Monitoring the complex relationship between the ice sheets, ice shelves and climate systems can help understand the climate, how it has changed in the past, and how it may change in the future.

Ice sheets play a significant role in maintaining the balance of the global climate system. The ice sheets contain 99% of the freshwater on Earth and they form by year on year snow that compresses and turns into dense, solid ice. The colossal amount of compressed freshwater that the ice sheets contain means that when ice sheets melt, they impact on the salinity of the oceans and the global sea levels. Scientists predict that if the Greenland Ice Sheet melted sea level would rise about 6 metres. If the Antarctic Ice Sheet melted, sea level would rise by about 60 metres.

In addition to see level rise, the ice sheets also impact the climate and have further knock-on effects due to the feedback loops in the climate system. For more information on how melting ice sheets effect climate dynamics ,check out Professor Konrad Steffen and Professor Andrew Shepherd's lecture on The Role of the Greenland Ice Sheet in the Climate System, part of Imperative Space’s The Frozen Frontier: Monitoring the Greenland Ice sheet from Space MOOC.

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What is an ice sheet?

Ice sheets form where snow falls in winter and doesn’t melt entirely over the summer.  Over thousands of years, layers of snow build up and compress older layers into solid ice. An ice sheet or continental glacier comprises of compressed ice on land that covers an area equal to or larger than 50,000 km2. Ice caps cover and area smaller than 50,000km2.

Currently two ice sheets exist: Antarctica in the Southern Hemisphere and Greenland in the Northern Hemisphere. During the last ice age, there were sheets and ice shelves in North America and Scandinavia.

The Antarctic Ice Sheet is the largest single mass of ice on Earth. It covers almost 14 million square kilometres and is made up of 30 million cubic kilometres of ice, and holds approximately 90 percent of the fresh water on our planet. If melted, the Antarctic Ice Sheet would raise sea levels by 61.1 meters.

The Greenland Ice Sheet covers approximately 82% of Greenland’s surface. If it were to melt, this ice sheet would raise sea levels by 7.2 meters. NASA’s Gravity and Recovery and Climate Experiment results estimate that 239 cubic kilometres of the ice sheet are melting each year.

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What is an ice shelf? 

Ice that extends from an ice sheet beyond the land and into the ocean is known as an ice shelf. Ice shelves are found only in Antarctica, Greenland, and Canada.

Scientists define the area or point in which the ice sheet becomes the ice shelf the grounding line. We will cover grounding lines later in this week.

Ice shelves form as glaciers and ice streams which flow and extend into ocean which is cool enough to allow the water to remain frozen. As the ice sheet extends into the ocean the ice detaches from the bedrock below and ocean water fills the cavity between ice and bedrock. The ice shelf floats and juts out into the ocean from the ice sheet.

Ice shelves can grow in size both by snow accumulation and by continental glacial flow. Ice shelves loos mass when areas break off into ocean waters. The thickness of ice shelves ranges from 100-1,000 meters.

Unlike ice sheets, ice shelves already float in water so they do not directly affect sea levels when they melt.

The world’s largest ice shelves are the Ross Ice Shelf and the Filchner-Ronne Ice Shelf in Antarctica. The Larsen Ice Shelf in Antarctica broke up into hundreds of small fragments in 1995 and 2002. Global warming trends may have been a factor in the breakup of the ice shelf.

Canadian ice shelves are attached to Ellesmere Island. The Ayles Ice Shelf broke up in 2005, the M’Clintock Ice Shelf broke from 1963 to 1966 and the Markham Ice Shelf broke up in 2008. The only Canadian shelves still existing are the Alfred Ernest, Milne, Ward Hunt and Smith ice shelves.

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How do we measure ice sheets and ice shelves? ‍

Ice sheets and ice shelves are remote and hard to reach. Nevertheless it is useful to monitor their size, mass and shape to better understand their dynamics, how they interact with the climate and how they contribute to sea level rise.

Important indicators that scientists must consider when reviewing the impact of ice sheets on sea level rise is mass balance. Mass balance is the total snow input and the total loss through melting, ablation, or calving. 

There are four ways that ice in the ice sheet may be lost:

• Ablation
• Surface melt
• Calving at the interface with the ocean
• Melting from contact with the ocean.

Mass gain occurs almost entirely by snowfall, although in a few areas rainfall on the snow can add a small fraction to the mass input. 

Scientists use three methods to ice sheet mass balance measurement:

• Mapping ice flow by using satellite images or Synthetic Aperture Radar (SAR) equipment on satellites
• Measuring volume change by measuring gravitational change using satellites such as ICESat and GRACE

It is also important to know the topography of ice sheets to be able to successfully model ice sheet dynamics. It can be used to validate the ability of a model to reproduce the present-day geometry. Ice sheet topography is also important in understanding ice flows as they travel downhill, so knowing the slope of the ice sheet helps us understand these dynamics.  

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What impact is climate change having on ice sheets? 

In Greenland, we see a trend of ice mass decline. From 1979 to 2006, summer melt on the ice sheet increased by 30%. Increased melt means that the glacier moves faster around the islands edges.

In Antarctica, we have seen a decrease in ice mass in the northern and western regions. This has been accredited to the ocean temperatures rising by 2.5 degrees celsius in the past 50 years. On the east coast of Antarctica, no clear trend has emerged. Scientists believe that Antarctica starting to lose ice but not yet to the same extent as found in Greenland.

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Why do we need satellites to measure ice sheets and ice shelves? 

 It is difficult and expensive to get to the ice sheets and ice shelves because they are vast and remote. Satellites such as CryoSat-2, GRACE and ICESAT enhance our understanding of the cryosphere by obtaining a large quantity of data that is impossible to gain through in-situ measurements.

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