in regards to sea level there are huge numbers of variables that can impact the data. which I am very skeptical gets controlled for.
plenty of manmade changes can have impacts beyond the global warming aspects. Dredging is a huge issue for consistent data near ports and coastlines. depending on the type of dredging some will just push the silt at the bottom out of the way. 1/2 of that is towards the land. there are also plenty of efforts to deepen ports, at some points this would artificially lower the sea level, but it would likewise cause it to rise in others. unless its controlled for, there is no way to know.
changes in vegetation can cause very large differences too. natural lagoons and mangroves do tons to control the tides. human development interrupts that and generally makes the existing conditions worse, including impacting the tides. in that case we are taking away area for the tides to go into, thus raising the water level. if not controlled for it impacts the data.
good old fashion trash has an impact. still a HUGE environmental issue worth fixing, but its not accounted for. its part of the "science" but its *probably* not factored for in this data.
I don't really understand the science but saltier water is heavier, even in the ocean you get layers of this. as the oceans get saltier, and more chemicals (again another huge environmental issue), we change those layers of water. with the denser water in many places taking up a larger percentage than it did before. this leaves less of the less dense water at the top to "absorb" the tides, resulting in higher tides as the same energy pushes less water. again its part of the science but its not controlled for.
some may say these are all still part of the man made issues leading to a global rise in water levels. but the thing is none of those is going change actual sea levels to cause us to drown or lose land, but it does change the data we take. but the change in that data can be used to bolster man made climate change.
There are two completely separate issues that are also never addressed. the earth's rotation is slowing. this means the moon has longer to pull the water in one direction. secondly the moon has natural cycles based on the time of the month where it is closer and further away from the earth. the moon is moving further away from the earth and average of like 3 centimeters a decade or something. taking that on its face you would think the moon at it closest, and furtherest is 3 centimeters further away. but its not. at some points in that cycle the moon is still the same distance away, but it gets even further away at its most distant point. this means the pendulum pull of the moon is swinging further and further out, which also has a greater impact on the tides.
we have also had a number of changes of our understanding of what is going on with the earth in the last couple years.
The latest study suggests Earth's plates today move twice as fast as they did 2 billion years ago – maybe because the mantle has got more runny
www.newscientist.com
1. the speed at which the tectonic plates move has increased, allegedly being faster than it ever has since the current land masses formed. This is still debated, and its debated what impact this has. More land vs less land vs just increased rate of change but same area of land.
2. we know there is at least twice as much water in the mantle that we had assumed. this plus the increased movement in the plates could result in some of the sea level increase being driven from below as well. again just a theory now, but its a variable that isn't addressed.
all of these are incredibly minute changes. but relative to the assumed changes humanity has caused they are quiet significant.