SeaRISE White Paper
Assessing Ice Sheet Contributions to Sea Level through the 21st Century
This white paper describing a community ice-sheet modeling effort was written in 4Q/2008. While the overall approach has not changed substantially, many of the details, such as the Data sets to be used and the actual Experiments to be run, have been modified based on frequent discussions with the participants. The notes from these telecons can be found at SeaRISE Telecon Notes
The IPCC Fourth Assessment Report highlighted both the importance of predicting future sea level change and the lack of understanding of ice dynamics to achieve such prediction. The series of IPCC reports demonstrates an evolving awareness of the immediate need for achieving a predictive capability for this ice sheet contribution to sea level and the necessity that this capability includes adequate treatment of the most dynamic elements of ice sheet behavior.
A recent workshop, held at Los Alamos National Laboratory (Lipscomb et al., Eos, 20 Jan. 2009in), focused on incorporating the best ice sheet models into coupled climate models, but it is clear that this major activity may not produce satisfactory results until after the deadline for input to the next IPCC assessment had passed. This realization led to the formation of an Assessment Cluster whose focus was to provide quantitative estimates of ice-sheet contributions to sea level for the 21st century.
Goals and Strategy
The goal of this effort is to provide quantitative estimates of ice sheet contributions to sea level for the 21st century, along with appropriate uncertainties.
Confidence in these estimates will be gained by subjecting a number of models to the same scenarios to reduce the impact of unrealistic characteristics of any single model on the aggregate predictions.
Given the wide range of potential contributions, a strategy has been designed to produce the most useful results in the relatively short period of time before the next IPCC report by determining an upper limit on sea level change over the next 100 to 200 years. Under this strategy, many of the early model experiments will employ extreme forcings or physical assumptions in order to identify this initial upper bound.
As our knowledge of the underlying physics and processes is improved through ongoing field studies, subsequent experiments that represent more likely scenarios will then be run to help lower the upper bound over the longer term. Experiments encompass ice-sheet responses to triggers similar to Larsen Ice Shelf disintegrations and rapid outlet-glacier thinning and retreat witnessed during the last 15 years. The likelihood of additional such events in the next century or two cannot be determined at this time, but might indirectly appear from results of other experiments.
All models will quantify their simulated ice-sheet responses relative to a control run using the same model. Such evaluation of the ice-sheet responses reduces model-specific biases and isolates the impact of the difference in forcing between the experiment and the control runs.
The effort includes both regional models and whole ice sheet models. The interactions are expected to be two-way: regional models will be used to help provide more reasonable forcings for selected whole ice sheet model experiments and whole ice sheet models will be used to define boundary fields that will enable regional models to refine the predicted responses of particularly dynamic areas.
Another anticipated benefit is that the results of this effort will help inform the implementation of dynamic land ice into a fully coupled Community Climate System Model (CCSM) and other Global Climate Models (GCMs).
The timetable is driven by an anticipated 5th IPCC report by 2013. This means that papers as input to this report must be submitted by 2011 and likely in press by November 2012. Initial model runs will be conducted in 2009. Results from these runs will be analyzed and discussed in early 2010, with the intention of a second generation of runs defined and completed by the end of 2010. The analysis, discussion and publication of these final runs will be completed in 2011, prior to the IPCC deadline.
Three model categories are defined:
- Whole Ice Sheet
- Ice-Stream/Ice-Shelf (including outlet glaciers)
The second and third categories are regional models and interact with the whole ice sheet models as outlined above. Each category is described in more detail below, along with the initial generation of experiments intended for each.