Arctic sea ice low – what does it really mean?

This is a cross-post from Carbon Brief

Arctic sea ice has hit the headlines over the last week. Last week the Polar Science Center of the Applied Physics Laboratory at the University of Washington found that Arctic sea ice volume has reached a new record minimum in 2010. This week, following an analysis of satellite data, researchers at the University of Bremen announced that Arctic sea ice extent has reached a \'historic low\' in 2011. Meanwhile, the National Snow and Ice Data Center (NSIDC), who normally release details of Arctic sea ice minima, has yet to determine that this year\'s summer minimum Arctic sea ice extent has been reached.

All of these different sources and claims can get pretty confusing. Just how do these different groups measure sea ice coverage? And why are they not all saying the same thing at the same time?

Arctic populations have been determining sea ice coverage visually for many years, so there are some some written records of ice extent over the last century. However, it is only since the launch of microwave energy-detecting satellites in the 1970s that scientists have been able to reliably quantify sea ice coverage. The satellites detect changes in microwaves emitted from the planet\'s surface. Ice, with its crystalline structure, emits more microwave energy than open seawater, so scientists can determine sea ice versus ocean coverage from the microwave data.

Some confusion arises from how the researchers present their data. Sometimes we hear about Arctic sea ice extent, other times sea ice volume, and sometimes even sea ice thickness. It is important to know which of these parameters is being discussed.

Carbon Brief have explained a bit about the difference between sea ice volume and sea ice extent in this blog. Here, we look at the question in a bit more detail.

The parameter that we tend to hear most about is Arctic sea ice extent - this is a measurement of the area of Arctic Ocean with at least 15% sea ice, and is determined from the satellite measurements discussed above. The National Snow and Ice Data Center (NSIDC) and IJIS (the International Arctic Research Center, working in co-operation with the Japan Aerospace Exploration Agency) post regular updates on Arctic sea ice extent.

The differences between results from separate research groups can arise as the groups may measure the microwaves using different frequency microwave sensors, and can reconstruct the ice coverage using different algorithms. This is why the sea ice reconstructions from researchers at the University of Bremen and the NSIDC differ slightly. The Bremen researchers use data from sensors that can study smaller area, but are more susceptible to interference melting or storm activity. The NSIDC sensors pick up a larger area, so are less detailed, but are not so susceptible to the effects of melting and storm activity. The two techniques do give good agreement though, and tend to show the same overall trends.

Mar Sept Arctic sea ice extent Cryoclim

The change in Arctic sea ice extent, 1979-2010. The graph shows the ice extent in March and September, as compared to the average sea ice extent for the whole period (the grey dotted line). Source: CryoClim

Arctic sea ice volume features less frequently in the news, but is another important parameter. The volume of sea ice is obviously more difficult to assess than the extent of sea ice, but it can be determined in two ways. It is either generated by combining computer modelling and the sea ice coverage determined by microwave satellite data (Pan-Arctic Ice Ocean Modeling and Assimilation System, PIOMAS - at the Polar Science Center), or more directly from altimetry measurements made by the CRYOSAT-2 satellite launched in 2010. The Polar Science Center posts regular updates on sea ice volume here.

PIOMAS Arctic sea ice volume anomaly

Arctic sea ice volume anomaly from PIOMAS updated once a month. Daily Sea Ice volume anomalies for each day are computed relative to the 1979 to 2010 average for that day of the year. The trend for the period 1979- present  is shown in blue. Shaded areas show one and two standard deviations from the trend. Error bars indicate the uncertainty of the  monthly anomaly plotted once per year. Source: Polar Science Center

Arctic sea ice thickness can also be determined using the CRYOSAT-2 satellite, or from the PIOMAS combination of modelling and microwave satellite reconstructions.

Although confusing, the fact that different research groups use different means of measuring the state of Arctic sea ice and may thus make slightly different assessments of this year\'s summer ice coverage minimum at slightly different times is relatively unimportant. The key point is that all of these different techniques show the same scenario - Arctic sea ice is declining, and that natural variation alone cannot explain it. Around half of the decline in Arctic sea ice since 1979 can be attributed to increases in greenhouse gases.

The media tend to focus on the Arctic sea ice minimum in September, and whether it has broken any records. However, this is misleading and draws attention away from more important phenomena. For example, as Real Climate highlighted in a recent blogpost, it is the major loss of Arctic sea ice during the early summer months of this year that will have the biggest climatic impact. Early summer ice-loss prevents the Arctic from remaining cool throughout summer, since the reflective ice is no longer present, allowing the sun to reach the open seawater and heat it. The warm seawater could delay the formation of ice during Autumn.

The early summer sea ice loss did, however, lead to much speculation in the blogosphere about the approaching September minimum, and whether it would break the record set by 2007\'s abnormally low sea ice extent. The unusual low of 2007 has since been ascribed to a very specific set of weather conditions, which exacerbated the increase in melting associated with global warming. Interestingly, as pointed out in an interesting blogpost at Climate Progress, similar weather conditions were present at the start of this summer, but the weather has not been so conducive to melting towards the end of summer, yet the Arctic sea ice extent has continued to hover around similar levels to those seen in 2007.

Posted by Verity on Wednesday, 14 September, 2011

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