The Atmosphere: Part 8: Climate Change: Sensitivity and Feedbacks Composition / Structure Radiative transfer Vertical and latitudinal heat transport Atmospheric circulation Climate modeling Suggested further reading: Hartmann, Global Physical Climatology (Academic Press, 1994)

A simple 2-box model Two equal mass boxes, temperatures T 1 and T 2 Radiatively relaxed to temperatures T e1 and T e2 with time constant τ r Temperatures mixed with time constant τ d

A simple 2-box model Two equal mass boxes, temperatures T 1 and T 2 Radiatively relaxed to temperatures T e1 and T e2 with time constant τ r Temperatures mixed with time constant τ d radiation

A simple 2-box model Two equal mass boxes, temperatures T 1 and T 2 Radiatively relaxed to temperatures T e1 and T e2 with time constant τ r Temperatures mixed with time constant τ d dynamics

A simple 2-box model steady state solutions { where. Two equal mass boxes, temperatures T 1 and T 2 Radiatively relaxed to temperatures T e1 and T e2 with time constant τ r Temperatures mixed with time constant τ d

with polar ice Assumes ice in high latitude box (T 2 < 263K) Equilibria of 2-box model Fix T e, vary γ

with polar ice Assumes ice in high latitude box (T 2 < 263K) Equilibria of 2-box model

with polar ice Assumes high latitude box is ice-free (T 2 > 263K) Assumes ice in high latitude box (T 2 < 263K) ice-free

Equilibria of 2-box model with polar ice Assumes high latitude box is ice-free (T 2 > 263K) multiple equilibria Assumes ice in high latitude box (T 2 < 263K) ice-free Ice-albedo feedback ➙ multiple equilibrium states

Equilibria of 2-box model Fix γ, vary T e T1T1 T2T2 ice-free with polar ice

A 1-D model (Budyko/Sellers) (T, ice edge, vary with latitude)

Climate forcings and sensitivity Function of solar constant, albedo (ice cover, cloudiness, vegetation, …) Function of temperature, water vapor, clouds, CO 2 and other greenhouse gases … in equilibrium, net incoming flux is Climate forcing due to i th process (all else held fixed) = δQ (e.g., [CO 2 ] doubling to 600ppm ➙ δQ = 4 Wm -2 ) }

Climate forcings and sensitivity Function of solar constant, albedo (ice cover, cloudiness, vegetation, …) Function of temperature, water vapor, clouds, CO 2 and other greenhouse gases … in equilibrium, net incoming flux is Climate forcing due to i th process (all else held fixed) = δQ (e.g., [CO 2 ] doubling to 600ppm ➙ δQ = 4 Wm -2 ) } ➙ climate sensitivity feedbacks

Radiative response only (no feedbacks) Suppose T e = T s - constant ( T e = 255 K) ➙ (Need doubling of CO 2 or 1.6% increase of solar constant to produce 1K warming)

Water vapor feedback -4 Wm-2K -1 +2Wm -2 K -1 w.v. feedback

Ice albedo feedback (overestimate — have neglected clouds) -4 Wm -2 K Wm -2 K -1

Cloud feedback Current climate: cloud amount ~ 50% 10% increase in A c ➙ would offset CO 2 doubling

Past T trend simulated by 3D coupled atmosphere-ocean models

Predicted ΔT for doubled CO 2 from 3 different coupled 3D atmosphere-ocean models