Lecture 8 Climate Feedback Processes

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Lecture 8Climate Feedback Processes

• GEU 0136

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Forcing, Response, and Sensitivity

• Consider a climate forcing
• (e.g., a change in TOA net radiation balance, dQ)
• and a climate response
• (e.g., a resulting change in the globally
  averaged annual mean surface air temperature,
  dTs)
• We can define a climate sensitivity parameter

• To know (i.e., forecast) expected climate change
  resulted from a forcing of DQ, simply multiply by
  lR
• Then the central question of know how
• What determine the magnitude of lR?

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Response, Sensitivity, and Feedback

• Sensitivity parameter depends on direct and
  indirect effects of forcing
• Changes in TS will also affect
• Outgoing longwave (sTe4)
• Planetary albedo (ice, snow, clouds)
• Water vapor absorption
• Total sensitivity must take all these indirect
  effects into account
• Some will amplify sensitivity, and some will damp
  sensitivity

S0 solar constant yj yj(S0)
DS0
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3 Basic Radiative Feedback Processes
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Stefan-Boltzmann Feedback

• Simplest possible model of planetary radiative
  equilibrium
• Outgoing longwave radiation will increase to
  partly offset any increase in incoming radiation

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Water Vapor Feedback

• As surface warms, equilibrium vapor pressure will
  increase (Clausius-Clapeyron)
• Increasing q increases LWdown (higher e), so Ts
  warms even more
• Air is not always saturated, but we can assume
  relative humidity remains fixed as Ts increases,
  and calculate new Ts from radiative-convective
  equilibrium

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Water Vapor Feedback (contd)

• Water vapor is a positive feedback mechanism
• OLR is only linear wrt TS, not quartic as
  predicted by BB curves

lR)FRH 2 lR)BB
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Ice-Albedo Feedback

• Cold temperatures make the surface turn white due
  to increased sea ice and snow cover on land
• White (high-albedo) surfaces reflect more SWdown,
  decrease energy absorbed , leading to colder
  surface temperatures
• Warmer temperatures tend to reduce planetary
  albedo, allowing more energy to be absorbed
• Positive feedback tends to amplify changes in
  TS resulting from any forcing

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Ice-Albedo Feedback

• SH ice sheet at pole, sea-ice from 50º to 80º
• NH sea-ice at pole, seasonal snow from 40 º
  northward

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Ice Age Changes

• Ice age surface albedo was much higher than
  present!

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Budyko Ice-Albedo Climate Model

• Solar rad is distribted according to latitude
• Energy transport is diffusive
• OLR is linear with TS
• Albedo switches between two values, depending on
  ice or no ice

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Budyko Ice-Albedo Climate Solutions

• Stronger sun causes ice edge to retreat to higher
  lat, vice versa
• Below 97 of current value, model produces a
  white Earth!

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Budyko Feedback Sensitivities, 1

• Ratio of meridional energy transport to longwave
  cooling
• Budyko used 2.6 modern measurements suggest 1.7
• Less sensitive using recent data

d g/B
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Budyko Feedback Sensitivities, 2

• Ice-free albedo decreases toward the poles to
  account for cloud masking of surface
• Ice transition makes less difference

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Tropical SST and LW Feedback

• Tropical SSTs didnt vary much during ice ages
  why?
• Near 300 K, LW cooling decreases very fast with
  increasing SST
• Positive feedback should make tropical SSTs
  sensitive and variable
• but theyre not!

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Longwave and Evaporation Feedbacks

• Tropical SST energy balance
• SWdown LWup H LE
  DF
• (200 W m-2) - (60 W m-2)
  (10 W m-2 ) (120 W m-2) (20 W m-2)

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Compensating Tropical SST Feedbacks

• Changes in LE with SST balance positive feedback
  with respect to longwave down

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Biophysical Feedback Daisyworld

• Consider a planet populated by two kinds of
  plants white daisies and black daisies.
• Write an energy balance for the planet, assuming
• (1) it emits as a blackbody
• (2) the albedo is an area-weighted average of the
  albedos of bare ground, white, and black daisies
• The daisies grow at temperature-dependent rates
  (optimum at 22.5º C, zero at 5 º and 40º), and
  also proportional to the fraction of bare ground
• The daisies also die at a specified rate c
• Solve for areas Ai and temperatures Ti of each
  surface (white daisies, black daisies, and bare
  ground)

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Daisyworld
h 0 transport is perfect
More generally,
h (S0/4) transport is zero
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Biophysical Feedback Daisyworld