Sea Levels Are Rising More Quickly Than in the Last Two Millennia

Here are five things to know about the rising tide
Erin Blakemore Smithsonian 23 Feb 16;

When scientists warn about climate change, they often use sea levels to illustrate the catastrophic effects of surging greenhouse gases. But just how much have human activities affected Earth’s sea levels? According to four new studies published in Proceedings of the National Academy of Sciences, the answer is dramatic indeed. the studies found that within the last 2,000 years, the sea levels rose more quickly than ever before.

“This isn’t a model,” one of the studies’ directors tells Warren Cornwall at Science. “This is data.” Each study emphasizes the effects human activities have on sea levels, and together they paint a sobering vision of a future with even higher seas. Here’s what you need to know about the new research:

Even small temperature changes make ocean levels rise

It’s tempting to think that small changes in temperature don’t make a big difference, but an analysis of global sea-level change over the past 3,000 years suggests otherwise. The study, which looked at a global database of sea-level reconstructions, concluded that sea level rises in the 20th century were faster than the last 27 centuries that preceded it.

After their initial data analysis, the researchers made a computer model that could project sea level backwards and forwards in time. They found that if global warming hadn’t bumped up by just 0.36 degrees Fahrenheit (0.2 degrees Celsius) during the 20th century, sea levels would have risen by just over one inch instead of about 5.5. In the future, the rise could be even more astonishing: One projection shows sea levels rising by 20.5 to 51.6 inches, and another shows sea levels rising by 9.4 and 24 inches.

Those numbers are scary—but match other scientists’ conclusions

Could such dramatic sea level rise calculations really be real? All signs point to yes. Another paper comes to nearly the same conclusion on the amount of sea level rise, and makes nearly the same projections on future sea level surges. This team’s models projected a rise of anywhere between 11 and 51.6 inches using a combination of past sea level and temperature measurements—numbers that look eerily familiar given the first study’s projections.

Ice sheets are sensitive to carbon dioxide levels

Okay, so the oceans seem to respond to even small temperature bumps. But what about ice sheets, which could contribute to sea level rise if they melt? It turns out they are quite sensitive, too. A third study shows that during the mid-Miocene period, when carbon dioxide levels were extremely similar to those that scientists project for the coming years, the ice responded dramatically to tiny shifts in carbon dioxide. In fact, the ice seemed to ebb and flow in sync with carbon dioxide levels.

Researchers use phrases like “highly sensitive” and “vulnerable” to refer to ice sheets’ responses to rising carbon dioxide. And they warn that given rises in current atmospheric carbon dioxide levels and projected increases, “reconstructions such as this one…imply an element of inevitability to future polar warming, Antarctic sheet retreat, and sea level rise.”

Scientists are getting better at simulating ice sheet levels

The final paper in the suite shows a big advance in simulating just how ice levels ebb and flow. The researchers from the third paper were able to come up with a new way to model how ice behaves—a model that could prove useful in future projections.

Bottom line: Brace yourself for rising sea levels

The data from the four papers lines up with another report just published by Climate Central, an independent organization of scientists and journalists devoted to reporting on climate. When that group ran the numbers on hourly water level records from U.S. tide gauges since 1950, they found that sea levels changed with global temperatures—and can almost certainly be attributed to human-caused climate change. They estimated that if not for climate change, a good three quarters of U.S. coastal flooding wouldn’t happen at all.

Can humans change the rising tide? Probably not: Other studies have found that even if carbon dioxide levels were stabilized, sea levels would continue to rise. But the seeming inevitability of rising seas is no reason to throw in the towel: Given the other severe consequences of even small jumps in temperature and carbon dioxide—famines and floods come to mind—it’s still worth it to keep reducing emissions.

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