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Volcanic CO2 Caused Ancient Episodes of Global Warming

In order to predict our climate future, scientists spend a lot of time looking into the past, trying to understand what conditions were like during times when the planet was much warmer or much cooler than it is today. The latest instance: a report published in the February issue of the journal Geosphere, offering a plausible explanation for swings between long stretches of greenhouse warming and dramatic cooling over millions of years.

The culprit, said lead author Cin-Ty Lee of Rice University in an interview, appears to be the ebb and flow of eruptions from volcanic mountain chains around the globe, which alternately pump heat-trapping carbon dioxide into the atmosphere and allow it to be re-absorbed by rocks and living things.

While these changes happen far too slowly to have any bearing on the current, human-caused episode of global warming, they do help scientists to understand the relationship between carbon-dioxide concentrations in the atmosphere and global temperature changes.

The new idea is that it’s a more sustained series of eruptions from volcanoes in strategic locations along the edge of continents that cause these long periods of warmth. 
Credit: flickr/NASA’s Marshall Space Flight Center

The main effect of volcanoes in the modern world is to cool the planet by throwing particles of sulfur dioxide high into the stratosphere, where they temporarily block the Sun. In the distant past, however, paleo-climatologists have long believed that volcanic activity was a major cause of global warming. Massive eruptions — far more powerful than anything going on today — can pump large amounts of heat-trapping carbon dioxide into the air. The problem with that theory is that those events don’t last long enough to explain periods of warmer climate that have lasted tens of millions of years.

The new idea Lee and his colleagues are proposing is that it’s a more sustained series of eruptions from volcanoes in strategic locations along the edge of continents that cause these long periods of warmth. When they subside, the CO2 eventually drops, and the planet plunges into a colder period, with year-round ice deposits at the poles — the situation we’re in today.

The genesis of this idea was, Lee said, “kind of random.” About four years ago, he dropped by an informal weekly lunchtime seminar at Rice known as “Looney Noons,” in which scientists talk about works in progress and speculative theories rather than their more polished work. The talk that day was about a mass extinction that wiped out a big fraction of the planet’s species about 93 million years ago, and a light bulb went off. “I remembered learning as an undergrad years before,” he said, “that there was a lot of volcanic activity at that time in the Sierra Nevada” — a range which, like the Cascades and the Andes, is only intermittently volcanic today.

According to geologists volcanic mountain ranges were both more extensive and more active during period starting about 140 million years ago. 
Credit: flickr/NASA Goddard Space Flight Center

That led Lee and another colleague to think about the difference between volcano types, and particularly about the unique characteristics of volcanic ranges on the edges of continents. These are generally created when plate tectonics shoves a slab of the Earth’s oceanic crust under its continental crust. The oceanic crust melts and sends hot magma up through the continental crust, where huge amounts of carbon-bearing rock, mostly limestone and marble, lie. “These rocks are made up of about 40 percent carbon, so as the magma bubbles upward and dissolves that rock, it frees up an enormous amount of carbon that eventually reaches the atmosphere,” Lee said.

According to geologists, these volcanic mountain ranges, known as continental arcs, were both more extensive and more active during period starting about 140 million years ago, as the giant continent known as Pangea broke up into the continents we know today. As they drifted apart, the continents’ leading edges — western North and South America, eastern and southern Asia — plowed over oceanic crust, triggering an increase in carbon-rich eruptions. That finally waned about 50 million years ago, at about the time when the planet went through a gradual but inexorable cooling.

Lee said if this theory is correct, these continental-arc volcanoes would only have a warming effect if there were plenty of carbonate rock deposits on continental margins. And since these deposits come, in part, from the shells of living organisms that once lived on continental shelves, this could only have happened after evolution had produced those organisms in great numbers, which happened just a bit over 500 million years ago.

“This is very speculative,” Lee said, “but it may be that the presence of life on Earth has actually changed the impact volcanoes have on climate.”

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