Foraminifera are aquatic creatures the size of the head of a pin that live for about a month, but have helped University of California researchers discover what may be the making and unmaking of ice ages.

Geology professors Howard Spero of UC Davis and David Lea of UCSB believe they have discovered evidence that carbon buildup can end or begin ice ages depending on climatic conditions. They have researched the connections between deep-sea ocean circulation, changes in the carbon cycle, and planetary climate, for more than a decade. According to Spero, the rapid increase in atmospheric carbon dioxide and its relation to other climatic events that occurred at the end of the ice age have never been adequately explained, but are now linked under one testable hypothesis.

Two years ago magnesium levels in the foraminifera calcium carbonate shells were found to correspond to the historical temperatures and carbon content in tropical waters. A rapid increase in carbon dioxide concentration, known as a carbon isotope event, has been connected with the climatic shift that terminated the last Ice Age approximately 16,000 years ago.

“By analyzing these microscopic shells and carbon-14 dating them we have linked the timing of the [carbon isotope] event with the warming of Antarctica and the tropical Pacific,” Spero said.

Atmospheric carbon dioxide levels and shifts in ocean circulation are closely related and can strongly affect the climate of the planet.

“The implications of the discovery are that ocean circulation and warming changes can occur extremely rapidly,” Spero said. “We have to consider what ocean circulation changes could produce another climatic shift.”

Humans increase the atmospheric carbon dioxide concentration by burning fossil fuels. Lea said the most important environmental factor that causes the planet to transition out of an ice age is a change in carbon dioxide concentration. Carbon dioxide in the atmosphere increased from 190 parts per million at the peak of the last ice age to 280 parts per million during the warm climate that preceded the Industrial Revolution. Since the Industrial Revolution, when humans began burning fossil fuels, the concentration has risen to more than 370 parts per minute.

“The warming that occurred between the last ice age and the Industrial Revolution was the largest warming on Earth ever,” Lea said. “The consequences of adding the same amount of Carbon and more [to the atmosphere] are going to be pretty large.”

By understanding the changes that occurred at the end of the last ice age, we can better understand how the climate system, the ocean system, and the biosphere interact and thereby gauge how the climate will respond to human influence on the planet, Lea said.

Atmospheric carbon dioxide levels affect ocean circulation which has a profound impact on Earth’s climate, said Matthew Schmidt, a second-year graduate student in interglacial paleo-oceanography who works with Spero.

Foraminifera shells are collected from samples of the ocean floor in the Galapagos region. Data taken from these samples shows the temperature and salinity of the ocean, which indicate the water density over time. Differences in water density allow for circulation, most importantly in the North Atlantic Ocean where “bottom” waters of all oceans form and deep-water circulation originates.

Schmidt said too much rain in the North Atlantic freshens the surface water and cuts off deep circulation, which can lead to another ice age. He attributed a rise in rainfall to a rise in atmospheric carbon dioxide. This rise leads to global warming, which causes cloud formation and rain.

Lea received a prestigious Guggenheim Fellowship to continue his research and will work with scientists in Cambridge next year, using the transition from the ice age as a “proving ground” for environmental ideas.

“We continue to delve into the geologic record of climate change for new information,” Spero said.