Intergovernmental Panel on Climate Change:
Fifth Assesment Report; Working Group One:
Chapter 13: Sea Level Change
Chapter Summary and Implications
13.4: Projected Contributions to Global Mean Sea Level
Sea Level change by heat uptake and thermal expansion
Currently the ocean absorbs more than 90% of the net energy increase that is accumulating in the climate system (Fig.13.8) and sea level rise due to thermal expansion of seawater is roughly proportional to this heat uptake (~0.11 m per YJ (10^24 J)). In the first part of the 21st century, ocean surface waters will dominate heat uptake and expansion. Later in the century, deeper ocean waters will begin to take up more energy as heat penetrates downward, and thermal expansion in these deep waters will dominant sea level rise. Due to the long time scales of ocean heat uptake, sea level rise by thermal expansion may occur for centuries to millennia after stabilization of radiative forcing. Ocean heat uptake may also cause changes to global scale circulation, which could have dramatic and uncertain consequences. Under the most extreme Representative Concentration Pathway (RCP) of 8.5, GMSL due to thermal expansion alone may exceed 2 m above pre-industrial levels by 2500. RCPs are the emission-based concentrations used to drive model projections. Currently the oceans are rising due to thermal expansion at a rate of ~0.38 m per °C of ocean temperature increase.
Figure 13.8: Projected heat uptake by the ocean during the 21st century for the AR5 RCP scenarios.
Glaciers
The projections of 0.06 to 0.15 m of Global Mean Sea Level Rise (GMSLR) rise by 2100 due to melting glaciers from AR4 were unfortunately based on an incomplete global glacier inventory and the models lacked certain dynamic processes. An improved Randolph Glacier Inventory (Arendt et al., 2012) provides the first complete global inventory of glacier location, area, and hypsometry. This new inventory along with improved model physics has much enhanced projections of glacier contribution to GMSLR. Process based models forced by temperature predict a minimum GMSLR of 0.07 m for RCP 2.6 and a maximum GMSLR of 0.26 m for RCP 8.5 by 2100 (Fig. 13.9). While predictions have improved significantly there still remain uncertainties regarding the glacier inventory (specifically glacier size cutoff) and transport of glacier meltwater on land before it reaches the ocean.
Figure 13.9: Model based predictions of Global Mean Sea Level Rise (GMSLR) during the 21st century from glaciers for the different RCP scenarios. Colors correspond to individual model projections and thin lines are individual AOGCM model runs.
Greenland Ice Sheet
The Greenland Ice Sheet (GIS) contributes to GMSLR through meltwater input from ice melt and by dynamic input of ice mass into the ocean. Both of these processes are expected to have significant effects on sea level rise during the 21st century. Fully coupled climate-ice sheet models consistently show an increasingly negative mass balance of the GIS over the 21st century leading to contribution of up to 0.17 m by 2100 (Table 13.4). The fully coupled model is required to take into account several feedback cycles that lead researchers to suggest a threshold level of warming (+3°C relative to pre-industrial) that will promote irreversible loss of the GIS to less than 30% of its present volume.
The dynamic contribution of the GIS to GMSLR occurs by one of three processes: 1) calving and ocean induced melt at marine-terminating outlet glaciers, 2) surface meltwater alteration of basal sliding conditions, 3) the indirect relationship between surface mass balance and dynamic ice flow. Sea level rise based on observations and modeling of these processes is likely to equate to 20 to 85 mm of GMSLR by 2100 for RCP 8.5.
Table 13.4: Modeled contribution of the Greenland Ice Sheet during the 21st century (given with 1 sigma uncertainties).
Antarctic Ice Sheet
Ice loss and contribution to GMSL rise happens by the same processes on the Antarctic Ice Sheet (AIS) as it does on the GIS. Under high emissions scenarios the surface mass balance is slightly positive (more accumulation than ablation). This is because under warmer climate projections, increased precipitation due to warmer air temperatures outweighs surface melt, leading to a positive mass balance, or gain of mass for the AIS, which leads to a small decrease in sea level over the next few decades. On multi-centennial time scales feedback cycles promote eventual mass loss and positive additions to GMSL. More so than the GIS, dynamic ice loss is the major contributor of sea level rise from the AIS. Under the same mechanisms as the GIS, changes in the AIS will likely lead to -20 to 185 mm of sea level rise during the 21st century. It is also likely that the dynamic contribution is severely underestimated and also likely that it will continue to increase beyond 2100.
Marine Ice-sheet instability and irreversible loss from the AIS
Much of the AIS terminates as grounded ice shelves fed by outlet glaciers draining the ice sheet. At the grounding line (the last point of contact with the ground as ice flows off the ice sheet), ice outflow generally increases with ice thickness. However, in a situation where the bed below the ice sheet deepens towards the ice sheet, small fluctuations in the grounding line can cause large changes in ice output (Box 13.2 Figure 1). A small climate forcing that moves the grounding line closer toward the ice sheet and into deeper water enhances melt and rate of ice output. This increased outflow is a positive feedback cycle that self-sustains until the bed begins to shallow toward the ice sheet once again. This situation has led to development of the Marine Ice-Sheet Instability (MISI) theory. It is thought that this mechanism may be responsible for the theoretical rapid disintegration of the West AIS. Accumulated theory, numerical models and paleo records all indicate irreversible loss of parts of the AIS due to an MISI mechanism. However, remaining uncertainties regarding theory, simulations, and resolution and length of paleo records prevent the prediction of the timing and magnitude of such irreversible loss.
Box 13.2 Figure 1: Diagramatic explanation of Marine Ice-Sheet Instability (MISI) theory and intiation of self-sustatining ice-shelf loss.
Anthropogenic Intervention in Water Storage on Land
Future affects of any potential human induced changes to water storage on land have not yet been studied much in the peer reviewed literature. Initial modeled predictions of GMSLR due to groundwater depletion range between 20 to 90 mm of SLR during the 21st century. On the other hand, impoundment of water ranges between 0 to 30 mm of GMSL decrease during the 21st century. Given the uncertainty in future socioeconomic scenarios, little confidence is attributed to these projected values.