A volcanic winter happens when global temperatures drop because tiny sulfuric acid droplets block sunlight and increase Earth's ability to reflect sunlight, called albedo, after a large, sulfur-rich volcanic eruption. The climate effects depend on how much sulfur dioxide (SO₂) and hydrogen sulfide (H₂S) are sent into the stratosphere, where these gases react with other molecules to form sulfuric acid (H₂SO₄) within a week. These sulfuric acid particles, called aerosols, reflect sunlight, cooling Earth's surface, while also trapping heat in the stratosphere for several years. Cooling can last even longer because of feedbacks between the atmosphere, ice, and oceans, which help keep the climate cool for a time after the volcanic particles have disappeared.
Physical process
A volcanic eruption releases molten rock materials, such as ash and gases, into the atmosphere. Most volcanic ash falls to the ground within weeks, affecting only nearby areas for a short time. However, sulfur dioxide (SO2) gas can rise into the stratosphere, where it forms sulfuric acid (H2SO4) aerosols. These aerosols can travel around the hemisphere where the eruption occurred in weeks and remain in the atmosphere for about a year. Their presence in the atmosphere can influence Earth’s temperature and climate for several years.
The way volcanic clouds spread in the stratosphere and their effect on climate depend on factors like the time of year the eruption happens, the location of the volcano, and how high the SO2 is injected into the atmosphere. If SO2 stays in the troposphere (the lowest layer of the atmosphere), the resulting H2SO4 aerosols are removed quickly by rain and last only a few days. H2SO4 aerosols from eruptions at higher latitudes (outside the tropics) have shorter lifetimes than those from tropical eruptions because they travel farther to be removed. However, eruptions at higher latitudes can have a stronger effect on climate in one hemisphere by keeping the aerosols confined there. Injections of SO2 during winter are less effective at cooling the climate compared to summer injections, as stratospheric aerosols are removed faster in polar regions during winter.
Sulfate aerosols in the stratosphere interact with sunlight by scattering it, creating visible atmospheric effects such as dimming of sunlight, colorful rings around the sun (called coronae or Bishop’s rings), unusual colors during twilight, and dark total lunar eclipses. Historical records of these effects provide evidence of past volcanic winters and date back to times before the Common Era.
Measurements of Earth’s surface temperature after major eruptions show that the size of an eruption, as measured by the Volcanic Explosivity Index (VEI) or the volume of erupted material, does not determine how much the climate cools. This is because larger eruptions do not always release more sulfur dioxide.
Scientists suggest that the cooling effects of volcanic eruptions may last longer than a few years, possibly for decades or even thousands of years. This long-term cooling is thought to result from feedback processes involving ice and ocean systems, even after the H2SO4 aerosols have disappeared.
In the first few years after an eruption, H2SO4 aerosols can cause significant global cooling. This cooling can lower the height of snowlines, allowing sea ice, ice caps, and glaciers to expand rapidly. As ocean temperatures drop and Earth’s surface reflects more sunlight (increased albedo), the expansion of ice and glaciers is reinforced, creating a cycle that maintains the cooling effect for centuries or longer.
Scientists propose that a series of large, closely spaced eruptions may have contributed to or worsened past cold periods, such as the Little Ice Age, the Late Antique Little Ice Age, stadials, the Younger Dryas, Heinrich events, and Dansgaard-Oeschger events. These events are linked to feedbacks between the atmosphere, ice, and oceans.
The weathering of a large amount of volcanic material released quickly is thought to play a role in Earth’s long-term silicate weathering process, which takes tens of millions of years. During this process, weathered silicate minerals react with carbon dioxide and water to form magnesium carbonate and calcium carbonate. These carbonates are removed from the atmosphere and stored on the ocean floor. Large volcanic eruptions can speed up this weathering process, reducing atmospheric carbon dioxide levels and helping to cool the planet.
The rapid formation of large igneous provinces composed of mafic rock can cause a sharp drop in atmospheric carbon dioxide, leading to extended icehouse climates lasting millions of years. An example is the Sturtian glaciation, the most severe and widespread ice age in Earth’s history. This event is believed to have been caused by the weathering of volcanic materials from the Franklin Large Igneous Province.
Past volcanic coolings
Studies using tree rings, historical records of dust veils, and ice core research have shown that some of the coldest years in the last 5,000 years were caused by large volcanic eruptions that released sulfur dioxide. For the past 2,000 years, hemispheric temperature changes from volcanic eruptions have been studied mainly using tree-ring data. Earlier in the Holocene, frost rings in trees that match high sulfate levels in ice cores indicate severe volcanic winters. For even older periods during the Last Glacial Period, oxygen isotope records that track annual changes help scientists measure volcanic cooling. This list is not complete, but it includes some important examples of cooling events clearly linked to volcanic aerosols, though the specific volcanoes responsible are often unknown.
During the Last Glacial Period, volcanic cooling events similar in size to the largest eruptions in the Common Era (such as Tambora and Samalas) are inferred based on oxygen isotope data. Between 12,000 and 32,000 years ago, the strongest cooling from eruptions was greater than the cooling caused by the largest eruptions in the Common Era. One eruption from this time, the Youngest Toba Tuff (YTT) 74,000 years ago, has drawn significant scientific attention due to debates about its climate effects.
The YTT eruption from the Toba Caldera is the largest known eruption in the Quaternary period and released 100 times more magma than the historical Tambora eruption. Evidence from polar ice cores around 74,000 years ago shows four possible atmospheric events that may be linked to YTT. Calculations suggest these events released 219 to 535 million tonnes of sulfate into the atmosphere, which is 1 to 3 times more than the Samalas eruption in 1257 CE. Computer models estimate that this amount of sulfate could have caused global temperatures to drop by 2.3 to 4.1 degrees Kelvin, with full recovery taking more than 10 years.
However, evidence about the cooling effects of YTT is not clear-cut. YTT occurred during Greenland Stadial 20 (GS-20), a 1,500-year cooling period considered the coldest and most extreme in the last 100,000 years. Some scientists suggest YTT may have worsened the severity of GS-20, but others argue the cooling was already happening before the eruption. In the South China Sea, temperatures dropped by 1 degree Kelvin over 1,000 years after YTT, but the Arabian Sea shows no clear impact. In India and the Bay of Bengal, cooling and dry conditions were observed above the YTT ash layer, though these changes may have started before the eruption. Sediments in Lake Malawi do not show evidence of a volcanic winter immediately after YTT, but the resolution of the data is uncertain due to mixing. Directly above the YTT layer, Lake Malawi sediments show a 2,000-year-long drought and cooling period. Greenland ice cores also show a 110-year period of rapid cooling following the YTT eruption.
The weathering of continental flood basalts, which erupted just before the Sturtian glaciation 717 million years ago, is believed to have triggered Earth's most severe glaciation. During this time, global temperatures dropped below freezing, and ice spread from low latitudes to the equator, covering nearly the entire planet. This glaciation lasted about 60 million years, from 717 to 659 million years ago.
Geochronology shows that the Franklin large igneous province, covering 5 million square kilometers, erupted just 1 million years before the Sturtian glaciation began. Other large igneous provinces on Rodinia, covering up to 1 million square kilometers, also erupted between 850 and 720 million years ago. Weathering of large amounts of fresh mafic rock from these eruptions likely caused runaway cooling and ice-albedo feedback. Chemical evidence shows massive amounts of weathered materials entered the ocean during these eruptions. Simulations suggest this process reduced atmospheric carbon dioxide by 1,320 ppm and cooled global temperatures by 8 degrees Kelvin, creating the most extreme climate change event in Earth's history.
Effects on life
Population bottlenecks — sudden drops in the number of individuals in a species — have been linked to volcanic winters by some scientists. These events can reduce populations to very low numbers, allowing evolutionary changes to happen more quickly in smaller groups. During the Lake Toba bottleneck, many species experienced major genetic changes due to the loss of genetic diversity. The Toba event may have decreased the human population to between 15,000 and 40,000 individuals, or possibly even fewer.