Arguments
Arctic Sea Ice Hockey Stick: Melt Unprecedented in Last 1,450 years
Posted on 24 November 2011 by Rob Painting
Many climate change "skeptics" obsess over the 'hockey stick', and their discussion inevitably leads back to 1998, when climate scientist Michael Mann first published his paper indicating that current global warming was anomalous in the last 1000 years or so. In plain language, Mann's work suggested that current warming was likely due to mankind's carbon dioxide pollution, not any as-yet-unidentified, or yet-to-be-discovered or observed natural phenomenon.
Despite the "skeptics" cherry-picked focus on a peer-reviewed paper more than a decade old, the science has moved on considerably since then. Paper after paper has basically affirmed that current warming is outside the bounds of natural variation, and therefore likely due to human activities. For example we have seen a sea level hockey stick, an underwater hockey stick, a South American hockey stick, an Arctic summer temperature hockey stick, a tropical glacier hockey stick, a North American mountain snowpack hockey stick, a glacier length hockey stick, and warming of Atlantic water into the Arctic hockey stick.
Into this league of hockey sticks, we have a just published scientific paper, (Kinnard [2011]), which shows that the Arctic sea ice retreat is also a hockey stick, and that the present rate of melt in the Arctic summer is unprecedented in the last 1,450 years. See figure 1. (Note that the hockey stick blade is facing down in this reconstruction).
Building a hockey stick
Because Arctic sea ice is influenced by both air and water temperatures, the study authors use a combination of Arctic ice core, tree-ring and lake sediments to reconstruct Arctic conditions over the last 2,000 years. As is often the case with these proxy reconstructions, the authors found the error bars in the reconstruction (the uncertainty) increased further back in time, due to a decreasing number of proxy records, and was not useful past 1,450 years ago.
When compared (validated) against historical sea ice observations it was found that the reconstruction not only had a dominant temperature-related signal, but that the proxy-based reconstruction also had a second signal which corresponded with variations in sea ice cover (extent), therefore confirming the 2nd network signal was a proxy for Arctic sea ice cover (as shown in figure 1).
Clearly there are periods in the reconstruction where rapid rates of ice loss occurred, but what stands out is that the length and rate of present day melt is unprecedented in the entire 1,450 year-long reconstruction. This is consistent with the Arctic summer temperature hockey stick (Kaufman [2009]) and warming of Atlantic water into the Arctic hockey stick (Spielhagen [2011]).
Arctic summer sea ice: going, going............
2011 saw the 2nd lowest summer sea ice extent on record (after 2007), and even more dramatically, this year saw the lowest ever recorded volume of Arctic summer sea ice.
This latest 'hockey stick' not only reinforces that current conditions in the Arctic are much warmer than the so-called Medieval Warm Period, but that sea ice is currently disappearing at a sustained speed that is unmatched in the last 1,450 years.
In the conclusions of Kobashi et al. quoted by Arkadiusz, the authors highlight the possible reasons of the discrepancies between Central Greenland and the Arctic as a whole. This should sound as a ringing alarm bell to those who'd like to use their reconstructions as indications of a more general trend. Indeed, the authors are clearly trying to put central Greenland temperatures in the context of the Arctic as a whole to improve the understanding of the former.
As already noted by Tom @50, Arkadiusz himself highlights the reason why his quote is irrelevant to the topic at hand, which makes me wonder why he extensively quoted it.
Also consider that the map is relative to 1961-90 and that the Greenland area was relatively cold during this period, even compared to earlier in the 20th Century.
That said, I'm with Skept.fr on being skeptical of these findings. The MWP doesn't just show smaller change than today, but change in the opposite direction to what we would expect. And the previous lowest minimum occurring during the Little Ice Age? It's counter-intuitive at least.
Could it be that cooling in Europe was due to some prolonged blocking pattern caused by, or resulting in, a warmer Arctic? I guess I'll have to check out the paper.. hope my Nature subscription hasn't expired yet.
In 1998, Mann, Bradley and Hughes published a reconstruction going back 600, not 1000 years. The millennial reconstruction was done in 1999.
These two papers often get confused, but usually by skeptics.
Here is a free pdf link to Flanner et al. 2011 mentioned by Tom Curtis.
One more hockey stick: Tingley and Huybers' bayesian approach The spatial mean and dispersion of surface temperatures over the last 1200 years: warm intervals are also variable intervals for which I still find only a manuscript, no publication.
Oh, hem, I'm not particularly skeptic of the paper we're discussing, I just quoted its findings. For the MWP or MCA in general, I think our level of understanding will progress, the paleoclimate community is very active. All studies conclude that the 1970-present period is unusually hot in the past thousand years on a hemispheric or global scale, it seems unlikely this result will substantially change. But the precise signature and mechanisms of past variability at centennial scales are still unclear and reconstructions may diverge for the amplitude of pre-industrial temperature changes (eg the 'spaghetti' schema in AR4). Mike Mann himself continuously refines his work (Mann 2009 differs in many details from Mann 1999) and so do the other teams working on past climates. Jan Esper 2010 made an interesting synthesis of the progress accomplished since the IPCC AR1 and the current research priorities of paleoclimate community.
Nothing exceptional about any of this.
But it has no relevance whatsoever to the question of whether the whole Earth was warmer at this point in time. Which is surely the point of why skeptics love to cite Greenland, grapevines in England etc ad infinitum but don't mention the North-East Pacific, Australia, etc during the same period. That is surely the point. It is what the global picture looks like that counts.