If you are looking for evidence of global warming, then a logical place to start is in the oceans. Any warming of the climate will ultimately lead to a net melting of ice reservoirs and a rise in global sea level.
Tide gauges are simple floats that move up and down as the level of the sea changes.
There are thousands of such devices distributed around the world, and some have been collecting data for over two hundred years. Useful gauges need to be located away from tectonically active regions and some urban areas where groundwater pumping contaminates the signal. Furthermore, the records need to be long enough to separate long-term trends from decadal fluctuations in sea level. The estimation procedure is straightforward. These trends are then corrected for 'known' geophysical effects, and 'global' sea-level rise is determined by simply taking the average of the corrected trends.