Sea level rise Totally Explained

Everything about Sea Level Rise totally explained

Sea-level rise is an increase in sea level. Multiple complex factors may influence this change.
Sea-level has risen about 130 metres (400 ft) since the peak of the last ice age about 18,000 years ago. Most of the rise occurred before 6,000 years ago. From 3,000 years ago to the start of the 19th century sea level was almost constant, rising at 0.1 to 0.2 mm/yr. Since 1900 the level has risen at 1 to 2 mm/yr; since 1993 satellite altimetry from TOPEX/Poseidon indicates a rate of rise of 3.1 ± 0.7 mm yr–1 . Church and White (2006) found a sea-level rise from January 1870 to December 2004 of 195 mm, a 20th century rate of sea-level rise of 1.7 ±0.3 mm per yr and a significant acceleration of sea-level rise of 0.013 ± 0.006 mm per year. If this acceleration remains constant, then the 1990 to 2100 rise would range from 280 to 340 mm, The collapse of the grounded interior reservoir of the West Antarctic Ice Sheet would raise sea level by 5-6 m.

The snowline altitude is the altitude of the lowest elevation interval in which minimum annual snow cover exceeds 50%. This ranges from about 5,500 metres above sea-level at the equator down to sea level at about 70° N&S latitude, depending on regional temperature amelioration effects. Permafrost then appears at sea level and extends deeper below sea level polewards.

As most of the Greenland and Antarctic ice sheets lie above the snowline and/or base of the permafrost zone, they can't melt in a timeframe much less than several millennia; therefore it's likely that that'll not, through melting, contribute significantly to sea level rise in the coming century. They can, however, do so through acceleration in flow and enhanced iceberg calving.

Climate changes during the 20th century are estimated from modelling studies to have led to contributions of between –0.2 and 0.0 mm/yr from Antarctica (the results of increasing precipitation) and 0.0 to 0.1 mm/yr from Greenland (from changes in both precipitation and runoff).

Estimates suggest that Greenland and Antarctica have contributed 0.0 to 0.5 mm/yr over the 20th century as a result of long-term adjustment to the end of the last ice age. The current rise in sea level observed from tide gauges, of about 1.8 mm/yr, is within the estimate range from the combination of factors above but active research continues in this field. The uncertainty in the terrestrial storage term is particularly large.
Since 1992 the TOPEX/Poseidon and Jason 1 satellite programs have provided measurements of sea level change. Current data are available. The data show a mean sea level increase of 2.9±0.4 mm/yr. However, because significant short-term variability in sea level can occur, this recent increase doesn't necessarily indicate a long-term acceleration in sea level changes.

Geological influences

At times during Earth's long history, continental drift has arranged the land masses into very different configurations from those of today. When there were large amounts of continental crust near the poles, the rock record shows unusually low sea levels during ice ages, because there was lots of polar land mass upon which snow and ice could accumulate. During times when the land masses clustered around the equator, ice ages had much less effect on sea level. However, over most of geologic time, long-term sea level has been higher than today (see graph above). Only at the Permian-Triassic boundary ~250 million years ago was long-term sea level lower than today. Long term changes in sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term.
During the glacial/interglacial cycles over the past few million years, sea level has varied by somewhat more than a hundred metres. This is primarily due to the growth and decay of ice sheets (mostly in the northern hemisphere) with water evaporated from the sea.
The Mediterranean Basin's gradual growth as the Neotethys basin, begun in the Jurassic, didn't suddenly affect ocean levels. While the Mediterranean was forming during the past 100 million years, the average ocean level was generally 200 meters above current levels. However, the largest known example of marine flooding was when the Atlantic breached the Strait of Gibraltar at the end of the Messinian Salinity Crisis about 5.2 million years ago. This restored Mediterranean sea levels at the sudden end of the period when that basin had dried up, apparently due to geologic forces in the area of the Strait.

Long-term causes	Range of effect	Vertical effect
Change in volume of ocean basins
Plate tectonics and seafloor spreading (plate divergence/convergence) and change in seafloor elevation (mid-ocean volcanism)	Eustatic	0.01 mm/yr
Marine sedimentation	Eustatic	< 0.01 mm/yr
Change in mass of ocean water
Melting or accumulation of continental ice	Eustatic	10 mm/yr
• Climate changes during the 20th century
•• Antarctica (the results of increasing precipitation)	Eustatic	-0.2 to 0.0 mm/yr
•• Greenland (from changes in both precipitation and runoff)	Eustatic	0.0 to 0.1 mm/yr
• Long-term adjustment to the end of the last ice age
•• Greenland and Antarctica contribution over 20th century	Eustatic	0.0 to 0.5 mm/yr
Release of water from earth's interior	Eustatic
Release or accumulation of continental hydrologic reservoirs	Eustatic
Uplift or subsidence of Earth's surface (Isostasy)
Thermal-isostasy (temperature/density changes in earth's interior)	Local effect
Glacio-isostasy (loading or unloading of ice)	Local effect	10 mm/yr
Hydro-isostasy (loading or unloading of water)	Local effect
Volcano-isostasy (magmatic extrusions)	Local effect
Sediment-isostasy (deposition and erosion of sediments)	Local effect	< 4 mm/yr
Tectonic uplift/subsidence
Vertical and horizontal motions of crust (in response to fault motions)	Local effect	1-3 mm/yr
Sediment compaction
Sediment compression into denser matrix (particularly significant in and near river deltas)	Local effect
Loss of interstitial fluids (withdrawal of groundwater or oil)	Local effect	≤ 55 mm/yr
Earthquake-induced vibration	Local effect
Departure from geoid
Shifts in hydrosphere, aesthenosphere, core-mantle interface	Local effect
Shifts in earth's rotation, axis of spin, and precession of equinox	Eustatic
External gravitational changes	Eustatic
Evaporation and precipitation (if due to a long-term pattern)	Local effect

Past changes in sea level

The sedimentary record

For generations, geologists have been trying to explain the obvious cyclicity of sedimentary deposits observed everywhere we look. The prevailing theories hold that this cyclicity primarily represents the response of depositional processes to the rise and fall of sea level. In the rock record, geologists see times when sea level was astoundingly low alternating with times when sea level was much higher than today, and these anomalies often appear worldwide. For instance, during the depths of the last ice age 18,000 years ago when hundreds of thousands of cubic miles of ice were stacked up on the continents as glaciers, sea level was 120 m (390 ft) lower, locations that today support coral reefs were left high and dry, and coastlines were miles farther basinward from the present-day coastline. It was during this time of very low sea level that there was a dry land connection between Asia and Alaska over which humans are believed to have migrated to North America (see Bering Land Bridge).
However, for the past 6,000 years (a few centuries before the first known written records), the world's sea level has been gradually approaching the level we see today. During the previous interglacial about 120,000 years ago, sea level was for a short time about 6 m higher than today, as evidenced by wave-cut notches along cliffs in the Bahamas. There are also Pleistocene coral reefs left stranded about 3 meters above today's sea level along the southwestern coastline of West Caicos Island in the West Indies. These once-submerged reefs and nearby paleo-beach deposits are silent testimony that sea level spent enough time at that higher level to allow the reefs to grow (exactly where this extra sea water came from—Antarctica or Greenland—has not yet been determined). Similar evidence of geologically recent sea level positions is abundant around the world.

Estimates

See IPCC TAR, figure 11.4 for a graph of sea level changes over the past 140,000 years.

Sea-level rise estimates from satellite altimetry since 1992 (about 2.8 mm/yr) exceed those from tide gauges. It is unclear whether this represents an increase over the last decades, variability, or problems with satellite calibration.

Church and White (2006) report an acceleration of SLR since 1870.

Recent studies of Roman wells in Caesarea and of Roman piscinae in Italy indicate that sea level stayed fairly constant from a few hundred years AD to a few hundred years ago.

Based on geological data, global average sea level may have risen at an average rate of about 0.5 mm/yr over the last 6,000 years and at an average rate of 0.1 to 0.2 mm/yr over the last 3,000 years.

Since the Last Glacial Maximum about 20,000 years ago, sea level has risen by over 120 m (averaging 6 mm/yr) as a result of melting of major ice sheets. A rapid rise took place between 15,000 and 6,000 years ago at an average rate of 10 mm/yr which accounted for 90 m of the rise; thus in the period since 20,000 years BP (excluding the rapid rise from 15-6 kyr BP) the average rate was 3 mm/yr.

A significant event was Meltwater Pulse 1A (mwp-1A), when sea level rose approximately 20 m over a 500 year period about 14,200 years ago. This is a rate of about 40 mm/yr. Recent studies suggest the primary source was meltwater from the Antarctic, perhaps causing the south-to-north cold pulse marked by the Southern Hemisphere Huelmo/Mascardi Cold Reversal, which preceded the Northern Hemisphere Younger Dryas

Relative sea level rise at specific locations is often 1-2 mm/yr greater or less than the global average. Along the US mid-Atlantic and Gulf Coasts, for example, sea level is rising approximately 3 mm/yr

U. S. Tide Gauge Measurements

Tide gauges in the United States show considerable variation because some land areas are rising and some are sinking. For example, over the past 100 years, the rate of sea level rise varies from about an increase of per year along the Louisiana Coast (due to land sinking), to a drop of a few inches per decade in parts of Alaska. The rate of sea level rise increased during the 1993-2003 period compared with the longer-term average (1961-2003), although it's unclear whether the faster rate reflects a short-term variation or an increase in the long-term trend.

Amsterdam Sea Level Measurements

The longest running sea-level measurements are recorded at Amsterdam, in the Netherlands - most of which lies beneath sea level, hence the name. Records from 1700 onwards can be found at http://www.pol.ac.uk/psmsl/longrecords/longrecords.html. Since 1850, a rise of approx 1.5mm/year is shown here.

Australian Sea Level Change

The London Royal Society calculates net sea level rise in Australia at 1mm/yr - an important result for the Southern Hemisphere. The National Tidal Center also graphs 32 gauges, some since 1880, for the entire coastline

Future sea level rise

In 2001, the Intergovernmental Panel on Climate Change's Third Assessment Report predicted that by 2100, global warming will lead to a sea level rise of 9 to 88 cm. At that time no significant acceleration in the rate of sea level rise during the 20th century had been detected. Subsequently, Church and White found acceleration of 0.013 ± 0.006 mm/yr².
These sea level rises could lead to difficulties for shore-based communities in the next centuries: for example, many major cities such as London and New Orleans already need storm-surge defenses, and would need more if sea level rose, though they also face issues such as sinking land.
Future sea level rise, like the recent rise, isn't expected to be globally uniform (details below). Some regions show a sea-level rise substantially more than the global average (in many cases of more than twice the average), and others a sea level fall. However, models disagree as to the likely pattern of sea level change.

Intergovernmental Panel on Climate Change results

The results from the IPCC Third Assessment Report (TAR) sea level chapter (convening authors John A. Church and Jonathan M. Gregory) are given below.

Further Information

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