Volcanic winter

A volcanic winter is a reduction in global temperatures caused by droplets of sulfuric acid obscuring the Sun and raising Earth's albedo (increasing the reflection of solar radiation) after a large, sulfur-rich, particularly explosive volcanic eruption. Climate effects are primarily dependent upon the amount of injection of SO2 and H2S into the stratosphere where they react with OH and H2O to form H2SO4 on a timescale of a week, and the resulting H2SO4 aerosols produce the dominant radiative effect. Volcanic stratospheric aerosols cool the surface by reflecting solar radiation and warm the stratosphere by absorbing terrestrial radiation for several years. Moreover, the cooling trend can be further extended by atmosphere–ice–ocean feedback mechanisms. These feedbacks can continue to maintain the cool climate long after the volcanic aerosols have dissipated.

Physical process An explosive volcanic eruption releases magma materials in the form of volcanic ash and gases into the atmosphere. While most volcanic ash settles to the ground within a few weeks after the eruption, impacting only the local area for a short duration, the emitted SO2 can lead to the formation of H2SO4 aerosols in the stratosphere. These aerosols can circle the hemisphere of the eruption source in a matter of weeks and persist with an e-folding decay time of about a year. As a result, they have a radiative impact that can last for several years. The subsequent dispersal of a volcanic cloud in the stratosphere and its impact on climate are strongly influenced by several factors, including the season of the eruption, the latitude of the source volcano, and the injection height. If the SO2 injection height remains confined to the troposphere, the resulting H2SO4 aerosols have a residence time of only a few days due to efficient removal through precipitation. The lifetime of H2SO4 aerosols resulting from extratropical eruptions is shorter compared to those from tropical eruptions, due to a longer transport path from the tropics to removal across the mid- or high-latitude tropopause, but extratropical eruptions strengthens the hemispheric climate impact by confining the aerosol to a single hemisphere. Injections in the winter are also much less radiatively efficient than injections during the summer for high-latitude volcanic eruptions, when the removal of stratospheric aerosols in polar regions is enhanced. The sulfate aerosol interacts strongly with solar radiation through scattering, giving rise to remarkable atmospheric optical phenomena in the stratosphere. These phenomena include solar dimming, coronae or Bishop's rings, peculiar twilight coloration, and dark total lunar eclipses. Historical records that documented these atmospheric events are indications of volcanic winters and date back to periods preceding the Common Era. Surface temperature observations following historic eruptions show that there is no correlation between eruption size, as represented by the VEI or eruption volume, and the severity of the climate cooling. This is because eruption size does not correlate with the amount of SO2 emitted.

Long-term positive feedback It has been proposed that the cooling effects of volcanic eruptions can extend beyond the initial several years, lasting for decades to possibly even millennia. This prolonged impact is hypothesized to be a result of positive feedback mechanisms involving ice and ocean dynamics, even after the H2SO4 aerosols have dissipated. During the first few years following a volcanic eruption, the presence of H2SO4 aerosols can induce a significant cooling effect. This cooling can lead to a widespread lowering of snowline, enabling the rapid expansion of sea ice, ice caps and continental glacier. As a result, ocean temperatures decrease, and surface albedo increases, further reinforcing the expansion of sea ice, ice caps, and glacier. These processes create a strong positive feedback loop, allowing the cooling trend to persist over centennial-scale or even longer periods of time. It has been proposed that a cluster of closely spaced, large volcanic eruptions triggered or amplified the Little Ice Age, Late Antique Little Ice Age, stadials, Younger Dryas, Heinrich events, and Dansgaard-Oeschger events through the atmosphere-ice-ocean positive feedbacks.

Weathering effects

The weathering of a sufficiently large volume of rapidly erupted volcanic materials has been proposed as an important factor in Earth's silicate weathering cycle, which operates on a timescale of tens of millions of years. During this process, weathered silicate minerals react with carbon dioxide and water, resulting in the formation of magnesium carbonate and calcium carbonate. These carbonates represent a removal of carbon dioxide from the atmosphere and its sequestration on the ocean floor. The eruption of a large volume of volcanic materials can enhance weathering processes, thereby lowering atmospheric CO2 levels and contributing to global temperature reduction. The rapid emplacement of mafic large igneous provinces has the potential to cause a swift decline in atmospheric CO2 content, leading to a multi-million-year-long icehouse climate. A notable example is the Sturtian glaciation, which is considered the most severe and widespread known glacial event in Earth's history. This glaciation is believed to have been caused by the weathering of erupted Franklin Large Igneous Province.

Past volcanic coolings Tree-ring-based temperature reconstructions, historical records of dust veils, and ice cores studies have confirmed that some of the coldest years during the last five millennia were directly caused by massive volcanic injections of SO2.

Hemispheric temperature anomalies resulting from volcanic eruptions have primarily been reconstructed based on tree-ring data for the past two millennia. For earlier periods in the Holocene, the identification of frost rings that coincide with large ice core sulfate spikes serves as an indicator of severe volcanic winters. The quantification of volcanic coolings further back in time during the Last Glacial Period is made possible by annually resolved δ18O records. This is a non-exhaustive compilation of notable and consequential coolings that have been definitively attributed to volcanic aerosols, although the source volcanos of the aerosols are rarely identified.

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