In September 2012 the Arctic sea ice reached its lowest ever observed extent.
Note this article has now been updated following the September sea ice minimum.
Arctic sea ice reached its seasonal minimum on 16th September 2012 at a record low value of 3.41 million square kilometres (US National Snow and Ice Data Center). This record low is some 0.76 million square kilometres lower than the previous daily record set on 18 September 2007. Satellite records began in 1979 and have shown a long-term decline in sea ice extent, at an annual rate of over 4% per decade. The seasonal minimum (September) ice extent has declined at the faster rate of 11% per decade, and this rate of decline has accelerated in the past 15 years. The last six years now make up the lowest six daily minimum extents in the 32-year record. This record minimum is 3.29 million square kilometres below the 1979-2000 average.
Understanding, monitoring and modelling sea ice cover is of interest to Met Office scientists as it plays a key role in our weather and climate. Sea ice decline is also iconic of climate change in the Arctic, while the presence of sea ice determines the accessibility of the Arctic ocean and can also affect European and global climate. Sea ice cover seasonally insulates the atmosphere from the ocean, preventing the exchange of heat and gases. The formation of sea ice during winter allows atmospheric temperatures to fall, due to a lack of sunlight, to colder than -30 °C. In addition, its formation extracts fresh water from the ocean, producing the cold saline bottom water which influences ocean circulation. As sunlight returns to the poles in summer, the ice melts back allowing the atmosphere to be warmed by the ocean and releasing fresh water to stratify the regional oceans. At the same time, when sea ice is lost the Arctic ocean can absorb solar radiation much more readily and warm. Sea ice decline has a feedback on the climate system - less sunlight is reflected back into space and so the planet warms, causing more sea ice decline. Observing, understanding and ultimately simulating these processes in weather and climate models is critical to developing more accurate weather forecasts and longer term climate projections.
Daily sea ice extent for the period 1979-2000 (including +/- 2 standard deviation range), and for the most recent 10 years, up to 16 September 2012 (data from NSIDC, Fetter et al, 2009). Enlarge
(a) Sea ice extent on 16 September 2012, (b) Regions of the Arctic referred to in the text. (Credit – National Snow and Ice Data Center) Enlarge
Detailed analysis and experiments will be required to determine the exact causes of the record low extent this year. However, it is likely that there are two underlying causes; the ongoing thinning of the ice, which preconditions the ice to the possibility of large summer losses, and the strong storm over the central Arctic in August. This is different to the record low of 2007 where one of the main causes of the record low ice extent was a high pressure dipole which persisted throughout the summer - a synoptic situation which we saw in early June this year and also during parts of the melt season in 2010 and 2011.
Long-term changes in Arctic sea ice are likely to have impacts locally in the Arctic as well as driving changes in European and global climate. As the sea ice decreases, the immediate atmospheric response is for a local warming of the lower Arctic atmosphere by the relatively warm Arctic Ocean. However, there is also evidence that depleted sea ice alters atmospheric circulation patterns outside the Arctic, throughout the following months and into winter. This appears to result in high pressure over the Arctic and low pressure over the mid-latitudes which in turn tends to drive more easterly winds across Europe, particularly in winter. While other factors are also involved in determining winter climate, this raises the risk of anomalously cold conditions over Eurasia. However, the relative importance of sea ice conditions and other factors in producing cold conditions are being investigated by Met Office scientists and others.
Daily minimum observed ice extents for the years from 1979, ordered from the lowest to highest extent. (data from NSIDC, Fetter et al, 2009) Enlarge Timeseries of September monthly mean sea ice extent from HadISST Enlarge
Current climate models indicate that a plausible date for the earliest ice free* summer in the Arctic is 2025-2030. The performance of climate models is evaluated continually and routinely assessed against available observations. Preliminary new results from the CryoSat satellite (combined with data from the previous satellite IceSat) indicate that ice volume between 2003 and 2012 has declined at a rate as least as large as previously indicated, and faster than many climate models. Detailed assessment of the observational evidence is therefore essential, and helps us to understand more about our climate and feedbacks, in addition to leading to improved models and projections. An improved understanding of Arctic processes may lead to revised projections for an ice free summer in the Arctic.
*Ice free is defined as ice extent less than 1 million square kilometres.