MOC Sensitivity to AnthropogenicMOC Sensitivity to AnthropogenicClimate Forcing in the CCSM:Climate Forcing in the CCSM:

Are There Any Abrupt SurprisesAre There Any Abrupt Surprises??

Peter R. GentPeter R. GentNCARNCAR

Blue: T85, ×1 Red: T42, ×1 Black: T31, ×3

CCSM3: Present Day Control Runs Maximum MOC in North Atlantic

T31, ×3

T42, ×1

T85, ×1

Control: Black

1% CO2 Run: Red, Cyan

2 × CO2: Green

4 × CO2: Blue

THC Response toIncreasing CO2

Year

1900 22002000 2100

CO

2 con

cent

ratio

n

20 century run

B1 stabilization(nominal 550 ppm)

A1B stabilization(nominal 750 ppm)

2300

Overshootscenario

Committed climate change

A1B

B1

1870

1870 control run

Future Scenario Runs Using CCSM3

Nominal 550 ppm stabilization

Nominal 750 ppm stabilization

A1B

B1

Year

Max

. mer

idio

nal o

vertu

rnin

g (S

v)in

Nor

th A

tlant

ic

Overshoot scenario

MOC Response to Future Scenarios

Sea

leve

l ris

e by

the

rmal

exp

ansi

on (

cm)

Year

Nominal 750 ppm stabilization

A1B

B1

Nominal 550 ppm stabilization

Overshoot scenario

Sea

leve

l ris

e by

the

rmal

exp

ansi

on (

cm)

Year

Nominal 750 ppm stabilization

A1B

B1

Nominal 550 ppm stabilization

Overshoot scenario

Sea

leve

l ris

e by

the

rmal

exp

ansi

on (

cm)

Year

Nominal 750 ppm stabilization

A1B

B1

Nominal 550 ppm stabilization

Overshoot scenario

Sea Level Response to Future Scenarios

SummarySummary

•• Gradual changes in anthropogenic forcingGradual changes in anthropogenic forcing⇒⇒ Gradual changes in North Atlantic MOCGradual changes in North Atlantic MOC

•• THC reductions, but no collapsesTHC reductions, but no collapses•• THC strength appears to be simply relatedTHC strength appears to be simply related

to the stability of the water columnto the stability of the water column•• Both in deep water formation regions andBoth in deep water formation regions and

in the overall North Atlantic MOCin the overall North Atlantic MOC

By end of 21st century SAT consistently warms in all scenarios

Abrupt transitions in Sea Ice10

6 km

2

Although forcing is gradual, abrupt transitions in sea ice do occurin some simulations.

September ice extent timeseries

Sea Ice Conditions

Simulated abrupt transitions in sea iceabrupt forcing (freshwater hosing) can result in abrupt ice changes

•Sea ice changes amplify climate response•Global teleconnections can result•Longevity of these changes are an issue

Sea ice change SAT Change

(From Vellinga and Wood, 2002;Vellinga et al, 2002)

N. Atlantic Fresh Water Input Expts. (CCSM2)

ConclusionsConclusions•• THC reductions due to anthropogenicTHC reductions due to anthropogenic

forcingforcing•• Europe always warms in future scenariosEurope always warms in future scenarios•• Abrupt sea ice changes in one A1BAbrupt sea ice changes in one A1B

ensemble memberensemble member•• Abrupt MOC changes are caused byAbrupt MOC changes are caused by

abrupt fresh water additions in the Northabrupt fresh water additions in the NorthAtlanticAtlantic

•• Europe most often cools in these expts.Europe most often cools in these expts.

Abrupt transitions in future climatescenarios?

Meehl et al, 2005Wigley, 2000

• Relatively gradual forcing.• Relatively gradual response in global air temperature