Misleading data is being used to claim sea levels are falling in Sydney and around the world

Claim: Sea levels in the Sydney area have fallen since 1914, representing “a true history of ocean rise”.

An October 27 Facebook post (direct link, archive link) shows a document that makes a number of claims about sea level rise, as well as a table of estimated sea level data for the Sydney, Australia area for several years from 1914 to 2019. Notably, the sea level reported for 2019 is lower than it was in 1914.

“Despite the deception and ignorance that claims the ocean is out of control, the opposite is true,” the caption reads. “Fort Denison, located in geologically stable territory on the southwest side of the world’s largest ocean, tells the true story of the rise of the ocean (sic).”

The post was shared more than 2,000 times in about three weeks.

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Our rating: False

Global average sea level has risen since 1914, as has the average sea level at Fort Denison, located in Sydney Harbour, Australia. The sea level data in the post does not show the annual average or average sea level at Fort Denison. The actual average annual sea level in the area in 2019 was higher than in 1914, according to government data.

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According to NASA, global average sea levels are rising due to global warming. This is because ocean water expands as it warms, and melting ice adds additional water to ocean basins.

Ben Hague, a climate scientist with the Australian Bureau of Meteorology, told USA TODAY that sea levels at Fort Denison have also risen since 1914. One analysis estimates that sea levels at Fort Denison rose about 0.07 inches (1.9 mm) per year from 1914 to 2018. But the agency doesn’t trust data collected before 1966, he said.

An analysis of data going back to 1966 shows that local sea levels have risen about 0.08 inches (2.17 mm) per year from 1966 to 2019 and 0.22 inches (5.59 mm) per year since 1993 through 2019, he said. The National Oceanic and Atmospheric Administration reports growth of 0.03 inches (0.8 mm) per year since 1886 at Fort Denison.

Daniel Fitzhenry, the man who claims to have created the document in a Facebook post, told USA TODAY the sea level readings listed were taken from Australian Bureau of Meteorology data. However, they do not reflect the average annual sea level for each year shown.

Instead, he said, they show the average sea level for the month in which the average sea level was highest that year. For example, in 1924, sea level was highest in April at 0.98 meters (38.5 inches), which is the value reported in the document for 1924.

All but two of the values ​​listed in the document correspond to the highest monthly average reported by the bureau for that year, but that’s not how annual mean sea levels are calculated, Haig said. Instead, all available hourly data for the year are used to calculate the average.

In 1924, the mean annual sea level, calculated from all available data, was lower than reported in the document—about 0.88 meters (34.6 inches). Additionally, when all data is included in the calculation, sea level at Sidney/Fort Denison 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, it would not mean sea level rise in the Sydney area is not occurring as sea levels fluctuate from year to year, Hague said.

“Like other weather and ocean variables, some years have above-average sea levels and other years below-average sea levels,” he said. “The Bureau therefore analyzes long-term patterns using linear trends or other methods that take into account data across all years, rather than simply calculating the difference between the first and most recent observation.”

Even if average sea levels near Sydney had fallen since 1914, that would not be evidence that global sea levels had fallen over that period, Hague said.

“Sea level and sea level changes are not uniform across the globe, so one specific location cannot be used to reliably represent global sea level,” he said. “Scientists typically use a vast collection of (global) satellite and tide gauge measurements in complex analyzes to determine global sea level.”

Although global sea level is rising on average, sea level may fall relative to land in certain areas where land is rising faster than the ocean due to glacial isostatic adaptation. This vertical movement of land occurs as land masses slowly recover from the weight of glaciers from the past ice age.

Other factors that cause differences in local sea levels include local currents and wind patterns, and changes in gravitational forces.

The Facebook user who shared the post did not provide evidence to support the post.

AAP also refuted the post.

Our Fact Checking Sources:

• Tom Mortlock, November 6, email exchange with USA TODAY

• Ben Haig November 10-14 email exchange with USA TODAY

• NOAA, April 19, 2022, Climate Change: Global Sea Level

• NOAA, accessed November 21, What is Glacial Isostatic Adjustment?

• NOAA, August 24, 2023, What is Subsidence?

• NOAA, August 10, 2017, Tracking Sea Level Rise and Fall

• NOAA Tides and Currents, accessed November 21, relative sea level trend 680–140, Sydney, Fort Denison 1 and 2, Australia.

• Planetary Vital Signs NASA, accessed November 21, Ice Sheets.

• NASA Vital Signs of the Planet, accessed November 21, sea level.

• Vital signs of the planet NASA, accessed November 21, Global Temperature.

• Carbon Brief, April 28, 2021 Melting glaciers have led to a “21% rise in sea levels” over the past two decades.

• Coastal Education and Research Foundation September 2020 Mean Sea Level Update: Australia

• Australian Government Bureau of Meteorology, accessed 21 November, Monthly sea levels at Fort Denison (Sydney) – 1914 to 2024.

• USA TODAY February 22, 2023 Fact Check: In Norway, land is rising faster than sea levels.

• USA TODAY, April 29: The climate science is sound. Satellite time lapse does not disprove sea level rise | Fact checking

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This article originally appeared on USA TODAY: Sea levels are rising in Sydney and around the world | Fact checking