The nation’s oldest tide gauge, housed in a wooden shack on a pier at Fort Point in San Francisco, recorded something unusual over the past 30 years.

While water levels in San Francisco Bay rise and fall every day with the tides, analysis of the gauge’s data shows that average water levels have fallen since 1980.

Sea levels in other parts of the world, meanwhile, have been rising by an average of 2 to 3 millimeters per year due to the effects of climate change.

The data from the bay are similar to those along the entire West Coast of the United States, which has been protected from sea level rise in recent decades by cold local waters and the effects of ocean currents.