The Case of the Alternating Ice Sheets
There has been a wave of triumphal announcements by climate change proponents recently, almost giddy over the summer shrinkage of the Arctic ice sheet. “Lowest level ever!” they proclaim, thought that is not quite true. Nonetheless, The Arctic pack ice has been receding over the last decade or so, but that is only natural. You see, there is a well known, if poorly understood, linkage between the ice at the north pole and the ice in and around Antarctica—and the ice around Antarctica is doing quite well. Satellite radar altimetry measurements indicate that the East Antarctic ice sheet interior increased in mass by 45±7 billion metric tons per year from 1992 to 2003. This trend continues today, reinforcing recent scientific investigations into this millennial scale oscillation between the poles. According to studies, this is how things have been for hundreds of thousands of years.
By now everyone who pays attention to climate matters has heard the news, the Nations Ice and Snow Data Center (NSIDC) has proclaimed a new record low for the Arctic ice sheet. The dweebs over at RealClimate are beside themselves with joy, smugly celebrating the impending ecological doom of all mankind. “Take to the lifeboats, the seas are a risin'.” Ok, maybe they are not quite that ecstatic, but this “record” is being used as a see-I-told-you-so to prop up anthropogenic global warming. Here is what the NSIDC had to say in their press release:
Arctic sea ice cover melted to its lowest extent in the satellite record yesterday, breaking the previous record low observed in 2007. Sea ice extent fell to 4.10 million square kilometers (1.58 million square miles) on August 26, 2012. This was 70,000 square kilometers (27,000 square miles) below the September 18, 2007 daily extent of 4.17 million square kilometers (1.61 million square miles).
NSIDC scientist Walt Meier commented, “By itself it's just a number, and occasionally records are going to get set. But in the context of what's happened in the last several years and throughout the satellite record, it's an indication that the Arctic sea ice cover is fundamentally changing.” Problem is the record is not all that meaningful. As I will explain, these folks are all missing the bigger picture.
The first thing to note is that this “record” is only valid for the period that we have had satellite observations, roughly 33 years beginning in 1978. There is no data for direct comparison before that, so you cannot even say with certainty that this is the lowest ice extent this century. Indeed, as was reported in “Greenland's Oscillating Glaciers,” the glaciers of Greenland hit a low back in the early 1930s that rivals current reports of glacial melting, an indication the Arctic pack ice might have been rather sparse in the summers back then as well.
Arctic sea ice over the past 33 years. Animation by D. Kelly O'Day.
The warmist apologists will say that those temperatures were warm but they were different, today we have warming all over. But that is not really true either, though there is not enough data to conclusively prove this argument one way or the other. Regardless of the ice coverage during the 1930s, if you go back farther to some of the historical climate optima it is hard to believe that the new “record” is, in fact, the most shrunken Arctic ice sheet ever (see “Driftwood On Ice”).
During the Holocene Climate Optimum, around 6,000 years ago, temperatures in the Arctic were 4°C higher than today and the Arctic Ocean may have been totally ice free during the summer. That this happened before makes the melting of the Arctic sea ice not a particularly bothersome thing; even the “endangered” polar bears managed to live through this balmy period in the high Arctic.
Even if we ignore the fact that there have been warmer periods in the Holocene climate record, there is a reason to not get upset by the apparent retreat of the Arctic ice sheet. That reason is explained in a paper by Stephen Barker and colleagues, entitled “800,000 Years of Abrupt Climate Variability,” that appeared in Science in 2011. Here is the abstract:
We constructed an 800,000-year synthetic record of Greenland climate variability based on the thermal bipolar seesaw model. Our Greenland analog reproduces much of the variability seen in the Greenland ice cores over the past 100,000 years. The synthetic record shows strong similarity with the absolutely dated speleothem record from China, allowing us to place ice core records within an absolute timeframe for the past 400,000 years. Hence, it provides both a stratigraphic reference and a conceptual basis for assessing the long-term evolution of millennial-scale variability and its potential role in climate change at longer time scales. Indeed, we provide evidence for a ubiquitous association between bipolar seesaw oscillations and glacial terminations throughout the Middle to Late Pleistocene.
According to the authors, ice core records from Greenland document the existence of repeated, large, abrupt shifts in Northern Hemisphere climate in the past. The last glacial cycle was characterized by rapid alternations between cold (stadial) and warmer (interstadial) conditions, cycles known as Dansgaard-Oeschger (D-O) oscillations. These oscillations led several scientists to propose a theory of inter-hemisphere climate linkage known as the seesaw model (see “Paleocean circulation during the Last Deglaciation: A bipolar seesaw?”).
The thermal bipolar seesaw model, first proposed by Wally Broecker, attempts to explain the observed relationship between millennial-scale temperature variability observed in Greenland and Antarctica. The speculative mechanism responsible for these oscillations is variation in the strength of the Atlantic meridional overturning circulation (AMOC). In trying to document the model Barker et al. encorporate orbital cycles, insolation, a number of different proxies and several mathematical techniques, yielding results shown in the figure below.
Ice core records from Greenland (GISP2) and Antarctica (EDC).
According to the seesaw model, a transition from weak to strong AMOC would cause an abrupt warming across the North Atlantic region (a D-O warming event) while temperatures across Antarctica would (in general) shift from warming to cooling. In other words, the thermal bipolar seesaw model states that there is an inverse relationship between temperatures in Greenland and the rate of change of Antarctic temperature.
“The northward heat transport associated with this circulation implies that changes in the strength of overturning should lead to opposing temperature responses in either hemisphere,” Barker et al. state. While this report is specifically about millennial cycles there are others who have proposed shorter term oscillations on the order of centuries or decades. Taking this relationship another logical step, shrinking ice in one hemisphere should imply growing ice mass in the other—the ice sheets alternate.
Is the ice mass growing in Antarctica? According to a study done by NASA scientists back in 2005 and published in Science, that is exactly what is happening down at the bottom of the world. Accumulation of snow in the interior of the continent is resulting in growth in the Antarctic glacial ice. The plot below shows the change in elevation between 1992 and 2003.
The important part of this plot is the long-term linear trend (black line), from which a steady increase in elevation since about 1995 is apparent (the red curve is an 11 year, least squares polynomial fit of questionable usefulness). The average rate of change from 1995 to 2003 is 2.2 cm/year after adjustment for isostatic uplift. This growth is even more dramatic when viewed on the map below.
Satellite radar altimetry measurements indicate that the East Antarctic ice-sheet interior north of 81.6°-S increased in mass by 45±7 billion metric tons per year from 1992 to 2003. Comparisons with meteorological model snowfall estimates suggest that the gain in mass is associated with increased precipitation. A gain of this magnitude is enough to slow sea-level rise by 0.12±0.02 millimeters per year. But it is not just the ice on the Antarctic continent that is showing signs of growth.
According to NASA's Earth Observatory, total Antarctic sea ice has increased by about 1% per decade since the start of the satellite record. “Whether the small overall increase in sea ice extent is a sign of meaningful change in the Antarctic is uncertain because ice extents in the Southern Hemisphere vary considerably from year to year and from place to place around the continent,” they report. “Considered individually, only the Ross Sea sector had a significant positive trend, while sea ice extent has actually decreased in the Bellingshausen and Amundsen Seas. In short, Antarctic sea ice shows a small positive trend, but large scale variations make the trend very noisy.”
Antarctic sea ice in winter and summer, 2011-2012.
The Arctic is an ocean basin surrounded by land. The Antarctic, on the other hand, is a large continent surrounded by ocean. Because of this geography, sea ice has more room to expand in the winter. But the ice also extends to warmer latitudes, leading to more melting in summer. The Antarctic sea ice peaks in September and retreats to a minimum in February, as can be seen from the seasonal maps above.
Earth's climate engine contains cycles within cycles, operating on timescales that often exceed a human lifetime. Really long cycles leave traces in sediment and glacial ice, short-term change can be witnessed first hand, but the intermediate cycles are difficult for even scientists to appreciate. Imagine if a year took a century to unfold and you were born in the dead of winter; the coming of summer would seem a frightening change, with temperatures rising dramatically and seemingly without limit.
It is easy to understand people getting frantic over shrinking ice sheets and melting glaciers, but such events are all natural and operate on timescales we are ill-equipped to comprehend. Nature will keep its own counsel without regard for overly excitable climate scientists. The rest of us should simply chill out.
Be safe, enjoy the interglacial and stay skeptical.