Using voluminous data left behind by our ancestors, modern scientists have discovered a correlation between volcanic activity and periods of bitterly cold weather in Ireland. They uncovered this connection while studying climate records produced by Irish monks between the years 431 and 1649 AD.

A team of scientists, led by historical climatologist Francis Ludlow from Harvard University, poured over an ancient collection of written records known as the Irish Annals. These notebooks were originally designed to track religious celebrations and observances. But over time, they were expanded to include other interesting information, including reports on social, cultural and political events, natural disasters and weather-related developments.

The Irish Annals Reveal Ancient Knowledge

Fortunately, the Irish Annals have been preserved for posterity. This has given modern historical researchers invaluable access to contemporary writings from a distant and otherwise obscure era. The Annals have revealed useful information about the society that produced it, but until Ludlow’s team came along no one had studied their weather and climate reports closely.

This is unfortunate, because the Annals turned out to be a goldmine of fascinating and enlightening climate-related data. Ludlow and his fellow researchers were particularly interested in reports of unusually cold or snowy conditions, marked by plunging temperatures, heavy icing of lakes and rivers, or frequent blizzards.

With 40,000 entries spread out over a period of 1,218 years, the climate historians had quite a daunting task to finish. When they were done, they’d identified approximately 70 blistering outbreaks of seriously cold weather, none of which would be expected during a typical Irish winter.

“It’s clear that the scribes of the Irish Annals were diligent reporters of severe cold weather, most probably because of the negative impacts this had on society and biosphere,” Ludlow says.

The next step in this research project was to cross-correlate these cold spells with volcanic activity. To facilitate this process, core samples were taken from glacial ice in Greenland. This frigid landmass is located about 2,600 kilometres to the north of Ireland, which is close enough for both Greenland and Ireland to be affected by the same volcanic eruptions.

Analyzing ice cores for volcanic activity is not difficult. When volcanoes erupt, they launch prodigious quantities of sulfates into the atmosphere, and when snow falls traces of these chemicals are carried back down to Earth, leaving telltale traces in the ice in locations where no melting occurs.

By counting down layers of snow fall contained within the Greenland ice samples, scientists were able to determine when each layer was created. This is possible because snowfalls in Greenland occur regularly and predictably, in summer and winter, and since each season’s snowfall creates a separate profile it was possible to identify them individually and assign them to specific years.

Using this method, the researchers found evidence for 48 volcanic eruptions during the period in question. And while the matches weren’t always perfect, there was a clear and statistically significant link between volcanic activity and severe weather events in Dark Age and Middle Age Ireland.

“Our major result is that explosive volcanic eruptions are strongly, and persistently, implicated in the occurrence of cold weather events over this long timescale in Ireland,” Ludlow says. “In their severity, these events are quite rare for the country’s mild maritime climate.”

The main reason for this effect is the sulfates that volcanoes produce. Ash injected into the troposphere and stratosphere by volcanoes, up to an altitude of 50 kilometres, will cool the Earth by blocking sunlight and preventing it from reaching the ground. But the sulfates (primarily sulfur dioxide) that emerge during volcanic eruptions have the greater effect. They first appear in the form of sulfurous gases, before c ombining with atmospheric water to form sulfate aerosols that reflect sunlight back into space.

It can take a few years for these aerosols to dissipate, and as long as they continue to float in the sky they will continue to cool the Earth below.

Ludlow’s study, which was published in the journal Environmental Research Letters in 2013, found that volcanic eruptions in the northern hemisphere were most strongly linked with bad weather in Ireland. But this was not always the case.

One interesting exception involved a massive eruption in the year 1600 of a volcano in Peru called Huaynaputina. This was an unusually explosive event, and it caused an outbreak of floods, famines and cold spells in multiple areas around the globe, including in Ireland. Peru and Ireland are nearly 10,000 kilometres apart, which highlights how powerful the effects of intense volcanic activity can be.

“The possibility that tropical eruptions may result in severe winter cooling in Ireland highlights the considerable complexity of the volcano-climate system, in terms of the regional expression of the response of climate to volcanic disturbances,” Ludlow explains.

Could Volcanic Activity Cancel Out Global Warming?

In examining the data reported by the monks, historians and aristocrats who compiled the Irish Annals, it became clear to Francis Ludlow’s team of scientists that volcanoes can have a significant impact on the weather.

But in evaluating this impact, we must be careful not to confuse weather with climate. In the modern age, we are dealing with a climate crisis that threatens to dramatically—and perhaps catastrophically—raise temperatures across the planet. Massive volcanic eruptions have the opposite effect, and it might therefore seem like volcanic activity could cancel out global warming. Should one of the so-called “super volcanoes,” such as the one below Yellowstone Park in the western United States, suddenly erupt, the countering effects could be especially significant.

But in reality, explosive volcanic activity could never overcome the effects of man-made global warming. Volcanic eruptions are transient events, and their effects on the weather are only temporary. Climate remains unaffected by this activity, and any cooling that follows even the largest volcanic explosion would soon be overwhelmed by the factors causing global temperatures to rise.

As for the super volcanoes, their eruptions occur very infrequently, with gaps of hundreds of thousands or millions of years between explosive events. Before they reach a critical stage they must go through a period of significant pressure buildup, which could last for a few thousand years, and none of the super volcanoes currently identified are anywhere close to erupting at the present time.

Given the tremendous power of volcanoes, and the record of their effects on climate in the past, they might seem like significant drivers of climate change. But fossil fuel use by humans has had a cumulative effect on climate that dwarfs anything volcanoes might be capable of producing.

Volcanoes may be incredibly destructive, but even at their worst they are incapable of matching the damage caused by human beings.

It is perhaps a symptom of human arrogance that we tend to look back on our ancient ancestors as simple-minded people who were ignorant of the wider-world, when the reality could not be further from the truth.

By Nathan Falde