Arctic Sea Ice: Going For A Long Time
According to some sources the extent of sea-ice in the Arctic is at a record low this summer meaning that ice-free summers are in prospect in perhaps a few decades, or possibly sooner. But as is often the case with climate change, and seems to be always the case in the reporting of climate change in the media, things are not as simple as that.
Arctic sea ice is undoubtedly declining in area, especially over the past decade or so. Some have pointed out that this is to be expected since climate models predict an enhanced response to global warming in polar regions and that the changes seen recently are in excess of what could be explained by natural factors.
Analysis of gridded global temperature change shows the Arctic regions to have warmed more than anywhere else on Earth in the past 30 years. Indeed, the warming of this region influences the global average temperature change. Many regions of the world outside the Arctic haven’t warmed much at all, including Antarctica (except for the Antarctic Peninsula which may have more to do with ocean current changes than anything else.)
According to some news reports Arctic ice has “melted to a level not recorded since satellite observations started in 1972 and certainly not experienced for at least 8,000 years.” The claim comes from scientists at Bremen University who issued a press release.
They say that on September 8 the sea-ice extent was 4.24 million sq km. the previous record they say was September 17 2007 which was 4.27 million sq km.
The minimum sea-ice extent figures are not always consistent. The Daily Mail for example says that the ice extent fell below 4.6 million sq km last week and that the record low in 2007 was 4.13 million sq km. The 4.13 figure comes from NSIDC figures.
The situation is best seen in the ice area as measured by the Nansen Environmental and Remote Sensing Center. This shows that 2011 to be broadly similar 2008 – 2010, although all years plotted show that they are low, 2007 is by far the lowest. One of the interesting things to note in the data is that a month after the melt starts the sea-ice extent is much closer to the 1979 – 2006 average and it’s a steeper than average melting in the mid-June to mid-July that is exceptional. Click on image to enlarge.
This graph should be considered when claims about a record-breaking year are made. An alternative view, and indeed a better description of the data is that 2011 joins 2008 – 2010 in participating in a slight, but significant, recovery from the record lows of 2007. So much for the media reporting!
The NSIDC has another way of looking at the data, plotting the area of ocean with at least 15% sea ice, showing 2011 to be similar to 2007, though I would liked to have seen error bars on this graph, and here.
Looking at the months July and August is interesting. Click on image to enlarge.
It shows a downward trend since the start of satellite data. Looking over the whole year there is a clear trend in the satellite data since it was first available in 1979.
When considering studies of ice variation in the polar regions, be it ice-sheets or sea-ice, one must remember that accurate satellite-based measurements have only been available since the late 1970s. This means that one must be careful about placing the satellite data into context. It would be too much to suppose that this long-term trend began in 1979 but how far back it goes is difficult to determine because the quality of data is poorer before the satellite era. There is some evidence that the decline has been occurring since 1953, and even further back from data compiled by the Arctic and Antarctic Research Institute. These charts are perhaps the oldest operational sea ice data in existence and show that sea ice extent in the Russian Arctic has generally decreased since the beginning of the chart series in 1933.
The importance of pre-satellite data is that it shows that the reduction in Arctic sea-ice has been going on for most of the 20th century, so this trend is hardly likely to be a response to anthropogenic influences. This is a conclusion that could be extended to the end of the century if the decline is consistent with that seen earlier in the century. The decrease has accelerated in the past few years and it is as yet, I maintain, unclear as to the cause. Is it a response to the warming of the region due to anthropogenic causes, or a non-linear response to the monotonic decline already seen, or variations in such influences as the Arctic Oscillation? Is it a dip from which a gradual recovery will take place? I don’t think anyone knows.