Evidence for Milankovitch theory (wikipedia!). Px272 Lect 3: Forcing and feedback Balance of solar...
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Evidence for Milankovitch theory
(wikipedia!)
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Px272 Lect 3: Forcing and feedbackBalance of solar incoming, and earth emitted outgoing radiation
Increments relative to pre-industrial era
Assumption that solar incoming ~ constant. Change is made in surface temperature as a consequence of forcing. i.e.
For a given radiative forcing we get a temperature change given by
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Climate sensitivity
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Figure TS.5
Anthropogenic Radiative ForcingsIPCC AR4 2007
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Aerosol direct forcingNature 438, 1138 (Dec 2005)
-22 Wm−
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Aerosol indirect forcing effect
-21.5 Wm−
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Figure TS.5
Radiative ForcingsIPCC AR4 2007
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G/house gas and aerosol buildup
•Rise of global Greenhouse gases with industrial era(IPCC S/PM 2001)
•CO2 emissions continue to accelerate (IPCC 2007)
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IPCC 2007
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(b) Sulphate aerosols deposited in Greenland ice
• Industrial sulphate aerosols appear to have peaked for Greenland record.
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Global warming potentials (GWP):
The ratio of the radiative forcing produced over a timescale of 100 years for 1kg of greenhouse gas, relative to 1kg of carbon dioxide.
Greenhouse gas GWP
Carbon dioxide (CO2) 1
Methane (CH4) 25
Nitrous Oxide (N20) 298
Hydrofluorocarbons (HFCs)
12-12000
Perfluorocarbons (PFCs)
5000-12000
Sulphur hexafluoride (SF6)
22200
Houghton (2009)
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Response and FeedbackAdd in some feedback
Feedback can either amplify, or damp the impact of changes in forcingPositive feedback has positive f, negative feedback has negative f.
Stability limit f<1 (some would claim this is exceeded).
f defined as
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Response and Feedback
Four key climate feedbacks:
- water vapour 0.5
- clouds (probably negative)
- ocean circulation (mainly retardation + redistribution)
- ice albedo (clea
λ ≈
rly positive)
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3
More direct approximation:
1. about half the emission
spectrum has temp. pinned
by water vapour saturation.
( IR emission) 12. 4
2 eS
TT
δ σδ
≈ ×
Water vapour feedback
2
Textbook version:
1. without this (and other)feedback would respond fully to
2. warmer surface leads to more (and hence higher) water vapour in atmosphere
3. H O powerful greenhouse gas so signif
e ST Tδ δ
icant positive feedback
0.5λ ≈
Water saturation(mid troposphere)<-Simulation Observed->
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Cloud Radiation Feedback
Potentially quite largeAltitude dependentNet highly uncertain
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Ocean Circulation Feedback
Huge heat capacity
- delays warming
Circulation driven by:temperature differencesalinity differencerotation of earth
- redistributes regionally:more winter heat to NW Europe from ocean transport than from sun
- global heat redistribution still dominated by atmosphere
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Ice Albedo Feedback
Ice highly reflective:
warming loss decrease of albedo increased forcing
positive feedback,
Distinguish:
loss of floating ocean ice:
thin, losing rapidly (IPCC 2007?), inc
λ→ → →
≈0.1
luded above. much slower loss of grounded ice caps: very thick, centuries+ to melt, major eventual contributors to sea level rise
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Arctic sea ice decrease
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(ca 2002)
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Changes in atmospheric composition
1990-2000
IPCC fig 3.4