The Arctic’s sea ice is much in the news right now, nationally if not locally, starting with a grim discussion of whether the next few months will see the frozen dome at the top of the world recede to a record minimum.

In the image above, derived from satellite data, the white areas still had ice as of Sunday. The orange line shows recent average ice cover for the same date – more precisely, a median of the ice extent as measured each June 17 over a 21-year base period from 1979 through 2000.

Below, a chart shows the same dramatic retreat but in a different way: The gray line corresponds to the 21-year median traced in orange above; the blue line shows how consistently this year’s ice cover has been below average.

And the dashed green line? Well, that tracks ice extent for much of 2007, the year in which sea ice fell to its record low so far. It stops short of the actual minimum, which typically arrives in September.

Near records, back to back

Because this year’s line crossed 2007’s so early in the melt season, and because its downward slope is getting steeper now, some scientists think the record is likely to fall this year. (It almost fell last year, too: The second-smallest ice cover on record was measured September 8 or 9, 2011.)

Walt Meier, one of the scientists who produce these data, discussed them with Discovery News and underscored these points:

We are seeing a lot of areas opening up within the Arctic Ocean, and along coastlines that normally are still ice covered. This time of year is potentially significant because we are approaching the summer solstice where the sun is at its maximum in the Northern Hemisphere.

Predicting Arctic ice cover is difficult because its retreat is subject to many variables, including wind patterns, cloud cover, ocean currents, even the amount of atmospheric soot drifting over the top of the world. There are natural, long-term cycles in ice extent as well.

But all of these factors combined do not diminish the role of global warming in driving a steady and substantial – if not perfectly linear – decline in Arctic ice cover. In 2007, the natural factors aligned with and accelerated the overall warming trend. In 2011 they did not, and the ice retreated almost as much.

Indeed, you could say that the measurements charted above may actually understate the long-term rate of change.

First off, the 21-year period of consistent, year-to-year satellite observations that form the baseline for this map and graph began after significant warming had already occurred, and thus after significant change in the ice patterns had begun.

Second, the use of “ice extent” as opposed to “ice area” as the chosen measurement somewhat enlarges the area of ice cover in the map and the graph (for a technical explanation of the difference – and much other interesting material on the subject – check out these FAQs from the National Snow & Ice Data Center, which manages the project.)

The feedback loop of retreating ice

Finally, and most important, retreating sea ice is captive to one of those self-accelerating feedback loops: as ice cover shrinks, reducing reflectivity at the ocean surface, seawater absorbs more solar warmth, which accelerates ice melt, which reduces reflectivity, and so on and on and on.

As for the impacts, I think everybody understands by now that the sea ice is critical habitat for polar bears, walruses and other Arctic wildlife. Also, that meltwater could disrupt the circulation of warm and cold seawater on a global scale.

Some have held out hope that opening up more sea surface might increase the polar ocean’s ability to absorb and sequester atmospheric carbon dioxide, offsetting the greenhouse effect somewhat and slowing the pace of global warming.

However, Swedish researchers reported over the weekend that Arctic waters are now losing some of this buffering capability, because carbon dioxide already in the ocean at those latitudes has risen to levels that may approach seawater’s carrying capacity. And at some point, they say – perhaps soon – these waters may actually begin returning carbon to the atmosphere.

For a summary of the Arctic’s changing climate and probable impacts, I recommend a special report that appeared in The Economist’s edition of last Saturday, which includes this passage:

Most climate models predicted that the Arctic Ocean could be ice-free in summer by the end of this century; an analysis published in 2009 in Geophysical Research Letters suggested it might happen as early as 2037. Some now think it will be sooner.

It is hard to exaggerate how dramatic this is. Perhaps not since the felling of America’s vast forests in the 19th century, or possibly since the razing of China’s and western Europe’s great forests a thousand years before that, has the world seen such a spectacular environmental change.  

And now for some (maybe) good news

On the positive side, perhaps, comes a discovery that thinning sea ice may be driving an explosion of phytoplankton growth in the Arctic – possibly at scales that could add new carbon-capturing capability to the waters there.

As reported in the Christian Science Monitor and elsewhere, scientists who were drilling through the ice last summer, to gauge various impacts of climate variability, were astounded to find these microscopic plants happily in bloom beneath a three-foot-thick ice shield.

Previously, phytoplankton were thought to require the unobstructed sunlight of virtually open water to carry on their photosynthesis. These phytoplankton, however, were not only functioning but reproducing at staggering rates beneath the ice – doubling in number more than once a day.

Plankton are the foundation of marine food systems, but whether these new blooms will be useful or harmful up the food chain isn’t known yet – it depends on population dynamics at each successive level in the sequence.

Nor is it entirely clear whether these under-the-ice communities are actually a new development in the Arctic ecosystems or just a new discovery. Kevin Arrigo, a scientist involved in the work, told the Monitor:

At this point we don’t know whether these rich phytoplankton blooms have been happening in the Arctic for a long time and we just haven’t observed them before. These blooms could become more widespread in the future, however, if the Arctic sea ice cover continues to thin.

[The newly discovered patterns] could make it harder and harder for migratory species to time their life cycles to be in the Arctic when the bloom is at its peak. If their food supply is coming earlier, they might be missing the boat.

Many cool pictures from the researchers participating in this ICESCAPE project are available in a flickr stream from NASA’s Goddard Space Flight Center. Additional maps and graphics illustrating the sea ice trends can be found in Julia Whitty’s Blue Marble blog at Mother Jones.