A volcanic winter is a drop in Earth's global temperatures that happens when sulfuric acid droplets block sunlight and increase Earth's albedo, which means more sunlight is reflected back into space. This occurs after a large, sulfur-rich volcanic eruption that releases a lot of sulfur dioxide (SO₂) and hydrogen sulfide (H₂S) into the stratosphere. These gases react with oxygen and water in the stratosphere to form sulfuric acid (H₂SO₄) within a week. The sulfuric acid forms tiny particles called aerosols, which have a major effect on Earth's climate. These aerosols cool the Earth's surface by reflecting sunlight, but they also warm the stratosphere by absorbing heat from Earth's surface. This cooling effect can last for several years. Additionally, natural processes involving the atmosphere, ice, and oceans can help keep the Earth cooler for a long time, even after the sulfuric acid aerosols have disappeared.
Physical process
An explosive volcanic eruption sends magma materials, such as volcanic ash and gases, into the atmosphere. Most volcanic ash settles to the ground within a few weeks, affecting only the local area for a short time. However, sulfur dioxide (SO₂) gas released during eruptions can form sulfuric acid (H₂SO₄) aerosols in the stratosphere. These aerosols can spread across the hemisphere of the eruption source in weeks and remain in the atmosphere for about a year. Their presence affects how much sunlight reaches Earth, causing climate changes that may last several years.
The movement of volcanic clouds in the stratosphere and their effect on climate depend on factors like the season of the eruption, the location of the volcano, and how high the SO₂ is injected. If SO₂ remains in the troposphere, the resulting H₂SO₄ aerosols are removed quickly by rain and last only a few days. H₂SO₄ aerosols from eruptions in higher latitudes (outside the tropics) have shorter lifetimes than those from tropical eruptions because they travel farther to be removed. However, extratropical eruptions may have stronger climate effects in one hemisphere because the aerosols stay confined there. Winter eruptions at high latitudes are less effective at cooling the climate than summer eruptions because aerosols are removed more quickly in polar regions.
Sulfate aerosols in the stratosphere scatter sunlight, creating visible atmospheric effects such as dimming of the sun, colorful light patterns (like coronae or Bishop’s rings), unusual twilight colors, and dark total lunar eclipses. Historical records of these events provide evidence of volcanic winters and date back to times before the Common Era.
After major eruptions, observations of surface temperatures show no direct link between the size of an eruption (measured by the Volcanic Explosivity Index or eruption volume) and the strength of climate cooling. This is because eruption size does not determine how much SO₂ is released.
Some scientists suggest that the cooling effects of volcanic eruptions can last longer than a few years, possibly for decades or even thousands of years. This extended impact may result from feedback loops involving ice and ocean systems, even after H₂SO₄ aerosols have disappeared.
In the first few years after an eruption, H₂SO₄ aerosols can cause significant global cooling. This cooling lowers 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 continues, creating a self-reinforcing cycle that can extend cooling for centuries or longer.
Some researchers propose that groups of large, closely spaced eruptions may have contributed to or worsened past climate events, 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 rapid release of large amounts of volcanic material can influence Earth’s long-term silicate weathering cycle, which operates over 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 deposited on the ocean floor, removing carbon dioxide from the atmosphere. Large eruptions can speed up weathering, reducing atmospheric CO₂ levels and lowering global temperatures.
The quick formation of large igneous provinces composed of mafic rock can rapidly decrease atmospheric CO₂ levels, leading to long-lasting icehouse climates that may last millions of years. An example is the Sturtian glaciation, the most extreme and widespread glacial event in Earth’s history. This event is thought to have been caused by the weathering of volcanic materials from the Franklin Large Igneous Province.
Past volcanic coolings
Tree-ring studies, historical records of dust in the air, and ice core research have shown that some of the coldest years in the past 5,000 years were caused by large volcanic eruptions that released sulfur dioxide into the atmosphere.
For the past 2,000 years, scientists have mainly used tree-ring data to study how volcanic eruptions affected global temperatures. Earlier in the Holocene period, scientists looked for frost rings in trees that matched high levels of sulfur found in ice cores, which helped identify times when volcanic eruptions caused very cold winters. For even earlier times during the Last Glacial Period, scientists used oxygen isotope records from ice cores to measure how much volcanic eruptions cooled the climate. This list includes major cooling events caused by volcanic activity, but the exact volcanoes responsible are often unknown.
During the Last Glacial Period, volcanic eruptions that cooled the climate as much as the largest eruptions in the Common Era (such as Tambora and Samalas) were identified by looking at oxygen isotope changes. In particular, eruptions between 12,000 and 32,000 years ago caused oxygen isotope cooling that was greater than the cooling from the largest eruptions in the Common Era. One major eruption from this time, the Youngest Toba Tuff (YTT) about 74,000 years ago, has sparked many scientific debates about its effects on the climate.
The YTT eruption from the Toba Caldera was the largest known volcanic eruption in the Quaternary period and released about 100 times more magma than the Tambora eruption. Scientists have studied polar ice cores from around 74,000 years ago and found four events with high levels of sulfate that might be linked to the YTT eruption. These events released between 219 and 535 million tons of sulfate into the atmosphere, which is 1 to 3 times more than the sulfate from the Samalas eruption in 1257 CE. Climate models suggest that this amount of sulfate could have caused global temperatures to drop by 2.3 to 4.1 degrees Celsius, and it would have taken more than 10 years for temperatures to return to normal.
However, evidence about how much the YTT eruption cooled the climate is mixed. The YTT eruption occurred around the same time as Greenland Stadial 20 (GS-20), a 1,500-year cooling period that was the coldest and most extreme in the past 100,000 years. Some scientists think the YTT eruption might have made GS-20 even colder, but others believe the cooling started before the eruption. In the South China Sea, temperatures dropped by 1 degree Celsius over 1,000 years after the YTT eruption, but the Arabian Sea showed no clear changes. In India and the Bay of Bengal, cooling and dry conditions began after the YTT eruption, but these changes might have started before the eruption. Sediment layers in Lake Malawi do not show evidence of a volcanic winter right after the YTT eruption, but the sediment layers are unclear due to mixing. Above the YTT layer in Lake Malawi, there is evidence of a 2,000-year-long drought and cooling period. Greenland ice cores show a 110-year period of rapid cooling that likely followed the YTT eruption.
The weathering of large volcanic rock formations called continental flood basalts, which erupted just before the Sturtian glaciation 717 million years ago, is believed to have caused the most severe ice age in Earth's history. During this time, Earth's surface temperatures dropped below freezing everywhere, and ice spread from the tropics to the equator, covering the planet. This ice age lasted nearly 60 million years, from 717 to 659 million years ago.
Geological dating shows that the Franklin large igneous province, covering 5 million square kilometers, formed just 1 million years before the Sturtian glaciation. Other large igneous provinces, each covering about 1 million square kilometers, also formed on the Rodinia supercontinent between 850 and 720 million years ago. Weathering of these large amounts of fresh volcanic rock led to a rapid drop in atmospheric carbon dioxide levels by about 1,320 parts per million and a global temperature drop of 8 degrees Celsius, causing 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 levels where evolutionary changes, which happen more quickly in smaller groups, lead to fast differences between populations. 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 as few as that.