Evening star., 1947-12-19
Stars, Men and Atoms
Sea Levels Rise Steadily
On Both Coasts of S.
Tidal Measurements of Past 20 Years
Indicate More Volume in Oceans
By Thomas r. Henry

AHulcdte Wore FOiume IN oceans
By Thomas r. Henry
Mean sea level on both the Atlantic and
Pacific coasts Of the United States has risen
approximately three-tenths Of foot In the
past 20 years.
This may be because there is more water In
the oceans or because the coasts are slowly
sinking
The movement, which started about 1928, has
Just become unmistakable in the tide Of
measurements Of the Coast and Geodetic Sur.
vey 1t was extremely difficult to detect says
H. A. Mariner, the Surveys tidal expert, because
SO many factors influence the mean height Of
the tides at any particular time. Local storms,
for example result in extremely high tides
which affect the mean for year or more.
Trend Steady for 20 Years
But through all the ups and downs over the
20-year period the steady upward trend, regard.
less OF local conditions. has continued until now
1t can be regarded as a statistical fact. Tho
steady rise is shown in tide gauge measurements
at Baltimore, Atlantic City, New York Boston
and Portland. The average Of three and-a-half
inches with fractional differences obtains for
all these stations.
The Pacific Coast gauges at San Diego, San
Francisco and Seattle give the same long -time
picture, although the sea level rise does not
appear to be quite as great. Reports from
Europe appear to show much the same stua-
ton, although no great reliance is placed on
them since the method Of taking observations
was not uniform. The rise appears only when
the measurements are made with great pre-
cision.
been started at several South Americs6.
stations but at least 20 years of observationas
will be necessary to detect the trend.
The upward turn Of the tide curve appears to
have started quite abruptly between ls2s-30.
Previously there had been years Of high mean
tide and years Of low mean tide, but they
followed no particular pattern. Through the
intervening years this pattern has been hidden,
because temporary local effects may be 10 times
as great as those due to the long time trend
The most plausible explanation is that there
Is more water in the oceans, due to the slow
melting Of the Arctic and Antarctic ice. That
the Arctic regions are growing slightly warmer
now Is generally accepted. There is some vague
evidence of the warming up of the Antarctic.
This may be the beginning Of process which,
1t is calculated, some day may place all the
coastal cities of the world at the bottom Of the
sea. Calculations are that when all the water
locked In the great polar ice caps is released
the mean sea level will be raised about 100 feet.
Theory Can't Be Tested
There Is no way at present, Mr.. Mariner
points out to test the correctness of this hypo-
thesis 1t is possible to maintain that the
continents, particularly the coastal plains, are
sinking. 1t is difficult to believe, however, that
this should have started suddenly, as did the
sea level rise
Some land areas are known to be rising, with
the resultant sinking of the adjacent sea level
This Is shown by tide gauges in southeast
Alaska and In Scandinavia. The rise is inter-
preted as part Of the rebound of the land from
the weight OF ice which was over them during
the last ice age about 10 thousand years ago.
Already the rise In the coastal sea levels
apparently is having considerable economic
effect. Storm tides are slightly higher, result
ing in much greater damage to shore property.
Tides, 1t is known, undergo a moon cycle of
about 10 years. This will cause them to become
slightly lower each year until about 1851 when
they will start up again. The upward trend of
the cycle, added to the fundamental upward
trend shown in the Coast Survey figures, may
mean some Of the highest storm tides in
history