Sea level is rising in relation to many of the world’s shorelines, and has been for decades. The main reason is that the volume of the ocean is increasing as a result of the melting of land ice and the warming of the sea itself. (Warm water expands, just as warm air does.)

Scientists once thought this volume increase had been going on, in fits and starts, for thousands of years. This widespread belief was often used as a debating point by climate-change skeptics, who argued that sea-level rise was nothing to worry about because it had existed throughout the history of human civilization.

But research in recent years has turned that notion on its head. The matter is not entirely settled, but some persuasive evidence points to the conclusion that the volume of the ocean was fairly stable for the last 2,000 years and began rising only recently, more or less in sync with industrialization. This is important because it suggests that sea level might be pretty sensitive to the greenhouse gases that humans are dumping into the atmosphere.

I made a brief mention of this issue in a long article on Sunday on sea-level rise but did not have the […]