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From the air in our oldest Antarctic ice core, we can see that CO2 changed in a remarkably similar way to Antarctic climate, with low concentrations during cold times, and high concentrations during warm periods (see Fig. 3). This is entirely consistent with the idea that temperature and CO2 are intimately linked, and each acts to amplify changes in the other (what we call a positive feedback). It is believed that the warmings out of glacial periods are paced by changes in Earth’s orbit around the Sun, but these small changes in climate are amplified, mainly by the resulting increase in CO2, and by the retreat of sea ice and ice sheets (which leads to less sunlight being reflected away).
We can see how remarkably closely Antarctic temperature and CO2 tracked each other, but what about global temperature? Here we can examine the last time the earth emerged from an ice age. Scientists have compiled other geologic records of temperature from around the world to calculate the average global temperature. On this global-scale, CO2 starts increasing before temperature (see Fig. 4.) (Shakun/Osman), thus temperatures are said to “lag” behind CO2. This demonstrates that CO2 was not only a feedback on natural c*****e c****e but in fact major forcing that drove the earth out the last ice age.
In our modern era, of course, it is human emissions of CO2 that are expected to kick-start the sequence of events. We see no examples in the ice core record of a major increase in CO2 that was not accompanied by an increase in temperature. Methane concentration also tracks the glacial-interglacial changes, probably because there were less wetlands in the colder, drier glacial periods.


Abrupt c*****e c****e
The c*****e c****es described above were huge, but relatively gradual. However, ice cores have provided us with evidence that abrupt changes are also possible. During the last glacial period, Greenland experienced a sequence of very fast warmings (see Fig. 5). The temperature increased by more than 10°C within a few decades. Other records show us that major changes in atmospheric circulation and climate were experienced all around the northern hemisphere. Antarctica and the Southern Ocean experienced a different pattern, consistent with the idea that these rapid jumps were caused by sudden changes in the t***sport of heat in the ocean. At this time, there was a huge ice sheet (the Laurentide) over northern North America. Most likely, freshwater delivered from the ice sheet to the North Atlantic periodically disrupted the overturning of the ocean, causing the t***sport of tropical heat to the north to reduce and then suddenly increase again. While this mechanism is unlikely to occur today’s world, it does show us that, at least regionally, the climate is capable of extraordinary changes within a human lifetime – rapid switches we certainly want to avoid experiencing.


Summary
Ice cores provide direct information about how greenhouse gas concentrations have changed in the past, and they also provide direct evidence that the climate can change abruptly under some circumstances. However, they provide no direct analogue for the future because the ice core era contains no periods with concentrations of CO2 comparable to those of the next century.
Fact file
• Ice core. Cylinder of ice drilled out of an ice sheet or glacier. Most ice core records come from Antarctica and Greenland.
• Ice cores contain information about past temperature, and about many other aspects of the environment.
• Atmospheric carbon dioxide levels are now 50% higher than before the industrial revolution. This increase is due to f****l f**l usage and changes in land-use.
• The magnitude and rate of the recent increase are almost certainly unprecedented over the last 800,000 years.
• Methane also shows a huge and unprecedented increase in concentration over the last two centuries.
References
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Osman, M.B., Tierney, J.E., Zhu, J. et al. Globally resolved surface temperatures since the Last Glacial Maximum. Nature 599, 239–244 (2021). https://doi.org/10.1038/s41586-021-03984-4
Marcott, S., Bauska, T., Buizert, C. et al. Centennial-scale changes in the global carbon cycle during the last deglaciation. Nature 514, 616–619 (2014). https://doi.org/10.1038/nature13799
NOAA Greenhouse Gas Data: Dlugokencky, E.J., J.W. Mund, A.M. Crotwell, M.J. Crotwell, and K.W. Thoning (2021), Atmospheric Carbon Dioxide Dry Air Mole Fractions from the NOAA GML Carbon Cycle Cooperative Global Air Sampling Network, 1968-2020, Version: 2021-07-30, https://doi.org/10.15138/wkgj-f215
North Greenland Ice Core Project members. High-resolution record of Northern Hemisphere climate extending into the last interglacial period. Nature 431, 147–151 (2004). https://doi.org/10.1038/nature02805
Parrenin, F. et al. Synchronous Change of Atmospheric CO2 and Antarctic Temperature During the Last Deglacial Warming. Science 339, 1060–1063 (2013).
Petit, J., Jouzel, J., Raynaud, D. et al. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399, 429–436 (1999). https://doi.org/10.1038/20859
Rubino, M. et al. Revised records of atmospheric trace gases CO2, CH4, N2O, and δ13C-CO2 over the last 2000 years from Law Dome, Antarctica. Earth Syst. Sci. Data 11, 473–492 (2019).
Shakun, J., Clark, P., He, F. et al. G****l w*****g preceded by increasing carbon dioxide concentrations during the last deglaciation. Nature 484, 49–54 (2012). https://doi.org/10.1038/nature10915
WAIS Divide Project Members. Precise interpolar phasing of abrupt c*****e c****e during the last ice age. Nature 520, 661–665 (2015). https://doi.org/10.1038/nature14401
Details
Author(s):
On this site: Thomas Bauska
Date:
30 June, 2022