1. Radiative forcing - measured at Earth‘s surface - corroborate the increasing greenhouse effect R.Philpona 1, B.Dürr 1, C.May 1, A.Ohmura 2 and M.Wild 2 1 Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Davos Doerf, Switzerland 2 Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology (ETH), Zürich, Switzerland Geophysical Research Letters, Vol.31, L03202, 2004

2. Radiative Budget of the Earth atmosphere Detailed overview of the earth atmosphere energy balance. Left hand side the incoming solar radiation, right hand side the outgoing infrared radiation.

3. Present (2002) Radiative Forcing

4. Radiative Balance At the top of the atmosphere, the expenditure all incident solar radiation must be accounted for the Earth as a whole, that is, the net incoming solar radiation must be balanced by the outgoing longwave radiation.

5. IPCC2001 - Definition of (RT) Radiative Forcing RT is the change in the net vertical irradiance (Wm -2 ) at the tropopause due to an internal change or a change in the external forcing of the climate system, such as, for example, a change in the concentration of carbon dioxide or water aerosols the output of the Sun. Usually RT is computed after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with all tropospheric properties held fixed at their unperturbed values. Forcing  |  F T s | - |  F T T | surface tropopause FssFss FsTFsT FTTFTT FTsFTs FTsFTs FSTFST FSLFSL The IPCC used the following definition, focusing on topopause level conditions:

6. The Alpine Surface Radiation Budget (ASRB) Network Measurements: - Temperature - Humidity - Longwave radiation(LW)  - Shortwave radiation(SW) 

7. Longterm measurements From 1980-2002:  T= +1.32(0.5)°C  H= + 0.51(0.2)gm -3 From 1995-2002:  T= +0.82(0.4)°C  H= +0.21(0.1)gm -3 1995-2002: LW downward radiation (LDR) 5 to 8 Wm -2 : +5.2(2.2) Wm -2 SW downward radiation (SDR) -0.5 to -6 Wm -2 : -2.0(3.7) Wm -2 Temperature abs. Humidity T[°C] H[ gm -3 ]

8. Corrections Theory: LDR increase with increasing greenhouse gases Problem: LDR increase also with increasing cloud amount, temperature and water vapour (humidity).  Corrections required to account for changing a.) cloud cover b.) temperature c.) humidity

9. Correction a.) Cloud Cover We define a CSI (Cloud Sky Index) index: Atmosphere as a grey body: emittance  A = LW  /  T a 4 Clear-sky emittance: - Brutsaert-formula:  AC =k(e a /T a ) 1/7 - Modified (altitude dependence):  AC (H)=  AD +k(e a /T a ) 1/8 withe a : water vapore pressure [Pa] T a : air temperature [K] k : location dependent coefficient from selected clear-sky cases  AD : altidude dependent clear-sky emittance for a dry atmosphere  CSI =  A /  AC (H)

10. Correction a.) Cloud Cover CSI =  A /  AC (H)CSI  1 : clear-sky, no clouds CSI > 1 : cloudy-sky, overcast  LDR clfr =LDR-LCELDR = +5.2(2.2) Wm -2 LCE = +1.0(2.8) Wm -2 (due to changing cloud cover)  LDR clfr = +4.2(1.9) Wm -2 (Total LW  change) SDR clfr = -1.0(3.7) Wm -2 (blocking of sunlight) Total effect: = 5.2(1.9) Wm -2

11. Correction b.) (  T) and c.) (  H) Realtive correction for  T and  H is obtained from GCM modelling based ECHAM-4 GCM: Measurements: 3.3% CO 2 increase (Central Europe)  1/3 of LDR increase (5.2 W/m 2 ) is due to greenhouse gases, i.e. 5.2 W/m 2 /3 = ~1.73 W/m 2 2/3 of LDR increase is due to temperature and humidity variation, i.e. i.e. 5.2 W/m 2  2/3 = ~3.46 W/m 2

12. Temperature Corretion b.) Temperature changes due to external warm air advection must be substracted from LDR cf. Correction : LDR clfr,tc = LDR clfr -  LDR t With temperature driven change in LDR:  LDR = 4  T a 3  t a  AC  Stefan Boltzmann constant T a: average temperature at the station  t a :temperature trend (  2/3) => LDR clfr,tc = 2.1 to 2.9 Wm -2 : mean: +2.4 (0.9)Wm -2 Temperature

13. Humidity Correction c.) MODTRAN simulation of LW  : +0.56 Wm -2 (500m) (water vapor 0.1 gm -3 increase): +1.7Wm -2 (3000m) Only 1/3 of measured vater vapor increase is due to green- house gases: modelled LW  increase: + 0.44 Wm -2  MODTRAN simulation: +1.58 Wm -2 LW  on average over 8 years due to greenhouse gases  Correction of LDR cf,tc with 2/3 of LW  increase due to humidity increase  remaining increase of LW  : +1.8 (0.8) Wm -2

14. Summary [Wm -2 ] LDR measurement - = Longwave Cloud Effect LDR cf (cloud free) LDR cf,tc (temp.correction) LDR cf,tc,uc (hum.corr.) +5.2(2.2)+1.0(2.8)+4.2(1.9) +2.4 (0.9) +1.8(0.8)

15. Conclusions Longwave flux increase +5.2(2.2) Wm -2 measured over 8 years. 1/3 of LDR increase is due to greenhouse gases After correction (clouds, temperature, humidity) cloud-free longwave flux increase +1.8 (0.8) Wm -2 due to greenhouse gases MODTRAN simulation predicts +1.58 Wm -2  direct observation of LW radiative forcing due to greenhouse gases