A first-of-its-kind analysis, “History of sea ice in the Arctic” (subs. req’d), by an international team of 18 top scientists led by Leonid Polyak concludes:
[E]pisodes of considerably reduced sea ice or even seasonally ice-free conditions occurred during warmer periods linked to orbital variations. The last low-ice event related to orbital forcing (high insolation) was in the early Holocene, after which the northern high latitudes cooled overall, with some superimposed shorter-term (multidecadal to millennial-scale) and lower-magnitude variability. 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.
The key point is that the Arctic loses ice when it is forced to lose ice. In the past that was driven by orbital changes, and now it is being driven by human emissions.
This Quaternary Science Reviews paper is based on a detailed study of “proxy records from the Arctic Ocean floor and from the surrounding coasts.” You can find a brief discussion of those methods in the Ohio State University news release here, which explains this is “the first comprehensive history of Arctic ice.” The analysis “re-examined the data from past and ongoing studies — nearly 300 in all — and combined them to form a big-picture view of the pole’s climate history stretching back millions of years.”
I asked the lead author, Leonid Polyak, of Ohio State’s Byrd Polar Research Center, when was the last time the Arctic was ice free. He replied:
The paleo data we have so far is very scant, so we can’t know for sure when the Arctic was ice free in the summer last time. To be conservative, the closest candidate is the early Holocene (roughly ~10 kyr ago), when the insolation in the Arctic was high due to the beneficial orbital configuration; however, the more data I see, the stronger is my impression that there was not that little ice at that time. The next best (actually, better) candidate is the Last Interglacial, about 125kyr ago, again due to orbitally-driven high insolation: the ice was likely very low, but we can’t say whether it was completely ice free in summer or not. There are also a few other major interglacials, which may have had a similar picture, in particular Marine Isotopic Stage 11, about 450 kyr ago. In any case we are talking about very rare events controlled by a forcing very different from today. If none of those intervals was really ice free, then a million year assessment would be correct.
The Quaternary Science Reviews piece, whose co-authors include Penn State’s Richard Alley and NSIDC’s Mark Serreze, explains why it was warm (and there was reduced ice) in the Arctic 11,000 years ago — and why it’s warm now with rapidly shrinking summer ice:
The present interglacial that has lasted approximately 11.5 kyr is characterized by much more paleoceanographic data than earlier warm periods, because Holocene deposits are ubiquitous and technically accessible on continental shelves and along many coastlines. Multiple proxy records and climate models indicate that early Holocene temperatures were higher than today and that the Arctic contained less ice, consistent with a high intensity of orbitally-controlled spring and summer insolation that peaked about 11 ka and gradually decreased thereafter….
Reviewed geological data indicate that the history of Arctic sea ice is closely linked with climate changes driven primarily by greenhouse and orbital forcings and associated feedbacks. This link is reflected in the persistence of the Arctic amplification, where fast feedbacks are largely controlled by sea-ice conditions.
So external forcings — primarily orbital in the past and primarily greenhouse gases now — start a process that is accelerated by polar amplification.
This was actually a special-themed issue of QSR, “Arctic Palaeoclimate Synthesis.” It has a good piece just on that fast feedback, “Arctic amplification: can the past constrain the future?” (subs. req’d), which concludes:
Arctic amplification, the observation that surface air temperature changes in the Arctic exceed those of the Northern Hemisphere as a whole, is a pervasive feature of climate models, and has recently emerged in observational data relative to the warming trend of the past century…. Here we evaluate the mechanisms responsible for Arctic amplification on Quaternary timescales, and review evidence from four intervals in the past 3 Ma for which sufficient paleoclimate data and model simulations are available to estimate the magnitude of Arctic amplification under climate states both warmer and colder than present. Despite differences in forcings and feedbacks for these reconstructions compared to today, the Arctic temperature change consistently exceeds the Northern Hemisphere average by a factor of 3–4, suggesting that Arctic warming will continue to greatly exceed the global average over the coming century, with concomitant reductions in terrestrial ice masses and, consequently, an increasing rate of sea level rise.
Now that should be alarming to anybody:
the Arctic temperature change consistently exceeds the Northern Hemisphere average by a factor of 3–4
And indeed the best recent models show staggeringly high Arctic warming this century if we stay on our current emissions path (see M.I.T. doubles its 2095 warming projection to 10°F “” with 866 ppm and Arctic warming of 20°F”).
As RealClimate noted in their useful discussion, “Polar amplification is thought to result primarily from positive feedbacks from the retreat of ice and snow.” Indeed, the popular explanation is that warming melts highly reflective white ice and snow, which is replaced by the dark blue sea or dark land, both of which absorb far more sunlight and hence far more solar energy.
But in fact Arctic warming is amplified for several additional synergistic reasons, which are worth knowing, as I discussed in “What exactly is polar amplification and why does it matter?”
As the International Arctic Science Committee (IASC) explains in their 2004 report, Impacts of a Warming Arctic (see figure here):
- In the Arctic, compared to lower latitudes, “more of the extra trapped energy goes into warming rather than evaporation.”
- In the Arctic, “the atmospheric layer that has to warm in order to warm the surface is shallower.”
- So, when the sea ice retreats, the “solar heat absorbed by the oceans in summer is more easily transferred to the atmosphere in winter.”
[And as one climate scientist explained to me, it can get incredibly cold above thick ice, but it can’t get much colder than freezing above open water.]
All this leads to more snow and ice melting, further decreasing Earth’s reflectivity (albedo), causing more heating, which the thinner arctic atmosphere spreads more quickly over the entire polar region, and so on and on.
And that in turn threatens a cascade of effects. As the scientists at The International Polar Year explained last year, this could “speed up melting of the Greenland ice sheet, accelerating the rise in sea levels,” and “Permafrost melting could also accelerate during rapid Arctic sea-ice loss due to an amplification of Arctic land warming 3.5 times greater than secular 21st century climate trends” (see “Tundra 4: Permafrost loss linked to Arctic sea ice loss”).
Yet the destruction of a significant fraction of the permafrost must be avoided at all cost, since the tundra feedback, coupled with the climate-carbon-cycle feedbacks that the IPCC models, could easily take us to the unmitigated catastrophe of 1000 ppm (see Tundra, Part 2: The point of no return). See also NSF issues world a wake-up call: “Release of even a fraction of the methane stored in the [East Siberian Arctic Shelf] shelf could trigger abrupt climate warming.”
One final point from the summary overview of the special issue of QSR:
Taken together, the size and speed of the summer sea ice loss over the last few decades appear anomalous compared to events from previous thousands of years, especially considering that changes in the Earth’s orbit over this time have made sea ice melting less, not more, likely.
Human-caused Arctic warming has overtaken 2,000 years of natural cooling, as a “seminal” 2009 Science study found” [see figure below]:
A Hockey Stick in Melting Ice
In short, “greenhouse gas emissions are overwhelming the system,” as David Schneider, a visiting scientist at the National Center for Atmospheric Research and one of the Science article’s co-authors put it.
So the next time some anti-science disinformer — or more likely, one of the doubters who has been duped by them — says past warmth undermines our understanding of human-caused warming, tell them, quite the reverse is true. The paleoclimate record provides us more cause to be worried, not less. We know natural forcings led to warming in the past, but human emissions of greenhouse gases are overwhelming the climate now, and threatening catastrophic levels of warming if we stay on our current emissions path.
- Science: CO2 levels haven’t been this high for 15 million years, when it was 5° to 10°F warmer and seas were 75 to 120 feet higher “” “We have shown that this dramatic rise in sea level is associated with an increase in CO2 levels of about 100 ppm.”
- Nature Geoscience study: Oceans are acidifying 10 times faster today than 55 million years ago when a mass extinction of marine species occurred