Why does global warming make sea levels rise?

Article by Isabel Key and Mina Frost.

3 causes of sea level rise:

1) THERMAL EXPANSION: The oceans take up over 90% of heat trapped by greenhouse gases [7]. This slows warming of the atmosphere [1], but it also causes sea level rise. Warmer water molecules push each other further apart, meaning warmer water takes up more space [10]. This is called ‘thermal expansion’ and it has caused about a third of sea level rise so far [7].

2) MELTING LAND ICE: Ice sheets naturally partially melt and reform on a yearly cycle [11]. Global warming is making more melt in summer and less reform in winter [6]. When ice on land melts, water runs into the sea, causing sea level rise [5].

3) MELTING SEA ICE: Ice floating on the ocean is known as ‘sea ice’. Ice is less dense than liquid water, so 1kg of ice takes up more space than 1kg of water [4]. This is why about 10% of an iceberg is above water [3]. This suggests that when an iceberg melts, the water produced only fills up the space that the iceberg took underwater before – melting sea ice should not increase sea level [5]. However, this is not quite true, because sea ice is freshwater (no salt), but the ocean is saltwater [4]. So, when 1kg of sea ice melts it adds 1kg of freshwater to the ocean [17,19]. The freshwater has a larger volume than seawater, so sea level rises a little [17,18,19]. However, this accounts for under 2% of sea level rise [13].

Which regions cause sea level rise? GREENLAND: ice sheets and glaciers are on land [14,15], and so add to sea level rise [20]. ARCTIC: is floating ice, so contributes very little to rise [13]. ANTARCTIC: no clear trend of sea ice extent over the last decades [9], perhaps due to natural variability, independent of climate change [12]. Parts of it are melting, such as the West Antarctic Ice Sheet [2,6]. In contrast, colder parts may gain ice [2], because as air warms, it holds more water vapor, so more snow can fall [2]. In the long term, however, we may see huge ice loss from Antarctic land ice [16,8], causing significant sea level rise.

Melting ice also indirectly further warms the Earth, and puts animals like polar bears at risk – see future posts.


[1] https://science.sciencemag.org/content/345/6199/897

[2] https://royalsociety.org/topics-policy/projects/climate-change-evidence-causes/question-14/

[3] https://beyondpenguins.ehe.osu.edu/issue/icebergs-and-glaciers/all-about-icebergs

[4] https://www.livescience.com/32110-why-do-icebergs-float.html

[5] https://www.jpl.nasa.gov/edu/teach/activity/whats-causing-sea-level-rise-land-ice-vs-sea-ice/

[6] https://unfccc.int/files/science/workstreams/research/application/pdf/5_wgiar5_hezel_sbsta40_short.pdf

[7] https://sealevel.nasa.gov/understanding-sea-level/global-sea-level/thermal-expansion

[8] https://iopscience.iop.org/article/10.1088/1748-9326/aa6512

[9] https://www.climate.gov/news-features/understanding-climate/understanding-climate-antarctic-sea-ice-extent

[10] https://www.bbc.com/bitesize/guides/zgr2pv4/revision/2

[11] https://ntrs.nasa.gov/search.jsp?R=20190025691

[12] https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014GL062231

[13] https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010GL042496

[14] https://discoveringantarctica.org.uk/oceans-atmosphere-landscape/ice-land-and-sea/ice-sheets-and-glaciation/

[15] https://www.researchgate.net/figure/Schematic-cross-section-of-Greenland-ice-sheet-hydrology-from-the-ice-margin-to-the_fig1_258310129

[16] https://nsidc.org/cryosphere/glaciers/questions/what.html

[17] https://nsidc.org/news/newsroom/20050801_floatingice.html

[18] https://skepticalscience.com/Sea-level-rise-due-to-floating-ice.html

[19] https://www.physlink.com/education/askexperts/ae389.cfm

[20] https://science.sciencemag.org/content/311/5763/963

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