SOTC: Contribution of the Cryosphere to Changes in Sea Level
Global sea level rose by about 120 meters during the several millennia that followed the end of the last ice age (approximately 21,000 years ago), and stabilized between 3,000 and 2,000 years ago. Sea level indicators suggest that global sea level did not change significantly from then until the late 19th century when the instrumental record of sea level change shows evidence for an onset of sea level rise. In the mid-19th century, the rate of sea level rise probably started exceeding the mean rate from the previous 2,000 years. Satellite altimetry observations, available since the early 1990s, provide more accurate sea level data with nearly global coverage and indicate that since 1993 sea level has been rising at a rate of about 3 millimeters per year. Sea level has risen since the early 1970s due to a combination of ocean thermal expansion and glacier mass loss (IPCC 2007 and IPCC 2013).
Contribution from the cryosphere
Which of the topics discussed so far in State of the Cryosphere have the potential to contribute to a rising sea level during a warming climate? Several — but some more than others.
- The seasonal snow cover melts during spring and summer and much of that water flows into rivers which eventually reach the sea. However, this is a process with a seasonal hydrologic cycle. With no net increase in seasonal snowfall over time, and no significant increase has occurred in recent decades, melting snow is not a factor that contributes to annual net sea level rise.
- Sea ice and ice shelves are already located in the ocean and thus do not have any further significant influence on sea level after they melt.
- As permafrost thaws, and the ice within the soil melts, an additional amount of liquid water becomes available but how much of this water actually reaches streams and rivers, and eventually the sea, is not well known at this time.
- The most significant contributors to sea level within the current climate are glaciers.
Current conditions: contribution from melting glaciers
Global sea level is currently rising as a result of both ocean thermal expansion and glacier melt, with each accounting for about half of the observed sea level rise, and each caused by recent increases in global mean temperature. For the period 1961-2003, the observed sea level rise due to thermal expansion was 0.42 millimeters per year and 0.69 millimeters per year due to total glacier melt (small glaciers, ice caps, ice sheets) (IPCC 2007). Between 1993 and 2003, the contribution to sea level rise increased for both sources to 1.60 millimeters per year and 1.19 millimeters per year respectively (IPCC 2007).
Antarctica and Greenland, the world's largest ice sheets, make up the vast majority of the Earth's ice. If these ice sheets melted entirely, sea level would rise by more than 70 meters. These ice sheets had long been believed to be in equilibrium, but more recent studies indicate growing ice sheet imbalance, with more mass leaving the ice sheet than is replaced by snowfall, especially in West Antarctica. See Ice Sheets for more information.
In contrast to the polar regions, the network of lower latitude small glaciers and ice caps, although making up only about four percent of the total land ice area or about 760,000 square kilometers, may have provided as much as 60 percent of the total glacier contribution to sea level change since 1990s (Meier et al. 2007).
How glaciers' contribution to sea level is computed
Global mass balance data are transformed to sea-level equivalent by first multiplying the ice thickness (meters) lost to melting by the density of ice (about 900 kilograms per cubic meter), to obtain a water equivalent thickness, and then multiplying by the surface area of these "small" glaciers (about 760,000 square kilometers). This provides an annual average mass balance of approximately -0.273 meters for the period 1961 to 2005. When dividing the mass balance value by the surface area of the oceans (361.6 million square kilometers), the final result is 0.58 millimeters of sea level rise per year. The Glacier Contribution to Sea Level graph demonstrates how the contribution from melting glaciers began increasing at a faster rate starting in the 1990s. This is in agreement with high-latitude air temperature records. The IPCC (2013) stated that it was "very likely" (at least 90 percent confidence) that the mean annual global rate of ocean level increase was 1.5 to 1.9 millimeters between 1901 and 2010, 1.7 to 2.3 millimeters between 1971 and 2010, and 2.8 to 3.6 millimeters between 1993 and 2010.
The IPCC Fifth Assessment Report contained significant uncertainty in its projections for glacier contributions to sea level rise over the course of the 21st century. An update (Radić et al. 2014) incorporated a newly released inventory showing the outline of nearly every glacier on Earth, 14 global climate models, and two emission scenarios: RCP4.5 (intermediate emissions) and RCP8.5 (high emissions). The intermediate-emission scenario indicated a sea level rise of 155 ± 41 millimeters, and the high-emission scenario indicated a rise of 216 ± 44 millimeters, though the authors conceded that large uncertainties remain.
Potential contribution of ice sheets
Destabilization of Antarctic and Greenland ice sheets has the potential to raise global sea level, but sea level would not rise uniformly everywhere. That is because ice itself has mass that exerts gravity on the global ocean. A loss of mass from the ice sheet would cause nearby ocean levels to fall, but because overall sea level would rise, the increase in sea level would be even higher farther away from the ice sheet. Consequently, ice sheet contribution to sea level rise—even if it were the same amount—would have different impacts, depending on whether the contribution came from Greenland or Antarctica (Bamber and Riva 2010).
Last updated: 13 July 2015