Global warming contributes to rising sea levels in two ways. First, hotter summers, warmer winters, and earlier springs are causing glaciers and ice sheets to gradually melt. The increased runoff from polar lands is causing sea levels to rise. Second, thermal expansion, the natural expansion of water as it heats up, is causing the ocean to take up more space, which also leads to rising sea levels.

Scientific calculations show decades of more ice losses than gains. On average, most of Earth’s mountain glaciers are continuing to melt. The Earth’s polar regions are especially vulnerable to global warming because temperatures in the Arctic and Antarctic are rising at twice the rate of the world average. Arctic and Antarctic sea ice volume and extent have been declining since record-keeping began in the late 1970s and prior.