Evidence for Milankovitch theory

(wikipedia!)

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

Climate sensitivity

Figure TS.5

Anthropogenic Radiative ForcingsIPCC AR4 2007

Aerosol direct forcingNature 438, 1138 (Dec 2005)

-22 Wm−

Aerosol indirect forcing effect

-21.5 Wm−

Figure TS.5

Radiative ForcingsIPCC AR4 2007

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)

IPCC 2007

(b) Sulphate aerosols deposited in Greenland ice

• Industrial sulphate aerosols appear to have peaked for Greenland record.

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)

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

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)

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->

Cloud Radiation Feedback

Potentially quite largeAltitude dependentNet highly uncertain

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

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

Arctic sea ice decrease

(ca 2002)

Changes in atmospheric composition

1990-2000

IPCC fig 3.4