The Earth hasn't always been partially covered with ice. During the Eocene Epoch 50 million years ago, the planet was actually ice-free. So what happens if the glaciers melt today? What about the polar ice caps? What would actually happen if all that ice was gone? How would it change the world?
Scientists have a pretty good idea of how glaciers melting would change the world because, well, they already are! The effects of glaciers melting and polar ice caps dwindling is easy enough to observe and study, and scientists have been doing so for a long time. The state of the planet if all the ice melted is fun to speculate about but likely wouldn't happen for thousands and thousands of years (if it happens at all). This list includes some of the impacts that such a thaw would have on the world today.
If all the glaciers and ice caps melted, Florida would be gone forever, buried under the resulting 216-foot rise of ocean water (some say it could be even worse). America would get a sex change, basically. It's by far the most dramatic change to the map of the lower 48 theorized by National Geographic in their 2013 feature "If All the Ice Melted," and that's saying a lot:
The entire Atlantic seaboard would be underwater, as well, and the Gulf Coast would be (soggy) toast. All coastal cities, essentially, would get the Waterworld treatment, including London, New York, and Tokyo.
Steven Dutch with the University of Wisconsin-Green Bay estimates that melting the polar ice caps would add 2/3 of a second to our days. Woohoo! As Dutch wryly observes, that would give us more time to "pile sandbags along the coasts."
How would melting ice caps lengthen our days? Dutch "shows his work" here, but the basic idea is that "redistributing" all of that water would affect the Earth's so-called "moment of inertia" in such a way that the Earth's rotation would slow down slightly (very slightly). Dutch admits that this exercise might even be overestimating things a bit: all the ice below sea level, after all, would just melt and be replaced by sea water (much like melted ice in a drink doesn't cause it to overflow).
It's not just all about rising water: there's a whole host of other Biblical catastrophes in store for us if the ice caps melted. Melting the ice that covers Antarctica could cause massive earthquakes worldwide, for example, according to Anthony Fordham, editor of Popular Science Australia, in his Idiot's Guides: Science Mysteries Explained. Fordham's idiot-friendly analogy is a Ping-Pong ball with a dent in it. The Earth is the ball and the dent is the pressure caused by the enormous sheet of ice that's currently on top of Antarctica. Remove the ice and the crust of the Earth would pop out, causing intense seismic activity all over the globe.
That's not all: this shake-up could cause active volcanoes in Antarctica to erupt, as well.
This one's a bit more theoretical, but it's a solid point: Dr. Hal Wanless from the University of Miami says the rising floodwaters could cause large parts of civilization to basically stop functioning. “We’re going to see civil unrest, war. You just wonder how—or if—civilization will function," he told National Geographic in 2013. Hypothetically, instantaneous melting of the polar ice caps would put large cities like Miami and London underwater, causing a massive refugee problem.
Even in a more realistic scenario, Wanless imagines that by the next century, millions of people will have to move due to rising sea levels, and they'll have to move at roughly the same time, likely causing chaos.