As the climate has warmed, Antarctica and Greenland have lost enough ice in the last 16 years to fill Lake Michigan, according to results from a new NASA mission.
Put another way, more than 5,000 gigatons of ice has melted (a gigaton equals one billion metric tons or enough to fill 400,000 Olympic-sized swimming pools), which drove up sea levels around the world.
The findings show how the massive ice sheets at the far ends of the planet will affect millions of people on coastlines everywhere.
"It will reach us eventually here, even though it's really far away and hard to think about," said Helen Fricker, a glaciologist with the Scripps Institution of Oceanography at the University of California-San Diego. "How much ice we are going to lose, and how quickly we are going to lose it, is a really key thing that needs to be understood, so that we can plan."
The data was measured by ICESat-2, a NASA satellite launched in 2018, which uses lasers to take detailed measurements of ice. It's the sequel to a previous satellite, ICESat, that gathered data from 2003 to 2009. Using information from both missions, researchers were able to quantify the massive scale of melting.
The mission is also shedding light on what's driving the melting. Antarctica's ice, now sitting on land, makes a slow progression to the ocean. When it reaches the coast, it floats, creating ice shelves skirting around the continent. Those shelves are natural barriers that slow the rate of ice loss, but as they melt in a warming ocean, that barrier is shrinking.
"It's like an apple tart and the ice shelves are like the wall of pastry around the edges of the tart," says Fricker. "And if those walls are too thin or they're not baked well enough, then the filling will ooze out."
Warmer air temperatures around Greenland are directly melting its ice, as well as causing ice to calve off into the ocean. Greenland holds enough ice to raise global sea levels as much as 23 feet and other NASA studies have found melting is accelerating.
Combined, melting ice from both places has caused about half an inch of sea level rise, around a third of what's been seen over the 16-year period. Sea levels are also rising because warmer water expands.
"The ocean expands first and then the ice responds to the warming ocean and warming air," said Robin Bell, a research professor at the Lamont-Doherty Earth Observatory. "It's like, you're going to feel warmer in your backyard before the ice cube melts on the table."
With ICESat-2 still circling the planet, scientists say it will continue to provide crucial information, helping refine their understanding of how the planet is changing.
"We've all been waiting for this new dataset," Bell said. "Our goal is to be able to tell every coastal community what they can plan on [in] the coming decades. To be able to do that, we both need to measure how the ice is changing but also understand better why it's changing."
With oceans predicted to rise as much as 8 feet by the end of the century, governments around the world are looking for detailed projections of how fast that will happen.
"There's a lot of infrastructure and airports and people that live right on the ocean," Fricker said. "And these people are going to feel the effects of sea level rise that's resulted because the ice sheets have melted."