There are two main reasons why sea level is rising as the world gets warmer. First, as ice sheets and glaciers melt, they send ice and water pouring into the oceans. But another reason is that water, like most substances, expands as it heats up — and as greenhouse gases warm the atmosphere, some of that heat is slowly warming the oceans as well. Scientists understand the expansion of water really well; you can look it up in a textbook. It is much harder to predict what will happen to the ice, though.