How Many Islands?
Islands are intrinsically impermanent. The more stable oceanic islands last a relatively brief time of 5-10 million years. Some islands drown, a result of erosion, subsidence of the ocean crust, or rising sea level. Sea levels are related in part to the amount of water bound up in the polar ice caps or released into the oceans; and the size of the polar ice caps is related to a variety of factors including variations in the positions of continents, the orientation of the earth's axis, and the amount of cloud cover.
Sea level is fairly high now; it was lower during the Little Ice Age, circa fourteenth to nineteenth century, and even lower about 18,000 years ago. A lowering of sea level brings back into view drowned islands.
Ongoing volcanism continues to add to existing islands and create new ones; an example is Surtsey, off the southern coast of Iceland, which came into existence with a submarine volcanic explosion on November 14, 1963, and has continued to accrete surface area as the ongoing lava flows cool. There are also islands that appear intermittently.
Because islands come and go, the number of islands in existence cannot be established except in relation to a proscribed time period—a human generation, or a century or two. With the discovery of some islands in the Russian Arctic in the mid-twentieth century, however, it is thought that no islands remain to be discovered in our time. Satellite and ship-based scanning equipment is now being used to search for islands whose positions appear on nautical maps but which have themselves disappeared, and to identify underwater sites of new island formation.