Global Warming and Rising Sea Levels
Background
Global warming is an increase in the mean temperature of the globe as a result of human activity. The primary cause of global
warming is the emission of greenhouse gases, which build up in the Earth’s atmosphere. As they build up, it gets harder
for heat to escape into space so it is reflected back onto the Earth and trapped within the atmosphere. This results in world wide
heating. This heat has the capability to melt many of the world’s ice reservoirs and to heat the ocean. These factors
together contribute to worldwide increases in sea level.
In his Encyclical Laudato Si', Pope Francis addresses the effect of global warming on the rising sea levels and calls
humanity "to recognize the need for changes of lifestyle, production and consumption, in order to combat this warming or
at least the human causes which produce or aggravate it" (Franciscus, 2015). The global mean sea level (GMSL) has been
rising since 1880 and has experienced a slight acceleration that started around the 1980s (CMAR, 2015). The global mean
temperature has also been increasing in a less steady fashion (NOAA, 2015). By comparing the data one can see a positive
trend in both GMSL and global mean temperature which indicates that global warming may be a large contributor to the rise
in global sea levels (Figure 1). Additional evidence comes in the form of the contributors to the rising GMSL. They are
thermal expansion of the ocean and melting of glaciers and ice sheets in Antarctica and Greenland. All of these causes
are related to an increase in temperature likely resulting from global warming. |
Figure 1. Rise in Global Mean Sea Level and Global Mean Temperature from 1880 to 2013 (CMAR, 2015; NOAA, 2015). |
Thermal Expansion
The largest contributor to GMSL rise is thermal expansion of the oceans. As the ocean heats up, it increases in volume.
From 1971 to 2010 the rate of change in GMSL rise fell within the range of 0.4 to 0.8 mm/year in areas of the ocean
less than 700 meters deep as a result of thermal expansion. For deeper regions the range was 0.5 to 1.1 mm/year (IPCC, 2013).
Using this information, many models have been created to suggest projected expansion of the ocean and how much it will
cause the sea level to rise. Models suggest that in the worst case scenario, thermal expansion will add two meters to the
pre-industrial GMSL by 2500 (IPCC, 2013).
Glaciers
Even in early assessments, glaciers have been recognized as a significant contributor to GMSL rise. From 2003 to 2009, glaciers contributed between 0.64
and 0.79 mm/year to the level of the oceans. There is only medium confidence in models that predict their contribution to GMSL rise because the percentage
of glaciers that are well-observed is very low (IPCC, 2013). However, some closely observed glaciers exemplify the severity of global warming. One such
glacier is the Muir Glacier. The temperature in Glacier National Park, where it is located, has risen 1.33 °C since 1900 (USGS, 2013). As a result, the
glacier retreated 20 km between 1941 and 2004 (Figure 2) (TerraNature, 2006). This is not isolated to Alaska, glaciers are melting everywhere from
Tibet (Figure 3) to Argentina (Figure 4).
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Figure 2. Melting of Muir Glacier, Alaska between August 13,
1941 and August 31, 2004 ( NASA, 2015). |
Figure 3. Melting of Kyetrak Glacier, Tibet between 1921 and 2009 ( Breashears, 2010). |
Figure 4. Melting of Upsala Glacier, Argentina between 1928 and 2004 ( N.a., 2011). |
Greenland & Antarctica
To observe the contributions of the Greenland and Antarctic ice sheets to the rising GMSL three techniques are used: the mass budget method, repeat
altimetry, and gravimetric methods that measure temporal variations in the Earth’s gravity (IPCC, 2013). Observations using these techniques indicate
that the rate of contribution from Greenland has increased from 0.09 m/year (over the time period 1992-2001) to 0.59 mm/year (over the time period
2002-2011) (IPCC, 2013).
Greenland is expected to be a big contributor to GMSL rise over the next century. The glaciers in Greenland that flow into the ocean are grounded very
deep below sea level (NASA, 2014). The deeper waters come to Greenland via ocean currents from the tropics. This warmer water is more effective at eroding
the glaciers than the cooler water in the upper levels of the ocean. Severe undercutting of glaciers could result in their collapse. If the current trends
in Greenland’s ice loss continue, it could raise the global sea level by 85 mm (IPCC, 2013).
Antarctica is, surprisingly, not currently a significant contributor to GMSL rise. Antarctica loses ice mainly by calving, or the breaking off of ice
shelves located on its periphery. Since these shelves are already submerged, they do not cause the sea level to increase. These shelves collapse partially
because of meltwater ponds that form as a result of regional melting. As shelves break off, the warming ocean moves closer to parts of the continent that
are grounded above sea level. If these parts of Antarctica begin to melt and/or slide into the ocean they alone could raise the GMSL by 185 mm (IPCC, 2013).
Consequences
If the GMSL rises just one meter, the consequences could be disastrous. Some potential effects of sea level rise are damage or destruction of one third of
Earth’s estuaries, wetlands, coral reefs, and deltas, interruption of coastal fisheries, extensive flooding in low-lying countries, cities, and barrier
islands, and saltwater contamination of freshwater coastal aquifers (Miller and Spoolman, 2013). Although only coastal areas will be directly affected by
the flooding, millions of people will need to be relocated. For example, Bangladesh, a low-lying country in Asia, could see an increase of four meters in
sea level by 2100 (Miller and Spoolman, 2013). If only one meter of rise is seen, approximately 17% of the country could flood and displace 20 million people (Figure 5) (YLD, 2015).
Figure 5. Projected flooding in Bangladesh if the GMSL rises by 1 meter ( YLD, 2015). |
References
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from http://e360.yale.edu/slideshow/the_himalayas_melting_glaciers/8/7/.
Cistro Marine and Atmospheric Research (CMAR), 2015. Reconstructed GMSL from 1880 to 2013.
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Franciscus, 2015. Laudato Si’:
On Care For Our Common Home. Vatican Press, Vatican City, 18-19 pages.
Intergovernmental Panel on Climate Change (IPCC), 2013. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the
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Environmental Science, Fifteenth Edition. Cengage Learning, Boston, 483 pages.
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National Oceanic and Atmospheric Association (NOAA), 2015. Global Land and Ocean Temperature Anomalies, January – December. Retrieved 3 December 2015 from
http://www.ncdc.noaa.gov/cag/time-series/global/globe/land_ocean/ytd/12/1880-2013.csv
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Consequences of Global Warming On Glaciers and Sea Levels. Retrieved 22
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Sullivan, Laura, Joe Swainson, and Fabio Bergamaschi, 2014. When Sea Levels Attack! Retrieved
12 October 2015, from http://www.informationisbeautiful.net/visualizations/when-sea-levels-attack/.
TerraNature, 2006. Severe Glacial Reduction in
Southern Alaska. Retrieved 22 September 2015, from http://www.terranature.org/glacierreduction_alaska.htm.
USGS, 2013. Retreat of Glaciers in Glacier National Park.
Retrieved 21 September 2015, from http://nrmsc.usgs.gov/research/glacier_retreat.htm.
Years of Living Dangerously (YLD), 2015. 17% Under Water. Retrieved 7 December 2015 from http://cdn.yearsoflivingdangerously.com/wp-content/uploads/2014/02/Ep108_Bangladesh_SeaRise.gif.
Last updated December 8, 2015
