Are Sea levels rising due to global warming?
Many scientists claim that man-made emissions of carbon dioxide and other gases have caused global temperatures to increase since the Industrial Revolution and will cause further increase in the 21stcentury, with numerous negative side effects, one of those being a rise in sea levels. The IPCC’s 2001 report predicts that global average sea level will rise by 10 to 80 centimeters by the year 2100.
Predicted consequences include coastal flooding, incursion of salt water into coastal freshwater supplies, and a host of other effects such as a change in ocean currents in the Arctic Ocean.
Let’s first have a look at what happened up until now.
Melting iceberg in Antarctica
As far as data shows the rate of sea level rise increased to about 15 centimeters per century around the mid 1800s. It appears that this rate has remained constant for the past 150 years.
This increase mostly results from thermal expansion of ocean waters, due to increased water temperatures and to the melting of glaciers and polar icecaps.
Currently there are nearly 29 million cubic kilometers of icecaps and glaciers in the world. A large portion of this ice is floating around the Arctic Ocean and in the seas near Antarctica. The melting of this sea ice will have no impact on sea level rise, because the mass of the floating ice is identical to the mass of the water it will turn into when it melts.
However, the melting of grounded ice in Antarctica and Greenland and of glaciers in various mountain regions of the world will cause sea levels to rise.
Is sea level rise caused by human activity?
Fig. 1: Rate of sea level rise (in cm/century) vs. year (from 18000 BC to 2000 AD); derived from graph by Lambeck cited in IPCC’s Climate Change 2001
Those who oppose the hypotheses that global warming is due to human activity will argue that Earth is currently between ice ages, that rising sea level has been the norm for the last 20,000 years and that sea levels have often been rising a whole lot faster than they are at present, as shown in the graph to the right.
Others will argue that, although sea levels have often been rising much faster than they are at present, it’s also clear from the graph above that they have been quite stable over the past 2000 years, and then started rising suddenly since the Industrial Revolution. Geological observations indicate that during the last 2,000 years, sea level change was small, with an average rate of only 0.0–0.2 mm per year. This compares to an average rate of 1.7 ± 0.5 mm per year for the 20th century. Therefore it would be logical to conclude that sea level rise over the past 150 years is indeed caused by human carbon dioxide emissions.
Another argument used to claim that humans aren’t responsible for sea level rise is that observations suggest a constant rate of sea level rise for the past 150 years, while rate of man-made carbon dioxide emissions has increased over 100-fold. Why hasn’t sea level rise increased at the same rate as CO2 emissions?
This is explained by the fact that there’s nothing like a constant rise in sea levels. On the timescale of years and decades, sea level records contain a considerable amount of variability. Therefore it’s impossible to measure a direct correlation between sea level rise and CO2 emission increase. However, a range of evidence clearly shows that the rate of sea level rise increased between the mid-19th and mid-20th centuries, simultaneously with human CO2 emissions.
Some will further argue that most of the cumulative rise in sea level preceded the majority of cumulative carbon dioxide emissions. Global temperature change and sea level rise do correlate with each other, but not with human activities; thus they claim that both temperature and sea level are changing principally due to natural phenomena.
This type of argument simply ignores all the facts which show an increased rate of sea level rise since the mid-19th century when human industrialization began. For a clear picture just take a look at the graph to the left.
For me this is clear evidence that human caused global warming is the cause of an increased rate of rising sea levels.
