Volcanic eruption may be boosting global heat wave

A climate science debate is simmering over how much of 2023's record warmth is due to human-caused factors, chiefly the burning of fossil fuels, and the role of other influences.

Why it matters: It's critical to gaining a better understanding of what is driving the sudden spike in global average surface temperatures, accompanied by scorching and deadly heat waves, wildfires and flooding.

The big picture: July was Earth's warmest month on record, the oceans are historically warm; some of the summer's heat would have been "virtually impossible" without human-caused global warming, studies show.

• The combination of the burning of fossil fuels and an intensifying El Niño event in the tropical Pacific Ocean is viewed as the primary driver of 2023's extremes.
• However, climate scientists are looking at what is happening and observing shifts that are more sweeping and rapid than they expected, even though the overall warming trends, El Niño's development and associated impacts were well-forecast.
• "Despite this being anticipated, I think we're all having an instinctual reaction that we're in territory we've not seen before," said Michelle L'Heureux, who leads El Niño forecasting at the National Weather Service. For example, no previous El Niño had kicked off amid such high global ocean temperatures, she said.
• "As a forecaster, there is reassurance in the planetary system acting in ways that are recognizable based on past experience. Now I don't quite know how to react because this is an entirely new experience. And that is unsettling," she said.

Zoom in: Scientists are learning, on the fly, how much the combination of natural climate variability and human-caused greenhouse gas emissions are capable of.

• However, some scientists are pointing to two other factors as potential contributors to this year's myriad climate extremes. This is where things get more complicated: trying to parse the relative contributions from these other climate change influences.
• One is a reduction in pollution from marine shipping vessels, via rules that went into effect in 2020. Fewer emissions of sulfate aerosols, which are tiny particles produced by ship engines and other sources, is thought to be unintentionally leading to more warming over the oceans.
• Berkeley Earth, an independent climate monitoring group, notes in a report out today that these effects are mainly confined to major shipping corridors and are relatively small in global impact — on the order of about 0.02°C (0.036°F).

Between the lines: Another suspected accomplice in 2023's record warmth is the 2022 explosive eruption of the Hunga Tonga-Hunga Ha'apai undersea volcano in the southern Pacific Ocean, which injected about 150 million metric tons of water vapor into the stratosphere.

• Water vapor is a powerful greenhouse gas, and computer model simulations show this stratospheric injection could enhance warming through the end of the decade.

The intrigue: Hunga Tonga's eruption boosted the stratosphere's water content by about 15%, according to Berkeley Earth, without adding larger amounts of cooling sulfur dioxide, as other volcanoes have done.

However, the eruption alone cannot fully explain the record warmth of 2023, climate scientists told Axios.

• One study published in Nature Climate Change in January found that Hunga Tonga's eruption may increase global average surface temperatures in 2023 by up to 0.035°C (0.063°F).
• There is considerable uncertainty in this calculation, including how this warming might vary regionally.
• Forthcoming research on a Hunga Tonga-like eruption came up with numbers that some have interpreted as indicating a greater temporary volcanic influence on the global climate.

Yes, but: That study did not specifically look at the actual eruption, and has not yet been peer-reviewed.

What they're saying: Climate scientist Zeke Hausfather summarized the volcano's contribution to 2023's record high temperatures this way: "So, not nothing, but only a small part of the story in the exceptional warmth the world is experiencing," he told Axios via email.

• Andrew Dessler, a climate scientist at Texas A&M University, also thinks the significance of the volcano's warming role pales in comparison to human-driven climate change and other influences.
• Dessler likens climate change to a "rising tide" that is elevating the baseline temperatures from which heat waves begin. "This means that our heatwaves become hotter, essentially 'floating' on the 'rising tide' of our warming world," he said via email.

The bottom line: "I think what is shocking people is how bad the impacts seem to be. We're passing thresholds in our world that people didn't realize were there," Dessler said.

• "While it seems sudden and unusual, it's been coming for a very long time."