Melting glaciers are having an impact on Earth’s landscape more complex than, and sometimes counter to, the effects of rising sea levels. While the effects of rising sea levels are becoming more widely known and understood, another phenomenon called “glacial isostatic adjustment” is activating volcanoes, leading to changing topography, and in some cases, falling sea levels. These counterintuitive effects are continuations of the effects of the end of the Last Glacial Maximum 19,000 years ago. The massive ice sheets of that era put massive amounts of pressure on the Earth’s surface, which displaced parts of the crust and deforming mantle substance. Areas directly under these ice sheets were pressed down, while areas of land nearby were often pushed up by the movements caused by the glaciers. These areas are called “forebulges”, and were sometimes pushed as high as 300 feet.
One primary example of such forebulges are in the central and southern portions of the United States, which were forced upwards by the weight of the Laurentide Ice Sheet over Canada and the northern US. In many parts of the world these forbulges are still visible today, although the ice sheets began to melt almost 12,000 years ago.
Since then, parts of the world which had been weighed down by ice sheets are rising. Areas that were once forebulges are now sinking. For example, Scotland is rising after its time under ice, while England is sinking almost an inch every year. As Canada rises about 4 inches a decade, the eastern US is sinking at a rate about half the rate that sea levels are rising. One study noted that Washington D.C may drop up to six inches over the next century as a result of this forebulge collapse.
In areas like the eastern US the effect of this isostatic adjustment is accelerating the pace of changes from rising sea levels, but in other areas that are rising, it is having the opposite effect. As the land rises, coastal areas in Alaska have gained land.
The redistribution of weight can even cause Earth to tilt on its axis as it moves towards finding its balance, and is even slowing the rotation of the planet, as glacial melt pools near the Earth’s center.
Furthermore, massive glaciers had the effect of suppressing earthquakes, and a 2008 study from the journal Earth and Planetary Science Letters shows that as the pressure on plates is released, pre-existing faults can become active again.
The effects of melting glaciers can be far reaching, and much more complex than simple rising sea levels.