By Matt Burdett, 16 March 2018
On this page, we look at the impact of climate change on water stored in ice and oceans, and changing sea levels.
Rising sea levels
The rising of the average level of the sea is already occurring. As with most indicators of climate change, there are years when the sea levels drop slightly, but the trend is upwards. The graphs below shows that levels have been increasing consistently not just in the age of satellite observations (the upper graph) but also since the start of the industrial age (as shown in the lower graph which relies on observations at ground level). It is predicted that sea levels will rise around 50 cm to 130cm by 2100 if greenhouse gas emissions are not rapidly curbed (Kopp et al, 2016).
- Variation in sea levels based on satellite observations. Source: NASA, 2018a.
- Variation in sea levels based on ground observations (coastal tide gauge records). Source: NASA, 2018a.
Causes of sea level change
Sea levels change for two main reasons: thermal expansion, and melting ice. Interestingly, the ice that is part of the hydrosphere can be known by a sub-category: ‘cryosphere’.
Thermal expansion is responsible for around a third of sea level rise. As water warms, it expands its volume. This volume expansion occurs differently depending on the pressure that the water is at (deep water is under greater pressure and will expand more than water at low pressure) and the starting temperature of the water (cold water expands less than warm water, and water under 4 degrees centigrade contracts). Therefore, sea level rise is hard to predict depending on which areas of the sea warm up the most.
Melting ice caps
The ice caps are the areas of ice found in the polar regions. If all the ice in these regions melted, it could raise global sea levels by up to 70 metres! (NSIDC, n.d.) There are two types of ice at the polar ice caps:
- Sea ice
- Land ice
Sea ice is ice that is floating in the sea. The Arctic ice is almost all sea ice, because the Arctic is actually an ocean – it has no land underneath it. Melting sea ice doesn’t significant cause sea level rise because when it is frozen it displaces roughly the same amount of water as the volume of the water it contains. Imagine an ice cube in a glass of water on a hot day. When the ice cube melts, the water level stays the same. However, there is some increase in sea levels because the ice is freshwater, which is less dense than saltwater. Therefore, when the freshwater melts, it takes up more volume than the equivalent weight of seawater. It has been suggested that melting sea ice could contribute about to about 4 cm of rising sea levels (NSICD, 2005). On the diagram below, you can see on the lower graph that the ice remains at roughly sea level because any extra ice will cause the icecap to sit more deeply in the water, and it will spread out more. However, it can build up more over Greenland because it is sitting on land, so can’t sink deeper into the sea.
- Cross sections showing the differences in the ice caps at the south and north polar regions. Source: BAS [British Antarctic Survey], n.d.
The melting of the land ice over Antarctica is much more significant. This is water that has been locked up as ice that is up to 3 km thick. Therefore if this ice melts, it will add to the volume of the sea and cause the sea level to rise. Though the actual level of sea level rise is very hard to determine, if all the ice melted some estimates suggest global sea level rises of around 4 metres (Davies, n.d.). This is not the same as suggesting how much actual sea levels rise will rise, because not all the ice will actually melt.
Note 1: On the diagram above, it doesn’t show the height above sea level of the land that the ice sits on; see the diagram below for that.
Note 2: Some Antarctic ice sits as an ‘ice shelf’ which is ice that is floating on the sea but connected to glaciers on the land, so not all of the Antarctic ice is land ice.
- Vertical cross-section through Antarctica. Source: NASA / LIMA, n.d.
As shown on the two graphs below, the amount of both sea and land ice is decreasing. The upper two graphs show Arctic sea ice and land ice, while the lower graph shows Antarctic land ice.
- Ice in the Arctic: Average area covered by sea ice in September (the end of the Arctic summer). Source: NASA, 2018b.
- Ice in the Arctic: Variation in the mass of the Greenland ice sheet since 2002. Source: NASA, 2018c.
- Ice in the Antarctic: Variation in the mass of the Antarctic ice sheet since 2002. Source: NASA, 2018c.
- Hidden Creek Glacier, 1916 and 2004, in Denali National Park and Preserve in Alaska. Source: Adema, Karpilo and Molnia, n.d.
The melting of glaciers is occurring all around the world, not just at the poles. Although the amount of ice in glaciers is relatively small compared to that in the ice caps, it has significant impacts. The graph below shows that although the amount of ice by both volume and area that is held in non-polar glaciers is small, glacial melt is causing up to 60% of current sea level rise. This may be because of they are melting faster than ice at the poles, and that they are entirely made up of land ice which has a much bigger impact on sea levels, as explained above.
- Comparison of volume (white), area (grey) and percent contribution to sea level rise (red) by small glaciers and ice caps [that are not at the poles], and the Greenland and Antarctic Ice Sheets. Source: NCIDC, 2018. Based on Meier et al. 2007.
Adema, Karpilo and Molnia, n.d. Melting Denali: Effects of Climate Change on Glaciers. Denali National Park & Preserve. https://www.nps.gov/articles/aps-v6-i1-c2.htm Accessed 28 February 2018.
BAS [British Antarctic Survey], n.d. Polar Extremes Cross Sections. https://discoveringantarctica.org.uk/oceans-atmosphere-landscape/atmosphere-weather-and-climate/key-factors-behind-antarcticas-climate/polar_extremes_cross_sections-2/ Accessed 4 March 2018.
Davies, n.d. If all the ice in Antarctica were to melt, how much would global sea level rise? How quickly is this likely to happen? http://www.antarcticglaciers.org/question/ice-antarctica-melt-much-global-sea-level-rise-quickly-likely-happen/ Accessed 4 March 2018.
Kopp et al., 2016. Common Era global sea-level variability. Proceedings of the National Academy of Sciences Mar 2016, 113 (11). http://www.pnas.org/content/113/11/E1434 Accessed 4 March 2018.
NASA, 2018a. Sea level. https://climate.nasa.gov/vital-signs/sea-level/ Accessed 2 March 2018.
NASA, 2018b. Arctic sea ice minimum. https://climate.nasa.gov/vital-signs/arctic-sea-ice/ Accessed 2 March 2018.
NASA, 2018c. Land ice. https://climate.nasa.gov/vital-signs/land-ice/ Accessed 1 March 2018.
NASA / LIMA, n.d. Meet Antarctica. https://lima.nasa.gov/antarctica/ Accessed 4 March 2018.
NSIDC [National Snow and Ice Data Center], 2005. Melting of Floating Ice Will Raise Sea Level. https://nsidc.org/news/newsroom/20050801_floatingice.html Accessed 4 March 2018.
NSIDC [National Snow and Ice Data Center], 2018. Contribution of the Cryosphere to Changes in Sea Level. https://nsidc.org/cryosphere/sotc/sea_level.html Accessed 4 March 2018.
Environmental impacts of climate change: Water: Learning activities
- How much might sea levels rise by 2100? 
- Describe the changes to sea levels over the past two decades. 
- Identify the two main causes of global sea level change. 
- Outline how thermal expansion causes sea level change. 
- Explain why ice melting in the Antarctic is likely to have a bigger impact on sea levels than ice in the Arctic. 
- Describe the changes in the volume of ice in the polar regions since 2000. 
- Why are melting non-polar glaciers having such a big impact on sea level rise? 
Devise and conduct experiments to demonstrate the impact of thermal expansion and melting ice. Remember to consider the role of evaporation in your experiment (e.g. use a lid for your container), and also consider:
- The difference between saltwater and freshwater
- Ice that is floating in water or sitting above it
- Expansion and melting rates at different temperatures
© Matthew Burdett, 2018. All rights reserved.
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