Finding the world's oldest ice core is a bit of a Goldilocks problem, Hubertus Fischer says.

There are many places where the ice is too thick, or perhaps too thin. The bedrock beneath could be too warm, melting important layers from below, or the ice could have folded and buckled, mixing up the layers in the core.

That's one reason Fischer and his colleagues, part of an international effort called the International Partnerships in Ice Core Sciences, or IPICS, have published a sort of "how to" guide for scientists seeking to find the location that might be just right to drill a 1.5-million-year-old ice core.

Their paper, called "Where to find 1.5 million yr old ice for the IPICS 'Oldest-Ice' ice core," was published yesterday in the European open-access journal Climate of the Past.

"What we hope to do is galvanize other people to be interested in the topic and help join us in doing all the geophysics research to help us find the right place," said Edward Brook, a geochemist at Oregon State University who was also a co-author on the paper.

At nearly double the age of the current oldest ice core, which goes back 800,000 years and was also found in Antarctica, the yet-to-be-drilled "Oldest Ice" core (researchers have given it a proper name) is a sort of holy grail for researchers studying past climates.

That's because for the last 800,000 years or so, the Earth's climate has switched fairly regularly between ice ages and warmer periods, at about 100,000-year intervals.

Hunting for ancient atmospheres
Prior to that, though, the cycles were different. Ice ages and warmer periods alternated about every 40,000 years.

Scientists know this because they've seen evidence of it in sediment cores, which go back further in time.

"We do know from studying ocean cores that a little bit before 800,000 years ago, the characteristics of the climate changed very dramatically," Brook said.

What they don't know is why.

Researchers like Brook and Fischer say the atmosphere of the past might offer some clues. And the only way to get samples of the atmosphere 1.5 million years ago is in the tiny bubbles of air trapped inside ancient ice.

"We would really like to know what the atmosphere was doing when we went through this major transition in cycles," Brook said. "For example, if CO2 were double what it was recently, that would tell us something about how the atmosphere reacts to CO2 levels."

Paleoclimate research such as this is one way researchers can understand how changes in carbon dioxide concentrations lead to changes in the Earth's temperature, a measure often referred to as climate sensitivity.

Data from sediment cores in lakes and oceans, which go back millions of years, offer some clues. Currently, evidence from those cores does not point to carbon dioxide levels being significantly higher at the time of the ice age cycle shift, Brook said.

On the other hand, the uncertainties in inferring atmospheric CO2 concentrations from sediment cores are relatively large, Brook said.

The 'domes' of the South Pole
An ice core provides a more direct measurement, not only of carbon dioxide but of other gases in the atmosphere at that time.

Now, researchers have the task of using the guidance outlined in Fischer's recent paper to home in on the oldest ice core record.

A next step, Fischer said, is to complete aerial radar surveys of certain regions, to narrow down a place where researchers should go out with hand-held radar to get higher-resolution data. He hopes a deep drilling project can be underway within three to five years.

The paper highlights parts of the Antarctic interior close to the major domes, which are the highest points on the ice sheet, as likely candidates to find the core.

"We haven't really pointed out a final location yet. What we've pointed out are major regions where it would be worthwhile to look into more detail," Fischer said. "It's still quite large, but compared to the size of Antarctica, that's small. It's manageable."

After an area is selected for a hand-held survey, scientists could do a quick reconnaissance drilling mission there to measure heat fluxes from the bedrock and see whether the ice there seems a likely candidate for the oldest core.

Richard Alley, a glaciologist at Pennsylvania State University who was familiar with the paper, wrote in an email that the authors have outlined a "sensible way forward."

"This is likely to be the best approach to finding undisturbed, in-order ice from great age," he wrote.

Alley also pointed out that scientists might be able to find ice older than 800,000 years by prospecting for it in places like the Allan Hills blue ice area in Antarctica, where old ice has risen to the surface. These samples would not contain an entire 1.5-million-year record but might allow researchers to extend the ice core history with discrete chunks of old ice found there.

"I suspect that a combination of the two approaches, looking for the old continuous records where indicated by the new paper, and looking for old ice where it is shallow but likely discontinuous, offers the greatest opportunity of success," Alley said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500