Understanding the past to know more about our future: study finds spikes in carbon dioxide levels correlated with end of last glacial cycle

Scientists often look to the past to understand what will happen in the future.  This is possible because we expect the same natural laws at play then will be the same now and in the years to come.  This is especially helpful when it comes to understanding the Earths’ complex, coupled cycles that dictate changes in atmospheric concentration, weather, etc.  Understanding the carbon cycle and how it relates to temperature and glacial cycles is especially crucial as we try to understand what our fossil fuel emissions will lead to in terms of temperature rises, climate extremes, and other factors.  A new study just out in Nature sheds some important light on this relationship between CO2 and glacial cycles in the last ice age – information that can help us understand what to expect in our own future of increased carbon emissions.

Figure courtesy of earthobservatory.nasa.gov

Figure courtesy of earthobservatory.nasa.gov

There have been plenty of past studies looking at carbon levels during the end of the last ice age about 25,000 to 10,000 years ago.  But these samples have come from regions that have now allowed for precise temporal resolution of CO2 changes.  For example, according to the article, Greenland samples have high levels of impurities that prevent analysis of atmospheric CO2 levels.  Antarctica samples have less impurities but are not able to be used with high temporal precision during the deglaciation periods for a variety of reasons.

Figure courtesy of waisdivide.unh.edu

Figure courtesy of waisdivide.unh.edu

Due to these drawbacks, the authors of the study moved to the West Antarctic Ice Sheet where steady annual snowfall leads to precisely, temporally layered levels of CO2 gas trapped in the ice. This allows the authors to connect CO2 levels and temperature chronologically to sub-centennial accuracy!  Very helpful for understanding more detailed relationships between CO2 fluctuations and temperature/deglaciation.  Here’s what they found…

The samples showed a general 80 ppm CO2 increase from about 23000 to 9000 years ago, similar to other cores, that coincides with the end of the last glacial period.  However, the data also showed much quicker bursts in CO2 rises, never seen before due to the precise resolution of the West Antarctic data.  These can be seen below (ignore the IRD stack data):

Figure courtesy of [1]

Figure courtesy of [1]

Focus on the lower (CH4) and middle (CO2) plots, which show the levels of each molecule over the course of thousands of years corresponding to the deglaciation period (x-axis).  The vertical colored bands show regions of a hundred years or show with sharp rises in either CO2 or methane levels, followed by relative plateaus for the hundreds of years afterward.  We have assumed up to this point that the CO2 and CH4 increases related to the end of the last ice age were relatively gradual over the course of this 15,000 year window, so this data elucidates a new dynamic in the Earth’s carbon cycle that we currently don’t understand.

The more gradual changes in CO2 beginning around 18,000 and 13,000 years ago reflect what scientists thought was happening throughout the entire time period (about 10 ppm CO2 increase per year).  The study also found these smaller increases correlated with a reduction in the strength of what is known as the Atlantic meridional overturning circulation (AMOC), a major Atlantic current that brings warm water north into the Arctic, where it cools and sinks and travels southward, as shown below.  The red indicates the warm northward current:

Figure courtesy of eoearth.org

Figure courtesy of eoearth.org

So this gradual increase is understood, but another mechanism must explain the fast uptakes around 16,000, 14,500, and 11,5000 years ago.  Changes in the AMOC on 100-year time scales may also be leading to these faster rises in CO2, however much more research will be required to understand their cause/mechanism.

One last point – some may say that this gives support to the fact that the recent increases in CO2 over the past hundred years or so is not anthropogenic.  This is likely not the case!  We have fast amounts of evidence that show that this most recent increase in CO2 correlates with our fossil fuel emissions.  It would be interesting to see if there are changes recently in the AMOC that mimic what happened 10,000 years ago to see if circulation changes are contributing to our present, incredible rise in CO2, however it is very unlikely it is the sole reason.

References

1)

ResearchBlogging.org

Marcott, S., Bauska, T., Buizert, C., Steig, E., Rosen, J., Cuffey, K., Fudge, T., Severinghaus, J., Ahn, J., Kalk, M., McConnell, J., Sowers, T., Taylor, K., White, J., & Brook, E. (2014). Centennial-scale changes in the global carbon cycle during the last deglaciation Nature, 514 (7524), 616-619 DOI: 10.1038/nature13799

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