Intergovernmental Panel on Climate Change:
Fifth Assesment Report; Working Group One:
Chapter 13: Sea Level Change
Chapter Summary and Implications
Frequently Asked Questions:
13.1: Why does local sea level differ from the Global Average?
13.2: Will the Greenland and Antarctic Ice Sheets contribute to Sea Level change over the next century?
When calculating and projecting sea level change, values are often reported in terms of Global Mean Sea Level (GMSL) change. However, it is well known that regional changes in sea level can vary dramatically from the global mean due to a number of factors. In a local sense, the movement of the land surface or sea surface relative to each other can cause significant change in local and regional sea level.
For example, the glacial isostactic rebound (rebound of the crust after removal of a load) associated with the melting a large ice sheet over Stockholm, Sweden is causing the land surface to rise, which lowers local sea level. The opposite is also possible, like in Manila where intense extraction of ground water is causing the land to subside, or lower, which causes local sea level to rise (Figure FAQ 13.1). Another factor affecting regional sea level is density, which is largely controlled by temperature and salinity. These parameters are not uniform across the world’s oceans and can vary substantially based on freshwater input, evaporation, and ocean circulation patterns. Mass redistribution between land and ocean (such as the melting of glaciers and ice sheets) can locally alter earth’s gravity, offsetting regional sea level.
The combination of the above processes, as well as others, modulates the expression of regional sea level change and also makes modeling and future scenarios complex and difficult.
FAQ 13.1 Figure 1: Satellite altimetry records of global sea surface change from 19993-2012. Also shown are observation records of local sea level change at various points around the globe from longer-term tide gauge measurements (grey lines), for comparison the mean global sea surface change is also shown (red line).
Being the largest ice masses on the Earth, the Greenland (GIS) and Antarctic (AIS) ice sheets contain within their ice mass roughly 7 and 57 m of sea level equivalent, respectively. If these ice sheets were to catastrophically disintegrate into the ocean, we would most definitely see dramatic sea level increases around the globe. Through paleoclimate records it is also known that mass changes to the GIS and AIS have altered global sea level in the past. However, the question of whether or not the GIS and AIS will contribute significantly to global sea level change over the next century is more complex.
In order to gauge their impact, one must first understand the processes by which these ice sheets grow (remove water from ocean) and shrink (add water to ocean). Both ice sheets grow through the process of accumulation (snowfall) and shrink by ablation (surface melt) and outflow (either to a floating ice shelf or by iceberg calving). Currently the GIS loses approximately half its accumulation through ablation and half by calving. The AIS on the other hand loses a majority of its mass through calving and submarine melt at its margin, and negligible amounts to surface melt. Fluctuations in mass flux then depend on net accumulation or net loss, which is dependent on several processes both internal and external to the ice sheets.
In Greenland, mass loss due to surface ablation has doubled since the early 1990s and it is projected to continue to increase into the next century. Along with this increase, dynamic outflow is also projected to increase along with increasing surface melt. With these increases, mass loss will continue to outweigh accumulation on the GIS, leading to a positive addition to sea level. With the projected climate conditions, surface ablation from the GIS is projected to contribute between 10-160 mm of sea level rise, while the outflow contribution is projected to be 10-70 mm by 2100 (FAQ 13.2 Fig. 1).
Antarctica differs slightly from Greenland in that it loses almost all of its mass to outflow because surface temperatures are too low to allow much of any ablation. Because of this, some projections indicate that increase precipitation over the next century could actually increase accumulation and cause a drop in sea level. However, one must take the dynamic loss of ice from the AIS into account as well. Iceberg calving and submarine melt are expected to increase in the coming decades and the possibility of catastrophic grounded ice shelf collapse (Marine Ice Sheet Instability theory) would far outweigh any small increases in accumulation. With these factors in mind, the AIS is projected to contribute -20 to 185 mm of sea level change by 2100 (FAQ 13.2 Fig. 1).
FAQ 13.2 Figure 1: Illustrative synthesis of GIS (left panel) and AIS (right panel) mass balance and outflow as well as projected sea level contribution out to 2100. Individual plots show the approximate individual contribution of GIS outlet glaciers to sea level rise. Outlet glaciers on the AIS are ranked by percentile and related to sea level rise.