Ice Cores and Past Climates

Posted on September 30, 2010

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Paleoclimate topics:

Calibrating climate models.  Drilling ice cores in Antarctica.  Antarctic ice core data, etc.

Like some sap in a sentimental song, Sam, the planet Earth has gone through changes.  And that’s because, as you might expect, lots can happen in 4.5 billion years.

Continents float about, join, unjoin, oceans realign, natural levels of greenhouse gases rise and fall.  The Sun itself has changed temperature.  It’s hotter now than it was in the distant past.  The wobble of the Earth’s axis and its orbit—two independent factors, by the way—have changed as well.  And all of this shuffling and shifting about has had consequences in climate.

We were talking before, Sam, about modeling past climates.  One advantage to Earth possessing such a long and eventful past is that we have a long, involved, and evolving climate history to work with, and to measure against climate models.  We get to test the models, to see if they work consistently across many actual climate scenarios, and we get to refine and calibrate them in the process.

“What do you mean by calibrate?”

There are all kinds of factors influencing climate.  It’s useful to know how important each is in a given circumstance so that we can refine our predictions.  It’s like you have an equation, a plus b over c, etc.  Calibrating consists of finding increasingly more accurate values for a, b and c, etc., so that your equation—or your model—produces a useful and/or meaningful answer.

But we really can’t evaluate the models unless we have other kinds of evidence to weigh them against.  I’m going to start talking about some of those other kinds of evidence today.

“Oh goody,” says Sam.

I like an enthusiastic spirit, Sam.  It takes an enthusiastic spirit to go to Antarctica to drill holes in the ice.  Consider Vostok ice station, located two miles above sea level in the bleak interior of the highest, windiest, coldest and driest continent.  One day in July at Vostok, in the heart of southern hemisphere winter, it went down to minus 89.2o C (-128.6o F), which is the official record, although some say that another July dip in temperature of 91o C below (-132o F) was the actual coldest temperature.

You wouldn’t want to try Vostok out in the Thought Experiment Room, Sam?

“No.”

Chilly Willy wouldn’t like it.  But the scientists go there to drill because the Antarctic ice sheets are old, and preserve information from the time when they were formed.  Every ice sheet began as a snowfall that didn’t melt, which was covered over with another snowfall that didn’t melt, layer by layer, year by year, millennium by millennium, for at least 425 million years at Vostok, for even longer periods at drills sites such as Dome C.  At Vostok Russian and French scientists have extracted an ice core 3623 metres long—more than two miles—which provides a climactic record spanning four complete glacial periods.  The Dome C scientists have extracted a core not quite as long, but containing a record over even a longer span.

Here is some of the information which the Antarctic ice core gives us.

From Hansen, Storms of My Grandchildren, p. 37

The graph on the top shows the ice core temperature data from Vostok, starting 425 thousand years ago and ending in 1750.

The second graph shows CO2 concentrations over the same period.  They extract atmospheric information from ice core data, too, Sam.  Tiny bubbles of air trapped in the ice tell us what the CO2 levels were like at the time the ice froze.

“What about the third graph?  You’re not going to tell me that you can tell sea levels from ice core data too.”

No, Sam.  You’re right there.  The information from the third graph is from another source, which I’ll discuss in more detail next time.  But it’s amazing how information from a totally different source matches up so well.

In fact, Sam, pause for a moment and look at those graphs.  See how CO2 levels, Antarctic temperature and global sea level all closely correlate with each other over time.  Isn’t that suggestive to you, Sam?

“I think you need to explain more.”

I will, Sam, I will.  Simple Sam wouldn’t be Simple Sam if you weren’t also being stubborn.  Right, Sam?  —  Now, no eye rolling.  Aren’t you supposed to be too mature for that?

___________

Here are some peer-reviewed sources in the area of Paleoclimatology.

Kennett, J. P. (1977) Cenozoic evolution of Antarctic glaciation, Circum-Antarctic Ocean, and their impact on global paleoceanography, Journal of Geophysical Research—Oceans and Atmospheres 82, 3843–3860.

J. R. Petit, J. Jouzel, D. Raynaud, N. I. Barkov et al. “Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica,” Nature 399 (June 3, 1999):  429-36.

Jansen, E., Overpeck, J. T., Briffa, K. R., Duplessy, J.-C., Joos, F., Masson-Delmotte, V., Olago, D., Otto-Bliesner, B. L., Peltier, W. R., Rahmstorf, S., Ramesh, R., Raynaud, D., Rind, D., Solomina, O., Villalba, R., and Zhang, D. (2007) Paleoclimate, Climate Change 2007: The Physical Science Basis Contribution of Working Group I to the Fourth Assessment Report of Intergovernmental Panel on Climate Change (Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M., and Miller, H. L., Eds.), Cambridge University Press, Cambridge.

Rind, D. (2002) Climatology – the sun’s role in climate variations, Science 296, 673–677.

Sijp, W. P., England, M. H., and Toggweiler, J. R. (2009) Effect of ocean gateway changes under greenhouse warmth, Journal of Climate 22, 6639–6652.

Posted in: Climate School