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P1.7 Measurements of the Radiative Surface Forcing of Climate P1.7 MEASUREMENTS OF THE RADIATIVE SURFACE FORCING OF CLIMATE W.F.J. Evans*, Northwest Research Associates, Bellevue, WA / Trent University, Peterborough, Ontario and E. Puckrin, Defense R&D Canada-Valcartier, Val-Belair, Quebec 1. INTRODUCTION The measurements have been obtained using commercial Fourier-transform One of the most pressing infrared (FTIR) spectrometers. These environmental issues encountered in this measurements have been used to quantify present century is that concerning the global the radiative flux associated with a number of warming of the earth’s surface. This change greenhouse gases. It is this radiative flux that may have a detrimental impact on society as provides an additional source of warming a whole, and particularly on human health, for the planet’s surface, and ultimately is ecology and, consequentially, on economical responsible for any change in climate. We viability. have provided the first direct measurements Over the past century the world has of the greenhouse effect for a number of been getting warmer; an average 0.7°C trace gases in the atmosphere. These gases increase has been observed in the global include trichlorofluoromethane (CFC-11), land-surface temperature records (IPCC, dichlorodifluoromethane (CFC-12), carbon 2001) and the past decade has been the tetrachloride (CCl4), methane (CH4), nitrous warmest yet since records were first kept. oxide (N2O), carbon monoxide (CO), nitric The potential impact that global warming has acid (HNO3), and tropospheric ozone (Evans on society is extensive, particularly through and Puckrin, 1994-1997; Puckrin et al., the associated change in climate that has 1996). Not only do these results prove that an been predicted by many sophisticated climate increase in the greenhouse effect is real, and models. The areas that may be affected the that trace gases in the atmosphere are most by global warming include: water and adding a significant radiative burden to the coastal resources, health, agriculture, energy budget of the atmosphere, but they fisheries, forests and energy (Canada’s also provide a means of validating the Second National Report on Climate Change, predictions that are made by global warming 1997). models (Ellingson et al., 1991). This last point In order to investigate this global is crucial since the temperature increases threat, an ongoing program of measurements predicted by the various climate models can of the downward atmospheric infrared vary by several degrees; even a change of radiation, otherwise known as the green- 0.7°C can have significant consequences on house radiation of the atmosphere, was different parts of the globe. The cause of the undertaken at Trent University in large uncertainty in the models resides in the Peterborough, Ontario (44oN, 78oW). difficulty of accurately predicting the climate . feedback mechanisms that are associated *Corresponding author address: W.F.J. Evans, with the interaction of oceans, vegetation, Northwest Research Associates, 14508 NE 20th Street, and clouds and water vapour with the Bellevue, WA 98007; email: [email protected] greenhouse effect. In order to evaluate the contributions of the various gases to global warming, the concept of radiative forcing, or absorption of the upward longwave radiation from the The data is based on models by Dickinson earth’s surface by the atmosphere, has been and Cicerone (1986) and those which are formulated (Ramanathan, 1987; IPCC, 1994). included in the IPCC report (IPCC, 1990). The radiative forcing at the tropopause is Although there are extensive model then used as an input to drive climate models calculations of radiative forcing in the for the purpose of evaluating the global literature, there are few experimental warming for various gases. Model measurements of the greenhouse radiation calculations of this radiative forcing have which is responsible for global warming. In been conducted by several authors (e.g., what follows, measurements of the surface Dickinson and Cicerone, 1986; Hauglustaine greenhouse radiation are described and et al., 1994). Earlier estimates of the compared with the modelled estimates of greenhouse effect were in terms of the radiative forcing. overall loss of radiation to space or of the downward radiation at the surface. It may be 2. METHODOLOGY noted that, when convection just above the ground is small, the earth's surface actually The measurements of the downward responds to the downward greenhouse atmospheric thermal emission were collected radiation from the atmosphere at the surface; using a Magna 550 FTIR spectrometer or a over land, the temperature responds to the high resolution Bomem DA8 system; the greenhouse radiation in a few hours. The instruments were capable of resolutions of -1 -1 greenhouse radiation is typically about 0.25 cm and 0.02 cm , respectively. Both 150 W/m2; the modelled increases in the instruments incorporated a liquid-nitrogen- greenhouse radiation since pre-industrial cooled, narrow-band, MCT detector with a 1 2 times are about 3 W/m2. The increase in mm element. The downward zenith sky downward surface greenhouse radiation for a radiation from the clear sky was collected by particular gas is quite similar to the positioning a gold-coated mirror at the absorption of upward longwave radiation by emission port along the optical axis of the the atmosphere (radiative forcing) as has instrument. A stored-phase correction was been demonstrated by Sinha and Toumi applied to the measured interferogram before (1996). An estimate of the radiative forcing conversion was made to the spectral domain (or net change in absorbed upward flux at the in order to account for phase changes that -1 tropopause) for several gases is shown in were present at 750 and 2000 cm . The Table 1. thermal emission background of the instrument was characterized by measuring a negligible source of thermal radiation which Table 1: Radiative Forcing at the Tropopause consisted of a blackened dewar containing liquid nitrogen. The background Greenhouse Gas Radiative Forcing (W/m2) measurement was taken immediately prior to H2O ∼90 and after the measurement of the sky CO2 ~50 radiation to ensure that the spectrometer was thermally stabilized. CH 1.7 4 The calibration of the atmospheric N2O 1.3 measurements was performed by placing an Tropospheric O3 1.3 ambient blackbody source beneath the gold CFC-12 0.16 mirror, filling the field-of-view of the spectrometer. The temperature of the CFC-11 0.062 blackbody was monitored by a chromel- Note: The modelled forcing for the first five gases is based on alumel thermocouple. The atmospheric the work of Dickinson and Cicerone (1986), and the forcing attributed to the CFCs is based on the IPCC report (1995). emission measurements required 15-30 2 minutes of observing time. This resulted in a In order to extract the greenhouse flux typical root-mean-square noise value of from individual gases, the background about 5.0×10-9 W/(cm2 sr cm-1) in the mid- emission of the atmosphere was simulated infrared region. using the radiative transfer code, FASCOD3 The greenhouse radiation from (Clough et al., 1988). The simulations tropospheric ozone was measured by a incorporated the temperature, relative technique in which the base of cold clouds humidity and pressure profiles from was used as a target. The thermal emission radiosonde measurements obtained at from the warm atmosphere below the cloud Maniwaki, Quebec, a location 280 km distant was measured against the low background from Peterborough. A constant mixing ratio emission from the cold cloud base (Puckrin et profile of 360 ppmv was used for carbon al., 1996). The cloud also screened out the dioxide (IPCC, 1995) and the concentrations emission from the stratospheric ozone above of other background gases were taken from it, effectively restricting the sampling area to the AFGL Atmospheric Constituent profiles the lower troposphere. (Anderson et al., 1986) and scaled to current tropospheric concentrations (IPCC, 1995). 3. RESULTS AND DISCUSSION The line transition parameters for the molecules were provided from the 1992 A typical winter spectrum of the AFGL HITRAN database (Rothman et al., downward radiance in the 5-16 µm 1992). The model utilized an aerosol profile wavelength range is shown in Figure 1, with that was representative of the visibility the emission from several greenhouse gases conditions as monitored by the local weather identified. The spectrum was measured at a -1 office. An example using this procedure is resolution of 0.25 cm . illustrated for CFC-12. HO HO HO 2 2 8e-6 4e-7 2 CO2 )) )) -1 -1 6e-6 3e-7 sr cm sr cm sr 2 2 A 4e-6 2e-7 CFC11 CO HNO 2 CFC12 3 O 2e-6 HNO 3 C B Radiance (W/(cm Radiance 3 CH 4 Radiance (W/(cm 1e-7 N2O CO 0 600 800 1000 1200 1400 1600 1800 2000 0e+0 -1 Wavenumber (cm ) 900 905 910 915 920 925 930 935 940 Wavenumber (cm-1) Figure 1. A spectrum of the greenhouse radiation Figure 2. The extraction of the thermal emission at the surface measured for February, 1996, band of CFC-12 from the measured atmospheric showing the contributions of several greenhouse emission spectrum (curve A). Curve B represents gases. the simulated background thermal emission in the 3 moderately polluted springtime air for an absence of CFC-12. The subtraction of curve B ozone concentration of 60 ppbv. A from curve A shows the thermal radiation comparison with simulated fluxes predicted associated with CFC-12 (curve C). Curve A in by the FASCOD3 atmospheric transmission Figure 2 shows the measured downward code (Clough et al., 1988) for the actual thermal emission for January, 1994. Besides atmospheric conditions and recent - the emission from CFC-12 in the 900-940 cm atmospheric gas concentrations (Kaye et al., 1 region, other gases including nitric acid, 1994; IPCC, 1995) is shown in the last water vapour and carbon dioxide have column of Table 2.
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