Math in the Middle... of Oceans COASTAL GEOGRAPHY LESSON 6 Page 8

Sea Level Rise & Global Warming: Trends & Consequences
Recent history of sea-level change arouses great interest because of its sensitivity to climate
change and the effects it has on shoreline erosion. Throughout history, sea level has
been rising 1-3 mm/yr due to variety of factors that may have played a role in warming the
Earth’s surface. But within the last 300 years, the earth has been warming at an accelerating
pace.
Global warming has been implicated in sea level rise and is caused, in part, by the addition
of greenhouse gases, especially carbon dioxide, (CO2), and methane, (CH4), in the atmosphere.
Since the start of the Industrial Revolution in the mid-1750s, atmospheric carbon
dioxide has been increasing at a greater rate than at any other time in history as a result of
increased burning of coal and oil.
 

How do present-day atmospheric concentrations of CO2 compare with those at the start of the
Industrial Revolution?

ANSWER: Present CO2 levels have increased by approximately 50 ppm or 15%
since the mid-1750s.
Greenhouse gases work to cause global warming by forming an atmospheric boundary that
traps heat. About 70% of the sunlight that reaches the Earth penetrates the atmosphere
and heats it. This heat is then transformed into infrared radiation and radiated back into the
atmosphere. But not all sunlight is absorbed and transformed. Some of it is reflected back
into the atmosphere by surfaces with high reflective characteristics, or albedo. Snow,
desert sand and bare rock have high albedos and can reflect as much as 90% of the
radiation back into the atmosphere.
Atmospheric water vapor, carbon dioxide and methane strongly absorb infrared wavelengths
and causes the atmosphere to act very much like a blanket to keep in the heat.
Therefore, increasing levels of greenhouse gases contribute to the general long-term warming
of the atmosphere.
Recently, much of the earth’s rainforests is being burned to create pasture to support
livestock. In Brazil, forests as large as the state of Tennessee are being burned on a yearly
basis. While this practice in itself increases the CO2 levels in the atmosphere, other factors
that result from deforestation also increase the amount of greenhouses gases being generated.
As the overarching canopy of trees are removed, the forest floor receives large
amounts of sunlight never before encountered. This leads to the release of huge quantities
of methane and CO2. In addition, the massive increase in the number of dead trees leads
to a population explosion of termites. Termites, in turn, produce enormous quantities of
methane, which is now one of the fastest-growing greenhouse gases.

Carbon dioxide concentrations and other heat-absorbing molecules have increased by almost
25% since World War II. Carbon dioxide triggers enough warming to increase the
amount of water vapor that evaporates from the ocean. This extra water vapor in the
atmosphere traps nearly 90% of the infrared rays radiating back from Earth, causing the
Earth to become warmer
Within the past century, the earth’s temperature has already increased by 0.5O C. This
corresponds with a global mean sea-level rise of approximately 15 cm over the same time.
Climate change is expected to cause a further rise of approximately 20 cm by the year 2030
as temperatures increase by 1.5-4.5O C. Warming would cause sea level rise in two ways:
thermal expansion of water and melting of continental and alpine glaciers and ice caps. If
greenhouse gas emissions rise unchanged, sea level rise may reach 65 cm from current
levels by 2100 (approximately 6 cm/decade) and 20-140 cm by 2200.
 

Theoretical migration of the lagoon, island, and shoreface of a barrier island as sea
level rises.

How much coastal erosion from sea level rise took place took place during the last century along
the southeast U.S. coast? How much coastal erosion will take place by 2030? 2100?
 

ANSWER: Since we know the slope of coastal regions of the southeast U.S. is the
angular equivalent to 0.115, and we know the past and predicted future rises in sea
level, we can solve for shoreline retreat distance. Slope = sea level rise (rise) /
shoreline retreat distance (run). During the past century, we had a sea level rise of
15 cm, so: 0.115 = 15 cm / shoreline retreat distance, or, shoreline retreat distance
= 15 cm / 0.115 = 130 cm. By 2030, shoreline retreat distance will be 174 cm. And
by 2100, shoreline retreat distance will be 562 cm.

These increases in sea level rise will have many implications. First, and most obviously,
higher sea levels would threaten low-lying coastal areas and small islands, impacting millions
of square kilometers. Areas likely to be hardest hit will be poor countries such as the
Maldives, in the Indian Ocean, and small Pacific islands.

The economic impacts of coastal erosion due to sea level rise in the U.S. will be monumental.
For example, the southeast U.S. has 34,000 km of coastline. If sea levels rise 174 cm
by 2030, we could see losses of approximately 59,160 sq. km of shoreline. And with
renourshment sands costing anywhere from $1-$10 a cubic yard, renourishment projects
would run into the billions of dollars.
 

Rising sea levels will also cause groundwater in some regions to become more saline as salt
water moves into aquifers. Flows of estuaries, coastal rivers, and low-lying irrigation systems
would be affected and tidal wetlands and mangrove forests would face tremendous
erosion causing a loss of biodiversity. Damage caused by floods, storms, and cyclones
might worsen and tidal ranges in rivers and bays may be altered.