(A)
Global warming for past 15,000 years:
During the last ice age glaciation peak, glaciers were up to two miles thick.
Probably a mile of ice was over my property in southeastern Michigan, USA.
Sea level has already risen about 400 feet as ice melted after the last glaciation peak about 18,000 to 20,000 years ago.
If climate history repeats:
Sea level would continue to rise another 16 to 32 feet, reaching the same peak height it reached in the last interglacial period.
The weight of glacial ice pushes down hard on land surfaces.
That means as glaciers over land melt, the land is no longer carrying so much weight, so the land rises (rebounds).
Sometimes melting ice can cause oceanside land to rise faster than the local sea level is rising (which is like having a falling sea level).
(B)
Measuring sea level:
Tide gauges measure sea levels along shorelines.
Long-term tide-gauge data reflect a rising sea level trend of about +1.7 mm/year (about +0.067 inches/year).
Satellites measure sea levels of the global oceans, and have been available since the early 1990s.
Short-term satellite data reflect a rising sea level trend of about +3.2 mm/year (about +0.125 inches/year).
These two methodologies measure sea level in different places, and sea level is very difficult to measure:
(1) Sea level can be rising in some locations, while falling in others.
(2) Satellites show the rise of sea level is not uniform accross the globe.
(3) Sea level is difficult to measure to the nearest inch, so a claim that sea level rose a few millimeters in a specific year are meaningless!
(C )
Why is sea level important?
Sea level can be a local problem.
For each shoreline community, local residents may care about their shoreline, but not sea level in the middle of oceans.
Sea level would affect people who own (often expensive) property on shorelines, and portions of some cities that are close to sea level, or even below sea level (parts of New Orleans).
Many factors affect local sea level -- cities actually have more to fear from land sinking, than from sea level rising:
Land subsidence is a drop of Earth's land surface relative to sea level.
For coastal cities, land subsidence is usually caused by excessive groundwater extraction during and after rapid population growth.
Subsidence is causing parts of Jakarta, Ho Chi Minh City, Bangkok and many other coastal cities, to sink … and they may eventually sink to below sea level, which is a recipe for destructive floods, as many New Orleans residents found out in the 1920s and 1990s.
(D)
Sea level: Many questions, but few answers:
There appear to be multi-decade periods when sea level rise accelerates, alternating with multi-decade periods when sea level rise decelerates.
There are also multi-decade periods with global warming (such as 1910 to 1940, and 1975 to 2000), alternating with multi-decade periods with global cooling (1940 to 1975), or a flat average temperature trend (2000 to 2015).
One mystery is why there's no correlation between these short-term sea level rise trends and short-term average temperature trends !
Long-term, there was no acceleration of sea level rise in US tide gauge records during the 20th century … and that implies Wall Street employees will not be surrounded by flooded Manhattan streets any time soon, (so will not have to take boats to their office buidlings, as Al Gore once predicted !)
Another mystery is why the average global temperature rise since the mid-1800s of about +1 degree C. (+/- 1 degree) has not caused an acceleration of sea level rise over the past 100 years (in fact, the rate of sea level rise may have decelerated).