Climate Sensitivity, Forcings, And Feedbacks

1

Forcings and Feedbacks in the Climate System

Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of theIntergovernmental Panel on Climate Change, FAQ 1.2, Figure 1. Cambridge University Press. Used with permission. 2

Forcings and Feedbacks

Consider the total flux of radiation through the top of the atmosphere:

TOA solar IRF F F

Each term on the right can be regarded as function of the surface temperature, Ts, and many other variables xi :

1 2, , ,.....TOA TOA s NF F T x x xBy chain rule,

1

0N

TOA TOATOA s i

is i

F FF T x

T x

3

Now let’s call the Nth process a “forcing”, Q:

1

1

1

1

0N

TOA TOATOA s i

is i

NTOA TOA i

s s

is i s

F FF T x Q

T x

F F xT T Q

T x T

Then

1

1

1sR N

TOA TOA i

is i s

T

F F xQ

T x T

4

1

1

1

sR N

TOA i

i i s

T S

F xQS

x T

1

TOA

s

FLet S

T

Climate sensitivity

Climate sensitivity without feedbacks

Feedback factors; can be of either sign

Note that feedback factors do NOT add linearly in their collective effects on climate sensitivity

5

Examples of Forcing:

• Changing solar constant

• Orbital forcing

• Changing concentrations of non-interactive greenhouse gases

• Volcanic aerosols

• Manmade aerosols

• Land use changes

6

Solar Sunspot Cycle

Image courtesy of NASA.7

8

1600 1700 1800

Year

1900 2000

200

150

100

50

0

Sun

spot

num

ber

Image by MIT OpenCourseWare.

99

1850

1362

1364

1366

1368

1370

0.4

0.2

0.0

-0.2

-0.4

1900 1950 2000

Two reconstructions of total solar irradiance combined with measurements, where available(enclosing the greenshading) and two climate records (enclosing the orange shading) spanning roughly 150 years.

Year

Tem

pera

ture

ano

mal

y (o

C)

Sre

c W

m-2

Reconstructed irradiance Temperature records

(

(

Image by MIT OpenCourseWare.

Satellite measurements of solar flux

Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report ofthe Intergovernmental Panel on Climate Change, Figure 2.16. Cambridge University Press. Used with permission.

10

X-Ray Flux

This image has been removed due to copyright restrictions. Please see theimage on page http://sidstation.loudet.org/03-solar-activity/data/flux.png.

11

Normal solar cycle variations in solar radiation

This image has been removed due to copyright restrictions. Please see Foukal, et al. "Variations in

Solar Luminosity and their Effect on the Earth's Climate". Nature 443 (2006): 161-6. The image is

also on page http://blogs.edf.org/climate411/wp-content/files/2007/05/solar_energy.jpg.

12

Inferences based on Models of

Solar Variability

Climate Change 2007: The Physical Science Basis. Working Group I Contribution tothe Fourth Assessment Report of the Intergovernmental Panel on Climate Change,Figure 2.17. Cambridge University Press. Used with permission.

13

Climate Forcing by

Orbital Variations

Milutin Milanković, 1879-1958

This image has been removed due to copyright restrictions.Please see the photo on page http://www.detectingdesign.com/milankovitch.html.

14

Obliquity Cycle (41 k.y.)

Normal to ecliptic

Precession of the Equinoxes (19 and 23 k.y.)

Northern Hemisphere tilted away from the sun at aphelion.

Northern Hemisphere tilted toward the sun at aphelion.

Eccentricity Cycle (100 k.y.)

Image by MIT OpenCourseWare.

Climate Forcing and Response

Image courtesy of Global Warming Art.

15

Strong Correlation between High Latitude Summer Insolation and Ice Volume

This image has been removed due to copyright restrictions. Please see Figure 2E on pagehttp://www.people.fas.harvard.edu/~phuybers/Doc/integrated_science2006.pdf.

16

17

This image has been removed due to copyright restrictions. Please see the imageon page http://en.wikipedia.org/wiki/File:Carbon_History_and_Flux_Rev.png.

Variation in carbon dioxide and methane over the past 20,000 years, based on ice core and other records

Climate Change 2007: The Physical Science Basis. Working Group I Contributionto the Fourth Assessment Report of the Intergovernmental Panel on ClimateChange, Figure TS.2. Cambridge University Press. Used with permission.

18

CO2 and Climate

Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Reportof the Intergovernmental Panel on Climate Change, Figure 6.1. Cambridge University Press. Used with permission. 19

Recent History of Volcanic Eruptions

Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report ofthe Intergovernmental Panel on Climate Change, Figure 2.18. Cambridge University Press. Used with permission.

20

Image courtesy of US government. 21

Variation with Time of Natural Climate Forcings:

Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report ofthe Intergovernmental Panel on Climate Change, Figure 6.13. Cambridge University Press. Used with permission.

2222

Examples of Forcing Magnitudes:

• A 1.6% change in the solar constant, equivalent to 4 Wm-2, would produce about 1oC change in surface temperature

• Doubling CO2, equivalent to 4 Wm-2, would produce about 1oC change in surface temperature

23

Contributions to net radiative forcing change, 1750-2004:

Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report ofthe Intergovernmental Panel on Climate Change, Figure 2.20. Cambridge University Press. Used with permission.

24

24

Examples of Feedbacks:

• Water vapor

• Ice-albedo

• Clouds

• Surface evaporation

• Biogeochemical feedbacks

25

Estimates of Climate Sensitivity

1

1

1

sR N

TOA i

i i s

T S

F xQS

x T

1

TOA

s

FS

T

Suppose that Ts = Te + constant and that shortwave radiation is insensitive to Ts:

4 4 3 2 1, 4 3.8TOATOA e e e

s s

FF T T T Wm K

T T

1

20.26S K Wm

26

Examples of feedback magnitudes:

• Experiments with one-dimensional radiative-convective models suggest that holding the relative humidity fixed,

2 12 ,

0.5

TOA

s RH

TOA

s RH

F qWm K

q T

F qS

q T

This, by itself, doubles climate sensitivity; with other positive feedbacks, effect on sensitivity is even larger

27

Ice-Albedo Feedback

28

LATITUDE

Annual range of zonal monthly surface albedo estimates by 2o latitudinal belts.

ALB

EDO

RAN

GE

(%)

80

20

40

60

0

60 40 20 0

ANNUAL RANGE OF ALBEDO

Northern Hemisphere Southern Hemisphere

Image by MIT OpenCourseWare.

29

Ice

line

latit

ude

Solar flux (x present)

Eq

0.1 1 10 100 1000

0.9 1.0 1.1 1.2 1.3

30o

60o

90o

All Ice

No Ice

Neo

prot

eroz

oic

S=

0.93

Log pCO2 (x present)

Budyko-Sellers typeenergy-balance model

(1969)

Present

Image by MIT OpenCourseWare.

Feedbacks in Climate Models

Water vapor

Cloud Surface albedo

Lapse rate Water vapor + lapse rate

Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report ofthe Intergovernmental Panel on Climate Change, Figure 8.14. Cambridge University Press. Used with permission.

3030

From Dufresne and Bony, J. Climate, 2008

Equilibrium temperature change associated with the Planck response and the various feedbacks, computed for 12 CMIP3/AR4 AOGCMs for a 2 × CO2 forcing of reference (3.71 W m−2). The GCMs are sorted according to ΔTe

s.

This image (published on Journal of Climate by the American MeteorologicalSociety) is copyright © AMS and used with permission.

31

Changes in global mean cloud radiative forcing (W m–2) from individual models

32

2.0

1.0

0.0

-2.0

-3.0

21 3 11 12 10 16 20 15 147 9 8 6 5 4 23 22 19 17 18

-1.0

W m

-2

Model ID number

Cloud Radiative Forcing

Image by MIT OpenCourseWare.

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