A NOAA tide gauge.
A new study examines sea level changes in the tropical Pacific Ocean with the aim of determining how much of its rise is due to anthropogenic causes.
Part of the challenge of this work is that the Pacific experiences a natural oscillation in sea level that is caused by the Pacific Decadal Oscillation (PDO), a decade-long cycle in ocean currents and temperatures. The length of the PDO makes it difficult to distinguish an overall long-term pattern (if such a pattern exists) from the short-term variability.
Added to this is the fact that accurate sea level measurements, taken using satellite altimetry, only go back as long as we’ve been using satellites—a few decades.
Fortunately, satellites aren’t the only way to keep track of sea level. Previous studies have stitched together the satellite data with data from tide gauges, an older, less-accurate measurement device. This combination provides enough data to allow an overall trend to be extracted.
Those earlier studies found no trend once the PDO was removed, implying that natural cycles may account for much of the sea level rise that we’ve observed—if there is a long-term trend, it has been impossible to distinguish it from the noise of this cycle. As a result, it has been assumed that the sea level will likely decline to close to where it started once it’s done rising with the current decade-long cycle.
The new study, however, discovered that even after removing the PDO, there is still a significant upward trend in ocean levels, at least in measurements of the region off the coasts of the Philippines and northeastern Australia.
This location suggests that the tropical regions of the Pacific are being warmed through their contact with the Indian Ocean. While they are also affected by shorter natural cycles, sea levels in the Indian Ocean have a measurable, long-term upward trend caused by human activity. The new study concludes, therefore, that anthropogenic forces are contributing to a long-term trend in the western tropical Pacific—they’re just doing so via the Indian Ocean.
The conclusion that the sea levels in the Pacific will return to somewhere close to where they started once the decade-long cycle enters its period of decline is therefore not correct—at least, not near the Philippines or northeastern Australia. The sea level there is likely to continue to rise, although the rate of rise may slow a bit once the cycle reverses. In the region near Indonesia, however, this study suggests that trends in sea levels will continue to be dominated by the PDO for some time.
The Earth’s climate is complex, and complete understandings of all the forces involved is challenging, especially when data sets are limited in scope, as they are here. The sea level will continue to rise, and as it does, the signal of the anthropogenic contribution in the Pacific will likely become clearer. But this new work indicates that it’s already there—if you know where to look.
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