John Button, of Timaru, asks :-

I have heard it argued that emissions from vehicles are insignificant as factors in environmental damage in comparison with the effects of volcanic eruptions and that therefore mankind is not responsible for alleged climate change. How correct is this?

Sara Mikaloff-Fletcher, an atmospheric scientist at NIWA, responded.

Volcanoes emit carbon dioxide, the most important greenhouse gas produced by human activities. How do these volcanic emissions compare with emissions from vehicles and other fossil fuel sources? Fossil fuel emissions are very well known from economic data on how much fuel is sold and consumed and basic combustion chemistry. Human beings emitted 36 billion metric tonnes of carbon dioxide in 2013 from fossil fuel burning and cement production. Earth’s volcanoes have been estimated to produce between 0.13 and 0.44 billion metric tonnes of carbon dioxide per year. Therefore, volcanic emissions of carbon dioxide are very small compared to human emissions.

The Earth is a big place, so you might wonder if it is possible that geologists have missed some volcanic emissions from remote locations. We can also use measurements of carbon dioxide in the atmosphere to determine whether it has accumulated more quickly than we would expect from human made emissions alone and how atmospheric carbon dioxide levels change after a major volcanic eruption.

Scientists have been measuring carbon dioxide in the atmosphere since 1958. While there are atmospheric carbon dioxide observing sites all over the world, the longest continuous record in the Southern Hemisphere is at Baring Head, New Zealand, which is near Wellington.

These measurements reveal that only about half of all the carbon dioxide we produce remains in the atmosphere, while the other half is absorbed by the oceans, plants, and soils. Earth’s natural systems have slowed climate change, rather than speeding it up.

In the period since atmospheric carbon dioxide measurements began, there have been three major explosive volcanic eruptions: Agung (1963), El Chichón (1982), and Pinatubo (1991). After each of these volcanic eruptions, the rate of carbon dioxide accumulation in the atmosphere slowed.

How could these volcanic eruptions cause carbon dioxide to decrease in the atmosphere, when volcanoes emit carbon dioxide? Volcanoes emit sulphur dioxide and other chemicals that cool the atmosphere and change patterns of cloud and rainfall. These changes affect the way carbon cycles through plants and soils, leading to additional carbon dioxide uptake by natural systems.

Thank you again for your question. The complex and often unexpected way that the carbon cycle responds to changes like volcanic eruptions is a big part of what makes this field of research so fun and exciting to me.