Rapid Changes in Glaciers and Ice Sheets and Their Impacts on Sea Level

Subcommittee members

Konrad Steffen (ice sheet, climate, coordinate)
Shawn Marshall (modeling)
Bob Thomas (ice sheet)
Peter Clark (paleo ice sheet)
Graham Cogley (glaciers)
Eric Rignot (ice sheet)
Jason Box (SMB modeling)
David Holland (ocean ice interaction)
Marika Holland (sea ice)

Chapter Outline

• Summary and Outlook
• Paleo-Record of Ice Contribution to Rapid Sea Level Change (Peter)
• Assessment of Current State
• Techniques of assessment of total mass balance (Bob)
• Mass budget
• Altimetry
• Gravity changes
• Polar Ice Sheets (Bob / Eric / Koni)
• Greenland
• Antarctica
• Glaciers and ice caps (Copley)
• Potential Mechanism of Rapid Ice Response
• Ocean-ice interactions (David Holland)
• Ice shelf/ice stream interactions (Shawn / Eric / Bob)
• Basal lubrication and glacier flow (Shawn / Bob / Koni)
• Sea level feedback (Peter)
• Summary and Recommendations (all)
• Process observations and monitoring
• Improving understanding
• Model development
• Recommendations
• Reconcile estimates of ice sheet mass balance derived using different approaches, and determine whether recent increases in mass losses are anomalous or reflect improvements in observational techniques.
• Identify causes for the apparent recent increases in mass loss to enable development of improved glacier models.
• Extend ongoing measurements of ice-thickness transects to cross each major outlet glacier in Greenland and Antarctica.
• Complete the World Glacier Inventory through sustained support for the Global Land Ice Measurements from Space (GLIMS) program.
• Utilize the Ice, Cloud, and land Elevation Satellite (ICESat) laser and, once launched, CryoSat-2 radar altimeter satellites—complemented by aircraft altimetry—to survey changes in the surface topography of the ice sheets; and based on experience gained, develop a suitable follow-on satellite.
• Utilize the Gravity Recovery and Climate Experiment Satellite (GRACE) and appropriate follow-on missions to infer changes in the mass of the glaciers and ice sheets.

Abrupt Changes in Atmospheric Methane

• Key Findings
• Recommendations (box)
• Introduction
• Methane and Climate
• Greenhouse forcing
• Modern methane budget
• Potential for large changes in atmospheric methane
• Motivating questions
• History of Atmospheric Methane
• Anthropogenic increase
• Last 800,000 years from ice cores
• Abrupt methane changes during the last ice age
• Potential mechanisms for abrupt change in atmospheric methane
• Introduction
• List four focus areas
• Justification for four focus areas
• Marine hydrate destabilization
• Review size of potential source
• Mechanisms to destabilize hydrates
• Review geologic data relevant to past hydrate release
• Quantity of methane released
• Climate impact
• Sidebar: LPTM
• Review results from models addressing marine hydrate release
• Quantity of methane released
• Climate impact
• Conclusion about potential for abrupt release of methane from marine hydrates
• Terrestrial hydrate (permafrost)
• Review size of potential source
• Mechanisms to destabilize hydrates
• Review geologic data relevant to past hydrate release?
• Review results from models addressing marine hydrate release?
• Quantity of methane released
• Climate impact
• Conclusion about potential for abrupt release of methane from marine hydrates
• Expansion of wetlands due to Arctic warming
• Review carbon dynamics relevant to methane emissions.
• Arctic wetland expansion
• Observations and projections
• Sidebar: Arctic terrestrial feedbacks
• Conclusion about potential for abrupt release of methane from wetlands
• Other Considerations
• Changes in methane sink strength
• Other?

Meridional Overturning Circulation Change and Influence on Climate

• Key findings
• Abrupt collapse of MOC is very unlikely.
• Recommendation
• Questions
• What are the factors that control the overturning circulation? - (see for guidance kuhlbrodt article  Rev Geophysics; inquire about contributing authorship) SEEK OUTSIDE COUNSEL
• What is the present state of the MOC? JOHNS
• Inverse models, Bryden, RAPID, Talley
• What is the evidence for change in the overturning circulation in the past? LYNCH-STIEGLITZ
• YD, Heinrich, 8.2K event, Holocene, DO events
• How well do the current coupled ocean-atmosphere models simulate the overturning circulation? MORRILL
• Look to IPCC Chapter 8 for initial guidance
• Key issue: how well are sill overflows represented?
• Simulation of LGM MOC from PMIP (recent paper has a synthesis – see Weber, Climate of the Past, Vol 3, p. 51, 2007)
• Can transients be used for assessments?
• What are the global and regional impacts of a change in the overturning circulation? DELWORTH
• Heinrich vs LGM
• Instrumental record (Atlantic SST impacts, possibly MOC driven; hurricanes, monsoons)
• Models: impact of MOC changes; Jungclaus results (hosing in the context of a greenhouse warming climate)
• Other effects; ecosystems, carbon cycle, nutrients??
• What factors that influence the overturning circulation are likely to change in the future, and what is the probability that the overturning circulation will change? WEAVER
• IPCC section 10.4 as initial guidance
• Greenland ice sheet melting (Jungclaus and others)
• SPINOFF BOX: SEA ICE (Marika Holland)
• What are the observational and modeling requirements required to understand the overturning circulation and evaluate future change? DELWORTH COORDINATES
• Assimilation models; emerging technique for estimating MOC and time history
• Detection – need for optimal strategy
• Need for understanding decadal/multidecadal variability and decadal prediction systems
• Observing requirements for MOC changes
• Ice sheet models coupled into AOGCMs
• Better reconstructions of past MOC and freshwater forcing
• Improved resolution and physics, particularly with respect to deep water formation
• Improvements in how fresh water enters ocean from boundaries

NOTES:
Question 4: may be an issue with simulating MOC in ocean only model; may need coupled model. Changed question.

Hydrologic Variability and Change

• Key findings
• Recommendations
• Introduction -- Statement of the problem
• Causes of hydrologic variability over the historical record
• What is our modern understanding over the historical record -- oceans, atmosphere, land surface feedbacks   
• Dynamic vs. thermodynamic (atmospheric circulation vs. surface water and energy balance)
• Box on snowpack and streamflow trends
• Megadroughts from tree-ring records over the past 2,000 years
• Temporal and spatial properties of tree-ring reconstructed drought
• Box on mid-Holocene aridity in North America
• Duration of past droughts indicated by tree rings
• Spatial patterns of variability
• Scientific questions on causes
• Can the megadroughts during the Medieval Climate Anomaly (MCA) period be attributed to external forcing?
• Were the MCA megadroughts unique in the Holocene?
• Box on paleoflood and drought records from Holocene.
• Does future drying in the SW (and subtropics) arise from the same dynamical mechanisms as the Medieval drying or is it dynamically distinct?
• Ocean/atmosphere and land-atmosphere coupling
• Current conditions
• Last millennium or longer
• Scientific questions on causes
• Potential predictability – ENSO/North American precip links, etc.
• Potential instability as an indicator transitions a new climate behavior
• Can we predict the onset of anthropogenic drying with climate models? Signature of non-anthropogenic droughts vs. developing or yet to be realized signature of anthropogenic droughts.
• Is the tropical ocean-North America response to radiative forcing linear or does it have thresholds?
• Large-scale impacts associated with drought-promoting states of the ocean/atmosphere (e.g., enhanced floods and hurricanes)