The lower stratosphere, extending roughly from 10 to 50 kilometers above the Earth’s surface, experienced two distinct periods of significant warming in the early 1980s and early 1990s. Satellite and ground-based observations recorded large-scale atmospheric anomalies during 1982–1983 and again during 1991–1992. These temporary thermal spikes were not random fluctuations but the direct consequence of massive natural events injecting material high into the atmosphere.
How Volcanic Eruptions Warm the Lower Stratosphere
Large, explosive volcanic eruptions inject substantial amounts of gas directly into the stratosphere, bypassing lower tropospheric weather systems. The primary component responsible for atmospheric heating is sulfur dioxide (SO2), which is more influential than ash and dust particles. Once in the stratosphere, sulfur dioxide reacts with water vapor to form tiny, highly concentrated droplets of sulfuric acid, known as sulfate aerosols.
These aerosols are extremely small, allowing them to remain suspended in the stable stratospheric layer for several years, unlike tropospheric aerosols which are quickly washed out by precipitation. The sulfate aerosol layer alters the planet’s radiation budget in two ways. First, the aerosols are highly reflective and scatter incoming solar radiation (shortwave energy) back into space, reducing the amount that reaches the surface.
Second, and crucial for stratospheric warming, these particles efficiently absorb both incoming solar radiation and outgoing terrestrial infrared radiation (longwave heat) rising from the Earth’s surface. This dual-absorption mechanism transfers energy directly into the surrounding air molecules. This localized energy transfer causes the air temperature within the aerosol layer—the lower stratosphere—to rise significantly.
The 1982/1983 El Chichón Event
The first of these warming periods followed the eruption of El Chichón, a volcano located in Mexico, which experienced a series of powerful explosions in late March and early April 1982. While the overall volume of erupted material was relatively small, the volcano released an unusually high concentration of sulfur-rich gases. The eruption injected an estimated seven million metric tons (Mt) of sulfur dioxide into the lower stratosphere.
The resulting aerosol cloud was initially confined to the tropical and northern subtropical zone, spreading rapidly around the globe between the equator and 30°N latitude. This focused layer of sulfate aerosols caused a pronounced atmospheric heating effect. Measurements showed that the temperature in the lower stratosphere rose by as much as 4°C in the months following the eruption, one of the largest increases recorded since systematic observations began in the 1950s.
The El Chichón aerosol layer persisted for over two years, gradually dispersing and settling out of the stratosphere. The resulting thermal anomaly demonstrated the powerful climate-forcing potential of volcanic sulfur gases. This event provided scientists with an early case study for monitoring the atmospheric effects of a major tropical eruption using modern instruments.
The 1991/1992 Mount Pinatubo Event
The second, and more substantial, warming event was triggered by the colossal eruption of Mount Pinatubo in the Philippines in June 1991. This was the second-largest volcanic eruption of the 20th century and created the most significant perturbation to the stratospheric aerosol layer since the 1883 Krakatau eruption. Pinatubo injected an enormous plume of gas and ash up to 40 kilometers high, releasing roughly 17 to 20 million metric tons of sulfur dioxide into the stratosphere.
This quantity of sulfur dioxide was approximately two to three times greater than the amount released by El Chichón. The massive sulfur injection led to the formation of an exceptionally dense and widespread layer of sulfate aerosols that quickly spread across the globe. The resulting aerosol cloud created a climate forcing nearly twice as strong as that produced by El Chichón.
The enhanced absorption of radiation by this dense layer resulted in stratospheric warming that was slightly higher and more prolonged than the 1982 event. The lower stratosphere warmed by 2°C to 3°C within four to five months of the eruption. This strong and sustained heating significantly altered stratospheric circulation patterns and remained observable for several years. The magnitude of the Pinatubo event established it as the definitive modern benchmark for understanding the climatic influence of tropical volcanic eruptions.
Global Atmospheric Consequences of Stratospheric Aerosols
The widespread stratospheric aerosol layer produced by both eruptions had consequences extending far beyond the local heating of the lower stratosphere. A primary effect was temporary global surface cooling that followed each event. The highly reflective sulfate aerosols scattered incoming solar radiation back into space before it could reach the troposphere and the Earth’s surface.
This reduction in solar energy reaching the ground resulted in a measurable drop in global average surface temperatures. Following the Pinatubo eruption, the Earth experienced a temporary global cooling of approximately 0.4°C to 0.6°C for nearly two years. This cooling effect demonstrated the ability of large volcanic eruptions to temporarily counteract other warming trends.
Another significant consequence was the enhancement of stratospheric ozone depletion, particularly over polar regions. The surfaces of the sulfate aerosol particles provided sites for heterogeneous chemical reactions. These reactions activate chlorine-containing compounds released from chlorofluorocarbons (CFCs), enabling them to rapidly destroy ozone molecules. The Pinatubo aerosol layer contributed to mid-latitude ozone concentrations reaching their lowest recorded levels and caused the Antarctic ozone hole to increase to its largest size up to that time in 1992.