Recent Sea Level Trends
Global mean sea level increased by about three millimeters per year since 1993 due to ocean warming and melting of ice on land. The global average, however, does not necessarily reflect the rate of change along any given coastline.
The maps to the right show sea level trends from satellite altimetry (colored contours) and tide gauges (circles) over the last 10 years and the last 20 years. Local and regional trends can be as much as 5 times the global rate depending on location and time period.
The satellite trends reflect changes in the sea surface height only, with most of the spatial variation resulting from the influence of winds blowing over the ocean. The tide gauge trends are relative to a fixed point on land and reflect changes in water level plus local vertical land motion. The land near a tide gauge can move up and down for a variety of reasons, such as earthquakes or ground-water withdrawal.
How well does the tide gauge network sample the ocean?
One way to evaluate the effectiveness of the global tide network is to test the ability of the network to produce an accurate estimate of global mean sea level (GMSL). Tide gauges sample the global ocean at a small number of discrete locations (< 300) that do not necessarily sample the ocean in an optimal way. The figure below shows the UHSLC’s Fast-Delivery Network Performance Metric, which we define as the spatial sampling error in estimates of GMSL from tide gauges in the Fast-Delivery Network. In general, the error decreases in time, which suggests an improving network that more effectively samples the global ocean. Read more here.