The Earth’s Hidden Carbon
We know with depressing clarity how much CO2 humans add to the atmosphere. The carbon footprint of our planet, however, is much more elusive.
It all comes down to—and indeed out of—volcanoes. The Earth has a huge internal store of CO2, which, for most of its 4.5 billion year lifetime, has been steadily seeping through the crust. Volcanoes are the exhaust vents of our planet.
Each year, volcanic degassing sheds around 540 million tonnes of CO2* into our skies. That's a puff of smoke compared to the anthropogenic contribution (human activities release close to 30 billion tonnes)—but it's an important puff; understanding natural CO2 emissions is essential to place our own in context.
And that's where we hit a snag.
Carbon dioxide might be the hottest topic around, but surprisingly, we still have very little idea of how much our planet puts out. (The 540 million tonnes I quoted is just a best estimate). And the main reason behind this uncertainty is simple: of the world's 550 active volcanoes, only 6% have actually been measured. At all.
That's partly down to geography. The volcanoes we know best—the likes of Etna, Vesuvius and Mount St. Helens—tend, by no coincidence, to be close to large cities. Most of the unmeasured ones are more far-flung.
From the isolated islands of the North Pacific, to the dense jungle of the Indonesian archipelago, the majority of Earth's volcanoes are—shall we say—out of the way. The unseen CO2 seeping from these remote mountains is the key to a more complete understanding of the natural carbon cycle.
There are a number of ways that volcanologists can get to this evasive carbon. One is obvious—climb more mountains (with highly sophisticated gas-measuring gear, of course). Alternatively, satellite measurements, which have the potential to provide almost continuous, worldwide coverage, are a high hope for future CO2 detection.
But to confront the problem fully, volcanologists will have to look deeper.
In Deep Water
Measuring all those untouched volcanoes might seem like a daunting enough task—and it is. But in fact, that's only the beginning. I'm afraid I've been a bit misleading. When I mentioned the "550 active volcanoes", I left out an ending: On land. There's an elephant in the room—or rather, under the sea.
Eruptions on land get all the glory, but submarine volcanism is every bit as important. At mid-ocean ridges, magma wells up from the mantle, creating new seafloor which gradually spreads, driving the conveyor belt of plate tectonics. This isn't volcanic activity as most people know it—but it emits CO2 all the same. And if you thought measuring CO2 on land was difficult (and trust me, it is), beneath the waves—several kilometers beneath the waves, that is—it becomes almost impossible.
It's here then, in the gloomy depths, that the biggest mystery lies. In order to fully understand the Earth's elusive carbon footprint, volcanologists will have to look down—and perhaps acquire submarines.
* Burton, M. et al. (2013) Deep Carbon Emissions from Volcanoes. Reviews in Mineralogy & Geochemistry, volume 75