A Warmer Ocean and Sea Level Rise

WARMER and RISING OCEANS

          The world’s oceans are warming, and rising. This is a fact which has been reinforced in recent literature (Palmer et al. 2007). Temperature increases are due primarily to global warming from fossil fuel emissions and other industrial factors causing rises in greenhouse gasses. Global warming is correlated with ocean level rise by changing water’s density due to thermal expansion, and creating more water input by the melting of glaciers (Nicholls and Cazenave 2010). So, as we emit more CO₂, we see higher sea-levels (Torstensson et al. 2012). Warming and rising of the oceans will have many affects: biological systems change, ecological relationships can be altered, human communities will be affected and possibly wiped out altogether, and commercial industries will have to change. This page is dedicated to the recent literature on the topic of ocean warming and sea-level rise, specifically its causes and consequences.    

          Research: 

Recent research has shown that a long term (decades) global warming signals have emerged in the top 300 meters of the ocean, due primarily to human activities  (Lyman et al. 2010).  In fact, the upper layers of oceans absorb much of the energy our planet absorbs.  Furthermore, these warming patterns are increasing in both frequency and intensity.  From May of 2009 to April of 2010, the tropical Pacific absorbed more energy (warmed) more than has ever been recorded in history (Ashok et al. 2012). What’s more interesting, according to Ashok and colleagues, these temperature anomalies in the Tropical Pacific were not similar to other temporal events attributed to El Nino weather patterns. This indicates that the temperature changes are due to global temperature changes, rather than local weather patterns (Ashok et al. 2012).

          It is now much more common knowledge that melting glaciers are expected to contribute to sea level rises in the next few hundred years. But how much will the levels of the ocean actually rise? One study by Gregory and colleagues, suggests that sea levels could rise by 132 mm by the year 2100, with  a further 76 mm of rise due solely to the melting of the Greenland Ice sheet (Gregory and Oerlemans, 1998). These data are not far from other predictions of rises between 80 and 120 mm by the year 2100, while it is considerably lower than some predictions of 500 mm. (Nicholls & Cazenave, 2010,  Gregory and Oerlemans, 1998).  This sea level rise may not happen all at once, however. There is evidence suggesting that as global temperatures rise, so does precipitation over Antarctica, which will actually accumulate that water in the form of ice and slow sea rise. This has been predicted to only be a bump in the road on the way to higher sea levels.         

This graph (above) shows the relationship between the frequency of anomaly of thermosteric sea-level between 0-700 m and the year. (Basically, how often the density of the ocean changed due to temperature increase over time).  As you can see, the trend is that the mean ocean temperature has a warming trend over the past two decades. (Lyman, 2010).

Consequences:

          There are many associated consequences with the rise in sea levels and warming of the oceans due to global warming. One of the more popular examples is the large impact it will have on polar biology; these systems will change dramatically as ice sheets melt, causing possible trophic cascade events (Gillett et al. 2008). There are other studies showing that warming oceans have an effect on other biota as well. One of the most rapidly warming areas of our oceans, the Japan Sea, has seen a sharp shift in Yellowtail (Seriola quinqueradiata) habitat occupation (Tian et al. 2012). Other studies show that warming oceans are associated with altered phytoplankton communities, which are the base of the food chain in the oceans. Phytoplankton communities were shown to have altered photosynthesis rates, altered pigmentation, and increased growth rates with increases in temperature and CO₂ levels (Torstensson et al. 2012).  These algal blooms could cause major anoxic dead-zones, causing extreme declines in ecosystem functionality.  However, it should be noted that this positive correlation with phytoplankton and temperatures is still under debate. To this date, examples of direct effects of warming oceans to biological systems are disparate, if only due to lack of research. One further study suggests that many species in the waters off of Australia will be pushed out of their habitable range due to warming oceans, possibly causing major extinction chains reactions (Wernberg et al. 2011).

          These biotic alterations due to warming ocean temperatures could also greatly affect commercial fishing industries and food stocks. Major food shortages could occur if ocean fish stocks collapse. Aside from food, warmer oceans and their increased sea levels will cause heavy monetary damage as well. Many parts of the world are very vulnerable to sea level rises, where flooding and saltwater influxes threaten communities. Rising sea levels may also increase coastal erosion, and contaminate water sources. People like to build their homes along waterways, making the worlds’ major river and coastlines more susceptible to damages.  Real life examples can already be found in Tokyo, where the coasts have retreated up to 5 meters, Shanghai has lost 3 meters, and Bangkok has lost 2 meters. These cities now must all depend on considerable infrastructure to reduce impacts on flooding (Nicholls & Cazenave, 2010).

          In conclusion, we know the earth’s oceans are warming, at rates previously unseen. They are absorbing much of the energy from our planet and are expanding thermally as a consequence. Ocean levels are also rising due to melting glaciers and water tables that were previously frozen for hundreds of thousands of years. These changes in ocean temperatures and levels may have many negative consequences to biota, humans, and our economies. 

  This is a graphic (above) of areas of the world that will be most affected by sea-level rise due to global temperature increases. Areas in purple show coastal areas that will be impacted, while the blue shows the vulnerable islands areas. Sea levels are expected to rise 3.3mm/year on average globally. Warmer areas of the world will see more dramatic increases, up to 20 mm/year (see graph below) (Nichols and Cavenave, 2010).