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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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….?