RISING SEA-LEVELS With rising temperatures, mountain glaciers around the world and the ice sheets of Antarctica and Greenland are melting. Water, the product of melting ice, needs to go somewhere. While a bit may be absorbed into the soil and water table, most of it adds volume to rivers and makes its way to or drains directly into the oceans. When the ice sheet of the Arctic, already floating in the sea, melts it does not contribute directly to sea-level rise because its mass is already displacing volume in the water. It’s similar to the melting ice cubes in a glass of water, the glass doesn’t overflow once the ice melts. This is important to remember when talking about Arctic sea ice. Unfortunately, mountain glaciers and the massive ice sheets of Greenland and Antarctica are not already floating in the ocean, so when they melt, especially collectively, they contribute to a rise in seas level. (Decreasing Arctic seas ice is problematic for other reasons that are discussed later). By the way, scientists often refer to the elements of the ecosystem where water is frozen (ice) as the cryosphere.
It is not just additional water that causes seas to rise. As seas increase in temperature, they expand. Warmer water has greater volume. This is called thermal expansion. Up to a third of the sea-level rise is projected to occur due to thermal expansion.
Again, depending on the action or inaction that society takes collectively, the sea-level is predicted to rise to different levels.
Rising seas are not inherently a problem. The problem is that hundreds of millions of people now live in coastal areas that are projected to be flooded as the seas rise. The problem is the speed and degree with which this is happening and projected to happen. Most societies today have constructed cities with massive and expensive infrastructure. Very few humans are nomadic. We cannot just pick up Beijing and move it further inland as the waters rise.
So, let’s start by looking at what science predicts regarding the rate and degree of rising sea levels and then look at some specific cases. Remember the degree and rate of sea-level rise depends on the degree and rate of warming. Scientists predict that by 2100 the average sea level rise will be between a quarter and three quarters of a meter (1 meter = 3.28 feet) above a 1986-2005 average baseline, so between nearly a foot and 2.5 feet. Again, you might be thinking that is not that much. However, just a 0.1 meter rise results in a sea-level that puts 10 million people at risk. We are certainly going to hit the mark of an increase of 1.5° C in average temperature. More warming equals more glacial and ice sheet melt. More severe warming would result in a sea-level rise of several meters (see the figure above), altering the geography of human-inhabitable land and flooding cities that hundreds of millions of people currently call home. Again, depending on the amount of warming, by 2100, 190-630 million people will live below predicted flood zones (see some of the original research here). There are currently an estimated 1 billion people living in areas below 10m above current high tide levels that are at risk of flooding.
When you think of rising seas causing damage, don’t just think of houses under water. Long before that occurs, winter storms which generate more powerful waves, nuisance flooding during particularly high tides, and the damage higher seas can cause during tropical storms and hurricanes will disrupt communities, damage infrastructure, and require millions of dollars to repair or abandon. According to research released in early 2019, homes in the US along the eastern seaboard and parts of the gulf coast have lost over $15 billion in value due to tidal flooding caused by rising seas. Research shows that higher seas also do more damage during tropical storms and hurricanes. “[T]he historical 100-year flood level would occur annually in New England and mid-Atlantic regions and every 1–30 years in southeast Atlantic and Gulf of Mexico regions in the late 21st century (source).