Radiative forcing - measured at Earth‘s surface - corroborate the increasing

greenhouse effect

R.Philpona1, B.Dürr 1, C.May1, A.Ohmura2 and M.Wild2

1 Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Davos Doerf, Switzerland2 Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology (ETH), Zürich, Switzerland

Geophysical Research Letters, Vol.31, L03202, 2004

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.

Present (2002) Radiative Forcing

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.

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 |FTs| - |FT

T|

surface

tropopause

Fss Fs

T

FTTFT

s

FTs

FST FS

L

•The IPCC used the following definition, focusing on topopause level conditions:

The Alpine Surface Radiation Budget (ASRB) Network

Measurements:- Temperature- Humidity- Longwave radiation(LW) - Shortwave radiation(SW)

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[g

m-3

]

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 changinga.) cloud coverb.) temperaturec.) humidity

Correction a.) Cloud CoverWe define a CSI (Cloud Sky Index) index:

Atmosphere as a grey body: emittance A = LW / Ta4

Clear-sky emittance:- Brutsaert-formula: AC=k(ea/Ta)1/7 - Modified (altitude dependence): AC(H)=AD+k(ea/Ta)1/8

with ea : water vapore pressure [Pa]Ta : 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)

Correction a.) Cloud Cover

CSI = A/AC(H) CSI 1 : clear-sky, no cloudsCSI > 1 : cloudy-sky, overcast

LDRclfr=LDR-LCE LDR = +5.2(2.2) Wm-2

LCE = +1.0(2.8) Wm-2 (due to changing cloud cover)

LDRclfr = +4.2(1.9) Wm-2 (Total LW change)SDRclfr = -1.0(3.7) Wm-2 (blocking of sunlight)

Total effect: = 5.2(1.9) Wm-2

Correction b.) (T) and c.) (H)Realtive correction for T and H is obtained from GCM modelling

based ECHAM-4 GCM:

Measurements: 3.3% CO2 increase (Central Europe)

1/3 of LDR increase (5.2 W/m2) is due to greenhouse gases, i.e. 5.2 W/m2/3 = ~1.73 W/m2

2/3 of LDR increase is due to temperature and humidityvariation, i.e. i.e. 5.2 W/m22/3 = ~3.46 W/m2

Temperature Corretion b.)

Temperature changes due to external warm air advection must be substracted from LDRcf.

Correction : LDRclfr,tc = LDRclfr -LDRt

With temperature driven change in LDR:

LDR = 4Ta3taAC Stefan Boltzmann constant

Ta: average temperature at the

station

ta:temperature trend (2/3)

=> LDR clfr,tc = 2.1 to 2.9 Wm-2 : mean: +2.4 (0.9)Wm-2T

emp

erat

ure

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

Summary [Wm-2]LDR measurement

- =

Longwave Cloud Effect LDRcf (cloud free)

LDRcf (cloud free) LDRcf,tc (temp.correction) LDRcf,tc,uc(hum.corr.)

+5.2(2.2) +1.0(2.8) +4.2(1.9)

+4.2(1.9) +2.4 (0.9) +1.8(0.8)

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