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At the completion of this section, you should be able to:
- Explain how melting ice sheets, glaciers, and thermal expansion contribute to global sea-level rise and describe how this rise varies regionally.
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Melting ice sheets and glaciers on land are a significant concern because they hold vast amounts of fresh water. When they melt, that water ends up in the ocean. For instance, after the last ice age ended, melting ice sheets and glaciers contributed to a global sea level rise of about 400 feet (approximately 120 meters), which continued up until about 5,000 to 6,000 years ago. After that, sea levels remained relatively stable until modern melting trends were observed beginning in the late 1800s.
So, how much could sea levels rise if the existing ice sheets were to melt completely? Scientists estimate that if the entire Greenland ice sheet melted, it would release enough fresh water into the ocean to raise sea levels by about 20 feet (approximately 6 meters). If the Antarctic ice sheet were to melt entirely, the increase could be around 200 feet (about 60 meters). Such extensive melting would drastically reshape our planet!
Video: How Earth Would Look if All the Ice Melted (2:44)
We're a long way from seeing the complete melting of the ice sheets, given the immense size of these formations. The Greenland ice sheet still spans more than 600,000 square miles (more than three times the size of Texas), and the Antarctic ice sheet covers over 5 million square miles (roughly the area of the contiguous United States and Mexico combined). However, the melting that has already occurred is contributing to rising sea levels. Since 1993, when satellites began consistently tracking sea levels, there has been an increase of more than 80 millimeters (over 3 inches), as illustrated in the graph below. A longer-term record from tidal gauges shows that increasing sea levels began before 1993, culminating in a total increase of nearly 10 inches since the late 1800s.
Much of the rise can be attributed to melting ice sheets and glaciers, but thermal expansion of the warming ocean waters is contributing, too. Thermal expansion is the process by which water expands in volume as it heats up. As ocean temperatures rise, seawater takes up more space, pushing sea levels higher even without any additional water being added from melting ice. As with trends in atmospheric air temperature, complexities exist, however. For starters, there are short-term ups and downs (each year doesn't always have a higher mean sea level compared to the prior year), and sea levels aren't rising equally everywhere. Variations in ocean currents and local geography mean that sea levels in some parts of the world are rising more quickly than the global average, while in other areas sea levels have fallen or are remaining steady, even while the global average sea level increases. Furthermore, natural geologic factors affect sea level, too, such as the fact that the basins that hold Earth's oceans are constantly (albeit very slowly) changing shape. Scientists must take these long-term natural factors into account when calculating the rate of sea-level change due to global warming.
While a global average sea-level rise of 10 inches since the late 1800s may seem like no big deal, consider that 11 of the world's 15 largest cities are along coastlines. In the United States alone, about 40 percent of the population lives in densely-populated coastal areas. Even with the sea-level rise that has occurred so far, low-lying coastal areas of some large cities are flooding more frequently. Already in Miami, Florida, the highest tides of the year (called "king tides") are increasingly causing flooding in parts of the city.
Estimates show that king-tide flooding in Miami Beach has increased by four times since 2006. So, what may seem like a slow and minor sea-level rise is starting to have local and regional economic impacts. Continued warming and sea-level rise will likely cause more areas (and people) along the world's coastlines to become increasingly vulnerable to flooding.