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NASA data shows the ocean's gradual,Watch V Online inexorable rise.

"There's a lot of things we're really sure about," NASA climate scientist Kate Marvel said at the 10th annual Social Good Summit on Sept. 22.

"We're sure sea levels are rising," emphasized Marvel, a researcher at the NASA Goddard Institute for Space Studies.

Scientists are also sure that greenhouse gases -- notably carbon dioxide -- are trapping heat on the planet, downpours are getting more intense, and rising temperatures are driving increasingly extreme Western wildfires.

Yet sea level rise (often) isn't as conspicuous as Earth's dying glaciers, historic droughts, and plumes of smoke from escalating wildfires. When we peer down into the water sloshing against a harbor's walls, the ever-shifting tides might obscure the sea's creeping rise. Still, as the planet's relentlessly warming oceans expand and great ice sheets melt into the seas, the saltwater incessantly rises.

Since 1900, sea levels have, on average, gone up around 8 inches, and this rate is increasing. From one year to the next, the change is hard to see, as seas are currently rising by about one-eighth of an inch each year.

But the notion that sea level rise and warming oceans are a slow-moving, far-off consequence is a misconception, Marvel emphasized. The consequences, while continuing, are already visible — particularly when it comes to tropical storms and cyclones.

A prominent driver of sea level rise today, in addition to melting ice sheets, is the warming seas, which results in a well-understood phenomenon called "thermal expansion" (water expands when heated). The oceans certainly have a potent heat source today, as the seas absorb over 90 percent of the heat generated by human activities. Critically, these warming waters can intensify tropical cyclones, which include hurricanes. The violent storms suck up moisture and energy from warmer ocean surfaces.

"Sea surface temperatures are hurricane food," noted Marvel.

In the Atlantic Ocean, for example, while there isn't a trend of more hurricanes forming, there is evidence that hurricanes are growing stronger, and hurricane scientists expect this trend to continue in the coming decades. What's more, research has shown remarkably warm waters have outweighed other factors in allowing recent storms to intensify into powerful cyclones. "When you boil it down, with a warmer atmosphere and sea surface temperatures, under ideal conditions we would expect storms to be stronger," Colin Zarzycki, a storm scientist at the National Center for Atmospheric Research, told Mashable last year.

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And when any storm hits land, boosted sea levels mean an ever-worsening storm surge, which is the resulting swell of water as a cyclone's violent winds push sea water into coasts. Every cyclone today now has a greater potential for destruction and flooding.

"We’re talking about things like storm surge, something that is not necessarily so easy to walk away from," said Marvel.

(New York City, some six years after incurring the storm surges from Hurricane Sandy, remains largely unprepared for future storms and continues to repair itself.)

Storm surges, though, will become all the more threatening as seas levels continue to rise.

Under extremely optimistic (if not nearly impossible) scenarios wherein global society curbs Earth's warming at an ambitious 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-Industrial Revolution temperatures, the relatively conservative UN Intergovernmental Panel on Climate Change (IPCC) expects around two feet of sea level rise this century. But under higher carbon emission scenarios, waters could rise by over three feet by century's end, according to the IPCC. Though, some oceanographers note that sea levels could actually rise by a catastrophic 6 feet this century alone, depending on if and when the destabilized ice shelves in West Antarctica collapse into the ocean, and how quickly Greenland melts.

The consequences are ever salient. Especially for the over 600 million people who inhabit lower-lying areas. (In the U.S., nearly 40 percent of the population lives in coastal counties.)

"Change is coming whether we like it or not," said Marvel. Though, she added, "we have the ability to create the change that we want to see."