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&“If Mum finds that I’m going to the beach, I’ll be in
trouble.” I said to myself.
I walked &&51&
&and tried not to make any noise. A few minutes later, I got out of
the house and started &&52& &to the bus stop
quickly. After half an hour, I was at the &&53& &with
my friends, Jimmy and Bobby. We began to play volleyball &&54&
. Suddenly, Bobby &&55& &the ball too hard and
it flew into the sea. I thought my swimming skill was &&56& &,
so I decided to get the ball.
I jumped into the
water and started swimming &&57& &the ball. However,
after about ten minutes, I got cramp （抽筋）in my leg and I &&58& &swim. I tried hard
to control (控制) my
body and after several minutes I &&59& . I began to swim
back to the beach. But soon I felt too &&60& &to
swim any further.
Luckily, a boat soon reached and &&61& &me.
When I woke up, I was in a(n) &&62& &. After a few
minutes, my parents
walked in with angry and &&63& &faces. They were
angry with me for my going out of the house &&64& &they
were glad because the doctor said I was not badly hurt.
I will never forget that terrible &&65& &and
never want to do it again.
51. A. quickly&&&&&&&& B.
easily&&&&&&&& C.
loudly&&&&&&&&&&& D.
52. A. walking&&&&&&& B.
driving&&&&&&& C.
running&&&&&&&&&& D. riding
53. A. house&&&&&&&& B.
beach&&&&&&&&& C .
station&&&&&&&&&& D. school
54. A. angrily&&&&&&& B.
sadly&&&&&&&&& C.
quietly&&&&&&&&&&& D.
55. A. held&&&&&&&&&
B. threw&&&&&&&&& C.
hit&&&&&&&&&&&&&&
56. A. good&&&&&&&& B.
poor&&&&&&&&&& C.
new&&&&&&&&&&&&&
57. A. over&&&&& &&&&B. towards (朝着)& C.
under&&&&&&&&&&&&
58. A. needn’t&&&&&& B.
must&&&&&&&&&& C.
couldn’t&&&&&&&&&& D. could
59. A. failed&&&&&&& B.
lost&&&&&&&&&&& C.
hurt&&&&&&&&&&&&&&
D. succeeded
60. A. lazy&&&&&&&& B.
tired&&&&&&&&&& C.
short&&&&&&&&&&& &&D.
61. A. missed&&&&&& B.
saved&&&&&&&&& C.
followed&&&&&&&&&& D. left
62. A. hospital&&&&& B.
shop&&&&&&&&&& C.
boat&&&&&&&&&&&&&&
63. A. excited&&&&& B.
relaxed&&&&&&&& C.
worried&&&&&&&&&&& D.
64. A. and &&&&&&&&B.
so&&&&&&&&&&&& C.
or&&&&&&&&&&&&&&&&
65. A. experience&& B.
news&&&&&&&&&& C.
matter&&&&&&&&&&&&
ABCDB& BACDA &&& ^cooco.n< cooco因你而专业a
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备课中心教案课件试卷下载From Wikipedia, the free encyclopedia
For other uses of "Sea level", see .
This marker indicating sea level is situated between
Sea level is generally used to refer to mean sea level (MSL),
level for the surface of one or more of
from which heights such as
may be measured. MSL is a type of  – a standardised
reference point – that is used, for example, as a
and , or, in , as the
is measured in order to
and, consequently, aircraft . A common and relatively straightforward mean sea-level standard is the midpoint between a
at a particular location.
Sea levels can be affected by many factors and are known to have varied greatly over . The careful measurement of variations in mean sea levels can offer information about
and has been interpreted as evidence supporting the view that
is an indicator of .[]
The term above sea level generally refers actually to above mean sea level (AMSL).
Precise determination of a "mean sea level" is a difficult problem because of the many factors that affect sea level. Sea level varies quite a lot on several scales of time and distance. This is because the sea is in constant motion, affected by the tides, wind, atmospheric pressure, local gravitational differences, temperature, salinity and so forth. The best one can do is to pick a spot and calculate the mean sea level at that point and use it as a . For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement point.
Sea level measurements from 23 long
records in geologically stable environments show a rise of around 200 millimetres (7.9 in) during the 20th century (2 mm/year).
To an operator of a , MSL means the "still water level"—the level of the sea with motions such as wind
averaged out—averaged over a period of time such that changes in sea level, e.g., due to the , also get averaged out. One measures the values of MSL in respect to the land. Hence a change in MSL can result from a real change in sea level, or from a change in the height of the land on which the tide gauge operates.
In the UK, the
(the 0 metres height on UK maps) is the mean sea level measured at
between 1915 and 1921. Prior to 1921, the datum was MSL at the .
In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collapsed data about the sea level. It is used for a part of continental Europe and main part of Africa as official sea level.Elsewhere in Europe vertical elevation references (European Vertical Reference System) are made to the
elevation, which dates back to the 1690s.
Satellite altimeters have been making precise measurements of sea level since the launch of
in 1992. A joint mission of
and , TOPEX/Poseidon was followed by
in 2001 and the
on the Jason-2 satellite in 2008.
vertical reference systems in Europe
Height above mean sea level (AMSL) is the elevation (on the ground) or
(in the air) of an object, relative to the average sea level datum. AMSL height is used extensively in
uses) to determine the coverage area a station will be able to reach. It is also used in , where some heights are recorded and reported with respect to mean sea level (MSL) (contrast with ), and in the , and . An alternative is to base height measurements on an
of the entire earth, which is what systems such as
do. In aviation, the ellipsoid known as
84 is increasingly used to define heights, however, differences up to 100 metres (328 feet) exist between this ellipsoid height and mean tidal height. The alternative is to use a
based vertical
When referring to
features such as mountains on a , variations in elevation are shown by . The elevation of a mountain denotes the highest point or summit and is typically illustrated as a small circle on a topo map with the AMSL
shown in either metres or feet or both.
In the rare case that a location is below sea level, the elevation AMSL is negative. For one such case see .
1. Ocean. 2. .
3. Local . 4. . 5.
To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or , called the . In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's
field. In reality, due to currents, air pressure variations, temperature and salinity variations, etc., this does not occur, not even as a long-term average. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as (stationary) . It varies globally in a range of ± 2 m.
Historically, adjustments were made to sea-level measurements to take into account the effects of the 235 lunar month
and the 223-month[]
on the tides.
Sea Level sign seen on cliff (circled in red) at , .
Several terms are used to describe the changing relationships between sea level and dry land. When the term "relative" is used, it means change relative to a fixed point in the sediment pile. The term "eustatic" refers to global changes in sea level relative to a fixed point, such as the centre of the earth, for example as a result of melting ice-caps. The term "steric" refers to global changes in sea level due to
variations. The term "isostatic" refers to changes in the level of the land relative to a fixed point in the earth, possibly due to thermal
it implies no change in the volume of water in the oceans. The melting of
at the end of
is one example of eustatic . The
of land due to the withdrawal of
is an isostatic cause of relative sea level rise.
can track sea level by examining the rocks deposited along coasts that are very tectonically stable, like the east coast of North America. Areas like volcanic islands are experiencing relative sea level rise as a result of isostatic cooling of the rock which causes the land to sink.
On other planets that lack a liquid ocean,
can calculate a "mean altitude" by averaging the heights of all points on the surface. This altitude, sometimes referred to as a "sea level", serves equivalently as a reference for the height of planetary features.
It has been suggested that this section be
into a new article. () Proposed since June 2014.
Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by
are smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as . Some land movements occur because of
adjustment of the
to the melting of
at the end of the . The weight of the ice sheet depresses the underlying land, and when the ice melts away the . Changes in ground-based ice volume also affect local and regional sea levels by the readjustment of the
and local ocean
changes can affect LMSL as well.
Eustatic change (as opposed to local change) results in an alteration to the global sea levels due to changes in either the volume of water in the world oceans or net changes in the volume of the .
There are many factors which can produce short-term (a few minutes to 14 months) changes in sea level.
Periodic sea level changes
Diurnal and semidiurnal astronomical tides
12–24 h P
0.2–10+ m
Long-period tides
Rotational variations ()
14 month P
Meteorological and oceanographic fluctuations
Atmospheric pressure
Hours to months
-0.7 to 1.3 m
(may also follow long-term pattern)
Days to weeks
Ocean surface
(changes in water
and currents)
Days to weeks
6 mo every 5–10 yr
Up to 0.6 m
Seasonal variations
water balance among oceans (Atlantic, Pacific, Indian)
Seasonal variations in slope of water surface
Seasonal water density changes (temperature and )
(standing waves)
Minutes to hours
(generate catastrophic long-period waves)
Up to 10 m
Abrupt change in land level
Up to 10 m
Sea-level changes and relative temperatures
Various factors affect the volume or mass of the ocean, leading to long-term changes in eustatic sea level. The primary influence is that of temperature on seawater
and the amounts of water retained in rivers, aquifers, , glaciers,
and . Over much longer , changes in the shape of the oceanic basins and in land/sea distribution will also affect sea level.
Observational and modelling studies of
indicate a contribution to sea-level rise of 0.2 to 0.4 mm/yr averaged over the 20th century. Over this last million years, whereas it was higher most of the time before then, sea level was lower than today.
Sea level reached 120 meters below current sea level at the
19,000–20,000 years ago.
Each year about 8 mm (0.3 inches) of water from the entire surface of the oceans falls onto the
ice sheets as . If no ice returned to the oceans, sea level would drop 8 mm (0.3 in) every year. To a first approximation, the same amount of water appeared to return to the ocean in
and from ice melting at the edges. Scientists previously had estimated which is greater, ice going in or coming out, called the , important because it causes changes in global sea level. High-precision
in low-noise flight has since determined that in 2006, the Greenland and Antarctic ice sheets experienced a combined mass loss of 475 ± 158 Gt/yr, equivalent to 1.3 ± 0.4 mm/yr sea level rise. Notably, the acceleration in ice sheet loss from
was 21.9 ± 1 Gt/yr? for Greenland and 14.5 ± 2 Gt/yr? for Antarctica, for a combined total of 36.3 ± 2 Gt/yr?. This acceleration is 3 times larger than for mountain glaciers and ice caps (12 ± 6 Gt/yr?).
float on the surface of the sea and, if they melt, to first order they do not change sea level. Likewise, the melting of the
which is composed of floating
would not significantly contribute to rising sea levels. However, because floating ice pack is lower in salinity than seawater, their melting would cause a very small increase in sea levels, so small that it is generally neglected.
Scientists previously lacked knowledge of changes in terrestrial storage of water. Surveying of water retention by
absorption and by artificial reservoirs ("impoundment") show that a total of about 10,800 cubic kilometres (2,591 cubic miles) of water (just under the size of Lake Huron) has been impounded on land to date. Such impoundment masked about 30 mm (1.2 in) of sea level rise in that time.
Conversely estimates of excess global groundwater extraction during
totals ~4,500 km3, equivalent to a sea-level rise of 12.6 mm (0.50 in) (&6% of the total). Furthermore, the rate of groundwater depletion has increased markedly since about 1950, with maximum rates occurring during the most recent period (), when it averaged ~145 km3/yr (equivalent to 0.40 mm/yr of sea-level rise, or 13% of the reported rate of 3.1 mm/yr during this recent period).
ice caps on the margins of Greenland and the
melt, the projected rise in sea level will be around 0.5 m (1 ft 7.7 in). Melting of the
would produce 7.2 m (23.6 ft) of sea-level rise, and melting of the
would produce 61.1 m (200.5 ft) of sea level rise. The collapse of the grounded interior reservoir of the
would raise sea level by 5 m (16.4 ft) - 6 m (19.7 ft).
altitude is the
of the lowest
interval in which minimum annual snow cover exceeds 50%. This ranges from about 5,500
(18,045 ) above sea-level at the equator down to sea level at about 70° N&S , depending on regional temperature amelioration effects.
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 cannot melt in a timeframe muc therefore it is likely that they will not, through melting, contribute significantly to sea level rise in the coming century. They can, however, do so through acceleration in flow and enhanced .
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 ).
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 terrestrial storage term, thought to be highly uncertain, is no longer positive, and shown to be quite large.
Comparison of two
during the last 500 Ma. The scale of change during the last glacial/interglacial transition is indicated with a black bar. Note that over most of geologic history, long-term average sea level has been significantly higher than today.
At times during , the configuration of the continents and sea floor have changed due to . This affects global sea level by altering the depths of various ocean basins and also by altering glacier distribution with resulting changes in glacial-interglacial cycles. Changes in glacial-interglacial cycles are at least partially affected by changes glacier distributions across the Earth.
The depth of the ocean basins is a function of the age of
(the tectonic plates beneath the floors of the world's oceans). As older plates age, they becomes denser and sink, allowing newer plates to rise and take their place. Therefore, a configuration with many small
that rapidly recycle the oceanic lithosphere would produce shallower ocean basins and (all other things being equal) higher sea levels. A configuration with fewer plates and more cold, dense oceanic lithosphere, on the other hand, would result in deeper ocean basins and lower sea levels.
When there was much
near the poles, the rock record shows unusually low sea levels during ice ages, because there was much polar land mass on 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.
Over most of geologic time, the long-term mean sea level has been higher than today (see graph above). Only at the - boundary ~250 million years ago was the long-term mean sea level lower than today. Long term changes in the mean sea level are the result of changes in the , with a downward trend expected to continue in the very long term.
During the glacial-interglacial cycles over the past few million years, the mean sea level has varied by somewhat more than a hundred . This is primarily due to the growth and decay of ice sheets (mostly in the northern hemisphere) with water evaporated from the sea.
The 's gradual growth as the Neotethys basin, begun in the , did not suddenly affect ocean levels. While the Mediterranean was forming during the past 100 million years, the average ocean level was generally 200
above current levels. However, the largest known example of marine flooding was when the
breached the
at the end of the
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
forces in the area of the Strait.
Long-term causes
Range of effect
Vertical effect
Change in volume of ocean basins
(plate divergence/convergence) and change in seafloor elevation (mid-ocean volcanism)
0.01 mm/yr
Marine sedimentation
& 0.01 mm/yr
Change in mass of ocean water
Melting or accumulation of continental ice
10 mm/yr
o Climate changes during the 20th century
oo Antarctica
0.39 to 0.79 mm/yr
oo Greenland (from changes in both precipitation and runoff)
0.0 to 0.1 mm/yr
o Long-term adjustment to the end of the last ice age
oo Greenland and Antarctica contribution over 20th century
0.0 to 0.5 mm/yr
Release of water from earth's interior
Release or accumulation of continental hydrologic reservoirs
Uplift or subsidence of Earth's surface ()
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
-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 )
Local effect
Loss of interstitial fluids (withdrawal of
Local effect
≤ 55 mm/yr
Earthquake-induced vibration
Local effect
Departure from geoid
Shifts in , , core-mantle interface
Local effect
Shifts in , axis of spin and precession of
Evaporation and precipitation (if due to a long-term pattern)
Local effect
Comparison of two sea level reconstructions during the last 500 Ma. The scale of change during the last glacial/interglacial transition is indicated with a black bar. Note that over most of geologic history long-term average sea level has been significantly higher than today.
Sea level change since the end of the last glacial episode. Changes displayed in .
Sea level has changed over . As the graph shows, sea level today is very near the lowest level ever attained (the lowest level occurred at the - boundary about 250 million years ago).
During the most recent ice age (at its maximum about 20,000 years ago) the world's sea level was about 130 m lower than today, due to the large amount of
that had evaporated and been deposited as
and , mostly in the . Most of this had melted by about 10,000 years ago.
Hundreds of similar
have occurred throughout the .
who study the positions of coastal sediment deposits through time have noted dozens of similar basinward shifts of shorelines associated with a later recovery. This results in
cycles which in some cases can be correlated around the world with great confidence. This relatively new branch of geological science linking eustatic sea level to sedimentary deposits is called .
The most up-to-date chronology of sea level change through the Phanerozoic shows the following long-term trends:
Gradually rising sea level through the
Relatively stable sea level in the , with a large drop associated with the end-Ordovician glaciation
Relative stability at the lower level during the
A gradual fall through the , continuing through the
to long-term low at the Mississippian/ boundary
A gradual rise until the start of the , followed by a gentle decrease lasting until the .
Main article:
For at least the last 100 years, sea level has been rising at an average rate of about 1.8 mm (0.1 in) per year. Most of this rise can be attributed to the increase in temperature of the sea and the resulting slight thermal expansion of the upper 500 metres (1,640 feet) of sea water. Additional contributions, as much as one-quarter of the total, come from water sources on land, such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human needs.
Further information:
Using pressure to measure altitude results in two other types of altitude. Distance
is the next best measurement to absolute. Above mean sea level is abbreviated as AMSL. MSL altitude is the distance above where sea level would be if there were no land. If one knows the elevation of terrain, the distance above the
is calculated by a simple subtraction.
An MSL altitude—called
by pilots—is useful for predicting physiological responses in unpressurised aircraft (see ). It also correlates with engine, propeller and wing performance, which all decrease in thinner air.
Pilots can estimate height above terrain with an
set to a defined barometric pressure. Generally, the pressure used to set the altimeter is the barometric pressure that would exist at MSL in the region being flown over. This pressure is referred to as either
or "altimeter" and is transmitted to the pilot by radio from
(ATC) or an
(ATIS). Since the terrain elevation is also referenced to MSL, the pilot can estimate height above ground by subtracting the terrain altitude from the altimeter reading. Aviation charts are divided into boxes and the maximum terrain altitude from MSL in each box is clearly indicated. Once above the transition altitude (see below), the altimeter is set to the
(ISA) pressure at MSL which is 1013.25 hPa or 29.92 inHg.
MSL is useful for aircraft to avoid terrain, but at high enough altitudes, there is no terrain to avoid. Above that level, pilots are primarily interested in avoiding each other, so they adjust their altimeter to standard temperature and pressure conditions (average sea level pressure and temperature) and disregard actual barometric pressure—until descending below transition level. To distinguish from MSL, such altitudes are called . Standard terminology is to express flight level as hundreds of feet, so FL 240 is 24,000 feet (7,300 m). Pilots use the international standard pressure setting of 1013.25 hPa (29.92 inHg) when referring to flight levels. The altitude at which aircraft are mandated to set their altimeter to flight levels is called "transition altitude". It varies from country to country. For example in the U.S. it is 18,000 feet, in many European countries it is 3,000 or 5,000 feet.
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