To a majority of climate scientists, the question is not whether the earth’s land ice will melt in response to the greenhouse gases those people are generating, but whether it will happen too fast for society to adjust.
Recent research suggests that the volume of the ocean may have been stable for thousands of years as human civilization has developed. But it began to rise in the 19th century, around the same time that advanced countries began to burn large amounts of coal and oil.
The sea has risen about eight inches since then, on average. That sounds small, but on a gently sloping shoreline, such an increase is enough to cause substantial erosion unless people intervene. Governments have spent billions in recent decades pumping sand onto disappearing beaches and trying to stave off the loss of coastal wetlands.
Satellite evidence suggests that the rise of the sea accelerated late in the 20th century, so that the level is now increasing a little over an inch per decade, on average — about a foot per century. Increased melting of land ice appears to be a major factor. Another is that most of the extra heat being trapped by human greenhouse emissions is going not to warm the atmosphere but to warm the ocean, and as it warms, the water expands.
Because we lack good data to model these changes, there is a certain casino approach to planning for the future. If sea level rises only two feet by the year 2100, then the problems will be less severe--albeit still severe--than if it rises by five feet. "A developing consensus among climate scientists holds that the best estimate is a little over three feet." Thus far, there seems to be little in the way of planning for this change, but adjustments are going to have to be made and the sooner they occur, the fewer the negative consequences there will likely be, because the rise will affect many of the major cities of the world, not just lonely beaches.