Intergovernmental Panel on Climate Change:
Fifth Assesment Report; Working Group One:
Chapter 13: Sea Level Change
Chapter Summary and Implications
13.3: Contributions to Global Mean Sea Level Rise During the Industrial Period
Contribution from Thermal Expansion
Over the time period 1971-2010 the rate of thermal expansion of the surface ocean (0-700 m depth) in terms of sea level rise was ~0.6 mm/yr and the rate of expansion for the deep ocean ~0.2 mm/yr. Using satellite altimetry data over the period 1993-2010, the contribution of surface water expansion to sea level rise was ~0.8 mm/yr, and the deep ocean contribution was ~0.3 mm/yr. Externally forced AOGCM (Atmosphere-Ocean Global Climate Model) models have successfully reproduced the observed thermal expansion contribution estimates within uncertainty for the historical period, so there is high confidence in the AOGCM projections of sea level rise due to thermal expansion (Table 13.1).
In order to accurately predict future sea level scenarios, the forecasting models need to be validated. This is done by comparing observational estimates of contributions to those derived from model scenarios forced with estimated time-dependent anthropogenic and natural forcings. This section summarizes both the observed contributions and modeled results for the historical period of instrumentation (Fig. 13.4).
Contribution from Glaciers
The contribution of melting land-based ice (glaciers, ice caps, ice sheets) is typically calculated using direct measurements of surface mass balance or, on larger ice masses, using remotely sensed measurements of gravity anomalies. Surface mass balance is the net gain or loss of snow/ice due to the competing effects of accumulation and melt. The combined contribution of melting glaciers over the period 2003-2009 was ~0.71 mm/yr. A few well-observed glaciers are used to calibrate surface mass balance models of glaciers around the world, in order to model their contribution to GMSL. Model validation is difficult largely due to the lack of glacier records (only 17 glaciers worldwide have records longer than 30 years). Therefore there is medium confidence in the projections of glacier change in AOGCMs, due to a lack of available initial state data, despite a good understanding of the processes and mechanisms at work.
Contribution from Ice Sheets
The contribution of the Greenland and Antarctic Ice Sheets is governed by their mass balance as well as internal ice flow dynamics. Three methods are used to calculate ice sheet contribution: observed mass balance method, repeat altimetry (satellite and airborne), and gravimetric methods that measure changes in earth’s gravity due to mass fluxes. The majority of the observed data from the last two decades comes from airborne and satellite platforms. Over the period 2002-2011 the Greenland Ice sheet contributed ~0.59 mm/yr to sea level rise, while the Antarctic Ice Sheet contributed ~0.4 mm/yr. The combined contribution of the two ice sheets over the period 1993-2010 was ~0.6 mm/yr. Due to the increased complexity of ice sheet dynamics on outflow and the lack of detailed long-term observations, there is only medium confidence in the AOGCM model predictions of ice sheet contribution, though there is high confidence in a positive future contribution from combined ice sheets to GMSL.
Contribution from Water Storage on Land
In the most recent decades there is no observable trend in natural water or snow storage on land despite a changing climate. However, human induced land use changes (especially impoundment of surface waters and groundwater depletion) are responsible for ~0.23 mm/yr of sea level increase between 1971-2010. The building of dams and reservoirs increase land storage, keeping water out of the oceans were as the pumping of groundwater eventually adds water to the oceans.
Table 13.1: Breakdown of global mean sea level budget (mm/yr) over different time periods over the instrumental records.
Figure 13.4: Comparison of modeled and observed components of global mean sea level change since 1900 (a). Changes in glaciers (b), ice sheets, and land water storage (c) are a positive increase in sea level when mass is added to the ocean. D) shows the rate of change for total contributions (blue, purple), just thermal expansion (black) and land water storage (yellow).