In the past 150 years, human emissions have put a lot of carbon dioxide in the air. We now measure a concentration of about 400 parts per million. This is about 40% higher than at any time in the past 400,000 years. Of all of the conclusions of modern climate science; this is one of the most reliable. But, despite all of the evidence, some people persist in claiming that the recent rise in carbon dioxide is all natural— for example, they say that instead of it being caused by humans, it all came out of volcanoes. Now, it is quite true that volcanoes emit some carbon dioxide. Over very long periods of geological time those small amounts can add up to make a really significant change to the atmosphere.
However, over a couple of hundred years, the emissions aren’t large enough to make a difference. There are two main classes of volcano: there are the ones that erupt under the ocean and the ones that erupt into the air. Both kinds are linked to the goings-on at the boundaries of the tectonic plates and to the upwelling of hot rock from the Earth’s mantle; the layer below the Earth’s crust. The undersea volcanoes are by far the more numerous, making up about 90% of the world’s volcanoes, although few of us have ever seen them. These volcanic chains are where new ocean crust is produced. But undersea volcanoes don’t produce very much carbon dioxide—only about 100 million tonnes per year—about the same amount as an average US state emits. Humans produce about 350 times as much carbon dioxide as the undersea volcanoes do. Carbon dioxide not only gets produced at the oceanic ridges, it also gets consumed there. What happens is that the newly formed basaltic rock undergoes chemical changes when it contacts seawater. This reaction absorbs carbon dioxide from the water at a rate of about 150 million tonnes per year.
The mid-ocean ridge volcanic processes as a whole, therefore, probably consume more carbon dioxide than they emit. We are much more familiar with the kind of volcanoes that erupt into the air. The biggest chain of these is the so-called “Pacific Ring of Fire”. This is a belt running all the way around the ocean from New Zealand to Japan, then to Alaska and down to the Andes. Old oceanic crust is consumed at these places and they form volcanoes that produce much more carbon dioxide than the ones under the sea. The magma in these volcanoes comes not just from the Earth’s mantle, but also from the melting of the more carbon and water-rich rocks in the crust. One reason these types of volcano tend to be more explosive is because of the larger amount of water vapour and carbon dioxide in their magma. Mount Etna in Sicily is one of the most prolific carbon-dioxide producing volcanoes in the world. It produces about 13 million tonnes per year, but this amount is still only about half as much as what Sicily’s five million people emit from burning fossil fuels.
In addition, dormant volcanoes and volcanic lakes together emit as much carbon dioxide as the actively erupting volcanoes do. Altogether, volcanoes that emit carbon dioxide into the air produce much more than undersea volcanoes: about five times as much. Volcanic rocks on the surface undergo weathering and this chemical process absorbs carbon dioxide out of the air, about 180 million tonnes per year, that’s approximately one-third of the amount put into the air by volcanoes. So if we add up all the sources of volcanic carbon dioxide, we get 640 million tonnes per year. Once we subtract the carbon dioxide that the reactions with volcanic rocks consume, we are left with a net 310 million tonnes per year. This last amount is roughly equal to the human emissions from the country of Turkey, that’s less than one percent of all human emissions. Human emissions for the planet as a whole in 2012 were 60 to 120 times bigger than volcanic emissions. Carbon dioxide emissions from cement-making alone are 3 to 6 times bigger than those from volcanoes.
Not only are volcanic emissions much too small to account for the rising carbon dioxide levels in the air, but, over the past few thousand years, natural emissions and natural sinks must have been in rough balance. The carbon dioxide composition of the air started to change really quickly after the 1950s. We can readily explain this as being due to the greatly increased rate of consumption of fossil fuels after the end of the Second World War. On the other hand, if volcanoes had suddenly started to erupt many times faster in the second half of the twentieth century, we surely would have noticed. After all, volcanoes don’t just silently produce carbon dioxide, they also throw out huge quantities of ash and magma and they often cause havoc for humans living nearby. Only about 40% of the carbon dioxide emitted from any source remains in the air, the rest goes into the oceans and is taken up by plants on land.
If we add up the carbon dioxide emissions and convert them into concentrations in the air, we see that emissions from humans over the past hundred years fit the observations like a glove, but the volcanoes don’t even come close. People who incorrectly blame volcanoes for the change in the air take the fact that volcanoes do indeed produce some carbon dioxide and then they jump to the false conclusion that this amount is enough to explain the increase we have measured. And they haven’t done the basic arithmetic that shows that it isn’t nearly enough to make any real difference at all in such a short time period. We know what caused the recent rise in carbon dioxide concentrations. We did..