This is easy enough to test. Drive some pilings into bedrock at several locations in the seawater. Then measure the high and low tide marks on an ongoing basis. Average out the measurements you get and compare over time. If the average depth is going up over time then the sea level is rising where you are taking the measures. I’m sure there are a million other factors, but it’s basically that simple. Also, I have been going to the same beaches and harbors for 40 years and they sure look the same to me. Alarmist BS.
That might be true if not for subsidence; that is, the land mass itself can rise or fall. My home is 600 feet above sea level. When we had our well dug, seashell fragments were found at about the two hundred foot level.
Did the sea fall over 400 feet; or did the land mass get pushed up 400 feet. It's more complicated than just measuring water depth at a single location.
They use satellites nowadays. The latest and greatest satellite to study ocean topography is the Jason 3 that launched last Dec.
The next big launch is with the ICESAT 2 in 2017. This is the one that the GOP is trying to shut down.
If you ever get to Jamestown, visit the original site. Not the fancy museum. Some parts of the original fort are now underwater. Sea levels have been rising since at least the 1600’s. Actually ever since the inter glacial warm period began thousand of years ago. We are now right at about peak sea level height for this inter glacial period.
I hear you but...you’d need to account for possible tectonic plate movement. The posts may go up or down based on changes in the land. This is what makes these measurements much harder (and allow alarmists to take advantage of).