Hi-Ho, Yet Another Record For All-Time Low Arctic Sea Ice Extent - NSIDC Updates

[hatrack]

28 August 2007
Overview of current sea ice conditions

Sea ice extent continues to decline, and is now at 4.78 million square kilometers (1.84 million square miles), falling yet further below the record absolute minimum of 5.32 million square kilometers (2.05 million square miles) that occurred on September 20–21, 2005.

Current sea ice conditions: August 27, 2007

Figure 1 provides the updated map of sea ice extent for August 27, 2007. Sea ice extent now stands at 4.78 million square kilometers (1.84 million square miles). The magenta line shows the median August extent based on data from 1979 to 2000.



Current sea ice conditions in context

Figure 2 is the updated time series of daily ice extent for 2007, which can be compared to the time series for 2005 and to the 1979 to 2000 average. Compared to conditions described in our last entry on August 21, we have lost an additional 140,000 square kilometers (54,000 square miles) of ice, an area roughly the size of the state of New York.

The rate of loss from day to day has started to slow. As the sun gets lower in the sky with the coming of autumn, the amount of solar energy reaching the surface declines and the Arctic begins to cool. Nevertheless, another two to three weeks of further ice loss is likely.



A bit of detail on how we measure sea ice

All of our discussion, so far, has focused on ice extent. To revisit that concept: Ice extent is the region of the Arctic Ocean covered by sea ice with a concentration of at least 15%. The microwave sensor used to collect sea ice data sees Earth's surface in discrete regions, called “grid cells” of roughly twenty-five kilometers (sixteen miles) on a side. Within a given grid cell, one may have a mixture of sea ice and open ocean water, which varies between 0% (all water, no ice) to 100% (completely ice covered). In the calculation of ice extent, we simply sum up the area of grid cells that have an ice concentration of at least 15%.

Importance of sea ice concentration

Ice concentration can also tell us about changes taking place within the icepack. For example, take a look at Figure 3, which compares maps of sea ice for the same day, August 27, in both 2007 and 2005. First, you can see that the extent of the ice--anywhere in which you see white grading to blue--is clearly lower. You can also see that the ice edge has pulled further back from the shore of north Siberia, this year.

However, when you look at the concentration of the sea ice, with white indicating high concentration, grading to the dark blue of open water, you see an additional level of detail. You see that within the pack ice, ice concentration this year is much lower throughout much of the Arctic Ocean than it was in 2005, especially near the pole. In areas with low ice concentration, dark open water between ice floes absorbs more solar radiation, promoting further melt. These areas are hence susceptible to additional melting over the next few weeks.



Ice area

In light of the the importance of ice concentration within the pack, another way to analyze the health of the sea ice is to look at a slightly different measurement: sea ice area. Sea ice area is a way of figuring in ice concentration over the entire Arctic. To determine the total ice area, we multiply each twenty-five- by twenty-five-kilometer (sixteen-mile by sixteen-mile) grid cell by its ice concentration. For example:

50% ice concentration x (25 kilometer x 25 kilometer grid cell) = 312.5 square kilometers of sea ice area

After making the calculation for all grid cells that contain at least 15% ice concentration, we add up the results. The number we obtain is the ice area for the Arctic. The total ice area numbers for August 27 are 3.17 million square kilometers (1.22 million square miles) in 2007 compared to 4.33 million square kilometers (1.66 million square miles) in 2005. In terms of ice area, sea ice cover in 2007 looks even worse than sea ice extent. So, you might wonder, "Area seems like a more intuitive measurement. Why do scientists generally talk about ice extent instead of ice area?"

The answer is that scientists are cautious about summertime values of ice concentration and area. To the sensor, surface melt appears to be open water rather than water on top of sea ice. So, while reliable for most of the year, the microwave sensor is prone to underestimating the actual ice concentration and area when the surface is melting. One way scientists resolve uncertainty about ice extent and ice area is to compare measurements from the microwave sensor with measurements from other satellites. For example, on a clear day the Moderate Resolution Imaging Spectroradiometer (MODIS) can "see" sea ice. Take a look at Figure 3 and compare it to the MODIS image in our August 22 entry, which shows the region north of Alaska. Where the microwave sensor shows low concentration, the MODIS image shows ice floes and open water. So in this case, the MODIS image confirms the microwave sensor's finding of low ice concentration north of Alaska.

In conclusion, while sea ice extent is already very low in 2007, the decline in sea ice area also tells us that the remaining icepack is in poor shape compared to 2005.

EDIT

http://nsidc.org/news/press/2007_seaiceminimum/20070810_index.html

[KingofNewOrleans]

1. The NSIDC is a very professional and cautious agency

They aren't prone to hyperbole. They don't use exclamation points. They dot all the "i"s and cross all the "t"s.

That Sea Ice concentration map is almost unbelievable. Especially that ridge of weak ice just west of the pole that practically runs all the way to Ellesmere Island.

If the melt does last as long as the 2005 season (Sept 22--a particularly late end date), even the NSIDC may break down and use a little hyperbole.
(If they use an exclamation point, pack up your stuff and head for the hills ;-) )

[XemaSab]

2. Especially if it's written in the style of a

text message.

OMG! Hd 4 teh hillz!!! :o