23-Million-Year-Old Leaves Helped Solve an Antarctic Climate Mystery

As Antarctica melts, it's releasing centuries' worth of locked-away ice into the ocean and pushing sea levels higher. It's scary stuff. But it also isn't the first time this has happened. Looking back 23 million years ago, the Antarctic ice sheet was also shrinking quickly. Until now, scientists weren't totally sure they understood the cause of that ancient melt.

Now a team of researchers thinks they've figured it out. Based on an analysis of ancient leaves that would have sucked up CO2, they say that all those years ago, carbon dioxide levels spiked over a relatively short stretch of time. Meanwhile, over 100,000 years, the ice sheet melted from 125 percent larger than the size it is today, to less than half of that size.

It's important to understand all of this, given the pressures of climate change. Once all that ice was gone, it took millions of years for it to grow back again.

To get to the bottom of this, researchers looked at mummified leaves from the bottom of an ancient lake in New Zealand, which was "just an extremely lucky combination of circumstances," lead author Tammo Reichgelt of Columbia University told me in an email. The lake was deep and had low oxygen levels at its bottom, which means that any prehistoric leaves that sank down there stayed relatively well-preserved, despite being 23 million years old.

"We stained the leaf surface with violet dye so that it brought out microscopic features," he said, including stomata, the little openings through which a leaf sucks up CO2 and breathes out oxygen.

The carbon isotope composition of the leaves, and the stomatal data they retrieved, gave them estimates of carbon dioxide that would have been in the atmosphere 23 million years ago. The work is described in a paper in in Earth and Planetary Science Letters.

Over the course of 20,000 years, atmospheric CO2 levels climbed from about 500 parts per million (ppm) to between 750 and 1,550 ppm. They later went back to about 425 ppm. An accompanying release from Columbia notes that today's atmospheric CO2 has surpassed 400 ppm, and is still climbing, the highest levels ever experienced by humans.

So, what caused all of this carbon dioxide to dump into the atmosphere, millions of years ago? During glacial periods, carbon gets stored in different ways, Reichgelt told me, "such as permafrost, [or] burial of organic carbon in the ocean." What happens to pull all that carbon out into the atmosphere isn't totally clear, he said, but once it starts happening, "positive feedback mechanisms" means it keeps happening.

"Preceding the spike in CO2 levels 23 million years ago, there was a particularly large glaciation, so there may have been an unusual amount of carbon stored, which was then subsequently released," he said.

Antarctica seems to be particularly sensitive to this, with an ice sheet that "can be more easily lost than built up," according to Reichgelt.

We know that, 23 million years ago, some kind of critical "tipping point" was reached, and the Antarctic ice didn't recover for millions of years. Let's just hope we figure out how to deal with climate change before that happens again—or that we haven't hit it already.

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