1. Discussion of Abrupt Climate Change
Paleo-Polar Joint Working Group Meeting
June, 2006

2. CCSM Abrupt Change Task Team
Some history
CCSM Abrupt Change Task Team
Cross working groups, PaleoWG, PCWG, & OWG
DO events are the numbers associated with warmings. H events are the yellow bars which are defined by periods of ice rafted debris found in sediments. Some proxy data seems to show H events were coolings (coincident with IRD) and freshwater dumps (timing is bad as it appears to be often shortly after the IRD event).
I recommend making the point that we don’t think future abrupt change will resemble past abrupt change too much. Mostly because the Laurentide ice sheet is gone, which made the climate fundamentally different.
Cecilia’s focus is on DO events because other abrupt events like Heinrich events, while clearly abrupt changes in ocean sediment debris, I am not convinced of a consistent abrupt climate signal. There is no doubt about DO events though. However, I am only sure the DO event signal occurs on Greenland (Heinrich events do not have a abrupt signal on Greenland). There are abrupt signals elsewhere, but it is debatable if they are coincident with DO events and/or they may not be easily linked to climate change.
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?

3. Working Hypothesis (for now) for D-O events and future abrupt change: It’s 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
15ºC Winter warming by 2050 in A1B scenario ensemble mean! 8C
-2C
16C
106 km2
Abrupt summer sea ice decay in ~2030 in one ensemble member!
In A LGM climate! Modest sea ice changes (eg magnitude of the amplitude of the modern seasonal cycle of sea ice) can produce a 10C warming and 50% increase in precip on Greenland.

4. 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
Cariaco basin is a high temporal resolution marine core in the Caribbean off the Venezuala coast. Variations are primarily due to variations in summer rainfall and runoff. During both Heinrich event 1 and the 8.2 ka event, Cariaco basin shows reduced precipitation/runoff
Heinrich 1 is a large fw event associated with a large discharge of iceberges into the N. Atl.
A marine proxy in the subtropical north Atlantic indicated a rapid and almost complete shutdown of the THC during this event.
The 8.2ka event is the release into Hudson Bay from an ice-dammed superlake. The 8.2 event reflects a reduction of precipitation that occurred in yrs and lasted 200 years.
Drier Wetter
8.2 Event

5. 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 >100 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

6. Idealized Freshwater Hosing Simulations (CMIP)
1 Sv / 100 years, North Atlantic 50 to 70°N
8.2 ka
LGM
The top two figures show the surface air temperature change in CCSM3 simulations induced by imposing 1 Sv of water into the North Atlantic for 100 years. The left is for an interglacial period, 8.2 kyrs BP, and the right is for glacial conditions, 21 kyrs BP. Both show similar patterns of cooling but with notable exceptions. Cooling is greatest in the North Atlantic at the location of the fw hosing but extends farther NE at 8.2 due to less initial sea ice.
The LGM hosing simulation shows greater cooling in the SH.
There are also significant differences in the response in the subtropical North Atlantic and North America associated with
differences in the subtropical High response.
Bottom shows scaled (to 50 years of control before fw forcing) of max of Atlantic MOC and annual NH sea ice area. Initial rates of change differ depending on climate state (8.2 vs LGM) but scaled change ends up similar for both. In neither case, does the THC collapse. LGM hosing simulation has not recovered yet, even after 100 years of no hosing.
1 Sv
(courtesy of Bette Otto-Bleisner)

7. 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

8. 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
106 km2

9. 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?

10. 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?