There are two main forces that can drive sea levels higher. One is something called thermal expansion, which involves the expansion of ocean water as it warms. The other is an influx of additional water, ushered into the sea by melting ice sheets and glaciers. Scientists have long concluded that sea levels are rising. Just look at Miami. Or the Maldives. They’ve also discerned that major ice sheets are melting at a faster clip than previously understood.
What has been less clear, however, is whether the development is recent or not. Over the last several thousands of years, has the ocean risen and fallen and risen again? A new study, just published in PNAS, suggests that the ocean has been surprisingly static since 4,000 B.C..
But that changed 150 years ago. Reconstructing 35,000 years of sea fluctuations, the study, which researchers say is the most comprehensive of its kind, found that the oceans are experiencing greater sea rise than at any time over the last 6,000 years. “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,” lead author Kurt Lambeck, a professor at Australian National University, told the Australia Broadcasting Corporation. “I think that is clearly the impact of rising temperatures.”
How much has the sea risen over the past century and a half? A lot. And it’s surging faster than ever. “There is robust evidence that sea levels have risen as a result of climate change,” Australian government research has found. “Over the last century, global average sea level rose by 1.7 mm [0.067 inches] per year, in recent years (between 1993 and 2010), this rate has increased to 3.2 mm [0.126 inches] per year.” In all, the sea has risen roughly 20 centimeters since the start of the 20th century. “The rate of sea level rise over the last century is unusually high in the context of the last 2,000 years,” the Australian report added.
But it’s not just the last 2,000 years. It’s the last 6,000 years, according to this recent study. And now, the rising sea levels over the last 100 years, is “beyond dispute,” Lambeck told the Guardian.
Using data drawn from 1,000 ancient sediment samples from the shores of Australia and Asia and from islands in the Pacific and Indian oceans, the researchers pieced together the history of the seas by studying indicators of an era’s sea level, like tree roots or mollusks. They found a large ice melt 16,000 years ago, which leveled off 8,000 years ago. Then over the last 6,000 years, little changed.
Until now. “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 4 to 5 meters [13 to 16.4 feet] higher than today,” Lambeck told the Guardian. “The question is how fast that change occurs when you increase temperatures.” He added: “It’s like if you leave a block of ice on the table, it doesn’t melt instantaneously, there’s always a delay in the system.”
But once it hits, it’s hard to reverse he said. You can’t put the toothpaste back in the tube.
“All the studies show that you can’t just switch off this process,” he continued. “Sea levels will continue to rise for some centuries to come if we keep carbon emissions at present day levels. What level that will get to, we are less sure about. But it’s clear we can’t just reverse the process overnight.”