Since 1880, global sea levels have risen by an estimated 8-9 inches. At least as concerning, however, is the recent acceleration in the rate of sea level rise, likely driven by increasing global temperatures associated with higher greenhouse gas emissions. In fact, more than one-third of the total increase has occurred since 1993.
This acceleration is expected to continue throughout the remainder of the 21st century. Even under a low emissions pathway, it is highly likely that sea levels will have risen by at least 12 inches between 2000 and 2100. That estimate rises considerably with higher emissions, topping out at about 8 feet under the worst-case scenario.
Further, because greenhouse gases remain in the atmosphere for hundreds of years or longer, even achieving net zero emissions will not immediately stop sea levels from rising. However, it could slow the pace considerably while allowing atmospheric greenhouse gas concentrations to gradually decline.
As water becomes warmer, it expands, taking up more space. Because man-made greenhouse gases have caused global temperatures to rise and because oceans absorb the majority of that heat, the physical area covered by oceans has increased. This element of sea level rise would occur regardless of whether any additional water was being added through other sources.
As temperatures rise, large ice sheets melt at a faster pace, adding water to the oceans. The Greenland Ice Sheet covers 75% of Greenland’s land mass and has lost 3.8 trillion tons of ice over the last quarter-century. The Antarctic Ice Sheet, which has not melted at as fast a rate, is also of considerable concern because it covers an area larger than the United States. These two ice sheets combined contain 99% of the freshwater ice on the planet, enough to raise sea levels by over 200 feet if completely melted.
In addition to the massive ice sheets, the world contains thousands of smaller mountain glaciers. Because of their lesser size, these ice masses are more prone to near-term melting that will cause run-off of water into oceans. Together, they contain enough ice to raise sea levels by about 20 inches.
Some of the land-based water withdrawn by human activity (e.g., irrigation) from below the earth’s surface or from rivers, lakes and wetlands eventually reaches the ocean through run-off or evaporation, adding to sea levels.
Rising sea levels increase risk in a number of ways. Because many of the world’s largest cities are located along coastlines, a substantial portion of the world’s population is directly exposed to rising sea levels.
Among the most dangerous impacts are the often-deadly storm surges experienced during hurricanes. Climate change delivers a double impact in this area, as hurricanes have the potential to become more powerful due to warmer ocean temperatures. This greater storm intensity, coupled with higher sea levels, can increase both the frequency and reach of storm surges.
In addition to this acute risk, chronic risks are likely to increase as well. For example, some U.S. coastal areas have experienced an increase of over 300% in high tide flooding in the last 50 years. Both the acute and chronic risks of sea level rise threaten coastal infrastructure such as roads, subways, water supplies, and waste treatment facilities, among other things.
Beyond the impact on highly developed urban areas, sea level rise also threatens the economic well-being and safety of less developed areas, particularly small island nations. These areas often lack the resources possessed by wealthier countries to mitigate the impact of flooding through engineering-based solutions.
"Climate Change: Global Sea Level"
"Is the Sea Level Rising?", National Oceanic and Atmospheric Administration.
"Greenland's Rapid Melt will mean More Flooding"
"Quick Facts on Ice Sheets", National Snow and Ice Data Center.