Guest review article by David Middleton
We’re often bombarded with headlines line this:
Arctic sea ice continues its downward spiral
At 4.6 million square kilometres in coverage, this year’s sea ice minimum is the sixth lowest on record.
The problem with headlines like this is the fact that the “record” only dates back to 1979. How meaningful could “the sixth lowest on record” be, when the record isn’t even forty years long?
NSIDC does feature an estimate of Arctic sea ice extent that goes back to 1953 based on the “Hadley data set”…
Arctic sea ice before satellites, NSIDC
Based on this analysis, Arctic sea ice extent was anomalously high from 1953 to 1978 and has shrunk to anomalously low since 1979. This is not very meaningful from a geological perspective. In a previous post, I cited McKay et al., 2008 as an example of a geological perspective on Arctic sea ice extent.
The Arctic was probably ice-free during summer for most of the Holocene up until about 1,000 years ago. McKay et al., 2008 demonstrated that the modern Arctic sea ice cover is anomalously high and the Arctic summer sea surface temperature is anomalously low relative to the rest of the Holocene.
Over most of the Holocene, >50% sea ice coverage occurred from 5.5 to 9 months each year. During the “Anthropocene”, >50% sea ice coverage has ranged from 9 to 12 months each year.
Steve Mosher noted in the comments that, “The Chukchi Sea IS NOT THE ARCTIC !!!! It is part of the arctic.”
This is very true… However, sediment cores are pretty-well limited to where they have been drilled. In another recent thread, tty brought Stein et al., 2017 to my attention. After reading through it, I decided that there was enough material there to take a crack at a general characterization of Holocene sea ice conditions relative to modern times.
PIP25: “Miracle on Ice”
In a pioneering study by Belt et al. (2007), the ability to (semi-)quantitatively reconstruct paleo-sea ice distributions has been significantly improved by a biomarker approach based on determination of a highly branched isoprenoid (HBI) with 25 carbons (C25 HBI monoene = IP25). This biomarker is only biosynthesized by specific diatoms living within the Arctic sea ice (Brown et al., 2014) and appears to be a specific, sensitive and stable proxy for Arctic sea ice in sedimentary sections representing Late Miocene to Recent times (Stein et al., 2012, 2016; Belt and M€uller, 2013; Stein and Fahl, 2013; Knies et al., 2014). The presence of IP25 in the studied sediments is direct evidence for the presence of sea ice.
For more semi-quantitative estimates of present and past sea ice coverage, M€uller et al. (2011) combined the sea-ice proxy IP25 and phytoplankton biomarkers in a phytoplankton- IP25 index, the so-called ‘PIP25 index’:
PIP25 = IP25/([IP25/(IP25 + ([phytoplankton marker] x c))
with c is the mean IP25 concentration/mean phytoplankton biomarker concentration for a specific data set or core.
Stein et al., 2017
The image below is of a Late Quaternary sea ice reconstruction for the western Yermak Plateau from Belt, et al., 2016
Generally speaking, the PIP25 index correlates to sea ice extent as follows:
- >0.7 = Perennial (year-round) ice cover
- 0.5-0.7 = Seasonal ice cover
- 0.1-0.3 = Reduced ice cover
- <0.1 = Ice-free year-round
Most of the Arctic has had seasonal sea ice cover over the past 30 years, with a relatively small area of perennial sea ice.
Perennial sea ice is roughly equivalent to the recent minima. Seasonal sea ice is more or less the difference between the maxima and minima.
So… How does this compare to the rest of the Holocene?
A geological perspective on Arctic sea ice extent
Kinnard et al., 2008 estimated Arctic sea ice extent minima and maxima back to about 1880 from a variety of methods. A comparison of this with a PIP25 reconstruction of the North Icelandic Shelf sea ice extent suggests that modern sea ice extent is somewhere between that of the Little Ice Age (LIA) and Medieval Warm Period (MWP), probably closer to the MWP.
Stein et al., 2017 compared sea ice extent reconstructions using PIP25 indices from four cores around the current area of perennial sea ice.
A comparison of the Fram Strait core to the North Icelandic Shelf indicates that the sea ice extent of the MWP was anomalously higher than the vast majority of the Holocene. Prior to the onset of Neoglaciation, Arctic sea ice extent appears to have ranged from nearly ice-free to the low end of seasonal coverage.
Andy May’s Arctic climate reconstruction exhibits the same general pattern as the four PIP25 sea ice extent reconstructions:
Prior to the onset of Neoglaciation, Arctic sea ice ranged from nearly ice-free to reduced. Since the onset of Neoglaciation, it has ranged from seasonal to perennial.
I plotted Andy May’s excellent Arctic climate reconstruction as an overlay of the Chukchi Sea core from Stein et al, 2017 to demonstrate the relatively decent correlation between Arctic temperatures and sea ice conditions. Note that all four core locations exhibit perennial sea ice during the Late LIA… And only during the Late LIA.
While it is clear that Arctic sea ice extent has diminished since 1979, it’s also important to note that this reduction must be viewed in the context of the overall Holocene. Holocene Arctic sea ice extent reached its Holocene maximum between 1600 and 1850 AD.
Figure 4 from Belt et al., 2013
So any reductions are from the maximum values of this geologic epoch. The LIA was clearly the coldest phase of the Holocene and modern sea ice conditions are much closer to the LIA than they are to the pre-Neoglaciation period.
Alley, R.B. 2000. The Younger Dryas cold interval as viewed from central Greenland.
Quaternary Science Reviews 19:213-226.
Belt S.T., Müller J. The Arctic sea ice biomarker IP25: A review of current understanding, recommendations for future research and applications in palaeo sea ice reconstructions. (2013) Quaternary Science Reviews, 79 , pp. 9-25.
Hoff U, Rasmussen TL, Stein R, Ezat MM, Fahl K. Sea ice and millennial-scale climate variability in the Nordic seas 90 kyr ago to present. Nature Communications. 2016;7:12247. doi:10.1038/ncomms12247.
Kinnard, C., Zdanowicz,C.M., Koerner,R .,Fisher,D.A., 2008. A changing Arctic seasonal ice zone–observations from 1870–2003 and possible oceanographic consequences. 35, L02507.
May, Andy. A Holocene Temperature Reconstruction Part 4: The global reconstruction. Watts Up With That. 2017.
Polyak, Leonid, Richard B. Alley, John T. Andrews, Julie Brigham-Grette, Thomas M. Cronin, Dennis A. Darby, Arthur S. Dyke, Joan J. Fitzpatrick, Svend Funder, Marika Holland, Anne E. Jennings, Gifford H. Miller, Matt O’Regan, James Savelle, Mark Serreze, Kristen St. John, James W.C. White, Eric Wolff. History of sea ice in the Arctic, Quaternary Science Reviews, Volume 29, Issues 15–16, 2010, Pages 1757-1778, ISSN 0277-3791, https://doi.org/10.1016/j.quascirev.2010.02.010.
Stein, R. , Fahl, K. , Schade, I. , Manerung, A. , Wassmuth, S. , Niessen, F. and Nam, S. (2017), Holocene variability in sea ice cover, primary production, and Pacific‐Water inflow and climate change in the Chukchi and East Siberian Seas (Arctic Ocean). J. Quaternary Sci., 32: 362-379. doi:10.1002/jqs.2929