Melting polar and glacial ice and thermally expanding ocean water have accelerated sea level rise to the highest rate in at least 6,000 years according to a new study in the Proceedings of the National Academy of Sciences. Using data from ancient sediment samples from around Asia and Australia, researchers looked back at 35,000 years of sea level history, finding that over the last 6,000 years little changed — until 150 years ago.
Using indicators of the era’s sea level, like location of ancient tree roots and mollusks, the scientists’ reconstruction found no evidence that sea levels fluctuated by more than about eight inches during the relatively stable period that lasted between 6,000 and about 150 years ago. Then, since the onset of the industrial revolution, sea levels have already risen by about that same amount. The scientists attribute climate change and rising temperatures that cause polar and glacial ice to melt and thermal expansion of the oceans as the primary cause for the rapid and extremely unusual increase in sea level. Water expands as it warms, and there is enough warming water in the ocean to cause a significant impact on sea levels.
The 35,000-year time frame was chosen because it represents an interglacial period with warmer global temperatures separating ice ages. The current Holocene interglacial has persisted since the end of the Pleistocene, about 12,000 years ago, with interglacial periods generally experiencing intervals of approximately 40,000 to 100,000 years. The researchers found that ice started melting 16,000 years ago and stopped about 8,000 years ago, but sea levels didn’t start to slow down until about 6,000 years ago.
“We know from the last interglacial period that when temperatures were several degrees warmer than today there was a lot more water in the oceans, with levels around four to five meters higher than today,” lead author Kurt Lambeck, a professor at Australian National University, told the Guardian. “The question is how fast that change occurs when you increase temperatures.”
Lambeck said that the sea level increase of the past 100 years is “beyond dispute” and that “what we’ve seen is unusual, certainly unprecedented for these interglacial periods.” He also said this is a process that can’t just be turned off and that “sea levels will continue to rise for some centuries to come even if we keep carbon emissions at present day levels.”
“What we see in the tide gauges, we don’t see in the past record, so there’s something going on today that’s wasn’t going on before,” Lambeck said. “I think that is clearly the impact of rising temperatures.”
Sea level rise will cause major damage and displacement along coastlines throughout the world. One recent study found that major U.S. cities along the coast will see ten times more flooding by mid-century. In the U.S., nearly five million people live in areas less than four feet above high tide, an elevation that is especially susceptible to rising sea levels that could increase that much by the end of the century.