It is not possible to gauge sea level rise simply by comparing two images of a location side-by-side, experts told Reuters, despite claims made online.
A widely shared post compares two images of Fort Denison, Sydney, with a caption that reads: “Unprecedented climate change has caused sea level at Sydney Harbour to rise approximately 0.0cm over the past 140 years” (here).
“Think the photos speak for themselves,” one user said in the comments.
Other examples online, which have been circulating since at least 2018, are viewable (here), (here), (here).
It is not possible to accurately measure sea level rise just by looking at two images of the same location at different periods of time, experts told Reuters. The claim has also previously addressed by Climate Feedback (here).
“Sea level is fluctuating on many different time-scales from semidiurnal, mixed, and diurnal tidal variations to the long-term sea level change due to climate change,” Beata Csatho, professor and chair of geology at the University at Buffalo and member of the National Aeronautics and Space Administration (NASA) Sea Level Change Team, told Reuters.
“Long-term sea level change is determined by removing all short-term variations from tide-gauge measurements or ocean altimetry records based on a set of models, such as ocean tide, load tide, inverse barometric effect (the pressure the atmosphere pushes the ocean down), etc.,” she said.
The images of Fort Denison (here) capture the ocean elevation “only at two particular moments,” Csatho said.
“Imagine that the first picture was taken at high tide and the second one was is taken at low tide. The apparent change could look like dropping sea level, while actually the sea level is slowly rising! The complexity of sea level elevation changes necessitated the use of a network of continuously recording tide gauging around the world, now supplemented by satellites that are constantly monitoring sea level,” Prof Csatho added (here).
The photographs may reflect “different tidal stages,” Gary Griggs, Distinguished Professor of Earth and Planetary Sciences, University of California, Santa Cruz, told Reuters.
An absolute global rate of sea-level rise has been recorded since 1993 by orbiting satellites, Prof Griggs said.
“This rate has averaged 3.42 mm/yr. but over the past decade or so has increased to 4.77mm/yr. over the past 10 years,” he said.
“Local rates, whether Sydney or San Diego, provide sea level changes relative to land. Where the land is rising or sinking will produce different local rates. So, in Sydney, the land has been rising over the period of tide gauge measurements producing lower than global sea level rise rates,” he added.Although Reuters was not able to verify when the two images used in the social media posts were taken, data on sea level rise has been collected at Fort Denison since 1886 (here). Between 1886 and 2010, sea-level rise averaged at 0.65 mm per year, according to the National Oceanic and Atmospheric Administration (NOAA).
Permanent Service for Mean Sea Level (PSMSL) data for sea level rise at Fort Denison is viewable here .
The Australian government Bureau of Meteorology sea level data for Fort Denison can be seen here .
Other images of Fort Denison are viewable here .
VERDICT
False. It is not possible to accurately measure sea level rise just by comparing two photographs.
This article was produced by the Reuters Fact Check team. Read more about our fact-checking work here .
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