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FIG. 2. (a) Calculated surface temperatures and (b) planetary albedos as a function of pCO2 and fCHy4. The assumed so- lar luminosity is 80% of the present value (S/So = 0.80). The greenhouse gases included in the calculation are CO2, CH4 and H20. Having corrected the problem with CH, absorption, we used the new model to repeat the calculations shown in Fig. 5 of Pavlov et al. (2000). (The old calculations are not shown here.) These simulations were performed for a time of 2.8 Ga, when solar luminosity was ~80% of the present value. The solid curves in Fig. 2 show predicted mean global sur- face temperature and planetary albedo as a function of CO partial pressure (pCO2) and CH, volume mixing ratio (fCH4). Comparison with the old calculations shows that much of the greenhouse warming by CHy has been lost. In particu- lar, there is only a small, roughly triangular region in which The proposed CO and CH, concentrations presented here are able to keep early Earth above freezing, but given the constraints discussed above, the modeled climate is still not consistent with the geological evidence. If the Late Archean climate was nonglacial, as seems likely, then mean annual surface temperatures must have been as warm as today (288 Kk), or warmer (Kiehl and Dickinson, 1987; Pavlov et al., 2000), which suggests that still more greenhouse gases were pres- ent in the atmosphere. Indeed, Knauth and Lowe (2003) pro- posed that the Archean climate was hot (60-70°C) through-

Figure 2 (a) Calculated surface temperatures and (b) planetary albedos as a function of pCO2 and fCHy4. The assumed so- lar luminosity is 80% of the present value (S/So = 0.80). The greenhouse gases included in the calculation are CO2, CH4 and H20. Having corrected the problem with CH, absorption, we used the new model to repeat the calculations shown in Fig. 5 of Pavlov et al. (2000). (The old calculations are not shown here.) These simulations were performed for a time of 2.8 Ga, when solar luminosity was ~80% of the present value. The solid curves in Fig. 2 show predicted mean global sur- face temperature and planetary albedo as a function of CO partial pressure (pCO2) and CH, volume mixing ratio (fCH4). Comparison with the old calculations shows that much of the greenhouse warming by CHy has been lost. In particu- lar, there is only a small, roughly triangular region in which The proposed CO and CH, concentrations presented here are able to keep early Earth above freezing, but given the constraints discussed above, the modeled climate is still not consistent with the geological evidence. If the Late Archean climate was nonglacial, as seems likely, then mean annual surface temperatures must have been as warm as today (288 Kk), or warmer (Kiehl and Dickinson, 1987; Pavlov et al., 2000), which suggests that still more greenhouse gases were pres- ent in the atmosphere. Indeed, Knauth and Lowe (2003) pro- posed that the Archean climate was hot (60-70°C) through-

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FIG. 1. Radiative flux comparisons for (a) surface downwelling and (b) top-of-atmosphere upwelling at constant CHyg (1 ppmv), and (c) surface downwelling and (d) top-of-atmosphere upwelling at constant CO» (360 ppmv). All calculations a: sume a midlatitude summer (MLS) atmospheric profile. In testing our climate model and finding the mistake men- tioned above, we compared our model with the radiative
FIG. 1. Radiative flux comparisons for (a) surface downwelling and (b) top-of-atmosphere upwelling at constant CHyg (1 ppmv), and (c) surface downwelling and (d) top-of-atmosphere upwelling at constant CO» (360 ppmv). All calculations a: sume a midlatitude summer (MLS) atmospheric profile. In testing our climate model and finding the mistake men- tioned above, we compared our model with the radiative
TABLE 1. CyH6 MIxING Ratios PREDICTED BY THE PHOTOCHEMICAL MODEL AT SPECIFIED CO) AND CHy LEVELS All values are ppmv.
TABLE 1. CyH6 MIxING Ratios PREDICTED BY THE PHOTOCHEMICAL MODEL AT SPECIFIED CO) AND CHy LEVELS All values are ppmv.
FIG. 3. Calculated altitude profiles for CHa, CoH., and other hydrocarbons for a 1-bar atmosphere with f{CO2 = 1000 ppmv and fCH, = 1000 ppmv.
FIG. 3. Calculated altitude profiles for CHa, CoH., and other hydrocarbons for a 1-bar atmosphere with f{CO2 = 1000 ppmv and fCH, = 1000 ppmv.
TABLE 2. GAUSSIAN WEIGHTS AND ABSORPTION CORRELATED K-COEFFICIENTS FOR C2H¢6 IN THE LONGWAVE
TABLE 2. GAUSSIAN WEIGHTS AND ABSORPTION CORRELATED K-COEFFICIENTS FOR C2H¢6 IN THE LONGWAVE
FIG. 4. Absorption cross sections for CyH¢ in the atmo- spheric window region. Dashed lines indicate the boundaries of the wavelength bins in our climate model.
FIG. 4. Absorption cross sections for CyH¢ in the atmo- spheric window region. Dashed lines indicate the boundaries of the wavelength bins in our climate model.
FIG. 5. Transmission as a function of path length for CoH. The solid curve is the calculated transmission, and the dashed curve shows the correlated-k approximation.
FIG. 5. Transmission as a function of path length for CoH. The solid curve is the calculated transmission, and the dashed curve shows the correlated-k approximation.
FIG. 6. Calculated hydrocarbon concentrations as a func- tion of fCH, for an atmosphere containing 1000 ppmv COp.
FIG. 6. Calculated hydrocarbon concentrations as a func- tion of fCH, for an atmosphere containing 1000 ppmv COp.
FIG. 7. Calculated C>H¢ concentration (dashed curve) and AT, (solid curve) as a function of CH4/CO> ratio at fCO2 = 300 ppmv. AT, represents the difference in calculated sur- face temperature resulting from the inclusion of CoH6.
FIG. 7. Calculated C>H¢ concentration (dashed curve) and AT, (solid curve) as a function of CH4/CO> ratio at fCO2 = 300 ppmv. AT, represents the difference in calculated sur- face temperature resulting from the inclusion of CoH6.
FIG. 8. Surface temperature as a function of pCO, and fCHy, with C2H¢ included. Other parameters are the same as in Fig. 2. The mixing ratio of CpH, is a function of both pCO and fCH4, and was calculated with our photochemical model. Numerical values for fC2H¢ are given in Table 1.
FIG. 8. Surface temperature as a function of pCO, and fCHy, with C2H¢ included. Other parameters are the same as in Fig. 2. The mixing ratio of CpH, is a function of both pCO and fCH4, and was calculated with our photochemical model. Numerical values for fC2H¢ are given in Table 1.
FIG. 9. Surface temperature and optical depth with in- creasing CHy at a CO mixing ratio of 50,000 ppmv. The solid curve includes the effects of organic haze; the dashed curve is for a haze-free atmosphere. The dash-dot curve shows the extinction optical depth of the haze at 500 nm (mid-visible).
FIG. 9. Surface temperature and optical depth with in- creasing CHy at a CO mixing ratio of 50,000 ppmv. The solid curve includes the effects of organic haze; the dashed curve is for a haze-free atmosphere. The dash-dot curve shows the extinction optical depth of the haze at 500 nm (mid-visible).
FIG. 10. Surface temperature with the effects of organic haze included. Other parameters are the same as in Fig. 8. The arrow indicates 100 PAL CO:, the hard limit based on paleosol interpretation. The expected high production of CH, during the Late Archean could have had a significant influence on climate. As illustrated in Fig. 9, once CH, became an important con- tributor to the greenhouse effect, the Archean climate should have become highly sensitive to the CH4/CO; ratio. If this ratio had been too high, then the haze would have been thick, which would have potentially led to a globally glaciated
FIG. 10. Surface temperature with the effects of organic haze included. Other parameters are the same as in Fig. 8. The arrow indicates 100 PAL CO:, the hard limit based on paleosol interpretation. The expected high production of CH, during the Late Archean could have had a significant influence on climate. As illustrated in Fig. 9, once CH, became an important con- tributor to the greenhouse effect, the Archean climate should have become highly sensitive to the CH4/CO; ratio. If this ratio had been too high, then the haze would have been thick, which would have potentially led to a globally glaciated
FIG. 11. Surface temperature contours plotted against log(pCOz) and log(pCHg). Black areas indicate surface tem- peratures below 250 K, grading into blue, cyan, green, yel- low, orange, and red as temperature increases to 300 K. Re- gions of the diagram associated with glacier-free conditions (> ~293 K) are colored orange or warmer, and the “hard pa- leosol limit” on pCOz (Rye et al., 1995) is represented by a vertical arrow at log({pCO ) ~ —1.52. The dashed oval rep- resents atmospheric compositions that best recreate late Archean paleosol and glacial data. Black dots indicate the in- dividual model results used to generate the contour plot.
FIG. 11. Surface temperature contours plotted against log(pCOz) and log(pCHg). Black areas indicate surface tem- peratures below 250 K, grading into blue, cyan, green, yel- low, orange, and red as temperature increases to 300 K. Re- gions of the diagram associated with glacier-free conditions (> ~293 K) are colored orange or warmer, and the “hard pa- leosol limit” on pCOz (Rye et al., 1995) is represented by a vertical arrow at log({pCO ) ~ —1.52. The dashed oval rep- resents atmospheric compositions that best recreate late Archean paleosol and glacial data. Black dots indicate the in- dividual model results used to generate the contour plot.
Related topics:
Ancient HistoryGeologyGeochemistryLand Surface TemperatureGreenhouse EffectTime FactorsAtmospheric Gases
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