Sea ice is frozen seawater that floats on the ocean surface, forming when the ocean water drops below the freezing point of saltwater, which is approximately -1.8 °C (28.8 °F). It is one component of the larger cryosphere, which consists of all of the snow and ice covered regions of the world, with some parts being frozen for hundreds of thousands of years. This frozen realm dramatically influences all of life on the planet, impacting global temperatures and acting as an indicator of climate change.
Credit: Christian Rohleder, German Weather Service (DWD) National Snow and Ice Data Center
Sea ice forms in two locations where conditions are cold enough for the ocean water to freeze, mainly the Arctic and the Antarctic, and the total sea ice extent covers about 15% of the world’s oceans during peak cold seasons, which totals approximately 25 million square kilometers (9.7 million square miles).
Arctic sea ice extent covers approximately 15.5 million square kilometers (6 million square miles) in late winter. Antarctic sea ice covers about 18.5 million square kilometers (7 million square miles). By the end of summer, the seasonal decrease is much more dramatic in the Antarctic, dropping to about 2.5 million square kilometers (1 million square miles), whereas the Arctic decrease is approximately 6.5 million square kilometers (2.5 million square miles) by comparison.
Sea ice thickness can vary considerably between the two regions, with the Antarctic ice ranging from 1 to 2 meters (3 to 6 feet) thick, and Arctic sea ice around 2 to 3 meters (6 to 9 feet) thick, with some Arctic regions reaching up to 5 meters (15 feet). Typically, the Antarctic sea ice does not grow to be as thick as the more landlocked region of the Arctic sea ice, since it doesn’t stay in the more open waters of the Antarctic as long as it does in the Arctic.
Acting as a blanket between the ocean and the atmosphere, sea ice helps prevent atmospheric warming. Sea ice regulates the transfer of heat between the atmosphere and the ocean, making it a crucial factor that influences the global climate and ocean movements. This interaction between the ocean and atmosphere dampens the seasonal cycle, delaying the cooling of the atmosphere in the autumn and the warming of the atmosphere in the spring.
Sea ice play a vital role in reflecting the sun’s energy back into space, known as the “ice albedo feedback”, which regulates ocean and air temperature, affects the global ocean circulation, and sustains wildlife habitats. Between 50-70% of the sun’s rays are reflected off the surface of the sea ice, and when covered with snow (which is typically all year round), reflects up to 90% of the sun’s rays, making the Arctic and Antarctic sea ice critical for keeping the polar regions cool and helping to moderate global climate. The open water of the ocean, by comparison, reflects only about 10% of the sunlight.
Schematic showing the “ice albedo feedback”
Sea ice also plays an important role in the polar ecosystems and communities, affecting the local wildlife, from microscopic phytoplankton and algae, to shrimp-like crustaceans such as krill, as well as polar bears, seals, arctic foxes, and even giant killer whales.
As sea ice changes and declines from global warming, this vast ecosystem of wildlife slowly loses their habitat and threatens their way of life. Indigenous peoples of the arctic also use sea ice for travel and hunting, filling a pivotal role in their lives and customs. Sea ice also protects these coastal communities from erosion by reducing the impact of ocean waves. Without the sea ice present, heavy storms can cause tremendous ocean waves that can severely damage the shoreline.
Sea ice thickness is an indicator of how old the ice might be. New ice is generally much thinner, breaks easily, and is more susceptible to melting, which can have a cascading effect across the entire ecosystem for a variety of reasons.
Studies show that multi year ice in the arctic has been declining over the last few decades more rapidly than previously thought, leaving more of the thinner seasonal ice than ever before and less of the thicker multi year ice. This thinner seasonal ice creates a chain reaction of more melting, by melting much faster, and breaking up and exposing more of the ocean surface, which is much darker than the ice sheets and absorbs more of the sun’s heat. This, of course, causes even more melting of the sea ice, creating what is known as a positive feedback loop.
Young, thin sea ice also has much more salt content (salinity) than mutli year ice, containing concentrated droplets of brine trapped between the ice crystals. This has many consequences, including the disruption of ocean currents, which in turn can affect the global climate. The water below thin sea ice has a higher concentration of salt, making it more dense. This salinated water sinks to the ocean floor and moves toward the equator, while the warmer mid-level water moves toward the poles, affecting the global ocean conveyor belt circulation.
The water below multi year ice is fresh and nutrient rich, floating above the heavier, briny water and feeding algae and phytoplankton, which in turn feed the fish, which feed seals and other mammals, which feed the polar bears and whales.
Credit: Maps by NOAA Climate.gov, adapted from Figure 3 in “Sea Ice” chapter of the 2018 Arctic Report Card, based on data provided by Mark Tschudi.
Sea ice thickness also affects the habitats of many mammals, and the rapid decline of the sea ice extent pushes some species closer to extinction. Polar bears and walruses use thick sea ice for travel, hunting, and mating. As the older, thicker ice sheets decline, these mammals are forced to travel further for food, and also affects their ability to reproduce. For example, melting ice means walruses sometimes have to travel up to 402 kilometers (250 miles) round trip to reach their feeding grounds. Foraging arctic foxes have been stranded on shore, unable to migrate across the ice. Peary caribou have been seen falling through the abnormally thin ice during migration. Even the unicorn of the sea, the narwhal, has become a potential indicator of arctic climate and environmental change by studying their migration and behaviour patterns.
The arctic is also home to thriving communities of Indigenous peoples who rely on the thick multi year ice for hunting, travel, and for protection against heavy storms that can potentially devastate the coastlines. Even the smallest changes in climate and environment can have a substantial influence on those who depend on local access to natural resources.
The importance of sea ice can’t be understated, and the study of sea ice thickness is vital to monitoring climate change so we can proactively plan for the future. There are many methods for measuring ice thickness, and with the rise of new innovations in technology and robotics, we are able to dig deeper and see further than ever before.
Let’s take a look at some of the more common methods for studying the conditions of sea ice and ice thickness in the arctic and antarctic.
Learn about how ROVs can aid in your arctic research
Satellite observations of sea ice conditions began back in 1979 and has changed the way we are able to monitor and study ice thickness, allowing for a broad visualization of the landscape and a documented visual history of change to build climate models and analysis from.
The two satellite methods of measuring sea ice thickness are radar altimetry and laser, or LiDAR (Light Detection and Ranging) altimetry.
Radar technology is primarily used in the European satellite CryoSat-2, which was launched by the European Space Agency during the Earth Explorer Mission that launched on April 8th 2010. It is used to measure how high the top of the ice is floating.
Because the radar captures such a broad area, it is able to compare the open sea spaces (leads) between the ice floes to the floating top portion of the ice (freeboard) to determine the estimated thickness of the ice below the water level (draft). The freeboard is approximately 1/9th of the entire ice thickness, so this measurement is used to determine the overall thickness of the sea ice.
Credit: The European Space Agency - https://www.esa.int
Because this technology is used through satellites (or aircraft), the footprint can be up to hundreds of meters, so it is averaging over larger areas. It is also limited to measurements directly under the satellite, so does not provide complete global coverage. Radar altimeters can see through the snow to the ice surface, but the weight of the snow on the ice sheets depresses the freeboard, and because the snow thickness is unknown, some guesswork is required to calculate the ice thickness.
Laser altimetry, also known as LiDAR, is used to measure the distance from the satellite to the ice surface, or the ocean surface if there is a lead in the sea ice. Unlike radar altimetry, lidar only reads to the snow surface, so to calculate the sea ice thickness, you need to guess the snow depth on the freeboard. The draft (ice below the surface) makes up about 90% of the total thickness, and the freeboard (ice above the surface), about 10%, minus the approximate snow depth. The snow depth can be estimated using the two methods in conjunction, since the radar reads through the snow to the ice surface, and lidar reads to the snow’s surface.
In 2018, NASA deployed ICESat-2 (Ice, Cloud and land Elevation Satellite), marking the second lidar mission since the launch of the first mission from 2003-2009. The data from this mission showed that the arctic sea ice had thinned by approximately 20% since the end of the previous mission, which conflicted with existing research that found that the sea ice had remained mostly consistent over the last decade.
However, a major limitation of using this type of technology is the lack of visibility through clouds, and since the arctic is cloud covered 90% of the time during summer, taking measurments during this season is practically impossible. Likewise, with radar altimetry, while able to see through clouds, becomes incredibly unreliable when the surface becomes wet from melting. This limits both of these methods, since accurate measurements are only possible in the winter months.
“I think we’re going to learn a lot from having these two approaches to measuring ice thickness. They might be giving us an upper and lower bound on the sea ice thickness, and the right answer is probably somewhere in between,” said Alek Petty, a sea ice scientist at NASA Goddard. “There are reasons why ICESat-2 estimates could be low, and reasons why CryoSat-2 could be high, and we need to do more work to understand and bring these measurements in line with each other.”
Electromagnetic Induction Sensors are used for smaller scale measurement variabilities in the ice thickness that are not possible using satellite, covering the distance of a few kilometers at a rate of a couple of kilometers per hour.
These devices can be carried on foot or dragged with a sled, and using a six foot long antenna, sends an electromagnetic wave that reflects at the intersection of ocean and ice because of the change in conductivity. By measuring the time it takes for the signal to return, you can calculate the distance, by the speed of the electromagnetic wave.
However, you can't separate the ice thickness from the snow depth, so a laser is need to measure the distance from the sensor to the snow or ice.
Measurements from aircraft use radar and laser altimetry, the same technologies as satellite measurements. They can only produce a limited coverage of the ice sheets, but have much higher spatial resolution than that of satellites. Electromagnetic induction sensors can also be used in aircraft in combination with laser altimetry to provide the distance to the ice surface.
Airborne measurements are typically used to provide information on the local variability of the ice thickness by conducting annual flights across repeated paths of sea ice. The image below shows a profile of ice thickness using laser altimetry to measure the freeboard (orange) and electromagnetic induction for the draft (blue).
A high resolution profile of ice thickness generated from an airborne inductance probe. Information credit: Stefan Hendricks, Alfred Wegener Institute
There are two methods of using sonar to measure sea ice thickness: stationary moorings and submarines, both of which use upward looking sonar.
Declassified submarine sonar data has been accessible since 1958 and is the longest record of sea ice thickness measurements available. Sonar is used to map the bottom of the ice (draft) by sending sound pulses and calculating the distance by the time it takes to come back using the speed of sound in water as a function of temperature/salinity.
Static upward looking sonar is conducted by mooring instruments to the seafloor that provide continuous measurements of the sea ice draft from specific locations. Measurements are stored until the instruments are recovered periodically.
Traditionally, scientists have measured sea ice thickness by using ’point-based’ measurements. This consists of using an auger or a hot water drill (which has a boiler that heats water and a hose with a nozzle) and drilling hundreds of holes in the ice floe and measuring the thickness by dropping a weighted tape measure into the holes.
This is still the most accurate and reliable method of measurement, and is often used with other technologies to confirm their findings. However, this method is also limited. Drilling is restricted to areas accessible by ship, it generally avoids the thickest ice, and is a very labour intensive and time consuming method to undertake.
Sea ice mass balance is a summary of the effects of weather and the temperature of the ocean on ice thickness and ice conditions, on both the freeboard and draft.
This is accomplished by using buoys embedded in the multi-year ice floes that use instruments to measure temperature sensors through the ice, an upward-looking sensor suspended beneath the ice, and a sensor above the ice looking downwards.
These mass balance measurements offer essential data on the heat exchange between the air, ice and ocean. The data gathered from these sensors provide insights into how much of the sea ice is being melted by the atmosphere above and how much from the ocean below, and is then sent via satellite back to the data archiving center.
This method can be expensive, though still more cost-effective than field campaigns, and can be prone to failure due to the harsh weather of the poles, and interaction with curious wildlife, such as polar bears.
Now, with recent advancements in robotics, and ROVs (remotely operated vehicles) in particular, scientists have more ways to inspect and analyze sea ice thickness in the severe environments of the Arctic. Let’s take a look at some case studies to see how ROVs are being used to aid research in such difficult conditions.
Kenneth H Dunton, Professor in the Department of Marine Science at the University of Texas at Austin and Project Lead with Beaufort Lagoon Ecosystems (BLE) Long Term Ecological Research (LTER) has been using two DTG3 ROVs for research on long-term ecosystem changes while studying lagoons and other aquatic sites along the northern Alaskan Arctic coast.
The team has studied a total of 530 km of coastline from their 3 research nodes along the Beaufort Sea Coast to track and understand two main characteristics of the complex and productive ecosystems of the Beaufort Sea lagoons:
How natural climate cycles influence coastal ecosystems in the Arctic How climate change effects (such as permafrost thaw, shifting precipitation regimes, and losses of sea ice) alter coastal ecosystems
While most studies focus on summer when there is no ice cover, by using ROVs, Kenneth and his team are able to work during all three study seasons of the year (frozen over, ice break-up, open water) allowing them to gain valuable insight into the underwater world, even in the crucial months of winter.
“We drill holes through the ice and deploy the equipment,” detailed Kenneth. “We need to see what is happening under the ice. Diving is very intensive and time-consuming. If we can avoid diving and use the ROV it saves a lot of time and money.”
Using ROVs for underwater arctic research also had many more benefits for Kenneth and his team, including:
Kenneth explained that the DTG3 allows researchers to navigate under the ice in very shallow water (4 m), between the frozen ice above and the seafloor below to collect data, deploy lines for net and bottom trawl sampling, and capture underwater views.
“It’s not open water, there’s a hard surface above and below,” explained Kenneth. “Deep Trekker was valuable in those conditions. I looked at a lot of ROVs, and Deep Trekker had the features I really needed. I was looking for simplicity, maneuverability, and instrument stability for precise navigation without visual aids.”
Homa Kheyrollah Pour, Canada Research Chair in Remote Sensing of Environmental Change and assistant professor at Wilfrid Laurier University's Department of Geography and Environmental Studies conducted research on global warming using a DTG3 ROV with her team in Délı̨nę, within the Tsá Tué International Biosphere Reserve in the Northwest Territories.
Her team, the Remote Sensing of Environmental Change Research Group (ReSEC Lab), had the goal of collecting data on the effects of climate change on Arctic lake ice phenology and thickness, and study the interaction between ice condition variability and lake water attributes and productivity.
Using the DTG3, the ReSEC team was able to:
Kheyrollah Pour noted that the durability and reliability of the DTG3 was crucial for their research. “We didn’t have any difficulty to be honest,” she shared. “Everything was good and smooth.” Despite the Arctic conditions the DTG3 battery lasted the entire day, allowing the team to make the most of their time up north. “We were impressed with how long it lasted in such cold conditions,” said Kheyrollah Pour.
Dr Alex Nimmo Smith and Mr Peter Ganderton from the Plymouth University Marine Physics Research Group used the Deep Trekker DTG2 ROV on a research trip to the Norwegian Arctic to perform underwater inspections and give a visual survey of the underside of the ice. The vehicle was deployed through ice holes in water of -2°C and was able to successfully perform the tasks, making the portable ROV ideal for the intended operations.
Watch Deployment In Action
Sea ice plays a key role in the regulation of global temperatures, and studying sea ice thickness allows us to monitor sea ice extent to better understand how and why ice conditions change over time, giving us the opportunity to proactively plan for the future. So how is Deep Trekker helping to save the Poles?
Deep Trekker ROVs are great tools that are fully modular, have high quality imaging capabilities, and work in temperatures of -10°C to 50°C (14°F – 122°F), making exploration in the harsh environments of the Arctic regions possible in ways far beyond what has been available to researchers in the past. Built to be user-friendly and easy to pilot, maneuvering underwater only requires a bit of practice for operators to pick up.
The smallest of the three, the DTG3, is a mini observation-class underwater ROV built with a lightweight design for remarkable portability that allows it to be quickly deployed for conducting underwater visual inspections. Using a three thruster design, it is piloted by manipulating the pitch or yaw for vertical and lateral movements, providing excellent maneuverability; however, it lacks the power and feature capabilities of its bigger brothers, such as DVL sensor upgrades or an integrated tool platform.
When taking on missions in the arctic, logistics can be a big part of the challenge. Having equipment that is extremely portable is important. The DTG3 is the most portable of them all, with a whole kit fitting into a single case, which is easy to strap on the back of a snowmobile.
For missions that need to have a ROV fight more current or hold more equipment, such as an imaging sonar for detecting gaps in the underside of the ice, water quality sensors for analyzing the chemical makeup of the water, and other advanced sensors, the PIVOT and REVOLUTION ROVs from Deep Trekker provide a more powerful platform with similar portability advantages to the DTG3.
The PIVOT and REVOLUTION are true observation class ROVs that use six vectored thrusters for both vertical and lateral movements, and come integrated with rotating tool platforms capable of completing a variety of tasks. Engineered with carbon fiber and stainless steel, both vehicles are built with a similar design, though the PIVOT is scaled down in size to provide improved portability and maneuverability in tight spaces.
Some of the most common modular addons for the PIVOT and REVOLUTION are:
All three ROVs operate from a rugged handheld controller that is proven in harsh arctic environments. There is no need to fumble around with laptops or consoles that are not able to withstand the cold and windy environments.
Whether you need to get eyes under the sea ice or take water or sediment samples, Deep Trekker’s ROVs are excellent choices for Arctic exploration and research. Built to be intuitive, an operator can learn to deploy and pilot the vehicles quickly and easily, which saves an enormous amount of time and money, and allows researchers to gain a deeper understanding of sea ice and the cold, majestic waters of the Poles.
For more information, you can check out our ROV Buying Guide with more detailed specs for each vehicle.
Underwater drones were first invented in the 1950s. Their design enabled...
American Aquaculture in Review | Deep Trekker Robotics
Underwater drones were first invented in the 1950s. Their design enabled...