Volcanic eruptions can affect flooding patterns worldwide

Everyone is aware that large volcanic eruptions have the power to cool the Earth. But here’s something you might not have anticipated: they can also affect floods – and not only locally, but globally.

New studies have discovered that when a volcano erupts and expels gas high into the air, it has the potential to alter global rainfall patterns in a way that soaks or dries out entire areas. And the location of the volcano – especially in relation to the equator – makes all the difference.

It’s not lava flows or ash clouds covering the sky. It’s about what occurs miles in the air, in the upper atmosphere, where invisible gas molecules alter the way that Earth transmits heat and moisture.

What happens after a major eruption

When a big volcano erupts, it tends to release tremendous quantities of sulfur dioxide gas into the stratosphere. There, the gas condenses into tiny particles that are referred to as aerosols.

The aerosols deflect sunlight and trap heat, cooling the Earth’s surface and heating the upper atmosphere. That temperature difference alters air’s circulation patterns around the globe.

The main player in all of this is something known as the Inter-Tropical Convergence Zone, or ITCZ. It’s a ring of thunderstorms and clouds that surrounds the Earth just north of the equator where trade winds from the north and the south come together.

This zone is responsible for heavy tropical rains, and it drifts north or south based on the season.

Volcanic eruptions can nudge the weather band out of its normal position. When that happens, areas that usually get soaked might dry up – and places that are normally dry could suddenly flood.

The volcano effect splits at the equator

To understand how this works, Princeton University researchers looked at three major eruptions over the past century: Santa Maria in Guatemala (1902), Agung in Indonesia (1963), and Pinatubo in the Philippines (1991).

Each one of these eruptions affected flooding in a different way. When Santa Maria erupted in the northern hemisphere, it caused a 25% increase in sites with peak flooding in the southern hemisphere’s tropics and a 35% increase in sites with decreased flows in the northern tropics.

Agung, in the southern hemisphere, had almost the reverse effect: 50% of stream gauges in the southern tropics showed reduced flooding, while 40% in the northern tropics showed an increase.

Both volcanoes had one thing in common – their gas plumes stayed mostly in their own hemispheres. That caused the ITCZ to shift away from the eruption, pulling rainfall and flooding with it.

The effects didn’t last forever. “The effects of the increased rainfall are generally strongest in the year after the eruption and lessen after several years,” said Gabriele Villarini, one of the study’s lead researchers.

An eruption that flips the script

Pinatubo was different. Its 1991 eruption sent aerosols into both hemispheres pretty evenly, which led to a different pattern.

Instead of shifting the ITCZ, the eruption cooled the planet more evenly and kicked off a different type of circulation shift. In this case, tropical regions in both hemispheres got less flooding – peak flows dropped at 20% of sites in the southern tropics and at 35% of sites in the northern tropics.

But in very dry places, things got wetter. About 35% of arid-region sites saw an increase in peak flows. This change was probably linked to what’s known as monsoon-desert coupling.

“Air sinks over Asian monsoon regions and rises over nearby arid regions. The rising air pulls moisture upward, causing greater rainfall in the arid areas,” said Hanbeen Kim, lead author of the research paper.

Not just a climate problem

What’s important here isn’t just the science – it’s what it means for people. These shifts in rainfall can lead to flooding disasters in places where no one expected it. And that matters for planning, especially as some scientists consider using artificial ways to cool the Earth that mimic volcanic effects.

The authors of the study said their results indicate that climate shifts – whether natural or human-induced – can cause a broad array of impacts. And it’s not only warming and cooling, but how water circulates across the Earth.

By connecting volcanic eruptions to flood patterns across the world, this study adds another piece in the puzzle of how we interpret Earth’s systems. It’s not just smoke and ash we need to watch out for – it’s how the invisible gases impact the skies above and the rivers below.

The full study was published in the journal Nature Geoscience.

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