Explosive volcanic eruptions inject sulfur gases into the stratosphere. These form sulfate aerosols (tiny droplets) that scatter and reflect incoming sunlight back to space, cooling Earth’s surface for months to years. A classic example is the 1815 Mount Tambora eruption in Indonesia, which contributed to the “Year Without a Summer” in 1816.
Dr. Zachary McGraw (Columbia University) and colleagues used a NASA climate model to compare tropical eruptions with extratropical ones (roughly 30°- 70° latitude).
Earlier work had suggested extratropical eruptions produce much stronger Northern Hemisphere cooling, up to about 7.5 times greater for similar sulfur output. The new simulations found only a modest difference at most (up to roughly twice the cooling), with no clear, strong location- based enhancement.
Reasons include:
- Tropical aerosols are transported into both hemispheres by atmospheric circulation, spreading the reflective cooling more evenly.
- Extratropical aerosols can stay concentrated in one hemisphere (often the Northern), potentially intensifying local effects, but their overall impact is limited by other factors.
- Aerosols from extratropical eruptions typically have shorter lifetimes (often up to ~9 months) versus tropical ones (up to ~3 years). This is linked to lower injection heights, removal by precipitation in wetter and cooler mid- latitude conditions, and poleward transport.
When the team compared model results with historical and proxy records (including numerous tropical eruptions after 750 CE), Northern Hemisphere cooling aligned more frequently with tropical eruptions. Extratropical events are rarer and therefore exert less overall influence on Northern Hemisphere temperatures.
In short, the work supports the idea that tropical eruptions play a larger cumulative role in regulating (cooling) Earth’s climate via the “volcanic thermostat” than previously emphasized relative to mid- latitude events. The findings refine understanding of how eruption latitude, season, sulfur amount, and aerosol lifetime shape climatic impacts.
Extratropical eruptions (roughly mid- latitudes, 30°- 70°) differ from tropical ones mainly in aerosol distribution, lifetime, cooling efficacy per unit sulfur, frequency, and overall climatic influence.
These differences come from the 2026 study by Zachary McGraw and colleagues (summarized in the Phys.org article and detailed in their Geophysical Research Letters paper).
Aerosol distribution and hemispheric effects
- Sulfate aerosols from extratropical eruptions tend to stay largely confined to the hemisphere of the eruption (often the Northern Hemisphere).
- Tropical aerosols are transported by stratospheric winds into both hemispheres, spreading the reflective cooling more evenly worldwide.
This confinement can produce some enhanced regional (e.g., Northern Hemisphere extratropical) cooling for extratropical events, but the effect is limited.
Cooling strength
Earlier analyses (including some paleoclimate comparisons) suggested extratropical eruptions could cool the Northern Hemisphere several times more strongly, up to ~7.5 times, than similar- magnitude tropical ones.
Large- ensemble NASA climate model simulations (spanning 5– 40 Tg of stratospheric sulfur, across seasons) found the ratio is consistently only about 1.4- 2 times (typically within a factor of two).
This is explained primarily by the single- hemisphere confinement of aerosols rather than any strongly disproportionate forcing. Paleoclimate evidence (tree rings and historical records) supports similarly modest differences.
Aerosol lifetime
Extratropical aerosols generally have shorter atmospheric lifetimes, often up to ~9 months, versus up to ~3 years for tropical ones.
Contributing factors include:
- Typically, lower injection heights into the stratosphere.
- Greater removal by precipitation (rain and snow) in the wetter, cooler mid-latitude climates.
- Poleward transport of aerosols from mid- latitudes.
Shorter lifetimes limit the duration and cumulative impact of the cooling.
Frequency and cumulative influence
Extratropical eruptions capable of significant stratospheric sulfur injection are comparatively rarer.
As a result, Northern Hemisphere cooling episodes in the historical and proxy record coincide more often with tropical eruptions.
Overall, extratropical events have exerted a smaller cumulative influence on Northern Hemisphere (especially extratropical) temperatures than tropical ones.
In summary, while extratropical eruptions can produce moderately stronger regional cooling due to aerosol confinement, they do not produce the dramatically enhanced effects once claimed, and their shorter lifetimes plus lower frequency mean tropical eruptions play the larger role in Earth’s longer-term volcanic “thermostat.” Effects also depend on exact latitude, season, sulfur mass, and injection height.
No Evidence for Strongly Enhanced Northern Hemisphere Cooling From Extratropical Versus Tropical Eruptions
This open- access research letter directly addresses claims that Northern Hemisphere extratropical (NHET) volcanic eruptions produce several times stronger cooling in the Northern Hemisphere extratropics than tropical eruptions of similar sulfur magnitude.
Key Points
- Models and tree- ring records show no evidence of greatly enhanced regional cooling from extratropical eruptions versus comparable tropical ones.
- Extratropical eruptions cool the extratropics no more than is expected from aerosols being confined to one hemisphere.
- Because large extratropical eruptions are rarer, they have historically caused less overall cooling of the Northern Hemisphere extratropics than tropical eruptions.
Abstract and Main Findings
Prior studies (Toohey et al., 2019; Burke et al., 2023) reported enhancement factors of ~3.2 times or even ~7.5 times for NHET cooling from extratropical versus tropical eruptions.
Using a large ensemble of GISS ModelE2.2 simulations (160 members total) spanning 5- 40 Tg of stratospheric sulfur (VSSI), two seasons (June and December), and locations on the equator versus 64°N, the authors find the extratropical- to- tropical NHET cooling ratio is consistently only 1.4- 1.9 times (within a factor of two).
This matches the physical expectation from aerosol confinement: extratropical aerosols largely remain in one hemisphere, while tropical aerosols spread into both. No additional amplification mechanism is required or supported.
Paleoclimate (tree- ring) evidence yields similarly modest ratios (overall estimates across reconstructions and simulations fall in the 1.4- 2.4 range).
The much larger prior factors are attributed to methodological issues, including reliance on sparse pre- 750 CE data, linear- regression approaches that prove unreliable in synthetic tests, and uncertainties in sulfur and temperature reconstructions for key events (e.g., Laki 1783, 536 CE, Okmok 43 BCE).
Owing to their relative rarity, extratropical eruptions exert a smaller cumulative influence on NHET temperatures than tropical ones. The findings challenge the idea that extratropical eruptions are of outsized importance for Northern Hemisphere climate.
Methods Highlights
- Prescribed aerosol forcing via a modified Easy Volcanic Aerosol (EVA) framework (adjusted for more realistic shorter lifetimes of summer extratropical aerosols).
- Interactive ocean and sea ice; NHET temperatures defined as land areas north of 30°N (to match tree-ring coverage).
- Ensemble approach isolates the forced response from internal variability.
- The EVA extratropical forcings are shown to be a high estimate of potential cooling relative to interactive aerosol models.
Plain- Language Summary
Volcanic eruptions cool Earth by injecting reflective sulfate aerosols into the stratosphere. Recent claims that mid- to high-latitude eruptions cool the Northern Hemisphere several times more than tropical ones of similar size are not supported.
Climate- model ensembles and temperature reconstructions show at most roughly twice the cooling, exactly what is expected when aerosols stay mostly in one hemisphere.
Large extratropical eruptions are also rarer, so tropical eruptions have had the greater overall impact even on Northern Hemisphere temperatures.
The paper includes supporting information with eruption lists, additional figures on temperature anomalies, regression reliability tests, and tree- ring comparisons.
Full text and SI are available via the DOI or the Columbia-hosted PDF.
Journal: Geophysical Research Letters
DOI: 10.1029/2026GL122071
First published: 15 October 2026
Authors: Zachary McGraw, Benjamin O. Goldman, Lorenzo M. Polvani
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