1. CO2 forcing induces semi-direct effects
Tim Andrews
School of Earth and Environment
Supervised by Piers Forster

2. Outline Introduction to radiative forcing concept Methodology
Direct forcing
Climate forcing
CO2 induced semi-direct effects
Geographical distributions
Summary
Semi-direct forcing

3. Introduction
A radiative forcing (F) is an externally imposed change in the radiative energy budget of the Earth’s climate system
The climate feedback parameter (Y) is an indicator of how strongly the climate responds to a given forcing
From a simple energy budget model we can relate the two through a linear approximation:
N = F + YT
where N = TOA radiative flux and ΔT = change in global-mean surface air temperature

4. Introduction At equilibrium (N = 0): F = -YTeqm
where ΔTeqm is the eqm surface temperature change
Knowing F and Y therefore allows a prediction of ΔTeqm
Want a forcing definition that has the same Y for all forcing agents

5. Radiative forcing definitions
(IPCC, 2007)
‘Direct forcing’
‘Climate forcing’
Each of the definitions has its own utility
(2) – (1) = forcing components of a stratospheric adjustment
(3) – (2) = forcing components of a tropospheric adjustment

6. Direct forcing of CO2
Stratospheric adjusted 2 x CO2 RF for various slab ocean GCMs
Dominated by FLN
Significant contribution from FLC; clouds lower the emitting temperature of the atmosphere, reducing the CO2 forcing

7. Climate forcing: Linear regression
Gregory et al. (2004) and Gregory and Webb (2007) showed that F,Y and T2x can be diagnosed by plotting N(t) as a function of T(t):
N = F + YT

8. Climate forcing: Linear regression
Gregory et al. (2004) and Gregory and Webb (2007) showed that F,Y and T2x can be diagnosed by plotting N(t) as a function of T(t): F
N = F + YT
Hence:
F = lim T->0 N
Y = gradient
T2x = N-intercept Y
T2x

9. Component form Ni = Fi + YiT Clear-sky shortwave Net
Cloud short / longwave
Clear-sky longwave
Ni = Fi + YiT

10. Climate forcing
Climate forcing allows for both stratospheric and any tropospheric adjustment (on the timescale of a few months or less)
Differencing the climate and direct forcing leaves the components of a semi-direct forcing, the result of rapid changes in tropospheric properties

11. Semi-direct effects Components of a semi-direct forcing:
CO2 alters the radiative heating of the troposphere, causing local changes to the vertical temperature profile with consequences for atmospheric properties – CO2 induced semi-direct effect
Each component can be associated with a different process

12. Semi-direct effects Cloud semi-direct effect induced by CO2:
Sign and anti-correlation of FLC and FSC suggests a reduction in cloud cover
FC is generally positive (a warming influence), maybe because the effect occurs more predominately in low level clouds

13. Semi-direct effects Other semi-direct effects induced by CO2:
FLN suggests response in atmospheric water-vapour and/or lapse rate
Contribution from FSN suggests rapid response in sea-ice fraction, both extension and retraction

14. Geographical distribution
Cloud semi-direct forcing components:
Excellent anti-correlation (particularly in tropics)
LW Cloud
SW Cloud
Large values in tropics, high convective clouds
Can take large values in mid-latitudes, low clouds

15. Geographical distribution
Clear-sky LW and SW semi-direct forcing components:
LW Clear
SW Clear
Generally negative, except in polar regions
Dominated by regions of sea-ice and snow cover

16. Summary & discussion
CO2 may induce semi-direct effects, analogous to dark aerosols
Forcing definitions should include any such effects by accounting for tropospheric adjustment
Clearly complicates the distinction between forcings and feedbacks
Time-dependent calculations show that the cloud feedback is weak (defined as its dependence on T) and that most of its radiative impact on climate should be considered as part of the forcing (the semi-direct effect)
Spread in T2x is usually attributed to a spread in Y, but should some be attributable to a spread in F….?