evidence: Ice sheets are the massive expanses of ice that cover Greenland and Antarctica, and which contain about two thirds of the freshwater on Earth. Climate change is already causing them to melt, and raising sea levels around the world. But if the Earth lingers at, or above, 2 degrees Celsius of warming, as it is on track to, that melting will steadily accelerate. Scientists warn that will cause parts of the ice sheets to collapse, sending massive amounts of water into the world's oceans. [...] Research suggests that the collapse of the West Antarctic ice sheet may already be underway. A massive glacier there, which covers an area about the size of the state of Washington, is melting quickly in response to climate change, and could splinter into the ocean in the coming decades.