Without Humans, Global Sea Levels Could Actually Be Falling

Without Humans, Global Sea Levels Could Actually Be Falling
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In the absence of humans, global sea level might have dropped a smidge last century, according to a detailed analysis of our past and future climate published today in the Proceedings of the National Academies of Sciences. Instead, sea level rose about 14cm — faster than it has in at least 27 centuries.

“The 20th century rise was extraordinary in the context of the last three millennia,” Robert Kopp, an Earth scientist at Rutgers and lead author on the new study, said in a statement. While many researchers have used models of the past to demonstrate the rapid swell of Earth’s tides in modern times, Kopp’s study is the first to draw on detailed geologic records from dozens of locales worldwide. The present, he can confirm, is historically weird.

And that weirdness is almost certainly due to industrial society. After developing statistical models of the relationship between temperature and sea level based on geologic records, Kopp and his colleagues decided to see what an alternate history would look like. They examined two models of the 20th century: one in which temperatures hovered around the 500 to 1800 AD average, and another in which a slight pre-industrial cooling trend continued.

Overall, in the absence of anthropogenic climate change, the study found that 20th century sea level rise likely wouldn’t have exceeded 7cm. And, Kopp said, “There’s a reasonable probability that sea level could actually have fallen.”

Kopp’s study also adds to the mounting pile of scientific evidence that the world’s coastlines are in store for a lot more sea level rise — 0.5 to 1.3m this century, if we continue pumping carbon into the air unabated. If that’s the case, a water-logged Miami will be the tip of the proverbial iceberg.

“125,000 years ago, when global mean temperatures were one to two degrees Celsius above today, sea levels were about 7 to 9 metres higher than present,” Kopp told Gizmodo. “That gives you an indication of the long-term commitment associated with a one to two degree warmer world.”

The message? Policy makers can no longer in good conscience ignore the threat of rising tides when planning for the future, given the trillions of dollars worth of infrastructure (not to mention human lives) at risk. In response to Hurricane Sandy, the Obama administration issued an executive order that future flood risk be factored into building standards for federally-funded construction projects in the US. But for cities, towns and even spaceports on the front lines, a much more comprehensive mitigation effort will be needed.

“You don’t want to act as though the worst case scenario is a certainty,” Kopp said. “But you also don’t want to lock in decisions that assume it isn’t going to happen at all.”


Image: Coastal flooding in Scituate, Mass., during a winter storm last January, via Dave Malkoff/Flickr