News•February 23, 2014

Small Volcanic Eruptions Add to Larger Impact on Climate

The recent slow down in global warming has been attributed to a number of factors, including excess heat being stored in the deep ocean and reductions of certain greenhouse gases. Now add volcanic eruptions to the mix of contributing factors.

A new analysis published in Nature Geosciences on Sunday shows that a series of relatively small eruptions since 2000 have likely helped dampened the rise in temperatures.

An eruption at Tavurvur, a volcano in Papua New Guinea. A 2006 eruption there is one of 17 smaller eruptions in the 21 century that have had a cummulative cooling effect on the Earth's average temperatures.
Credit: Wikimedia Commons

The slow down in global warming has been termed a “hiatus” by some because global average temperatures have plateaued in the past 15 years after rising sharply since the 1970s. Climate models have failed to replicate the slow down, which has led some skeptics to question their validity and how sensitive the Earth’s climate is to human greenhouse gas emissions.

Ben Santer, a scientist at Lawrence Livermore Laboratory in Berkeley, Calif., and lead author of the new study, jokingly referred to this as “the models-suck interpretation” in an interview.

“You have to look at all the evidence, though” he said.

That includes the focus of the new research, which analyzes recent volcanic eruptions on the climate system.

Volcanic eruptions have a well-known cooling effect on the Earth’s surface temperatures. Eruptions spew ash clouds full of tiny particles called aerosols high into the stratosphere where they effectively act as a giant mirror that reflects incoming sunlight back into space, cooling the Earth in the process. Other natural and manmade sources of aerosols include forest fires, emissions from coal plants, and dust kicked up from deserts.RELATEDStudy Ties Global Warming ‘Hiatus’ to Pacific Cooldown
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The eruption of Mount Pinatubo in 1991, considered the most recent major eruption, essentially made the stratosphere 10 to 100 times more opaque than normal, which resulted in 1°F of cooling across the globe over the 15 months after the eruption.

Scientists have long assumed that since Pinatubo, eruptions have had a negligible impact on the climate, including the 17 comparatively small eruptions since 2000.

In a sense, they’re right. On their own, each of those eruptions had an order of magnitude smaller impact on the amount of sunlight reflected back to space compared to Pinatubo, but together they had a larger impact.

Global-mean temperature (ºC) and CO2 (parts per million) for 1971-2012.Temperature is represented in terms of deviation from 1980-1999 average. Both are based on annual mean values.The temperature for the hiatus period is highlighted.
Credit: Scripps Institution of Oceanography.

“After Pinatubo, (stratospheric aerosols) didn’t decay to zero or a background level,” Santer said. “There were a ‘swarm’ of eruptions from 2000 onward. The cummulative effect was to reflect more sunlight back into space.”

The research suggests they increased the Earth’s ability to reflect sunlight by 4-7 percent annually from 2000-2009.

That crucial piece of information is missing from the latest climate simulations used to inform the Intergovernmental Panel on Climate Change’s (IPCC) reports. Those models have been able to accurately capture the impacts of Pinatubo and El Chinchon, another major eruption in Mexico in 1982. To see if they could successfully replicate the impacts of the 17 smaller scale eruptions since 2000, Santer and his colleagues updated two computer models used as part of the IPCC with the revised impacts.

They found that the updates brought models closer in alignment with real world temperature observations. The simulated warming trends were up to 15 percent smaller, though Santer said there are still some uncertainties. He said better observations and refinements to how the models integrate the eruptions could help reduce those uncertainties.

“Since none of the standard scenarios for evaluating future global warming include volcanic eruptions, this paper will help us quantify the impacts of future large and small eruptions when they happen, and thus better interpret the role of humans in causing climate change,” said Alan Robock, a leading expert on the intersection of volcanoes and climate at Rutgers University.

Robock, who was not associated with the study, said researchers need to continue to refine their understanding of pollution, greenhouse gas emissions, and natural factors since each has a role in shaping the climate.

A graph showing global average temperatures by decade from the 1880s through the 2000s.
Click image to enlarge.

Scientists have recently focused on trying to better understand the slow down in global warming. Though the term “hiatus” has been popularized, it’s somewhat of a misnomer. The 2000s were still the warmest decade recorded since the 1880s. And other signs that human greenhouse gas emissions are affecting the climate, such as rising seas and decreasing Arctic sea ice, haven’t slowed.

But the slow down in global temperatures has led scientists to reevaluate other causes beyond volcanoes. Possible culprits include heat uptake in the oceans, rising Chinese coal plant emissions, policies that have reduced CFCs — a greenhouse warming gas that also affects the ozone hole — and climate oscillations such as La Niña and other ocean temperature shifts.

Ultimately, it’s unlikely that one single factor is behind the slow down.

“All of these things make some contribution,” Santer said. “The scientific challenge is to figure out what percentage each contributes. It’s not an either/or situation.”