Sunday, September 16, 2007

Arctic Sea Ice: an ecosystem in collapse

It's September, and all you weather wonks and wonkettes know what that means...the summer melt is for all intents and purposes over, and it is time to visit the National Snow and Ice Data Center and assess the state of sea ice in the arctic. This years news is frightening. This summers melt was the most extensive of any year since 1978, when the data was gathered the first time.

Let's start by defining what sea ice is. Sea ice is any ice floating in the ocean that formed from sea water. (Icebergs are not sea ice - they are pieces of glacier that have broken free.) Sea ice forms and melts with the polar seasons. While both arctic and antarctic sea ice are important parts of the ecosystem and vital to birds and mammals that rely on them for habitat; arctic sea ice appears to have a greater impact on climate regulation via heat exchange, and is important in the regulation of the salinity of the ocean and the moisture content in the air. During the winter months, it insulates the relatively warm ocean water from the frigid arctic air. The amount of moisture in the air determines the amount of cloud cover and precipitation received.

The satellite data is now available for this years melt, and it reveals that the regional ice cover is the lowest it has ever been. Previously, the worst year for sea ice was 2002, when it was 4% lower than any point since 1978, and 14% lower than the mean for the years 1979-2000. (Serreze et al. 2003). Historically, the low-ice years have been followed by returns to near-normal conditions, but 2002 was different. It was followed by two more low-ice years that almost matched the melt record of 2002, then a new record low was set again in 2005. When the data was tabulated for 2005, it was determined that the summers-end sea ice was diminishing by approximately 8% per decade, and was projected to be gone entirely during the summer months by 2070. That date will be adjusted to a point much nearer when the final results of this years data are tabulated.

The graph above is an updated time series of daily ice for 2007, as compared to the previous record, set in 2005 and the 1979-2000 average. Since this graph was generated on September 3, an additional 180,000 square kilometers (69,000 square miles) of sea ice have melted, although the day-to-day loss has slowed considerably, and the absolute minimum has likely occurred, or will any day. The minimum cover has occurred as late in the year as September 25. On September 10, the ice cover stood at 4.24 million square kilometers (1.63 million square miles), falling yet further below the previous record absolute minimum of 5.32 million square kilometers (2.05 million square miles) that occurred on September 20–21, 2005. The Northwest Passage became accessible a few days ago, and is still open.

Models of greenhouse warming have long shown an "arctic amplification" effect, and that climate change will be most pronounced over the Arctic Ocean. The loss of sea ice is accelerating, and the phenomenon is driving itself. Look at the photo of sea ice on the right. Water is dark, and ice is white. White ice reflects the heat of the sun back to space, dark water absorbs that light/energy and it is stored in the oceans in the form of elevated temperature. Expanses of dark water absorb massive amounts of the suns energy through the summer, and this heats the upper ocean. As air temperature drops in the winter, that energy is released back into the atmosphere, which increases air temperature, which slows the formation of sea ice...and so it goes.

The health of an ecosystem can be assessed by how well it's top predator fares. In the arctic, the top predator is the polar bear, and polar bears are not faring so well these days. Two years ago, researchers spotted polar bears swimming as far as 95 kilometers from shore, and the percentage of bears sighted in the water jumped from 4% to 20%. The carcasses of drowned polar bears have washed ashore and been recovered, and on post mortem examination, they show signs of starvation. Instances of cannibalism among the bear population have been reported as well. The species has been officially added to the endangered list, but it is entirely possible that the polar bear has passed the tipping point and will not survive in the wild. An ecosystem that loses it's top predator is an ecosystem in collapse. The collapse of the arctic ecosystem is something we should all be nervous about, because the effects will be far reaching, not just localized or isolated.