A profoundly altered planet is what our fossil fuel driven civilization is creating, a planet where Sandy scale flooding will become more common and more destructive for the world’s coastal cities. By releasing carbon dioxide and other heat trapping gases into the atmosphere, we have warmed the Earth by more than a full degree Fahrenheit over the past century and raised sea level by about eight inches. Even if we stopped burning all fossil fuels tomorrow, the existing greenhouse gases would continue to warm the Earth for centuries. We have irreversibly committed future generations to a hotter world and rising seas.
In May the concentration of carbon dioxide in the atmosphere reached 400 parts per million, the highest since three million years ago. Sea levels then may have been as much as 65 feet above today’s, the Northern Hemisphere was largely ice free year round. It would take centuries for the oceans to reach such catastrophic heights again, and much depends on whether we manage to limit future greenhouse gas emissions. In the short term scientists are still uncertain about how fast and how high seas will rise. Estimates have repeatedly been too conservative.
Global warming affects sea level in two ways. About a third of its current rise comes from thermal expansion.. from the fact that water grows in volume as it warms. The rest comes from the melting of ice on land. So far its been mostly mountain glaciers, but the big concern for the future is the giant ice sheets in Greenland and Antarctica. Six years ago the Intergovernmental Panel on Climate Change issued a report predicting a maximum of 23 inches of sea level rise by the end of this century. But that report intentionally omitted the possibility that the ice sheets might flow more rapidly into the sea, on the grounds that the physics of that process was poorly understood.