Major eruptions and wildfires leave a clear signature on global climate

Big eruptions and massive wildfires can shove huge amounts of particles and gases into the atmosphere, sometimes shifting temperatures worldwide.

The tricky part is proving what any single event actually did, because the atmosphere is already full of overlapping signals.

A new study shows a way to filter out that background noise and spot the fingerprint of specific disasters – including how they warmed or cooled different layers of the atmosphere.

The work focuses on three major events: the 1991 eruption of Mount Pinatubo, the 2019–2020 Australian wildfires, and the 2022 eruption of the underwater volcano Hunga Tonga.

The researchers found that all three produced statistically significant temperature changes in the stratosphere, even though their effects closer to the ground were not always clear.

Three events, different ingredients

The three events were extreme, but in different ways, mainly because of what they injected into the sky and where it ended up.

Pinatubo blasted an enormous load of sulfate aerosols into the stratosphere. The Australian fires lofted smoke high enough to reach the upper troposphere and stratosphere.

Hunga Tonga, meanwhile, delivered a staggering amount of water vapor into the stratosphere – in what the researchers describe as the biggest atmospheric explosion seen by satellites.

“These events are extraordinary and very unique in terms of the different materials they inject into different altitudes,” said study lead author Yaowei Li, a visiting scientist at MIT.

“So we asked the question: Do these events actually perturb the global temperature to a degree that could be identifiable from natural meteorological noise? Could they contribute to some of the exceptional global surface warming we’ve seen in the last few years?”

The answer, broadly, is yes for the stratosphere but mostly no for explaining the recent spike in global surface warming.

Climate shifts above Earth

The study zooms in on the stratosphere, the layer above the troposphere. The troposphere is where we live and where human-driven warming has been strongest in recent decades.

The stratosphere behaves differently, and it can react sharply to big injections of aerosols, smoke, or water vapor.

In the results, Pinatubo showed the classic pattern scientists expect: warming in the stratosphere paired with cooling in the troposphere.

The Australian fires and Hunga Tonga also produced strong stratospheric signals. The fires caused warming, while Hunga Tonga caused cooling. But neither produced a robust, globally detectable tropospheric signal in the first two years after the event.

“Understanding the climate responses to natural forcings is essential for us to interpret anthropogenic climate change,” said Li.

“Unlike the global tropospheric and surface cooling caused by Pinatubo, our results also indicate that the Australian wildfires and the Hunga Tonga eruption may not have played a role in the acceleration of global surface warming in recent years. So, there must be some other factors.”

Filtering out climate noise

The heart of this paper is its method. Instead of staring at messy temperature records and guessing what caused what, the team used a signal-to-noise approach.

They started with satellite temperature measurements taken at different altitudes since 1979 and compiled a large dataset from 1986 onward.

Those measurements reveal long-term tropospheric warming and stratospheric cooling – trends largely driven by greenhouse gases. The researchers removed those long-term trends to flatten the baseline.

But even after that, natural variability still complicates things. Earth naturally swings between warmer and cooler phases through cycles like El Niño and La Niña, and the sun also adds a repeating influence on roughly an 11-year cycle. The team accounted for those patterns too and subtracted their effects.

Once those major “background” drivers were minimized, the remaining bumps and dips in the atmosphere were easier to match to major one-off events like a volcanic eruption or a giant fire season.

Pinatubo: The classic cooling case

Once researchers applied the method, Pinatubo behaved exactly as climate scientists have long described, providing a useful reality check.

The team found that the eruption cooled global tropospheric temperatures in the lower atmosphere by up to about 1.3°F (0.7°C), and the cooling lasted more than two years.

The reason is straightforward: sulfate aerosols in the stratosphere reflect incoming sunlight, so less solar energy reaches the lower atmosphere and surface.

At the same time, those aerosols warmed the stratosphere by absorbing heat radiated upward from below.

Because this matched many previous studies, the researchers felt confident applying the same filtering technique to the two more debated cases.

Wildfire smoke heats high

For the 2019–2020 Australian fires, the team found a striking stratospheric warming signal: up to about 1.4°F (0.77°C) globally, lasting around five months. But they did not detect a clear global tropospheric temperature change tied to the fires.

“In the end, we found that the wildfire smoke caused a very strong warming in the stratosphere because these materials are very different chemically from sulfate,” Li explained.

“They are dark-colored particles, meaning they are efficient at absorbing solar radiation. So a relatively small amount of smoke can cause a dramatic warming.”

Even though the fires injected “only” about one million tons of smoke particles into the upper atmosphere, that was far less mass than Pinatubo’s aerosols. But their ability to absorb sunlight made them powerful stratospheric heaters.

Water vapor changed the sky

Hunga Tonga’s signature was different again. The eruption injected an enormous amount of water vapor – nearly 150 million tons – into the stratosphere.

The team found this produced cooling in the middle-to-upper stratosphere of up to about 0.9°F (0.5°C), lasting several years.

That result is especially interesting because water vapor is often associated with warming in many contexts, but its effects depend on altitude, chemistry, and how it alters radiation and circulation in the stratosphere.

Surface warming still unresolved

One reason this paper matters is what it rules out. The past few years have been exceptionally warm globally, and it’s natural to wonder whether huge wildfires or a massive eruption could have altered the atmosphere enough to be partly responsible.

The study’s results suggest these two recent natural events don’t explain the persistent warmth in the lower atmosphere.

“The Australian fires and the Hunga Tonga eruption really packed a punch at stratospheric altitudes, and this study shows for the first time how to quantify how strong that punch was,” Li said.

“I find their impact up high quite remarkable, but the ongoing issue is why the last several years have been so warm lower down in the troposphere – ruling out those natural events points even more strongly at human influences.”

Thus, the dramatic phenomena we can see – smoke plumes, eruption clouds – really do shake the upper atmosphere. But the relentless warming where people live still points back to the slow, steady driver we already know too well.

The study is published in the journal Proceedings of the National Academy of Sciences.

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