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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12.340 Global Warming ScienceSpring 2012 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.