Any Reason to Expect a Tipping Point with Arctic Sea Ice?

by Emil Morhardt

Williamson et al. (2016) examined the satellite data looking for signs of a tipping point in Arctic sea ice loss, but found none (my Jan 1 post). About the same time, Notz and Stroeve (2016) looked at the same data and did a simple linear correlation between September Arctic sea ice area and cumulative CO2 emissions since 1850. Voila! There was a strong negative linear correlation between the two showing a sustained loss of 3 ± 0.3 square meters of September sea ice area per cumulative metric ton of CO2 emission. Their title summarizes the result clearly: Observed Arctic sea-ice loss directly follows anthropogenic CO2 emission. If this linear trend continues and there is no tipping point—and there is no reason to expect one—we can make a pretty good guess about the timing of the future of Arctic sea ice to the extent we can predict CO2 emission levels. At the rate we are going, September Arctic sea ice will be completely gone before mid-century (and global average temperatures will have risen more that 1.5ºC.) Furthermore, we can now get a feeling for how much our personal use of fossil fuels and the energy derived from the directly affects Arctic sea ice; the average CO2 release from personal use is several metric tons, 

This study is important because global climate models differ widely in their timing of the loss of sea ice for a given trajectory of CO2 emissions, and the combined models, while showing a similar linear trend over most of their range, underestimate the actual loss by over a third, estimating 1.75 ± 0.67 square meters of September sea ice area per cumulative metric ton of CO2 emission. The authors speculate that the models underestimate the increase of incoming longwave radiation (heat re-emitted downward by the CO2 is what I think they are referring to here) but note that at the moment this can’t be tested directly because of lack of sufficient direct observations.

Notz, D., Stroeve, J., 2016. Observed Arctic sea-ice loss directly follows anthropogenic CO2 emission. Science 354, 747-750.

Williamson, M., Bathiany, S., Lenton, T., 2016. Early warning signals of tipping points in periodically forced systems. Earth System Dynamics Discussions 6, doi: 10.5915/esd-5917-5313-2016.

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