For centuries, humanity relied on coastal landmarks and tide gauges to understand sea levels. But satellites changed everything. Since the early 1990s, orbiting instruments have provided precise, global records of ocean surface height.
These data revealed not only how seas are rising but also how predictions from decades ago were impressively accurate.
Satellites changed sea-level tracking
Study lead author Torbjörn Törnqvist is a professor in the Department of Earth and Environmental Sciences at Tulane University.
“The ultimate test of climate projections is to compare them with what has played out since they were made, but this requires patience – it takes decades of observations,” said Törnqvist.
He noted that the team was quite amazed at how good those early projections were, especially considering how crude the models were back then, compared to what is available now.
“For anyone who questions the role of humans in changing our climate, here is some of the best proof that we have understood for decades what is really happening, and that we can make credible projections.”
Sea-level rise differs across regions
Professor Sönke Dangendorf emphasized the importance of translating global patterns into regional forecasts.
“Sea level doesn’t rise uniformly – it varies widely. Our recent study of this regional variability and the processes behind it relies heavily on data from NASA’s satellite missions and NOAA’s ocean monitoring programs,” he said.
“Continuing these efforts is more important than ever, and essential for informed decision-making to benefit the people living along the coast.”
Satellites confirm acceleration
When satellites first began tracking sea levels in the early 1990s, they showed an average increase of about one eighth of an inch per year. It was only later that scientists confirmed this pace was speeding up.
By October 2024, NASA researchers announced that the rate of rise had doubled over three decades. That moment offered the perfect opportunity to compare real-world changes against projections made nearly thirty years earlier.
Close call with predictions
In 1996, the Intergovernmental Panel on Climate Change (IPCC) released its assessment report, just as satellite monitoring began.
The report projected about 8 centimeters of sea-level rise over 30 years. The actual outcome was 9 centimeters, nearly identical.
However, the models underestimated melting ice sheets by over 2 centimeters. Back then, the destabilizing effects of warming ocean waters on Antarctic ice were poorly understood. Greenland’s ice was also flowing into the ocean faster than anticipated.
Components of sea-level rise
The study shows that thermal expansion of seawater and melting of smaller glaciers were predicted fairly well. But contributions from Greenland and Antarctica were treated as negligible.
In reality, these ice sheets accounted for nearly a quarter of observed sea-level rise. Another overlooked factor was groundwater depletion, which transferred more water to oceans than expected.
Ignoring ice-sheets caused errors
Early IPCC reports assumed that the dynamic behavior of ice sheets could be ignored for decades. This assumption proved incorrect. Later assessments that excluded dynamic ice flow produced unrealistically low projections.
Once dynamic ice loss was included, estimates increased significantly. Modern assessments now highlight the “deep uncertainty” surrounding possible ice-sheet disintegration, which could drive sea levels far higher than expected.
Past models were surprisingly accurate
Thermal expansion was slightly overestimated, which balanced out the underestimated role of ice sheets.
This created projections that, by chance, matched reality more closely than their flawed assumptions should have allowed.
Still, the overall accuracy offers confidence in today’s more advanced models, especially since early reports successfully predicted atmospheric carbon dioxide levels.
Future climate projections
Predictions made in the 1990s have proven largely accurate, but the greatest challenges still lie ahead.
Ongoing uncertainty about ice sheets and human emissions makes continuous monitoring vital for helping coastal societies prepare for what lies ahead.
“Given the advances in both resolution and process understanding since the 1990s, the early success of the IPCC-SAR projection gives considerable confidence to climate projections for the future,” wrote the researchers.
“Meanwhile, the importance of continued monitoring of all relevant components of the climate system by key agencies cannot be understated.”
The study is published in the journal Earth’s Future.
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