Discussion of Abrupt Climate Change

Paleo-Polar Joint Working Group Meeting

June, 2006

CCSM Abrupt Change Task Team

Cross working groups, PaleoWG, PCWG, & OWG

Some Science IssuesWhat 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?

Some history

Working Hypothesis (for now) for D-O events and future abrupt change: It’s the Sea Ice!

Modest sea ice change can cause 10ºC warming and 50% more precipitation on Greenland in an LGM climate - Li et al 2005

Primary focus of the PCWG and OWG contingency

106 k

m2

Abrupt summer sea ice decay in ~2030 in one ensemble member!

15ºC Winter warming by 2050 in A1B scenario ensemble mean!

8C

-2C

16C

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

8.2 Event

Dri

er

Wet

ter

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

8.2 ka LGM

1 Sv

Idealized Freshwater Hosing Simulations (CMIP) 1 Sv / 100 years, North Atlantic 50 to 70°N

(courtesy of Bette Otto-Bleisner)

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

Abrupt sea ice transitions in future scenarios10

6 km

2

Three important factors driving abrupt transitions

• An increasingly “vulnerable” (thinner) ice cover

• A trigger - rapid OHT increases• Positive feedbacks - albedo and oht

changes

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?

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?