Feedback processes, threshold effects Part (a)

(1)

S.I. Seneviratne, ETH Zurich

Winter term 2006/07

Feedback processes, threshold effects Part (a)

Sonia Seneviratne

Institute for Atmospheric and Climate Science ETH Zürich

sonia.seneviratne@env.ethz.ch

2 24.10.2006 SIS 1. Introduction

31.10.2006 CS 2. Land surface processes in the global energy and water cycles (a) 07.11.2006 SIS 2. Land surface processes in the global energy and water cycles (b) 14.11.2006 CS 2. Land surface processes in the global energy and water cycles (c) 21.11.2006 Reserve date (Hydrologie-Seminar, ENSEMBLES)

28.11.2006 EJ Discussion of Exercises (1)

05.12.2006 SIS 3. Feedback processes, threshold effects (a) 12.12.2006 Reserve date (AGU)

19.12.2006 SIS 3. Feedback processes, threshold effects (b) 09.01.2007 EJ Discussion of Exercises (2)

16.01.2007 CS 4. Modeling of the coupled land-atmosphere system (a) 23.01.2007 SIS 4. Modeling of the coupled land-atmosphere system (b) 30.01.2007 SIS 5. Outlook, current research questions

SIS = Sonia I. Seneviratne, CS = Christoph Schär, EJ = Eric Jäger

Schedule

(2)

3 Lecture’s web page

http://www.iac.ethz.ch/staff/sonia/lecture

– Lecture notes

– Exercises (1+2) and example solutions

Land-Atmosphere-Climate Interactions

Chapter 3a

Feedback processes, threshold effects

Sonia I. Seneviratne and Christoph Schär, Institute for Atmospheric und Climate Science, ETH Zürich

Winter term 2006/2007

(3)

• Overview feedback processes

• water vapour feedback

• ice/snow - albedo feedback

• soil moisture - precipitation feedback

• soil moisture - temperature feedback

• vegetation - climate feedback (tundra - taiga ; grasslands - forests; CO ₂ effects)

• How to investigate feedbacks and coupling between variables?

• sensitivity experiments

• targeted experiments (e.g. GLACE)

• measures of coupling

• Next lecture: Thresholds, memory effects

Climate feedbacks and thresholds 6

• Feedbacks and thresholds are essential for the functioning of the climate system: They induce strongly non-linear effects (relevant for extreme events, scenario uncertainty for climate change)

• They contribute to a large extent to the

complexity of the climate system

(4)

7 Climate feedbacks

+ "

Positive feedbacks Negative feedbacks

Signal enhancement Signal damping

(Ruddiman 2000) (Ruddiman 2000)

Water vapour feedback 8

Initial change (e.g. [CO ₂ ] ! )

T !

Water vapour !

Greenhouse trapping of radiation !

Clausius-Clapeyron

(5)

(Peixoto and Oort, 1992)

A bs or pt io n [% ]

Wave length [µ]

CH

₄

N

₂

O O

₂

,O

₃

CO

₂

H

₂

O

shortwave longwave

Water vapour

feedback more than doubles the

sensitivity of surface temperature to

anthropogenic forcing

(e.g. Soden et al. Science 2005)

H

₂

O vapor is the most important greenhouse gas.

CO

₂

is the most important anthropogenic greenhouse gas.

Albedo 10

Albedo = Reflected portion of incoming shortwave radiation

Surface Conditions Albedo !

Clouds 100 m thick 0.4

500 m thick 0.7

Oceans, Lake Zenith angle 30° 0.05

60° 0.10

85° 0.6

Ice 0.25-0.35

Snow old-fresh 0.45-0.85

Grassland 0.2-0.3

Forest 0.1-0.2

Global mean 0.3

S S· !

Important radiation/temperature feedbacks linked with albedo:

• e.g. ice/snow - albedo feedback

(6)

11 Ice/snow - albedo feedback

• Strongest effect at limit of snow/ice cover

• Also relevant for global warming

• “Snowball Earth”:

Controversy

T #

Snow and ice extent ! ; Albedo ! Radiation

absorbed at surface #

Ice/snow - albedo feedback 12

IPCC A2 simulations, (2070-2100)-(1960-1990)

(M. Litschi, ETH)

(7)

Soil

moisture !

Evapo-

transpiration ! Precipitation !

Indirect effect

Soil moisture - precipitation feedback 14

Domain FR

(Schär et al. 1999, J. Climate)

(8)

15 Soil moisture - precipitation feedback

Domain FR

wet experiment:

- shallow boundary layer - enhanced concentration of fluxes of heat and moisture (shallow layer) $ enhanced convective instability

(Schär et al. 1999, J. Climate)

Soil moisture - precipitation feedback 16

Domain FR

wet experiment:

- shallow boundary layer - enhanced concentration of fluxes of heat and moisture (shallow layer) $ enhanced convective instability

- net radiative energy flux is larger in the wet

experiments!

(Schär et al. 1999, J. Climate)

(9)

Domain FR

(Schär et al. 1999, J. Climate)

wet experiment:

- shallow boundary layer - enhanced concentration of fluxes of heat and moisture (shallow layer) $ enhanced convective instability

- net radiative energy flux is larger in the wet

experiments!

Less energy input as net SW

Less energy loss as net LW

Altogether:

more energy available for SH and LH

Soil moisture - precipitation feedback 18

Soil

moisture !

Evapo-

transpiration ! Precipitation !

Vegetation control

Soil moisture memory

Negative ET feedback

(10)

19 Soil moisture - precipitation feedback

Soil

moisture !

Evapo-

transpiration ! Vegetation control

depth of reservoir depends on vegetation cover

Soil moisture - precipitation feedback 20

Soil

moisture !

Evapo-

transpiration ! Vegetation control

Importance of phenology

(11)

Soil

moisture !

Evapo-

transpiration !

Soil moisture memory

Soil moisture content

time

• Soil moisture content at any point in time is an integrated measure for precipitation and evapotranspiration of previous months: single precipitation event may not have a large impact

Soil moisture - precipitation feedback 22

Soil

moisture !

Evapo-

transpiration ! Precipitation !

"

If P ! feedback is not strong enough, ET ! will

ultimately lead to SM # : Possible negative

feedback from ET

(12)

23 Soil moisture - temperature feedback

T !! ET !! threshold

reached T !!!

(e.g. Seneviratne et al. 2006, Nature)

24 Standard deviation of the summer (JJA) 2-m temperature

SCEN CTL

CTL

_UNCOUPLED

SCEN

_UNCOUPLED

Seneviratne et al. 2006, Nature

Soil moisture - temperature feedback

Impact on summer temperature variability in Europe (present / future)

(13)

(“tundra - taiga effect”) (grasslands forests)

(Ruddiman 2000)

26 Vegetation feedbacks

“Tundra - Taiga” feedback

(maps from: http://www.radford.edu/~swoodwar/CLASSES/GEOG235/biomes/main.html )

Taiga Tundra

(Ruddiman 2000)

(14)

27 Vegetation feedbacks

(grasslands forests)

(Ruddiman 2000)

Vegetation - CO ₂ interactions 28

(Sellers et al. 1997)

Stomate density: 10‘000 - 100‘000 / cm

²

Exchanges of water and CO

₂

are coupled (plants lose water to

evapotranspiration as a byproduct of carbon assimilation)

(15)

(Sellers et al. 1997)

Stomate density: 10‘000 - 100‘000 / cm

²

Exchanges of water and CO

₂

are coupled (plants lose water to evapotranspiration as a byproduct of carbon assimilation)

Enhanced [CO

₂

] % enhanced water-use efficiency or enhanced assimilation?

Vegetation - CO ₂ interactions 30

(Sellers et al. 1997)

Stomate density: 10‘000 - 100‘000 / cm

²

Enhanced [CO

₂

] % enhanced water-use efficiency or enhanced assimilation?

More recent studies suggest:

only young plants, not mature trees

(e.g.

Korner et al. Science 2005)

Exchanges of water and CO

₂

are coupled (plants lose water to

evapotranspiration as a byproduct of carbon assimilation)

(16)

31 Vegetation - CO ₂ interactions

(Sellers et al. 1997)

Stomate density: 10‘000 - 100‘000 / cm

²

Some evidence for effect

^(e.g.

Leuzinger et al.

2005, Tree Physiology)

Enhanced [CO

₂

] % enhanced water-use efficiency or enhanced assimilation?

Exchanges of water and CO

₂

are coupled (plants lose water to evapotranspiration as a byproduct of carbon assimilation)

More recent studies suggest:

only young plants, not mature trees

(e.g.

Korner et al. Science 2005)

Vegetation - CO ₂ interactions 32

[CO

₂

] !

T !

ET ! SM #

"

e.g.: possible impact on soil moisture temperature feedback...

time

(17)

Soil

moisture !

Evapo-

transpiration ! Precipitation !

e.g.: possible impact on soil moisture precipitation feedback...

[CO

₂

] ! "

34 (Gedney et al. 2006, Nature)

Direct CO

₂

effect on runoff ?

Vegetation - CO ₂ interactions

(18)

35 (Ciais et al. 2005, Nature) NPP, 2003

Vegetation - CO ₂ interactions

Impact of hydrological cycle on carbon cycle...

Vegetation - CO ₂ interactions 36

[CO

₂

] !

T !

ET ! SM #

"

e.g.: possible impact on soil moisture - temperature feedback...

time

+

T++

(19)

• Feedbacks act to amplify or damp the original response to a forcing

• Very large number of interrelated feedbacks within the land-atmosphere coupled system

% high complexity

% high relevance in the context of climate change

Outline 38

• Overview feedback processes

• water vapour feedback

• ice/snow - albedo feedback

• soil moisture - precipitation feedback

• soil moisture - temperature feedback

• vegetation - climate feedback (tundra - taiga ; grasslands - forests; CO ₂ effects)

• How to investigate feedbacks and coupling between variables?

• sensitivity experiments

• targeted experiments (e.g. GLACE)

• measures of coupling

• Next lecture: Thresholds, memory effects

(20)

39 How to investigate feedbacks and coupling?

Feedback: V1 V2

Coupling: V1 V2

How to investigate feedbacks and coupling? 40

Feedback: V1 V2

Coupling: V1 V2

In a model framework, more straightforward to investigate coupling, i.e.

Coupling: V1’ V2’

Compare & V2 and & V1

(Sensitivity experiments)

(21)

(Shukla and Mintz, Science, 1982)

Precipitation

dry

wet 2-month experiment

Start: June 15, Maps for July

Wet: ET = ET

_pot

Dry: ET = 0 Soil moisture: Wet vs. Dry experiments

42 (Shukla and Mintz, Science, 1982)

Soil moisture: Wet vs. Dry experiments Precipitation

dry

wet 2-month experiment

Start: June 15, Maps for July

Wet: ET = ET

_pot

Dry: ET = 0

e.g.: Sensitivity to soil moisture anomalies

(22)

43 Soil moisture: Wet vs. Dry experiments Precipitation

dry wet

(Shukla and Mintz, Science, 1982)

Temperature

+ 20-30

^o

C

e.g.: Sensitivity to soil moisture anomalies

44 Soil moisture: Wet vs. Dry experiments Precipitation

dry wet

(Shukla and Mintz, Science, 1982)

Surface pressure

e.g.: Sensitivity to soil moisture anomalies

(23)

(Fischer et al.

2006, submitted)

• importance of spring soil moisture for summer 2003 heatwave

Quantitative approaches to estimate coupling 46

interactive land, interannually varying ocean climatological land, interannually varying ocean interactive land, climatological ocean

(Koster et al. 2000, JHM)

Variance of annual precipitation

climatological land, climatological ocean

(24)

47 Quantitative approaches to estimate coupling

interactive land, interannually varying ocean climatological land, interannually varying ocean interactive land, climatological ocean

(Koster et al. 2000, JHM)

Variance of annual precipitation

climatological land, climatological ocean Residual variance

(mostly chaotic behaviour of the atmosphere)

Variance due to interannually varying SST and soil

moisture: we might have a hold on these aspects

48 ALO: interactive land, interannually varying ocean AO: climatological land, interannually varying ocean AL: interactive land, climatological ocean

(Koster et al. 2000, JHM)

Variance of annual precipitation

A: climatological land, climatological ocean

SST(A, AL): no interannual variations

= SSTclim

SST(AO, ALO): prescribed, interannual variations = SSTvar

SM(A, AO): no interannual variations

= SMclim

SM(AL, ALO): interactive (different for each ensemble member), interannual variations: avg = SMvar

Feedback processes, threshold effects Part (a)

Winter term 2006/07

Feedback processes, threshold effects Part (a)

Sonia Seneviratne

Institute for Atmospheric and Climate Science ETH Zürich

sonia.seneviratne@env.ethz.ch

2

24.10.2006 SIS 1. Introduction

28.11.2006 EJ Discussion of Exercises (1)

05.12.2006 SIS 3. Feedback processes, threshold effects (a) 12.12.2006 Reserve date (AGU)

19.12.2006 SIS 3. Feedback processes, threshold effects (b) 09.01.2007 EJ Discussion of Exercises (2)

16.01.2007 CS 4. Modeling of the coupled land-atmosphere system (a) 23.01.2007 SIS 4. Modeling of the coupled land-atmosphere system (b) 30.01.2007 SIS 5. Outlook, current research questions

SIS = Sonia I. Seneviratne, CS = Christoph Schär, EJ = Eric Jäger

Schedule

3

Lecture’s web page

http://www.iac.ethz.ch/staff/sonia/lecture

– Lecture notes

– Exercises (1+2) and example solutions

Land-Atmosphere-Climate Interactions

Chapter 3a

Feedback processes, threshold effects

Sonia I. Seneviratne and Christoph Schär, Institute for Atmospheric und Climate Science, ETH Zürich

Winter term 2006/2007

• Overview feedback processes

• water vapour feedback

• ice/snow - albedo feedback

• soil moisture - precipitation feedback

• soil moisture - temperature feedback

• vegetation - climate feedback (tundra - taiga ; grasslands - forests; CO 2 effects)

• How to investigate feedbacks and coupling between variables?

• sensitivity experiments

• targeted experiments (e.g. GLACE)

• measures of coupling

• Next lecture: Thresholds, memory effects

Climate feedbacks and thresholds 6

• Feedbacks and thresholds are essential for the functioning of the climate system: They induce strongly non-linear effects (relevant for extreme events, scenario uncertainty for climate change)

• They contribute to a large extent to the

complexity of the climate system

7

Climate feedbacks

+ "

Positive feedbacks Negative feedbacks

Signal enhancement Signal damping

(Ruddiman 2000) (Ruddiman 2000)

Water vapour feedback 8

Initial change (e.g. [CO 2 ] ! )

T !

Water vapour !

Greenhouse trapping of radiation !

Clausius-Clapeyron

(Peixoto and Oort, 1992)

A bs or pt io n [% ]

Wave length [µ]

CH

N

O O

,O

CO

H

O

Water vapour

feedback more than doubles the

sensitivity of surface temperature to

anthropogenic forcing

(e.g. Soden et al. Science 2005)

H

O vapor is the most important greenhouse gas.

CO

is the most important anthropogenic greenhouse gas.

Albedo 10

Albedo = Reflected portion of incoming shortwave radiation

Surface Conditions Albedo !

Clouds 100 m thick 0.4

500 m thick 0.7

Oceans, Lake Zenith angle 30° 0.05

60° 0.10

85° 0.6

Ice 0.25-0.35

Snow old-fresh 0.45-0.85

• vegetation - climate feedback (tundra - taiga ; grasslands - forests; CO ₂ effects)

Initial change (e.g. [CO ₂ ] ! )