On million year time scales, does volcanism cause global warming or cooling? The answer is both, depending upon the longevity of the volcanism and its effluents. Prolonged volcanic emissions – over tens of thousands to millions of years – of greenhouse gas carbon dioxide will warm the earth’s surface. But magmatism and volcanism create continental crust. During volcanic episodes and after the magmatic system dies, this new crust consumes carbon dioxide in chemical weathering reactions. This draw down of atmospheric carbon dioxide can result in global cooling, as is inferred to have resulted in the Cenozoic beginning around 30 million years ago, after collision of the India -Eurasian plates. Volcanism and tectonic activity can both warm and cool the earth’s surface as magmatic arc systems grow and die.
Volcanism also ejects sulfur particles into the atmosphere. These particles block and reflect sunlight away and this albedo effect may result in cooling of the earth’s surface. Volcanic ash falling on both land and sea may act as a fertilizer, enhancing organic productivity and further drawing down and sequestering carbon dioxide through increased organic carbon burial. A recent paper published in Geology by Gerilyn Soreghan and colleagues (open access) points to a temporal coincidence between explosive eruptions and glacial conditions during the Late Paleozoic. The researchers suggest that the prolonged icehouse conditions from around 360 million years ago to 260 million years ago resulted from explosive volcanism and effects of sulfate aerosols.
This paper has prompted a thoughtful commentary (open access) by Rice University geologists Cin-Ty Lee and Sylvia Dee on the broader controls of volcanism and crustal weathering on global climate. On the particular question of whether the Late Paleozoic ice age was a result of sulfate ejections, they differ somewhat from the authors of the study. Cin-Ty Lee and Sylvia Dee point out that the residence time of sulfur particles in the atmosphere is just a few years. To maintain a global icehouse for a 100 million year period would require large explosive eruptions every few years over tens of millions of years.
They point to an example of another period of enhanced magmatic activity in the Cretaceous Period. Field evidence from the continental interior of the U.S. shows just about 200 eruptions over a 10 million year period. Only a few of these were large enough to have ejected significant amounts of aerosols into the stratosphere. That is not to say that sulfate aerosol albedo cannot cool the planet. But it may happen over shorter 1,000-10,000 year time scales. In case of the Late Paleozoic icehouse, they suggest that the pattern of cooling may hint at the causative factor. Numerous short-lived cooling events would be suggestive of explosive volcanism as the cause.
On longer time scales, carbon dioxide will play a larger role in modulating climate. Explosive eruptions and resulting sulfate particle emissions are only a small component of magmatic flux. On the other hand, carbon dioxide degassing is taking place even without eruptive activity. Long lived magmatic activity will result in a warming trend due to an increase in atmospheric carbon dioxide. Post magmatism, a drop in atmospheric carbon dioxide levels and cooling due to silicate weathering also takes place on longer million-year time scales.
An extract from Cin-Ty Lee and Sylvia Dee’s commentary:
More broadly, the nature by which volatiles are exchanged between planetary interiors and their surfaces is rich with complexity. The magnitude and style of magmatism not only controls volatile degassing but also erosion, weathering, radiative balance, and biological productivity. How magmatic processes change through time and with geodynamic states is an area ripe for interdisciplinary research and new discoveries. Soreghan et al.’s work is an example of how investigating these processes from deep time to the present, as well as on Earth and other planets, will force us to rethink how planetary systems operate.
The geologic record shows that enhanced phases of volcanic activity sustained over thousands of years can cause the earth’s climate to tilt towards a long lasting greenhouse or an icehouse. People who claim that the warming of the earth’s surface over the past few decades is due to natural causes like volcanic eruptions and not fossil fuel burning must understand the time scales and amounts involved. Even big volcanic eruptions that occur every few years emit only a few million tons of CO2. Awkwardly, for anthropogenic warming deniers, these eruptive events may result in a short term cooling phase due to the effects of sulfate aerosols. A recent survey puts the total global emissions due to volcanic eruptions and non-eruptive degassing of magma to be about 0.3 billion tons per year. In contrast, human activity is putting 30-40 billion tons of CO2 in the atmosphere every year.