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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Arctic sea ice extent in the past

What the science says...

While there have been times in the distant past when Arctic sea ice extent was lower than today's, the current sea ice extent is the lowest in the past several thousand years.

Climate Myth...

Arctic sea ice extent was lower in the past
"...there [is] anecdotal and other evidence suggesting similar melts from 1938-43 and on other occasions." (John Christy via Jonathan Leake)

It's important to note that we expect the Arctic to have been cooling over the past ~6,000 years due to the Earth's orbital cycles.  Thus if we look back far enough in the past, we can certainly find a period during which the Arctic was hotter and Arctic sea ice extent was lower.  However, this actually contradicts the argument that the current sea ice decline could be natural, because that long-term orbital forcing has not reversed, and thus cannot account for the sudden and rapid Arctic warming and concurrent sea ice decline. 

Kaufman et al. (2009) reconstructed past Arctic temperatures, and confirmed that the Arctic had been cooling for at least the past 2,000 years prior to the 20th Century, and found an Arctic temperature 'hockey stick' (Figure 1).

arctic hockey stick
Figure 1: Arctic temperature change reconstructed by Kaufmann et al. (2009) including data updated for corrigendum and including instrumental measurements for the Arctic region (60 to 90° N) from NASA.

Perhaps the authoritative paper on Arctic sea ice extent over the past 1,450 years is Kinnard et al. (2011), which used a combination of Arctic ice core, tree ring, and lake sediment data to reconstruct past Arctic conditions.  The results are shown in Figure 2.

Figure 2: Arctic sea ice extent over the past 1,450 years reconstructed from proxy data by Kinnard et al., with a 40-year low pass filter applied.  Note that the modern observational data in this figure extend through 2008, and thus it is a close approximation of current conditions, even though the extent is not as low as current annual data due to the 40-year smoothing.

Based on the Kinnard results, Arctic sea ice extent is currently lower than at any time in the past 1,450 years. 

Polyak et al. (2010) looked at Arctic sea ice changes throughout geologic history and noted that the current rate of loss appears to be more rapid than natural variability can account for in the historical record.

"The current reduction in Arctic ice cover started in the late 19th century, consistent with the rapidly warming climate, and became very pronounced over the last three decades. This ice loss appears to be unmatched over at least the last few thousand years and unexplainable by any of the known natural variabilities."

Last updated on 19 September 2013 by dana1981. View Archives

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Comments 1 to 5:

  1. Good information, thank you. There is nice irony in an argument put forward by a "sceptic" in reality providing evidence for AGW. I have a question of fine detail - not because I want to criticise, but to fine tune my understanding. The OP mentions changes in the Earth's "orbital" motion: is it really the orbital motion that's operating here, or is it the Earth's rotational motion? I can envisage the Earth's precession around its rotational axis as having exactly the effect discussed, and that font of all knowledge, Wikipedia, tells me that the period of Earth's precession is around 26,000 years, which seems to fit in with the argument.

  2. Hi. I came looking for historical Arctic sea ice coverage and saw this article for the first time. The figures looked wrong. You say the Arctic sea ice extent has rumbled along at about 10 to 10.5 million sq. km for 1450 years before suddenly plunging to 8.5 m sq. km about 1950 (maybe later - hard to tell with that resolution). But Cryosphere Today reports 12.8 m sq. km today, and the 1979-2008 mean they use is about 13.7 m sq. km. This means no modern decrease as shown on your graph. What's going on?

  3. Richard, the first problem is that you are treating two different things as if they were the same. Cryosphere Today reports sea ice area. The second graph above shows sea ice extent.

    The precise meaning of 'extent' varies from source to source, but the majority use something like 'the area of ocean surface with at least 15% ice coverage'. So, if you had a 100 meter x 100 meter section of ocean with 15% of it covered by ice that'd be a sea ice extent of 10,000 square meters, but a sea ice area of only 1,500 square meters.

    Second problem - You are comparing daily values to decadal values. Look at the Cryosphere Today charts again and instead of looking at the current (near winter maximum) values look at the annual minimums instead... 1979-2008 mean below 5 million sq km... OMG, that's far lower than anything in the chart above! There must have been a huge sea ice collapse! Or not. Obviously the ice extent and area vary wildly throughout the year. Since the chart above is showing the extent averaged over decades these inter-annual fluctuations are averaged out.

    Third problem - What constitutes 'the Arctic'? Just the 'Arctic circle'? The 'Arctic basin'? Any region north of the equator which has ever had sea ice coverage? Different data sets are looking at different regions.

  4. Richard Treadgold @2, Cryosphere Today is reporting an area 12.8 million km^2 as we approach the spring maximum in sea ice area (and extent).  Kinnard et al reports on the sea ice extent at the "late summer" (September).  As can be seen from the Cyryosphere today graph (below), the September Minimum is has recently been around 4 million km^2, ie, significantly less than approximately 8 million km^2 shown at the end of the Kinnard et al reconstruction. 

    The reasons the final reconstructed values are near double the comparable values in the Cryosphere today chart are because it is a reconstruction of extent, whereas the graph shows area; because the reconstruction is unlikely to have achieved resolution of a single month, and therefore shows a two or three month average; and because late summer sea ice extents have reduced significantly since the end of the reconstruction (1995).

  5. Thank you, gentlemen. Let me absorb this.

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