4.9: Sea-Level Change and Climate

Although tectonic processes and sedimentation are the principal agents that shape seafloor topography, changes in climate also affect certain features, particularly the continental shelves and coasts. Substantial climate changes have been observed during recorded human history. For example, much of North Africa and the Middle East, which is now a region of desert and near-desert, was a fertile region with ample rainfall during Greek and Roman times. However, such short-term changes are small in comparison to changes that occur during the hundreds of millions of years of a tectonic spreading cycle.

Climate Cycles

Over the past 1000 years, the Earth’s global average surface air temperature has varied by about 1.5°C. During the past 10,000 years, it has varied within a range of about 2°C to 3°C (Fig. 4-32). However, in the immediately preceding 2 to 3 million-year period, the Earth’s temperatures were as much as 10°C below what they are today. These colder periods are called ice ages or glacial periods. During the most recent ice age, the Earth’s climate alternated between periods of glacial maxima (when temperatures were about 10°C below those of today) and interglacial periods (when temperatures were close to those of today). During glacial maxima, the polar ice sheets extended to much lower latitudes than they do now. Several earlier major ice ages each lasted about 50 million years. Longer periods with warmer climates have occurred between ice ages. The most recent ice age may not yet be over, and the relatively warm climate of the past 10,000 years may simply represent a geologically short interglacial period.

Figure 4-32. The history of climate and sea-level changes during the past 18,000 years. (a) The global average surface air temperature was about 7–8°C lower than today until about 18,000 years ago and rose close to its present value by about 8000 years ago. (b) In response to the global temperature increase, sea level rose rapidly (about 120 m in total) as global climate warmed from about 18,000 to about 8000 years ago. More recently, sea level continued to rise slowly as global temperatures remained relatively stable. (c) The rate of sea level change increased to nearly 2 mm•yr^–1 about 12,000 years ago and has decreased since then.

Climate varies on timescales ranging from a few years to tens or hundreds of millions of years. Variations over the past 18,000 years are shown in Figure 4-32. These shorter timescale variations are superimposed on longer-term variations. Understanding the causes and consequences of the historical variations is important. Such knowledge may provide the key to understanding the effects and consequences of the extremely rapid (on geological timescales) global warming that has been predicted to occur and that appears to already have begun as a result of human enhancement of the atmospheric greenhouse effect.

Eustatic Sea-Level Change

When the Earth’s climate cools for a long time, ocean water cools and contracts and polar ice sheets expand as water is transferred from oceans to land in the form of glaciers and snow. Both of these processes cause the sea level to drop. Conversely, warm climate periods tend to heat and expand ocean water, and melting continental ice returns water to the oceans, raising the sea level. Such changes of sea level, called eustatic changes, occur synchronously throughout the world, although not all locations experience exactly the same amount of rise or fall. In contrast, sea-level changes caused by isostatic movements of an individual continent affect only that continent (CC2).

During the initial breakup of Pangaea, the climate was relatively warm, and it remained so until about 10 to 15 million years ago. During the warmest part of this period, about 75 million years ago, sea level was considerably higher than it is today, and as much as 40% of the Earth’s present land area was below sea level. For example, a shallow sea extended from the Gulf of Mexico far north into Canada and covered what is now the land between the Rocky Mountains and the Appalachian Mountains. In contrast, at the peak of the most recent glacial period, about 20,000 years ago, sea level was at least 100 m below its present level, exposing much more land area. Sea level has risen and fallen by various amounts many times as Pangaea has broken apart in the present spreading cycle, and coastlines have migrated back and forth accordingly.

Sea-Level Change and Continental Margin Topography

Erosion by rivers carves out valleys, and the rivers transport the eroded sediment downstream, where it is deposited in lower-lying areas or in the coastal oceans. Erosion by waves and winds at coastlines (Chap. 11) also tends to reduce topography and deposit the eroded sediment in the shallow waters of the continental shelves. In contrast, at water depths of more than a few meters, erosional forces in the oceans are generally reduced and sediment accumulation is dramatically reduced, except in proximity to rivers that transport massive sediment loads to the ocean (Chap. 6).

As a result of sea-level oscillations, the area between the edge of the continental shelf and an elevation several tens of meters above the present sea level has been subjected to alternating cycles of wind, river, and wave erosion at some times and sediment deposition at others. These processes have substantially modified the topography along the coasts. The effect is most apparent at passive margins. On most such margins, there is evidence of sea incursion and erosion throughout the area between the continental shelf edge and areas far inland from the current coastline. This evidence includes buried deposits that contain freshwater and shallow-water marine organisms and flat or low-relief topography.

The continental shelf, which has been progressively covered by the rising sea during the past 19,000 years, is cut across by numerous shelf valleys. Most of the valleys were carved out by rivers during the last ice age, when sea level was lower. Many submarine canyons are extensions of shelf valleys (Fig. 4-33).

Figure 4-33. The continental shelf is often cut across by shelf valleys, some of which connect to steep-sided submarine canyons that cut across the continental slope.

Isostatic Sea-Level Change

In addition to eustatic processes, isostatic processes induced by changing climate can affect coastal topography. During an ice age, massive ice sheets accumulate over many parts of the continental crust. The weight of the ice forces the continental crust to sink lower into the asthenosphere (CC2). This process also depresses (lowers the level of) the continental shelves. For example, Antarctica’s continental shelf is currently depressed about 400 m lower than most other shelves by the weight of its ice sheets. At the end of an ice age, when the ice melts, a depressed section of a plate slowly rises until it again reaches isostatic equilibrium. However, isostatic leveling is much slower than eustatic sea-level change. Many areas of continental crust, including the coasts of Scandinavia and the northeastern U.S. and Canada, are still rising in response to the melting of glaciers that occurred several thousand years ago.

Glaciers shape topography in other ways as well. They often cut narrow, steep-sided valleys. Many valleys left after glaciers melted have been submerged by rising sea level to become deep, narrow arms of the sea known as fjords (Chap. 13).

Sea Level and the Greenhouse Effect

Although oscillations of sea level are normal occurrences in geological time, human civilization has emerged during a long period of relatively stable sea level (Fig. 4-34). 

Figure 4-34. Global average surface air temperature and sea level variations within the past 1400 years. (a) Since 600 CE, global average surface air temperature has varied by about 1.5°C, but is currently rising at about 1°C per century. Sea level has varied by about 20 m and appears to follow the trend of temperature changes. (b) Since about 1920, air temperature has started to rise more quickly, as has sea level. However, there is much short-term variability, and the apparent upward trend may not be sustained. (c) Since 1993, instrumental records of temperature show large variations in temperature, but a general increasing trend of 0.16°C•yr^–1. During the same period, highly-accurate satellite measurements show that sea level rise may be accelerating and is now estimated to be about 3 mm•yr^–1. It is not known how far temperature and sea level will rise in the future. It is known that the temperature increase and sea level rise are due to a combination of natural change and the enhanced greenhouse effect but it is not clear what the relative contributions of these two factors has been. This raises the question as to whether the rate of sea level rise could accelerate substantially if the current rising temperature trend is sustained

If the predicted greenhouse warming of the planet by as much as 2°C during the first half of this century does indeed occur, the higher average global temperature will cause the ocean water to continue to warm and expand, and the current polar ice sheets to continue to melt. As a result, sea level could rise by several meters or more. Warming and melting of ice sheets are slow processes that will continue possibly for centuries even if we were to completely stop adding to the concentration of carbon dioxide in the atmosphere. However, the most recent predictions based on the best available scientific information are that sea level will rise by about 1 m, and likely more, by the end of this century. A sea level rise of only a few tens of centimeters would inundate vast areas, including major coastal cities. Therefore, the history of coastal modification during periods of sea-level rise is of more than academic interest.