Monitoring sea level is complicated. It is not the same in different parts of the world because the world is not a perfect sphere, because the Earth’s gravity field is complex, because the land is rising and falling, and because of the effects of ocean currents, wind and climate.
Sea Level measurements using tidal gauges show a rise in sea level of about 20 cm in the 20th century at a rate that has been constant since 1910. Over the last decade or so there have been numerous research papers that have suggested there might be decadal and longer period fluctuations in some tide gauge records. To look closely at the sea level data Chambers et al (2012) looked at global sea level records using several different techniques, finding a 60-year periodicity. Click on image to enlarge.
Nobody understands what could cause fluctuations in the sea level over decadal timescales. Some have suggested that there is a link to changes in volcanic aerosols, global surface temperature, solar influences and land water storage.
Interestingly, the 60-year oscillation appears in every ocean basin. The tidal gauges in the North Atlantic, western North Pacific, and Indian Ocean have similar phase (within 10 years) and amplitude when regionally averaged. The western South Pacific gauges around Australia and New Zealand also have a significant 60-year oscillation, although one that lags by 10–15 years. There is no obvious 60-year cycle in Hawaii or on the west coast of the U.S.
One of the puzzles surrounding sea level measurements is the larger rates of increase returned by satellite altimetry. The researchers note that the upturn in sea level rise due to a 60-year oscillation with a minimum between 1980 and 1990 is consistent with the increased trend obtained from satellite altimetry. This suggests that the high satellite altimetry sea level rates are temporary.
Some recent studies have suggested the recent change in trends of global or regional sea level rise reflects acceleration, something that has been reported in the media. This claim must be re-examined in light of a possible 60-year fluctuation. While technically correct that the sea level is accelerating in the sense that recent rates are higher than the long-term rate, there have been previous periods were the rate was decelerating.
Until we understand whether the multi decadal variations in sea level reflect distinct inflexion points or a 60-year oscillation we should be cautious about saying that the global sea level is accelerating.