Cause and Effect: Did an underwater volcanic eruption add to rising temperatures

“It was one of the largest eruptions we have observed in the satellite era,” Stuart Jenkins, a scientist in the Department of Physics at the University of Oxford, told HT in an email interview. “It was particularly explosive; its pressure wave was observed in stations all around the world.”

The volcano wasn’t just unusual in its force and debris field, which blanketed nearby islands with ash, triggered destructive tsunami waves, and caused lightning that lasted nearly 12 hours. The plume it emitted was saturated with water vapour lending it an extraordinary character that now has scientists in a frenzy.

Why, you ask?

Unlike most volcanic eruptions that have a temporary cooling effect on the planet, this one seems to have done just the opposite, at least temporarily.

Usually, volcanic plumes have a higher concentration of sulphur compounds and carbon dioxide than water vapour. The sulphur dioxide in volcanic plumes works to bring down surface temperatures, even if temporarily, due to a phenomenon that is somewhat opposite to the greenhouse effect.

“This is the first volcano in the observational record that may warm rather than cool the surface,” said Luis Millan, a research scientist at NASA’s Jet Propulsion Laboratory (JPL), who spoke to HT over another email interview.

Water vapour is a greenhouse gas that tends to trap heat from the sun. The Tonga eruption blasted nearly 146 million tonnes of water vapour into the stratosphere, the largest quantity ever observed.

On the other hand, gas plumes with high sulphur compounds “lead to the formation of sulfate aerosols that can reflect sunlight back to space, which can result in temporary cooling of the surface” Millan explained.

Why the volcano threw up such amounts of water vapour is explained by the fact that the Hunga Tonga-Hunga Ha’apai’s caldera — the hollow that forms after a volcano spews out magma — is 150m below sea level.

Impact on climate

When the first assessments about what composed the plume became known, social media groups — both of experts and enthusiasts — were rife with speculation. Some went as far as to say that the oceans were warming due to underwater volcanic activity.

But temperature trends suggest otherwise.

According to NASA data, the five hottest Julys since 1880 have all happened in the past five years.

The records set this year have anyway been in “uncharted territory”.

“Our research suggests that Tonga’s global contribution to surface temperature changes is very small (peaking at around +0.04 degrees C),” Jenkins, who was the lead author of the paper ‘Tonga eruption increases chance of temporary surface temperature anomaly above 1.5 °C’ which was published in the journal Nature Climate Change, said.

In the paper, Jenkins and three other authors wrote, “over a multiyear period, Tonga will cause a temporary increase in global surface temperatures due to this large water vapour increase and lack of counterbalancing sulfate aerosol perturbation”.

The effects could be more pronounced, “particularly in the latitude band where Tonga is found (20 degrees south of the equator)”, Jenkins told HT.

India falls in this band.

How long can the effect last?

“The effects will only last for around five years since this is the lifetime of the water vapour when put into the stratosphere,” said Jenkins.

Millan said he expects an even shorter effect. “In this case, temporary means transient, that is, the effect will only last perhaps for a couple of months and then everything will return to pre-Hunga Tonga conditions,” he said.

Jenkins also rejected the correlation of the recent temperature extremes with the eruption.

“Recent global weather patterns more widely — for example the extreme sea surface temperatures in the Atlantic basin — are unlikely to have been caused substantially by the Tonga eruption. They are better explained as a mixture of a strong developing El Nino in the tropical pacific, and the long-term impact of human-induced warming,” he said.

“It is worth noting that this is happening before we see the full warming impact of the El Nino event, which typically plays out in the second year after it develops,” WMO’s secretary-general, Petteri Taalas, said in a statement on September 7, when the UN agency confirmed summer 2023 as the hottest ever.

What else did Tonga do?

There is one long-lasting impact that the eruption might have on climate: the unusually early re-emergence of the Ozone hole over Antarctica.

The Antarctic Ozone hole, an atmospheric phenomenon, normally starts to form in mid-to-late August, as the Sun rises over the South Pole, and closes towards the end of November.

In 2023, the development started in early August following some of the lowest minimum total column ozone values for the Southern Hemisphere in the last four decades throughout July, the European weather monitoring agency Copernicus said.

“The excess water has briefly cooled the stratosphere as well as altered the stratospheric circulation patterns. It also led to ozone depletion inside the water vapour plume in the immediate days after the eruption,” Millan said.

Water vapour breaks down in the stratosphere, releasing reactive hydrogen oxide molecules that destroy ozone. These molecules also react with chlorine-containing gases, converting them into forms that destroy ozone as well. So, the wetter the stratosphere, the less the ozone, and the bigger the ozone hole.

“Scientists have long warned what our fossil fuel addiction will unleash,” UN secretary-general Antonio Guterres said last month.

The June-July-August season — summer in the Northern Hemisphere — was the warmest ever recorded, with an average temperature of 16.77°C, which was 0.66°C above the 1990-2020 average, according to the European Union’s Copernicus Climate Change Service (C3S) and the World Meteorological Organization (WMO).

“The effects (of the Tonga eruption) are expected to be much smaller than those from climate change related to burning fossil fuels,” Millan said. “The record-breaking global temperatures are just a preview of what may happen if we do not take more bold and ambitious climate action.”