According to the National Snow and Ice Data Center, Arctic sea ice extent in April was the 5th lowest on record. Averaged over April 2010, Arctic sea ice extent was only 14.15 million sq. km. Arctic ice in the first two weeks of April didn’t change very much in its extent. That changed in the 2nd half of the month. Increasing solar radiation and thin ice for this time of year has led to a decrease in the ice extent. That decrease isn't out of the norm, but the already low extent present at the beginning of the month means today's extent is anomalously low.
The change in April ice extent has been measured at -2.6% per decade by the NSIDC. What that means is as of the end of April 1978, the Arctic had 15.5 million sq. km. of sea ice while April 2011's extent was, as stated above, only 14.15 million sq. km. And this year's end of April extent was slightly butressed by a relative lack of change for the better part of 2 weeks or so. Conditions could be even worse than they are. 2008-2010 April's were characterized differently than April 2011. After posting record low extent values every year from 2004-2007, the past three Aprils saw a rebound in extent values. The past three Aprils looked more like the extents of the 1990s. Alas, 2011 looks a lot more like 2004 than 2010.
Arctic Pictures and Graphs
The last `State of the Poles` post I did was only a couple of weeks ago. That said, things certainly look different since that time. I'll start with current conditions (yesterday), centered on the North Pole:
Compare this with April 19th's satellite representation, also centered on the North Pole:
The big difference between these two pictures is the obvious and decrease in sea ice concentration in multiple regions across the Arctic. You can also see the ice edge receding from where it was on the 19th of April. Since they were some of the last areas to freeze, it is unsurprising to see melting and melted ice in places like the Hudson Bay, the Newfoundland Sea, the Greenland Sea, the Bering Sea and the Sea of Okhotsk. It is very surprising to me to see lower ice concentration values near the North Pole. When I first saw an indication of this a couple of weeks ago, I was hesitant to believe it since the satellite data can look different day-to-day based on the angle from it to the Earth and other factors. As the days have gone by, this condition hasn't disappeared, however, which leads me to think that there really are areas of thin ice where thicker ice normally can be found, even at the end of April.
As a whole, here is what the Arctic ice extent looks like in time-series form through May 4th:
You can easily see the rapid decrease in ice extent in the past couple of weeks that followed relatively stable conditions (for the Arctic area as a whole) at the beginning of the month. As I stated above, this rapid decrease is normal for this time of year, as you can see from the slope of the `Average` time series (dark gray line in the light gray envelope). The big difference is the poor condition of ice extent at the beginning of April - 2nd lowest in recorded history. In the past couple of days, the 2011 time series line has approached and met the 2007 time series line. The 2007 time series was added to this plot to provide additional context. Since the record low extent was recorded in 2007, it is useful to compare current values and trends to that year's.
Antarctic Pictures and Graphs
Here is a satellite representation of Antarctic sea ice conditions from May 4th:
For comparison purposes, here is the similar picture from April 19th:
New sea ice continues to form along the edges of the ice mass from April. This is occurring at a normal rate. The extent is still below normal, however, due to the record low extent set back in February. If ice was freezing at a much higher than average rate, the difference between this year's extent and normal would be decreasing.
Here is the time series graph of Antarctic sea ice extent with the +/- 2 standard deviations in light gray and the climatological mean in dark gray through May 4th:
This graph shows that ice is freezing at a somewhat faster than average rate. The difference between conditions at the beginning of April and the end of April is good to see.
Here are my State of the Poles posts from April and March.
You can find the NSIDC's May report here.
Cross-posted at WeatherDem - the blog.