Over the course of around 100,000 years, glaciers advance and retreat across the world due to the way the Earth “wobbles” during its orbit around the Sun. On million-year-long timescales, however, another mechanism determines whether Earth is in an icehouse state, where glaciers have proliferated across much of the planet's surface, or a greenhouse state, where they have retreated and the poles can be forested and warm: plate tectonics, the motions of vast masses of continental and oceanic rock that shape our world.

A new, groundbreaking study in Science has taken a look at one of this mechanism's key features – volcanism – and tracked how it’s changed the global climate over the course of 720 million years. Using their own fieldwork and the results of nearly 200 previous peer-reviewed studies, they have come to the conclusion that huge effusions of carbon dioxide have occurred at least three times in the ancient past, which had a profound warming effect on the planet.

Remarkably, they imply that volcanoes have often been the force that drives the planet from an icehouse state to a greenhouse state. Without this warming trigger, life on Earth would have evolved very differently.

^{A zircon, seen here through an optical microscope, is a common component of continental arc volcanism, the type that often produces vast amounts of carbon dioxide.  Denniss/Wikimedia Commons; CC BY-SA 3.0}

For their research, they looked at volcanoes that form when a dense oceanic plate slides under or “subducts” beneath a less dense continental plate. This drives complex volcanism that often taps into deep reservoirs of carbon. When these volcanoes erupt, they tend to produce a lot of carbon dioxide gas, which escapes into the atmosphere and warms it.

Zircon, a resilient mineral produced during massive asteroid impacts, is also produced by this type of volcanism more than any other variant. By looking at the sedimentary deposits of this so-called continental arc volcanism, and tracking how much zircon was produced across the last three-quarters of a billion years, the researchers could produce estimates of how pronounced this volcanism – and its associated carbon dioxide output – was.

“Ultimately, we find that during intervals of high zircon production we have greenhouse conditions,” said Ryan McKenzie, a postdoctoral researcher at Yale University and lead author on the paper, said in a statement. “As zircon production diminishes, we see a shift into our icehouse conditions.”

This study confirms a theory long-held by many geosciences, and links neatly into the overall effect that plate tectonics has on climate change: When the world has finished assembling supercontinents, arc volcanism dies off somewhat, and the world cools. When supercontinents break apart, arc volcanism is kick started and the world begins to warm.

^{The proportion of new zircons (red line) correlate with increased volcanism, increased carbon dioxide emissions, warmer worlds (red shaded regions) and the break-up of massive continents. McKenzie et al./Science}

Volcanism in the long-term may warm the world, but as another study published this week showcased, the gargantuan movement of plates can also cut off the magmatic supply to these volcanoes, effectively smothering them. In addition, when oceanic crust is forced into the sky, it acts as a huge sink for carbon dioxide, pulling it out of the atmosphere in erosional processes and cooling the world.

Climate change deniers often bring up volcanism as their “get out of jail free” card, claiming they will always decide the fate of the world’s climate. This is categorically wrong: As this new study points out, the climate change-inducing volcanism they researched only takes place on scales of tens of millions of years.

In the short term, all even the most violent volcanic eruptions will do is emit enough sunlight-reflecting sulfur-dioxide-turned-sufuric-acid-droplets to take away the next year’s summer.