The Earth has been in the Holocene era, a warmer period following an ice age, for about 12,000 years. During this epoch, there were approximately 22 sudden colder phases that lasted for several centuries, leading to the expansion of glaciers.
"Until now, there was no convincing explanation for this phenomenon. Something always seemed to be missing," said Alice Paine from the University of Basel, funded by the SNSF. Influences such as southward-drifting icebergs, fluctuations in solar activity, or changes in ocean currents were suspected.
The postdoctoral researcher, along with an international team, has now delved deeper into another idea: volcanic activity. Their study published in the journal Nature Communications* indicates that eruptions very likely triggered the prolonged cold periods. The researchers also provide a compelling explanation for the underlying mechanism.
Simultaneous Volcanic Eruptions and Glacier Growth
The team compiled information from various sources about major volcanic eruptions during the Holocene. There were 51 such events, many of them in the West Pacific along the so-called Pacific Ring of Fire. This region is home to many active volcanoes because tectonic plates collide and overlap there, which can lead to vast, explosive eruptions.
The timing of the volcanic events was determined with great precision using ice cores from the Arctic and Antarctic, as well as dating organic materials trapped in ash deposits. An updated dating of volcanic rock provided additional insights.
The researchers then compared this data with the occurrence of the prolonged cold periods in the Holocene. The dating of these phases was primarily based on information about the maximum extent of glaciers, determined by analyzing moraines, the rocky material pushed by the ice tongues.
The result: More than eighty percent of the glacier expansions coincided with at least one of the volcanic eruptions. "By combining the results of our statistical analysis with existing palaeoclimatic reconstructions, we are quite certain that this is not a coincidence. Moreover, the analysis also aligns with all previous understandings of the mechanisms," said Paine.
Temperature Conditions Become Unbalanced
According to these findings, a volcanic eruption sometimes disrupts the Earth's energy balance, causing global climate change: Sulfur particles are ejected into the atmosphere during the eruption, reflecting incoming sunlight. This cools the Earth's surface in the northern hemisphere.
The Arctic sea ice expands, reducing the ocean's warmth discharge to the atmosphere and altering ocean current patterns. In addition, a rain-rich low-pressure area around the equator shifts southward, enlarging the cold zone.
"All the effects amplify one another, working like a snowball effect," said Paine. Eventually, it becomes so cold that glaciers start to grow, especially in higher altitudes like Switzerland.
However, certain conditions need to be met for this to occur. If the eruption is too weak, sulfur particles do not reach the upper atmosphere. If it is too strong, the dust column may collapse under its weight, and comparatively little sulfur reaches the higher atmosphere.
Paine emphasizes that this phenomenon should not be confused with what happened in the "Year Without Summer" in 1816. Back then, the eruption of the Tambora volcano in present-day Indonesia led to a temperature drop of several degrees, causing global crop failures and famines. However, the effect lasted only a year until the dust particles were removed from the atmosphere.
In contrast, the effects of volcanic eruptions causing long-term cold phases last much longer. "The sulfur particles only set the snowball rolling. But their impacts on the Earth’s system persist for decades or even centuries, long after they have been washed out of the atmosphere."
Forecasts Are Difficult
A big question is whether such a cold phase caused by volcanoes could occur again in the near future. "For the events we examined, the climate was in its natural state," the researcher notes. Currently, though, it is in an unprecedented state due to human influence.
Some researchers believe the climate could now be more easily tipped by events like volcanic eruptions. Others argue an eruption would be no more than a drop in the ocean compared to the massive atmospheric changes currently occurring. "The more our climate changes, the more complex the processes become," says Paine. "There is still a huge knowledge gap to fill here."
Press Contact:
Alice R. Paine
Department of Environmental Sciences
University of Basel
Bernoullistrasse 32
4056 Basel
Tel.: +41 61 207 2880
E-Mail: alice.paine@unibas.ch