The accumulation of ice at the Earth’s poles was a process that took hundreds of thousands of years. Snow fell, solidified into glaciers and expanded into ice sheets stretching millions of square miles. The icy expanses of Greenland and Antarctica hold about 75 percent of the planet’s fresh water. Yet every year they lose more ice than they gain from snowfall. The question of just how quickly that ice is melting continues to vex scientists. And the answer could mean the difference between gradual and catastrophic sea level rise. For this month’s Environmental Outlook: the latest research into Earth’s ice sheets.