Discussion of Abrupt Climate Change

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Discussion of Abrupt Climate Change
Paleo-Polar Joint Working Group Meeting June, 2006
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CCSM Abrupt Change Task Team
Some history
Cross working groups, PaleoWG, PCWG, OWG
Some Science Issues What causes the abrupt
warming - Dansgaard-Oeschger events? What is the
response of the global climate system to abrupt
coolings? Will there be something like abrupt
change in the future?
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Working Hypothesis (for now) for D-O events and
future abrupt change Its the Sea Ice!
Primary focus of the PCWG and OWG contingency
Modest sea ice change can cause 10ºC warming and
50 more precipitation on Greenland in an LGM
climate - Li et al 2005
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Heinrich and other freshwater events in the North
Atlantic
Primary Focus of the PaleoWG contingency
Understanding global response of climate system
to freshwater forcing using CCSM and paleoclimate
records

• Mechanism and feedbacks in coupled system
• Transmission of signal by ocean and atmosphere
• Rates of change and recovery
• Dependence of response on background climate
  state and associated forcings

Heinrich Event 1
Drier Wetter
8.2 Event
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Experiments of the PaleoWG

• ? Equilibrium simulation for 8.2 ka to provide
  control and initial conditions for paleo-hosing
  experiment.
• Idealized paleo-hosing simulations to understand
  the response and recovery, spatially and
  temporally, depending on background climate
  state
• ? LGM, 100-year FW pulse of 1 Sv plus gt100 years
  of recovery
• ? 8.2 ka, 100-year FW pulse of 1 Sv plus years
  of recovery
• Deglacial, still in design stage
• More in-depth study of 8.2 event, particularly
  exploring range of realistic freshwater estimates
  of amount of water and location of impulse

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Idealized Freshwater Hosing Simulations (CMIP) 1
Sv / 100 years, North Atlantic 50 to 70N
8.2 ka
LGM
1 Sv
(courtesy of Bette Otto-Bleisner)
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Experiments of the PCWG/OWG

• Idealized hosing experiments (as discussed by
  Cecilia Bitz)
• ? Modern, instantaneous FW pulse 1/3 of
  Greenland ice cap plus recovery period
• LGM, same FW pulse but 100 yr recovery
• Ocean Flush - Initially weaken ocean stability
  with thermohaline off so it resumes abruptly
• Simulations to diagnose mechanisms in IPCC future
  scenarios
• Simulations to assess whether future scenario
  mechanisms may play a role in different control
  climates

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Abrupt sea ice transitions in future scenarios

• Three important factors driving abrupt
  transitions
• An increasingly vulnerable (thinner) ice cover
• A trigger - rapid OHT increases
• Positive feedbacks - albedo and oht changes

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Science Issues Discussion of what should be the
next steps?

• Some Possibilities
• Sensitivity studies to further explore
  interesting behavior in current simulations
• Simulations of important freshwater events of the
  past (Meltwater Pulse 1A, Younger Dryas)
• Understand impact of Greenland ice cap meltback -
  past and future
• Explore sensitivities to climate state, location,
  amount and duration of freshwater forcing
• Further exploration of threshold behavior in
  sea ice
• Others?

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Priorities/Needs

• What should be the priorities for additional work
  on abrupt change?
• Possible protocol abrupt change experiment?
• What new model capabilities are needed?
• Are other resources needed?