Greenland Warming at Last Deglaciation: the Younger-Dryas Not So Cold

by Emil Morhardt

I’ve been blogging recently about papers that claim the thousand-year cessation of global warming in the midst of the last deglaciation—known as the Younger-Dryas (Y-D)—was triggered by a comet. Buizert et al.’s (2014) paper on Y-D temperature changes doesn’t address the comet question, but another equally interesting one: why did the sudden reversal of temperature 12,800 years ago (whatever it was triggered by) cause the temperature to plunge clear back to what it was before any warming had started? That’s what the relative deuterium and oxygen-18 concentrations from the Greenland Ice Sheet ice cores imply—more about that in a moment. Nevertheless, it seemed unlikely because at the time of the Y-D, a considerable amount of CO2 had accumulated in the atmosphere and Antarctica was warming apace. The answer, according to this paper is that temperatures did not cool down so much after all; things cooled off for sure, and warming was delayed for another thousand years, but at the depth of the Y-D cooling most of Greenland was on the order of 4˚C warmer than it had been 4,000–5,000 years before—but still quite cold.

The problem, according to Buizert et al. is the uncertainty intrinsic in the most widely used paleothermometers: the ratio of the heavier oxygen-18 in the Greenland ice cores to that of the much more common oxygen-16, and similarly, the ratio of the heavier deuterium, to the much more common hydrogen. These relative concentrations can be measured from ice cores with high accuracy, but as has been known all along, it is not only the temperature at the site where the precipitation occurred (and the ice core was obtained)  that determines the ratios. They are also influenced by the relative concentrations in the source water body where evaporation took place (the lighter isotopes are preferentially evaporated, and gradually get depleted in the source waters as ice sheets grow, for example), and how long it took to transport the water to the site and the precipitation conditions along the way (the heavier isotopes precipitate out first).

These scientists used an entirely different paleothermometer developed by Severinghaus et al. (1998): the relative concentration of the heavy nitrogen-15 to the much more common nitrogen-14 in air bubbles trapped in the ice. The method of determining temperature is subtle but is related to the isotopic fractionation that occurs under various ice conditions that allow the lighter isotopes to leak out preferentially, which is also influenced by the temperature. The method is also well explained in Kobashi et al. (2008) from the Severinghaus lab.

Buizert et al. conclude that the nitrogen method gives a much more realistic indication of the Y-D cooling, and the Y-D temperatures they calculate for Greenland are reinforced by previous global climate modeling for the same period.

Buizert, C., Gkinis, V., Severinghaus, J., He, F., Lecavalier, B., Kindler, P., Leuenberger, M., Carlson, A., Vinther, B., White, J., 2014. Greenland temperature response to climate forcing during the last deglaciation. Science 345, 1177-1180. Abstract available from the Science website.

Kobashi, T., Severinghaus, J.P., Kawamura, K., 2008. Argon and nitrogen isotopes of trapped air in the GISP2 ice core during the Holocene epoch (0–11,500 BP): methodology and implications for gas loss processes. Geochimica et Cosmochimica Acta 72, 4675-4686.

Severinghaus, J.P., Sowers, T., Brook, E.J., Alley, R.B., Bender, M.L., 1998. Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice. Nature 391, 141-146.

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