Sea-level rise is the
most feared impact
of global warming.
There has been
unnecessary alarm,
caused by poor data,
bad analyses,
and wild guess
computer model
projections.
There are changes
in global sea level
( eustatic sea level ),
and changes in
local relative sea level.
Sea-level changes
are measured
relative to a defined
reference level,
but remember that
Earth’s surface is
dynamic, not static.
Seas rise.
Seashore land subsides ( sinks ).
A tide gage can't tell the difference.
Past sea-levels
are measured,
or inferred from,
geological evidence.
Modern observations
use tide gauges.
Since the early 1990s,
satellite data have been
available too.
Either melting ice and/or
a warming ocean,
might cause sea levels
to rise.
The current
ice age
reached its
peak glaciation
about 20,000
years ago,
with sea level
at about -400 feet
( -120 meters )
lower than today.
Geological sampling
shows rapid melting,
up to +26 mm per year
over short periods
between about 15,000
and 10,000 years ago,
after which the
rate of rise declined
to +1mm to +2 mm per year.
The +26 mm per year rises
are linked to breakout floods
into the oceans, from large
Northern Hemisphere
pro-glacial lakes.
No large
glacial meltwater
lakes exist today,
so such high
rise rates
are unlikely
to be repeated.
In addition,
the Antarctic
ice cap
has expanded,
not decreased,
in the past 20 years.
In his papers,
Nils-Axel Mรถrner
( 1983, 2004, 2011 )
established a maximum
possible glacial
eustatic rate of change,
of +10 mm per year,
or one meter per century.
Local relative sea level
is traditionally measured
at ports, using tide gauges,
some of which have records
extending back to the
eighteenth century.
After correcting
for subsidence
( land sinking ),
or land uplift,
the longer-term
tide-gauge
records show a
twentieth century
sea-level rise of
+1 to +2 mm per year.
The UN's IPCC ( 2001 )
estimated an average rate
of eustatic rise
between 1900 and 2000
of +1.6 mm per year.
Global average sea level
has been rising gently
for the past 100+ years,
by simple observation.
The precise rates of change
are an open question.
There was no
significant increase
in the rate of sea-level rise,
in contrast to climate model
projections for an increase
of the rate during the
twentieth century.
If the late twentieth century
global warming was as extreme
as the IPCC claims it has been,
why can that global warming
not be detected in
global sea-level data ?
The rate of increase
in atmospheric carbon dioxide
levels grew dramatically
after 1950
( from a 1900–1950
mean rate of rise of +0.33 ppm/year,
to a 1950–2000 mean rate of rise
of +1.17 ppm/year ).
But the mean global sea-level
rate of rise did not
trend upwards after 1950.
Since the early 1990s,
sea-level measurements
have been made by
microwave radar
and laser ranging
from various
orbiting satellites,
including the U.S.
TOPEX-Poseidon,
the European
Remote-Sensing
Satellite (ERS),
Geosat Follow-On (GFO),
EnviSat, and
Jason series.
Satellites
and tide gauges
do not measure
the same thing.
Tide gauges measure
relative to a fixed
land benchmark.
Satellites measure
relative to a
mathematical model
of the shape of
Earth’s gravity field ( geoid ),
that's not well characterized,
and varies over time.
With satellite
“sea-level change,”
up to 50%
of the change
results from
geoid changes
-- as a result,
satellite
measurements
are more than
+3mm per year.
Another problem
with satellite
measurements
is that significant
differences occur
with different sensors
used by different
research groups.
Satellite altimetric
measurements that show
+2 mm per year,
and especially those showing
greater than
+3 mm per year,
are likely to be wrong
( versus +1 to +2 mm per year
from tide gauge data. )
The important
question
is not,
“Is sea level rising?”
Geological, tide gauge,
and satellite records
all agree it is.
PS:
Note that when
sea level rises,
corals grow up to the
higher sea level,
easily keeping pace
with the sea-level rise.
We have many
coral islands
in the world,
despite a
sea level rise
of more than
+400 feet
( +120 meters )
in the past
20,000 years.
When sea-level rise stops,
the coral grows sideways.
Atolls are not a "dipstick"
to measure sea-level rise.