Arctic sea ice is frozen sea water, which makes it part of Earth’s cryosphere and an integral part of the global climate system. It freezes, grows, declines and melts in the ocean where, for most of the year, it is usually covered with snow. Arctic sea ice tends to form in September and increases in extent during the winter months, reaching a maximum in March. Thereafter, as temperatures rise, the ice melts. The melting season usually starts in late March and ends during September. However local conditions predominate, so some areas may be freezing at the same time as others are continuing to melt, or vice versa.
Why Is Sea Ice Important?
Arctic sea ice is a critical component of the polar biome for four main reasons.
(a) First, it cools the Arctic Ocean, thus reinforcing the cooling effect of the polar region on Earth’s temperature and climate system. Cold Arctic water is also a key driver of the thermohaline circulation – the deep ocean currents that redistribute colder and warmer waters around the world, while moderating climate, sequestering carbon dioxide and enriching surface water with nutrients wherever they upwell.
(b) Second, sea ice has a bright white surface which reflects 50-70 percent of sunlight back into space – 90 percent if covered in snow. As long as sea ice exists, its surface reflectivity or “albedo” helps to mitigate climate change and limit the rise in global temperature. Unfortunately, when ice melts, the consequent loss of reflectivity becomes a climate feedback which boosts warming, since the dark ocean surface is then exposed which reflects almost no light at all.
(c) Third, sea ice helps to reduce the effects of ocean winds, which cause greater mixing of seawater and faster underwater ice melt. In addition, winter sea ice helps to slow down glaciers flowing into the ocean, thus helping to preserve the mass balance of ice sheets, such as the Greenland ice sheet, from further reduction.
(d) Fourth, sea ice forms a natural habitat for seals, polar bears and other creatures, thus helping to maintain polar biodiversity. For example, krill, a keystone species of the marine food web, congregate on the underside of sea ice for shelter and for the phytoplankton they prey on, who also gather there.
According to satellite measurements and ground observations, over the past thirty years Arctic summertime sea ice has declined dramatically in both extent and thickness. This corroborates other observations of a warming Arctic, such as elevated temperatures, decline of the Greenland ice sheet, a rising number of Arctic fires, and thawing of permafrost below the surface the ground. For details of glacial ice in the southern hemisphere, see our in-depth article on the Antarctic Ice Sheet, the largest store of ice on the planet.
- Why Is Sea Ice Important?
- What Is Sea Ice Extent?
- What’s The Difference Between ‘Sea Ice Extent’ And ‘Sea Ice Area’?
- What Is The Arctic Sea Ice Minimum?
- What Is The Arctic Sea Ice Maximum?
- Does Sea Ice Raise Sea Levels When It Melts?
- What Are The Latest Arctic Temperatures?
- What Are The Latest Sea Ice Measurements?
- Latest News of Summer Ice Retreat
- How Do Scientists Forecast The Future Of Sea Ice in the Arctic?
- What Does The IPCC Say?
- When Will The Arctic Be Ice-Free In Summer?
What Is Sea Ice Extent?
Sea ice extent describes those areas of the ocean with a minimum of 15 percent ice coverage. It’s an important metric because satellites can measure it much more precisely than other variables like thickness or age.
What’s The Difference Between ‘Sea Ice Extent’ And ‘Sea Ice Area’?
During the winter, sea ice tends to coalesce into a single mass. But during the rest of the year, gaps – such as leads, polynyas or other areas of open water – appear where the ice has melted or fractured. And at times during the summer, only a few ice floes can be seen, along with smaller pieces of drift ice. “Sea ice area” measures only the ice, not the sea water. So, for example, if 1 sq km contains half water and half made up of pieces of ice, then the “area” is 0.5 sq km. By contrast, “sea ice extent” (which is always a larger number than area) measures the ice and the water in the vicinity. So, in our example, the “extent” would be 1 sq km.
What Is The Arctic Sea Ice Minimum?
The Arctic sea ice minimum is the point during the year, when sea ice extent is at its lowest. It occurs at the end of the summer melting season. For example, the lowest Arctic sea ice minimum ever recorded occurred on September 16, 2012.
In recent years, with rising temperatures across the region, ice melt has continued for longer, so the sea ice minimum has been occurring later. This impacts directly on global warming, since the sun is hottest during the summer, and the Arctic needs as much ice as possible in order to reflect it. Furthermore, if the melting season is getting longer then more ice is melting. Although this may re-freeze during the winter, it means that the average age of the ice is falling. This is bound to affect the overall structure and integrity of the Arctic ice pack.
What Is The Arctic Sea Ice Maximum?
The Arctic sea ice maximum is the point during the year, when sea ice extent is at its highest. It happens at the end of the winter season. Scientists are finding that less sea ice has been freezing during the winter, so the ice is already “weak” when the summer melting season arrives. This may be due to the age issue, mentioned above, or the fact that the Arctic ocean is getting warmer, or a combination of both. 1
How Do Scientists Measure Arctic Sea Ice?
Until the 1970s, the only reliable way of monitoring the thickness, volume and extent of sea ice in the Arctic Circle was by taking physical samples and through personal observation. With the advent of satellites (like NASA’s 1978 Scanning Multichannel Microwave Radiometer (SMMR) satellite) all this changed. Today, climate scientists and glaciologists rely on data from the Defense Meteorological Satellite Program (DMSP) satellite, as well as the European Space Agency CryoSat-2 (launched 2010) and the NASA ICESat-2 satellite, short for Ice, Cloud and land Elevation Satellite (launched 2018).
Does Sea Ice Raise Sea Levels When It Melts?
No. Any ice that is already in the sea (like icebergs, ice floes, floating ice shelves) does not raise sea levels when it melts. Why not? Because the volume of water it displaces as ice, is pretty much the same as the volume it adds to the ocean when it melts. We say pretty much, because a recent study claims that the volume of melting sea ice is fractionally greater. The study calculates that if all the existing sea ice melted, it would lead to a global sea level rise of about 4 cm (1.5 inches). 2
What Are The Latest Arctic Temperatures?
Back in 2012, the Arctic region was at its warmest for 4,000 years. 3
Today, we know that the region is warming twice as fast as the global average. 4 According to another study, published in Geophysical Research Letters, temperatures in the region are higher now than they have been for 44,000 years, perhaps even for 120,000 years. 5 See also: What’s the Difference Between Climate and Weather?
What Are The Latest Sea Ice Measurements?
Over the past 41 years, the Arctic has lost about 1.9 million square kilometers (734,000 square miles) of (December) ice, based on the difference between 2019 and 1979. This is roughly equivalent to the entire combined size of Alaska and California, and represents an average loss of 3.6 percent per decade.
Over the past 10 years, Arctic sea ice extent has been persistently low, marked in particular by the record low minimum of September 2012. In total, 8 of the 10 lowest September ice levels have occurred during the past decade, with the 13 lowest occurring in the last 13 years (2007-2019). Taking a slightly longer view – always best when evaluating the details of a climate system – the extent of ice during the 1990s was lower than the 1980s, with the 2000s being lower than the 1990s, and the 2010s lower than the 2000s.
The thickness and volume of ice in the Arctic also remained low during the 2010s, according to estimates from the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS), based on data from the European Space Agency (ESA) CryoSat-2 satellite. A key characteristic of Arctic ice today is its youth. Ice more than 4-years old, which used to account on average for 30 percent of the Arctic Ocean winter ice in the 1980s, has now (Dec 2019) almost disappeared.
In 2019, sea ice extent in the Arctic Ocean reached its minimum for the year on September 18 – a total of 4.15 million sq km (1.6 million miles). This is the second-lowest minimum ever recorded, after the record low of 3.39 million sq km (1.3 million sq mi) in 2012.
Also in 2019, average sea ice volume was the 2nd lowest on record with 13,500 cubic km – about 600 cubic km above the 2017 record.
Latest News of Summer Ice Retreat
The German research vessel RV Polarstern conducted a 12-month survey of Arctic sea ice, ending in October 2020. It witnessed how the summer ice declined to just under 3.74 million sq km (1.44 million sq miles). This represents its second lowest ever extent in the modern era. Only in 2012 did the pack ice retreat further – 3.41 million sq km. According to researchers, the downward trend (September) is roughly 13 percent per decade.
By comparison, the freezing and melting of sea ice in Antarctica appears to have more to do with natural variability, including annual and decadal cycles in the surrounding Southern Ocean, than with man made global warming.
How Do Scientists Forecast The Future Of Sea Ice in the Arctic?
For example, modeling studies helped to dampen speculation after the previous record low sea ice extent in 2007. This record-breaking low point of 4.13 million square kilometers (1.59 million square miles) – a massive 24 percent drop on the previous record in 2005 – was interpreted by some scientists as a sign that a sea ice “tipping point” had been reached. Fortunately, subsequent modeling studies 6 indicated that this was unlikely.
Seemingly, Arctic sea ice responds (in a linear way) to rising levels of carbon dioxide, so it’s likely to be our greenhouse gas emissions that determine whether or not we have ice-free summers in the Arctic Ocean. 7 Our current emission levels indicate that ice-free conditions are probable sometime over the coming decades. However, rather like the Indian Ocean Dipole amplified the Australian wildfires in 2019-2020, studies show that natural climate variability will also play a big role. 8
What Does The IPCC Say?
In their recent Special Report on the Oceans and Cryosphere in a Changing Climate (2019), the Intergovernmental Panel on Climate Change (IPCC ) gave an overview of the current science. It included the following points:
- As of 2019, sea ice in the Arctic is melting 12 days earlier after the winter and melting for 28 days longer in the early Autumn, since 1979. An average of 3 days and 7 days longer, respectively, per decade.
- Historical reconstructions of sea ice extent as far back as 1850, based on ship reports, charts, aeroplane surveys and other sources, suggest that ice loss over the past two decades is unprecedented in at least 150 years.
- Arctic sea ice is getting younger. Today, ice aged five years or more accounts for 2 percent, down from 30 percent in 1979. Over the same period, sea ice aged less than one year increased from roughly 40 percent in 1979, to 60–70 percent today.
- Arctic sea ice has thinned. Multiple satellite observations show reductions in Arctic Basin ice thickness from 2000 to 2012 of almost 60 centimeters (2 feet) per decade. Emerging evidence suggests that this loss of thickness is unprecedented over the past one hundred years.
- Roughly half of the Arctic summer sea ice loss is accounted for by man-made climate change. The remainder is likely caused by natural variability.
When Will The Arctic Be Ice-Free In Summer?
As yet, there is no scientific consensus as to when the summer Arctic is likely to be ice-free. The Third U.S. National Climate Assessment (NCA) published May 6, 2014, states that the Arctic Ocean is expected to be ice free in summer before 2050. Climate models that best replicate historical trends forecast a ‘nearly ice-free’ summer Arctic by the 2030s. 9 Based on the projections offered by several different models, one study puts the early limit for a sea ice free summer Arctic at around 2040. 10 The most pessimistic forecast is from Professor James Anderson of Harvard University who believes Arctic sea ice will disappear by the early 2020s. 11 If true, and it seems a little unlikely, it would mark a significant change in the surface of Planet Earth.
- August trends for 1982–2017 show sea temperatures rising about 0.5°C per decade over large areas of the Arctic basin. See: IPCC. “Special Report on the Ocean and Cryosphere.” (2019)
- “The melting of floating ice raises the ocean level”. Noerdlinger, Peter D. et al; (July 2007). Geophysical Journal International. 170 (1): 145–150.
- “Recent melt rates of Canadian arctic ice caps are the highest in four millennia”. Global and Planetary Change. 84: 3–7. Fisher, David; Zheng, James; Burgess, David; Zdanowicz, Christian; Kinnard, Christophe; Sharp, Martin; Bourgeois, Jocelyne (2012).
- “Permafrost and the Global Carbon Cycle.” T. Schuur. NOAA Arctic Essays. November 22, 2019.
- “Unprecedented recent summer warmth in Arctic Canada”. Geophysical Research Letters. 40 (21): 5745–5751. Miller, G. H.; Lehman, S. J.; Refsnider, K. A.; Southon, J. R.; Zhong, Y. (2013).
- “Recovery mechanisms of Arctic summer sea ice.” Geophysical Research Letters. Tietsche, S., D. Notz, J. H. Jungclaus, and J. Marotzke. 2011.
- Observed Arctic sea-ice loss directly follows anthropogenic CO2 emission. Science. Notz, D., and J. Stroeve. 2016.
- “How predictable is the timing of a summer ice-free Arctic?” Geophysical Research Letters. Jahn, A., J. E. Kay, M. M. Holland, and D. M. Hall. 2016.
- “Pacific Ocean Variability Influences the Time of Emergence of a Seasonally Ice-Free Arctic Ocean.” Geophysical Research Letters, 2019. J. A. Screen, C. Deser.
- “When will the summer Arctic be nearly sea ice free?” James E. Overland, Muyin Wang. Geophysical Research Letters. Volume 40, Issue 10, Pages 2097-2101. 28 May 2013. First published: 04 March 2013
- “We Have Five Years To Save Ourselves From Climate Change, Harvard Scientist Says.” Jeff McMahon. Forbes.com Jan 15, 2018.