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 last ice age reached its peak
about 20,000 years ago,
with sea level at about -400 feet
(-120 meters) lower than today.
Geological sampling shows
rapid melting, up to 26mm per year
over short periods between
about 15,000 and 10,000 years ago,
after which the rate of rise
declined to 1–2mm per year.
The 26mm per year rises
are linked to breakout floods
into the oceans, from large
Northern Hemisphere
pro-glacial lakes.
No large meltwater lakes exist today,
so such high rise rates of rise
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 10mm per year,
or 1.0 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 +1–2 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, a
nd 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 +1–2 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.