Rock doc: New ice cores shed more light on past climate change

By June 19, 2013

Late in the last century scientists published reams of data about Earth’s climate derived from ice cores taken from Greenland and Antarctic glaciers. By drilling down into the ice with hollow bits (think of using a spinning pipe as a drill) workers were able to pull columns of ice up to the surface. The material brought to light in this way was very special for several reasons.
First, the ice cores show annual layers going back in time. That means scientists can count backwards through time from the surface downward, a bit like school kids can count the rings of a tree, measuring out history year by year.
The ice in the cores had originally been snow at the surface, snow that was buried by later snows and slowly became compacted to form a coarse icy material called “firn” and then glacial ice itself. The ice has tiny bubbles of air in it, air that was trapped in the snow layers in times gone by. This means that scientists can analyze the small air samples that are sealed in the ice and determine the composition of the air that was blowing around the Earth in past millennia.
Chemical analyses of the air in the glacial ice show that our planet’s atmosphere has gone through cycles of changes over thousands of years. In particular, the concentration of greenhouse gases like carbon dioxide and methane (the main ingredient in natural gas) has varied over time. The specific dates of the changes can be determined by counting up the layers in the ice core — a pretty nifty trick.
If the only thing the ice cores told us was how much Earth’s air has changed, that would be interesting in itself. But scientists are also able to analyze the ice and from it infer a couple of things.
First, the ice is made of hydrogen and oxygen atoms, which together form the familiar water molecule of high school science class. But atoms are not all created equal. Some oxygen atoms weigh more than other oxygen atoms – we say that oxygen has different isotopes. The great thing about that fact is that using the ratios of the isotopes of oxygen found in the ice cores, scientists can calculate past temperatures. Again, by counting up the annual layers in the ice core, we can quite precisely date when temperature changes occurred. Beyond that, some other information about weather and climate can be deduced by things like how much dust is in a particular layer of ice. In short, the ice cores from Greenland and Antarctica have told us a lot about climate change going back hundreds of thousands of years.
As interesting as the ice core work in the polar regions has been, it’s been rather limited geographically. That is to say, we are interested in climate all over the Earth, and in particular where we live, more than we are interested in climate change just at the poles.
Now new information about climate is starting to be unearthed from two ice cores taken from Peru’s Quelccaya Ice Cap. These two cores have nearly 1,800 annual layers in them. That means they are not anywhere near as long as the polar ice cores, but they are in an interesting place — at a low latitude rather than at high latitudes.
“These ice cores provide the longest and highest-resolution tropical ice core record to date,” said Prof. Lonnie Thompson in a press release from Ohio State University. Thompson is the lead author of a study of the Peruvian ice core. “In fact, having drilled ice cores throughout the tropics for more than 30 years, we now know that this is the highest-resolution tropical ice core record that is likely to be retrieved,” he said.
The Peruvian ice core appears to show the influence of El Nino changes at sea. This gives us a new way to study temperature fluctuations in part of the Pacific Ocean over nearly two millennia.
Climate is complex and a great deal of effort is required to learn how it has varied in Earth’s past. But step-by-step we are learning a great deal more than we knew when this Rock Doc first read about the Ice Age in elementary school.
Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University.

E. Kirsten Peters

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