How do we know the ocean temperature has increased in the last 50 or 100 years?

B/c AL Gore said so. He knows everything, including the fact that the more taxes we pay, the better off those on welfare become! Yay!

NOAA and other world wide weather observation agencies have been recording weather Data for over 100 years. This data includes sea surface temperatures. In comparison to the records they've kept from 100 years ago, average temperatures have increased.

Heres an image that shows visually the changes in sea surface temperatures since 1971-2000, you can see here that even in 30 years that has been increases that stretch all the way up to 3 degrees Celsius.

http://www.ncdc.noaa.gov/img/climate/research/2005...