Max-Planck-Institute for Meteorology: The influence of volcanic aerosol on climate depends on temperature

Stratospheric sulfate aerosol, as produced by large volcanic eruptions, cools the Earth and has been discussed as a climate engineering method. However, the understanding of how this aerosol affects the planet's radiation budget has been incomplete until now. Using a simple model, researchers at the Max Planck Institute for Meteorology have broken down the key processes and shown that the cooling effect depends on the Earth’s surface temperature.

Photograph of the Earth shot from the International Space Station with visible volcanic ash cloud — The atmospheric plume after the eruption of the underwater volcano Tonga, pictured from the International Space Station. Credit: JSC/NASA

Large volcanic eruptions can inject sulfur dioxide high into the atmosphere. In the stratosphere, a layer that begins at an altitude of about 15 kilometers, this gas oxidizes to form sulfuric acid, which then condenses to form a sulfate aerosol. The aerosol reflects visible solar radiation and thus cools the Earth, as has been observed in the past after major volcanic eruptions.

Although this effect is understood in principle and is being considered as a potential radiation management technology on a warming Earth, uncertainties remain about some basic aspects. Sulfate aerosol does not only have a cooling effect, it also traps heat, similar to the greenhouse gas carbon dioxide.

Using a simplified model of the tropical atmosphere, researchers at the Max Planck Institute for Meteorology (MPI-M) have investigated how this greenhouse effect of the sulfate aerosol depends on climate and atmospheric conditions. The lead author of the study, Ravikiran Hegde, conducted the research while working in the Global Circulation and Climate group of Hauke Schmidt, as part of the Max Planck Society's cooperation program with the Indian Institutes for Science Education and Research (IISERs).

The sulfate aerosol forcing in detail

The team separated the aerosol's interaction with Earth's thermal (long-wave) radiation into three separate, but related components: first, the absorption and re-emission of thermal radiation; second, heating of the transition region between the lowest atmospheric layer – the troposphere – and the stratosphere by these processes; and third, the increase in water vapor concentration in the stratosphere due to this warming.

Four-panel figure showing radiative fluxes vs. surface temperature (Ts). (a) Shortwave flux: two blue lines (Tg10, Tg20) decrease; red line (0.5×CO₂) near zero. (b) Longwave flux: blue lines increase; red line nearly constant. (c) Net flux: all lines negative but increasing with Ts. (d) Ratio FLW/FSW: both blue lines decrease with Ts. Legend identifies Tg10 (light blue), Tg20 (dark blue), and 0.5×CO₂ (red). — Temperature dependence of the radiative forcing of carbon dioxide (red) and sulfate aerosol (blue).
CC BY 4.0 Hegde et al. (2025) DOI: 10.5194/acp-25-3873-2025

The study shows that the effect of sulfate aerosol on climate depends on Earth’s surface temperature, and the aspect of absorption and re-emission of thermal radiation dominates. The warmer the Earth's surface, the more radiation is absorbed by the aerosol, increasing its greenhouse effect and reducing the overall cooling effect.

This is known to occur with carbon dioxide as well, but the temperature dependence is stronger for sulfate aerosol. This difference results from the absorption of different wavelengths of thermal radiation by the two substances.

Impact on Earth’s self-regulation

The way in which the planet regulates a forcing depends on the state of the aerosol and thus on the state of the atmosphere as well. A warmer Earth emits more thermal radiation into space, which cools the planet – a negative feedback mechanism known as the Planck response. However, sulfate aerosol in the stratosphere masks some of this emission, making the feedback less pronounced.

The results of the study have important implications for climate engineering considerations, as they highlight how global warming may modulate the effect of stratospheric sulfate aerosol on climate. However, the researchers also point out that other aspects, such as clouds and circulation, must be taken into account to obtain a complete picture.

Original publication

R. Hegde, M. Günther, H. Schmidt und C. Kroll. Surface temperature dependence of stratospheric sulfate aerosol clear-sky forcing and feedback. Atmos. Chem. Phys., 25, 3873–3887, 2025. DOI: 10.5194/acp-25-3873-2025

Contact

Ravikiran Hegde
Max Planck Institute for Meteorology
ravikiran.hegde@we dont want spammpimet.mpg.de

Dr. Hauke Schmidt
Max Planck Institute for Meteorology
hauke.schmidt@we dont want spammpimet.mpg.de

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