Tide gauges can help measure sea level change, but their limited locations and short records make it hard to pinpoint trends. Now researchers are evaluating the instruments' limitations.

At stations all around the world, tide gauges constantly measure sea level as it rises and falls. Data from these gauges inform many estimates of average global sea level rise in the 20th century.

However, inferring average sea level change across the globe from these local measurements isn’t as straightforward as it may seem. Ocean dynamics, land motion, and changes in Earth’s gravity and rotation can all skew local sea level trends, and there are very few tide gauges that have collected data for more than 50 years. All these factors make it difficult to isolate the amount of average global rise that reflects global ocean warming and ice mass loss.

A new study by Thompson et al. evaluates how local and regional processes affect the amount of historical global sea level rise inferred from tide gauge records. They conclude that the best tide gauge records tend to underestimate the average rate of 20th century global sea level rise.

The scientists used a sample of the 15 longest, highest-quality tide gauge records—the records that are used most often by researchers when estimating global changes in sea level. To determine the potential for bias in their sample, the team used “ice melt fingerprints” from a global set of glaciers and ice caps, as well as the Greenland and Antarctica ice sheets. These fingerprints describe the unique global pattern of sea level change caused by deviations in local gravity and Earth’s rotation when a large ice mass melts. They also utilized simulated patterns of dynamic sea level change due to variability in winds and ocean circulation. Finally, using the uncertainty in these patterns as a guide, they generated 10 million possible combinations of the ice melt fingerprints and dynamic patterns to determine how the locations of the gauges within these patterns affected the ability to estimate global average sea level rise.

The study found that this set of the best tide gauge records underestimates the rate of sea level rise due to melting of Northern Hemisphere glaciers and ice caps, which are thought to be the dominant source of melt during the 20th century. The analysis also showed that this negative bias is not likely to be overcome by contributions from ice sheet melt and ocean dynamics.

As a result, the authors place a lower bound on 20th century sea level rise of about 1.4 millimeters per year during the 20th century, and the most likely “true” global rate was closer to 1.7 millimeters per year. (Geophysical Research Letters, doi:10.1002/2016GL070552, 2016)

—Leah Crane, Freelance Writer