Silly, but for some reason I can’t stop wondering about this… If we removed all the man-made things from the seas, oceans, waterways etc. (i.e. ships, boats, oil rigs etc.), how much would the water level rise? I’m guessing our impact can’t be more than a drop in the ocean (haha) but watch me be wrong. I recently read an article on Archimedes’ Buoyancy Principle and that started me thinking about displacement. I don’t know the first thing about any aspect of this question, but maybe someone here can help?
If you remove stuff that’s floating, the water level will drop, not rise.
The same for the stuff that is not floating. But anyway, apart from having confused rise and fall, shiroijin is surely right to conjecture that the change in sea level would be negligible. Indeed, I would lay long odds that it would be undetectable.
The direct effect of human activity on sea level has actually been negative, due to all of the dams we have built:
Ah geez, Smeghead, that’s what I meant. Thanks for correcting me. :smack:
And Michael63129 - If we included dams in the equation (being they are man-made) then you and njtt are right in that we don’t have any influence at all!! It’s like we don’t matter - as if we ever did.
Thanks for answering guys. I really appreciate it!
Well we matter to the environment, including the oceans, in lots of ways, just not this one.
(And I think you mean if we excluded dams. This starting to show a pattern!)
But again, it’s not 30% of what’s in the ocean, but 30% of the drop in a bucket that we control. The ocean is mainly replenished through rain, and the water in those dammed lakes and reservoirs evaporates into clouds like any other.
Actually, it is 30% of the observed sea level rise, which is due to melting ice sheets and warming water (thermal expansion) which has nothing to do with human activity, not directly (direct = dams, structures in water). Of course, if all of the ice sheets melt, sea levels would rise by around 200 feet, making this negative contribution negligible (200 feet is also still a small percentage of the total water in the oceans, at least if you don’t live near the coast).
Also, evaporation from dammed lakes doesn’t mean anything - it is the water in those lakes that was prevented from flowing back into the ocean, so unless every dam was emptied you would have that water lost from the oceans.
No no njtt! This time I had my head on straight. I meant including the dams with the boats, ships, oil rigs that we were removing from the water. I promise I’m usually much more clear. 
I’ve since found something here that dealt with a portion of my question (http://boards.straightdope.com/sdmb/archive/index.php/t-101643.html) and it said:
“Underwater Tankers with Nuclear Energy for Russian Arctic Shelf
http://www.kiae.ru/rus/inf/new/new4.htm
The proposed nuclear underwater icebreaking tanker has the following features:
Water displacement 44,000 cubic meters
So say there are the equivalent of 10,000 Russian underwater tankers out there on the world’s oceans. That’s 10,000 x 44,000. That’s 440,000,000 cubic meters. Yes? I did it on the calculator, it came out 44 followed by 7 zeros.
There are 1.35 x1017 cubic meters of water in the world’s oceans.
http://witcombe.sbc.edu/water/physicsearth.html
Lake Superior has a volume of 12,100 cubic km, which is 12,100,000 cubic meters. Yes? No?
http://www.great-lakes.net/lakes/ref/supfact.html
So if you removed all 10,000 Russian underwater tankers, it would be the same as removing about 37 Lake Superiors from the oceans. I can’t visualize that as having much of an impact on a body of water that holds Ten to the Seventeenth Cubic Meters.”
…with another poster giving a quick correction…
“Lake Superior has a volume of 12,100 cubic km, which is 12,100,000 cubic meters. Yes? No?
No. You need to multiply by 1000^3, because it’s cubic, not just 1000. Thus 12,100 cubic km is 12,100,000,000,000 cubic meters. So you’d need 275,000,000 Russian tankers just to fill Lake Superior, which is absolutely miniscule compared to the ocean.”
And that’s just hypothetical tankers and still not much of a change! And to quote a board member named Interrobang!? “I hate when I do a search and still don’t find the earlier thread asking the same question. (I blame my search terminology, btw, not the board – at least this time.)”