Volcanoes are spectacular displays of the massive forces at work inside our planet, yet they are dwarfed by humans in at least one respect: their carbon dioxide emissions.
Despite statements made by climate change deniers, volcanoes release a tiny fraction of the amount of carbon dioxide emitted by human activities every year.
In fact, humans release roughly 135 times more carbon dioxide annually than volcanoes do, on average, according a new analysis. Put another way, humans emit in under three days the amount that volcanoes typically release in a year, according to the best estimates of volcanic emissions.
"The question of whether or not volcanoes emit more CO2 than human activity is one I get more than any question in my email from the general public," says Dr Terrence Gerlach, a retired volcanologist, formerly with the Cascades Volcano Observatory, part of the US Geological Survey. Even Earth scientists who work in other areas often pose him the question, he says.
To lay out a clear answer, Gerlach compiled the available estimates of CO2 emissions from all global volcanic activity on land and undersea and compared them with estimates for human emissions. He published the compilation in Eos, a publication of the American Geophysical Union.
Researchers estimate the amounts of carbon dioxide released by terrestrial volcanic eruptions by methods including remote sensing or flying through clouds of erupting volcanic gas, and by measuring certain isotope concentrations near undersea volcanoes. Carbon dioxide is dissolved in magma at great depths and is released as the magma rises to the surface.
"A lot of climate sceptics claim that volcanoes emit more CO2 than humans do," says Gerlach. "They never give any numbers, but the fact is you will never be able to find the volcanic gas scientist that will agree to that."
One example of these sceptic's claims is the 2009 book,Heaven and Earth: Global Warming, the Missing Scienceby Professor Ian Plimer of the University of Adelaide, who did not respond to Discovery News' requests for comment.
"The main reason, I think, that this myth persists," says Gerlach: "First of all, the emissions are extremely spectacular. When people see volcanic eruptions on television and it's awesome, and it's very easy for people to imagine that huge amounts of CO2 are being emitted to the atmosphere."
"However, these spectacular volcanic explosions that are so stunning on TV last only a few hours," he says. "They are ephemeral. In contrast, the sources of anthropogenic CO2 (smokestacks, exhaust pipes, etc) are comparatively unspectacular, commonplace, and familiar, and in addition they are ubiquitous, ceaseless, and relentless. They emit CO2 24/7."
Comparing the numbers
While there is uncertainty in the measurements - researchers estimate between 0.13 and 0.44 billion metric tons are released by volcanoes each year, with their best estimates between 0.15 and 0.26 billion tons - even the highest end of the range is dwarfed by anthropogenic emissions of 35 billion tonnes in 2010.
Gerlach noted that human land-use changes alone, which include deforestation, release 3.5 billion tonnes per year. Cars and light-duty trucks produce 2 billion tonnes; even cement production produces 1.5 billion tonnes. Any of these by itself is still several times higher than the annual emissions of all of the world's volcanoes.
Pakistan or Kazakhstan each produce about the amount of CO2 as volcanoes do each year, Gerlach notes in the article.
In yet another comparison, Gerlach reported that in order for volcanic emissions to match those made by humans, the 1980 Mount St Helens eruption would need to happen every 2.5 hours. The 1991, Mount Pinatubo eruption would need to occur every 12.5 hours.
"There is no way you can escape the fact that volcanoes are releasing a tiny amount of emissions right now," says Professor Bernard Marty of the Centre de Recherches Petrographiques et Geochimiques in Nancy, France. "There is no doubt about this."
"Even if you do the reverse and you compute how much volcanism should happen to match atmospheric levels, you end up with completely unrealistic eruption rates," he said.
Dr Marie Edmonds, a volcanologist at Cambridge University agrees. While volcanoes are the most important natural source of atmospheric CO2, she notes, "The results show clearly that the amount is 100 to 150 times less than anthropogenic amounts."