Volcanic gasses and ash emanate from the summit eruptive vent as a vast plume, and from surrounding fumaroles at Kilauea Volcano on 28 May 2009. The vent, which formed in March 2008, broke a 26-year-long period of no eruptive activity at Kilauea's summit.

Robin Wylie, is a doctoral candidate in volcanology, at University College London. He contributed this article toLiveScience's Expert Voices: Op-Ed & Insights.

The exploding hills really give the game away: We've always known the Earth is a smoker. The true extent of its habit, though, is only just beginning to surface.

Before the human species found its talent for pyromania, atmospheric levels of the Earth's greenhouse superstar, carbon dioxide (CO 2 ), were controlled, for the most part, by volcanoes.

Since our planet emerged from the debris which formed the solar system, some four and a half billion years ago, a lifetime supply of primordial carbon has been locked away in the mantle — against its will. Partnering with oxygen and smuggled as a dissolved gas in liquid rock, it breaches the surface at our planet's volcanic airways: CO 2 , then, has been seeping into the planet's atmosphere for as long as there has been one.

Until the end of the 20th century, the academic consensus was that this volcanic output was tiny — a fiery speck against the colossal anthropogenic footprint. Recently, though, volcanologists have begun to reveal a hidden side to our leaking planet.

Exactly how much CO 2 passes through the magmatic vents in our crust might be one of the most important questions that Earth science can answer. Volcanoes may have been overtaken in the carbon stakes, but in order to properly assess the consequences of human pollution, we need the reference point of the natural background. And we're getting there; the last twenty years have seen huge steps in our understanding of how, and how much CO 2 leaves the deep Earth. But at the same time, a disturbing pattern has been emerging.

In 1992, it was thought that volcanic degassing released something like 100 million tons of CO 2 each year. Around the turn of the millennium, this figure was getting closer to 200. The most recent estimate, released this February, comes from a team led by Mike Burton, of the Italian National Institute of Geophysics and Volcanology – and it’s just shy of 600 million tons. It caps a staggering trend: A six-fold increase in just two decades.

These inflating figures, I hasten to add, don't mean that our planet is suddenly venting more CO 2 .

Humanity certainly is; but any changes to the volcanic background level would occur over generations, not years. The rise we’re seeing now, therefore, must have been there all along: As scientific progress is widening our perspective, the daunting outline of how little we really know about volcanoes is beginning to loom large.

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Quiet monsters

The exhalations of our planet can be spectacularly obvious. The fireworks, though, are only part of the picture. We now know that the CO 2 released during volcanic eruptions is almost insignificant compared with what happens after the camera crews get bored. The emissions that really matter are concealed. The silent, silvery plumes which are currently winding their way skyward above the 150 or so active volcanoes on our planet also carry with them the bulk of its carbon dioxide. Their coughing fits might catch the eye — but in between tantrums, the steady breathing of volcanoes quietly sheds upwards of a quarter of a billion tons of CO 2 every year.

We think. Scientists' best estimates, however, are based on an assumption. It might surprise you to learn that, well into the new century, of the 150 smokers I mentioned, almost 80 percent are still as mysterious, in terms of the quantity of CO 2 they emit, as they were a generation ago: We've only actually measured 33.

If the 117 unsampled peaks follow a similar trend, then the research community's current projection might stand. But looking through such a small window, there's no way of knowing if what we have seen until now is typical or not. It's like shining a light on a darkened globe: randomly, you might hit Australia, and think you’d seen it all – while on the edge of your beam, unnoticed, would be Asia. Our planet's isolated volcanic frontiers could easily be hiding a monster or two; and with a bit of exploration, our estimate of volcanic CO 2 output could rise even higher.

You'd think that would be enough. That might be my fault — I tend to save the weird stuff until the end. Recently, an enigmatic source of volcanic carbon has come to light that isn't involved with lava — or even craters. It now seems that not only is there CO 2 we can't get to, there's some we can't even see.

Carbon dioxide is always invisible, but its presence can be inferred in volcanic plumes — betrayed by the billowing clouds of water vapour released alongside it. Without the water, though, it's a different story. The new poster-child of planetary degassing is diffuse CO 2 — invisible emanations which can occur across vast areas surrounding the main vents of a volcano, rising through the bulk of the mountains. This transparent haze is only just beginning to receive proper attention, and as such we have very little idea of how much it might contribute to the global output.

Even more incredibly, it even seems that some volcanoes which are considered inactive, in terms of their potential to ooze new land, can still make some serious additions to the atmosphere through diffuse CO 2 release. Residual magma beneath dormant craters, though it might never reach the surface, can still 'erupt' gases from a distance. Amazingly, from what little scientists have measured, it looks like this process might give off as much as half the CO 2 put out by fully active volcanoes.

If these additional 'carbon-active' volcanoes are included, the number of degassing peaks skyrockets to more than 500. Of which we've measured a grand total of nine percent. You can probably fill it in by now — we need to climb more mountains.

The author's most recent Op-Ed was "Hawaiian Mythology Digs Deep into Volcanic Past." The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on LiveScience.