volcanic eruption. Long-term cooling effects are primarily dependent upon injection of
sulfide compounds in
aerosol form into the upper atmosphere—the
stratosphere—the highest, least active levels of the lower atmosphere where little precipitation occurs, thus requiring a long time to wash the aerosols out of the region. Stratospheric aerosols cool the surface and troposphere by reflecting solar radiation, warm the stratosphere by absorbing terrestrial radiation, and when combined with anthropogenic chlorine in the stratosphere, destroy ozone which moderates the effect of lower stratospheric warming. The variations in atmospheric warming and cooling results in changes in tropospheric and stratospheric circulation.