Average sea levels increasing in ‘true ocean rise story’ | Fact check

An Oct. 27 Facebook post (direct link, archive link) shows a document that makes a series of claims about sea level rise and includes a table of purported Sydney, Australia, area sea level data for several years between 1914 and 2019. Notably, the sea level listed for 2019 is lower than that of 1914.
“Despite the deceit and ignorance industry claiming out of control ocean rises the truth is the opposite,” reads the post’s caption. “Fort Denison based on geologically stable ground on the south western side of the worlds largest ocean tells the true ocean rise story (sic).”
The post was shared more than 2,000 times in about three weeks.
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Average global sea levels have risen since 1914, as have average sea levels at Fort Denison, located in Australia’s Sydney Harbor. The sea level data in the post does not show annual mean – or average – sea levels at Fort Denison. The actual annual average sea level in that area was higher in 2019 than in 1914, according to government data.
Global average sea levels are rising due to global warming, according to NASA. This is because ocean water expands as it warms and melting ice contributes additional water into ocean basins.
Ben Hague, a climate scientist at the Australian Bureau of Meteorology, told USA TODAY that sea levels have also risen at Fort Denison since 1914. One analysis estimated that Fort Denison sea levels rose roughly 0.07 inches (1.9 mm) per year from 1914 to 2018. However, the agency has limited confidence in the data collected before 1966, he said.
An analysis of data from 1966 onward shows local sea levels rose around 0.08 inches (2.17 mm) per year from 1966 to 2019 and 0.22 inches (5.59 mm) per year from 1993 to 2019, he said. National Oceanic and Atmospheric Administration reports a rise of 0.03 inches (0.8 mm) per year since 1886 at Fort Denison.
Daniel Fitzhenry, the person who says he created the document in the Facebook post, told USA TODAY that the listed sea level values were taken from Australian Bureau of Meteorology data. However, they do not represent the annual average sea level for each listed year.
Instead, he said, they show the mean sea level for whichever month had the highest mean sea level that year. In 1924, for instance, the area sea level was highest in April at 0.98 meters (38.5 inches) and that is the value listed for 1924 in the document.
All but two of the values listed in the document match the highest monthly mean value for that year reported by the Bureau, but this is not how annual mean sea levels are calculated, Hague said. Instead, all of the available hourly data over the year is used to calculate the average.
For 1924, the average annual sea level calculated from all available data is lower than what is reported in the document − around 0.88 meters (34.6 inches). Additionally, when all the data is included in the calculation, Sydney/Fort Denison sea level was higher in 2019 (1.01 meters) than in 1914 (0.98 meters), not lower.
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But even if 2019 levels were lower than 1914, that wouldn’t mean Sydney area sea level rise isn’t happening, because sea levels fluctuate year to year, Hague said.
“Just like other weather and ocean variables, some years have higher than average sea level, while other years have lower than average sea levels,” he said. “Therefore, the Bureau analyzes long-term patterns using linear trends or other methods that consider all years of data, rather than simply computing the difference between the first and most recent observation.”
Even if mean sea levels near Sydney had dropped since 1914, that wouldn’t show global sea levels had dropped during that timeframe, according to Hague.
“Sea level and its change is not uniform across the globe, so one particular location cannot be used to reliably represent global sea levels,” he said. “Scientists generally utilize an extensive collection of (global) satellite and tide-gauge measurements in sophisticated analysis in order to determine global sea level.”
Even though global sea levels are rising on average, it is possible for sea levels to drop relative to the land in certain areas where the land is rising faster than the ocean due to glacial isostatic adjustment. This vertical land movement occurs as land masses slowly rebound from the weight of past ice age glaciers.
Other factors that cause differences in local sea levels include the local current and wind patterns, as well as variations in gravitational forces.
The Facebook user who shared the post did not provide evidence to support the post.
AAP also debunked the post.
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