Twenty-four million years ago, glaciers were advancing over the land, leaving massive ice sheets and changing the earth’s surface. Carbon dioxide levels in the atmosphere were so low that climates changed from tropic to frigid. Scientists have wondered why the entire planet was not enveloped in ice, what kept the last ice age in check. The answers, they believe, can be found in interactions between volcanoes, rocks, and land plants.

During the last ice age, large volumes of CO2 were released into atmosphere by volcanoes but the effect was blunted by the weathering and erosion of volcano-ejected silicate rocks, which buries carbon-containing minerals in the earth’s crust. Researchers at Yale University, the Carnegie Institution for Science, and the University of Sheffield say that land plants were helping the process along. Their roots secreted acids that dissolved minerals and increased the amount of carbon dissolved in groundwater. When the erosion of rocks lowered the atmospheric levels of CO2 so low that the plants were starved, they could no longer help remove the carbon, allowing the atmosphere to stabilize until an equilibrium was reached.

The study published in the July 2 issue of Nature used simulations of the global carbon cycle and observations from plant growth experiments. The researchers found that when CO2 levels were so low plant growth was compromised, the plants could no longer help remove CO2 from the atmosphere as fast as volcanoes and other sources could provide it.

“Ultimately, we owe another large debt to plants” said co-author Ken Caldeira from the Carnegie Institution for Science. “Aside from providing zesty dishes like eggplant parmesan, plants have also stabilized Earth’s climate by inhibiting critically low levels of CO2 that would have thrown Earth spinning into space like a frozen ice ball.”

Looking back 74,000 years, another research team studied a specific volcano eruption to gauge its effect on the ice ages. The volcano, named Toba, ejected 1,000 times more rock than Mount St. Helens in the 1980s. It is believed to have cooled the climate by 10 degrees Celsius and triggered a global famine.

Located on Sumatra, an island in western Indonesia, Toba released sulfur dioxide along with carbon dioxide. The sulfur dioxide combined with water vapor to form sulfate aerosols, which blocked solar radiation.

 Running scenarios of eruptions even more powerful than Toba’s, researchers said the result of the massive eruption was a cooler climate that lasted only decades, not the 1,000-year period of extreme cold, and did not usher in an ice age. Alan Robock, climatologist of Rutgers University and leader of the team, reported their results in the May 27 issue of Journal of Geophysical Research, Did a volcanic eruption trigger an episode of widespread glaciation?

The researchers’ answer, no, gives us some reassurance that another massive eruption somewhere on the planet might not usher in a prolonged ice age. But it would cause misery for humankind. Global temperatures might lower by as much as 17 degrees Celsius for several years and recovery from that might take decades.

 The theme of the August 2009 issue of Beyond Penguins and Polar Bears is Icebergs and Glaciers. Readers can find background information on glaciers in the polar regions and their impact on the planet today. Check the archives for issues featuring rock and minerals and other topics related to global climate. Included in each issue of the online magazine for K-5 teachers are suggestions for lessons and classroom activities that combine science and literacy instructions.