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sea ice

Imaggeo on Mondays: Polar backbone (Arctic Ocean)

Imaggeo on Mondays: Polar backbone (Arctic Ocean)

This image was taken during the Arctic Ocean 2016(AO16) expedition that ventured to the central regions of the Arctic Ocean, including the North Pole. It shows a pressure ridge, or ice ridge, as viewed from onboard the deck of the icebreaker Oden. It was quite striking that the ice ridge resembled an image of a spine – sea ice being a defining characteristic of the broader Arctic environment and backbone to global climate interactions.

An ice ridge is a wall of broken ice that forms when floating ice is deformed by a build up of pressure between adjacent ice floes. Sea ice can drift quite quickly, and is driven by wind and ocean currents. Ridges are typically thicker than the surrounding level sea ice, being built up by ice blocks of different sizes. The submerged portion of the ridge is referred to as the “keel”, and the part above the water surface is called the “sail”. Ridges can be categorized as “first year” or “multi-year” features, with weathering affecting the morphology.

In the Arctic, such ridges have been measured to in excess of 20 m in thickness including keel and sail. As someone who studies plate tectonics, these collisional boundaries between plates of ice reminded me of a downscaled mountain-building setting.

The AO16 expedition ran from August to September 2016 and involved the Swedish icebreaker Oden and the Canadian icebreaker the Louis S. St-Laurent. A wealth of geological, oceanographic, meteorological data was collected. This period appeared to have coincided with the second lowest extent of sea ice coverage on record (tied with 2007), with around 4.14 million square kilometers.

The geological evolution of the Arctic Ocean in the regions closest to the margins of northern Greenland and the Canadian Arctic Islands are some of the most poorly understood. This is largely a function of the oceanic gyre system, which causes the thickest sea ice to build up in these areas making physical access difficult. From a maritime engineering perspective, the ice ridges pose a challenge and risk to icebreaking operations and navigation. Ice ridges may determine the design load for marine and coastal structures such as platforms, ships, pipelines and bridges, and are important for both ice volume estimations and for the strength of pack ice.

By Grace Shephard, geophysicist from the Centre for Earth Evolution and Dynamics (CEED) at the University of Oslo, Norway.

Geosciences Column: The complex links between shrinking sea ice and cloud cover

Sea ice breaking on the Chukchi Sea, Barrow, July 2014

The global climate system is complex. It is composed of, and governed by, a plethora of interconnect factors. Solar radiation, land surface, ice cover, the atmosphere and living things, as well as wind and ocean currents, play a crucial role in the climate system. These factors are intricately connected; changes to some can have significant effects on others, leading to overall consequences for the global climate.

Since the 1980s, sea ice has been decreasing gradually as a result of global warming. But the impact of retreating sea ice on the global climate system aren’ t yet fully understood. A new study published in the EGU’s open access journal, Atmospheric Chemistry and Physics, attempts to unravel the complex feedback systems between Arctic sea ice extent and cloud cover in the region.

The researchers, lead by Manabu Abe, of the Institute of Arctic Climate and Environmental Research in Japan, argue that shrinking sea ice extent in the Arctic is the cause for increased cloud cover in the region. This, in turn, further enhances the feedback processes of Arctic warming because it cause sea ice to retreat further.

Sea ice, is the ice that ‘grows’ as water in the poles is exposed to very low temperatures over long periods of time. Although some waters are covered by ice year round, most sea ice forms during the cold winter months and melts in the summer.

Global climate influences the annual growth of sea ice. This year, ocean waters in the Arctic are failing to freeze and sea ice isn’t forming as quickly as it normally would. Alarmingly, October 2016 registered the lowest sea ice extent since records began.

Scientists think that the unusually low amount of sea ice formed in the Arctic this year is the result of extraordinarily hot sea surface and air temperatures, which are essentially stopping the formation of ice on ocean waters.

But sea ice extent also influences global climate. Solar radiation is absorbed and reflected by the Earth’s atmosphere (including clouds) and surface. Ice is more reflective than water and land. So as ice cover across the globe decreases, so does the planet’s ability to reflect solar radiation, causing the Earth’s surface to warm further, which, in turn, causes more melting of ice. This is know as the ice-albedo feedback loop.

The effects of shrinking sea ice are not limited to surface warming. Ocean heat uptake and storage can be affected, as can be the formation of low-level cloud cover over the Arctic. While the surface of clouds reflect solar radiation, they also prevent heat from being lost from the Earth’s surface. That’s why, often, on overcast nights temperatures are higher than on clear nights.

A study back in 2012, proposed that increased cloud cover in the Arctic enhanced the radiation emitted by the atmosphere and clouds – known as longwave radiation (DLR) -, causing higher surface air temperatures in autumn. This would extend the sea ice melting season. But there is little data which measures radiation at the surface, making the claim controversial.

Other studies have used computer simulations of the global climate, to mimic the effects of reduced sea ice conditions on cloud cover. They show that the areas of open ocean created by the reduction in sea ice mean more moisture is transported from the ocean to the atmosphere, resulting in the formation of more clouds. But the simulations are not very good at representing polar clouds and so the results aren’t entirely reliable.

Now, Abe and his coworkers, used a new state-of-the-art climate simulation to try and shed light on the problem. They included data from as far back as 1850 in their study, as well as making it more robust by taking into account other factors, such as changing sea surface temperatures, greenhouse gases, aerosols and land use (from the 1980s to 2005), which might affect the formation of clouds.

Geographical map of the simulated linear trend in the total cloud cover (shaded) and sea ice concentration (contours) in (a) September, (b) October, and (c) November during the period 1976–2005. The units are decade. From M.Abe at al., 2016

Geographical map of the simulated linear trend in the total cloud cover (shaded) and sea ice concentration (contours) in (a) September, (b) October, and (c) November during the period 1976–2005. The units are decade. From M.Abe et al., 2016

The new simulation found that between 1976 and 2005, Arctic sea ice decreased through the summer and autumn months (which is corroborated by satellite observations). Meanwhile, cloud cover increased throughout autumn, winter and spring, reaching its peak in October.

The researchers argue that the link between the two trends is not coincidental. Reduced sea ice extent in the autumn months,coupled with a decrease in atmospheric temperatures, means more heat is exchanged from the oceans to the atmosphere, which fuels the formation of clouds. More clouds mean downwards longwave radiation (DLR) in October is increased by as much as 40 to 60% (compared with clear autumn skies). With less heat being reflected off the surface of the Earth, sea ice extent decreases further due to melting and so a feedback loop (not dissimilar to the ice-albedo loop) is established.

The results reinforce the findings of previous studies, but some questions remain unanswered. The scientists point out that, it is not only important to understand how much cloud cover increases by as a result of shrinking sea ice extent. In a warming climate, how increases in air temperature and humidity affect the vertical structure of clouds will play an important role in the sea ice-cloud feedback loop. The vertical profile of a cloud also strongly influences how and how much DLR is reflected back on the Earth’s surface, so there is a need for a better understanding of the feedback processes related to clouds too.

By Laura Roberts Artal, EGU Communications Officer

References

Abe, M., Nozawa, T., Ogura, T., and Takata, K.: Effect of retreating sea ice on Arctic cloud cover in simulated recent global warming, Atmos. Chem. Phys., 16, 14343-14356, doi:10.5194/acp-16-14343-2016, 2016.

Wu, D.L., and Lee, J.N.:Arctic low cloud changes as observed by MISR and CALIOP: Implication for the enhanced autumnal warming and sea ice loss, J. Geophys. Res.-Atmos., 117, D07107, doi:10.1029/2011JD017050, 2012

Geosciences column: Playing back the Antarctic ice records

Satellites are keeping tabs on the state of Arctic and Antarctic sea ice, and have observed considerable declines in ice extent in many areas since records began, but what do we know of past sea ice extent?

Ice cores keep an excellent record of climate change, but until recently, ice cores have not been used to quantify patterns in past sea ice extent because few reliable compounds are preserved in the ice. While methanesulphonic acid (MSA) has been used in the past, it is an unstable compound and is easily remobilised after it has been deposited. The amount of sea-salt sodium deposited in brine pools or as high salinity crystals known as ‘frost flowers’ that form on the ice surface can also be used to identify changes in sea ice extent. These deposits are difficult to distinguish from larger sources of sea-salt sodium (aerosols and sea spray) though, making it a poor proxy.

Recent research published in Atmospheric Chemistry and Physics may provide an answer. An international team of geochemists have identified two stable indicators of sea ice extent in ice cores: the halogens bromine and iodine.

The oceans are considered to the be the main reservoir of bromine and iodine, but satellite data show these halogens also have a strong link with sea ice at the poles. Furthermore, recent research suggests that algae that grow under sea ice are big contributors to atmospheric iodine. Sea ice provides a substrate for algae to grow on during the spring. It is thin enough for light to penetrate, allowing the algae to photosynthesise, and also allows compounds produced by the algae to reach the atmosphere, including iodine and iodine oxide. These peaks in iodine oxide production are spotted by satellites during the spring.

The underside of Antarctic pack ice. The brown-green algae are an important food source for krill, as well as being a source of atmospheric iodine. (Credit: Kills and Marshall, 1995)

The underside of Antarctic pack ice. The brown-green algae are an important food source for krill, as well as being a source of atmospheric iodine. (Credit: Kills and Marshall, 1995)

During interglacials, the extent of sea ice is much smaller than during a glacial period. This means that the thin sea ice (where iodine is released into the atmosphere) is much closer to the coast. Air bubbles trapped in compacting continental snow preserve these atmospheric gases, which can be sampled in an ice core at a much much later date. Thus, when there is more iodine in the ice core, the extent of sea ice is small; when there is less, the thin edge of the ice sheet was much further from the coast.

Satellite measurements also show the amount of bromine oxide in the atmosphere peaks during the polar spring. This is because the increase in light level stimulates a series of photochemical reactions that convert bromine salts into bromine and bromine oxide, releasing it into the atmosphere. These events are known as bromine explosions and result in an increase in atmospheric bromine.

How bromine ions (bromide) makes it way from the ocean to the atmosphere and onto the surface of Antarctica (where it is later compacted by layers of snow, forming ice, and drilled to produce the Talos Dome ice core). Blue, red and green lines indicate aerosol-phase bromide, gas-phase bromide and hydrogen bromide, respectively. (Credit: Spolaor et al., 2013)

How bromine ions (bromide) makes it way from the ocean to the atmosphere and onto the surface of Antarctica (where it is later compacted by layers of snow, forming ice, and drilled to produce the Talos Dome ice core). Blue, red and green lines indicate aerosol-phase bromide, gas-phase bromide and hydrogen bromide, respectively (click for larger). (Credit: Spolaor et al., 2013)

Because the extent of sea ice is reduced during interglacials, bromine explosions must occur closer to the Antarctic coast, meaning more bromine will be trapped in the ice sheet during an interglacial period. Using sodium as a proxy for the amount of sea salt in the ice core, Andrea Spolaor and her team were able to work out how much of the bromine was in the atmosphere at the time. Since bromine explosions result in an an increase in atmospheric bromine, but have no effect on sodium, these events can be identified when the ratio of bromine to sodium in the ice core is high.

Now we can work out the extent of past sea ice, what lies ahead? The first part of the IPCC 5th Assessment Report released earlier today concerns the physical changes in the Earth’s climate and what we can expect in the future; the future of our changing planet will be discussed at this year’s Geosciences Information For Teachers workshop at EGU 2014, and you will be able to find a great discussion of climate change, its history and its impacts in the next issue of GeoQ – stay tuned!

By Sara Mynott, EGU Communications Officer

Reference:

Spolaor, A., Vallelonga, P., Plane, J. M. C., Kehrwald, N., Gabrieli, J., Varin, C., Turetta, C., Cozzi, G., Kumar, R., Boutron, C., and Barbante, C.: Halogen species record Antarctic sea ice extent over glacial–interglacial periods, Atmos. Chem. Phys., 13, 6623-6635, 2013.

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