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Interannual variability of the extratropical northern hemisphere and the potential vorticity wave guide
Atmospheric Science Letters (2001) doi:10.1006/asle.2001.0035
Global warming potentials modi®ed for surface radiative forcing for use in surface energy balance models W. F. J. Evans and E. Puckrin Environmental Resource Studies, Trent University, Peterborough, ON, Canada Abstract: The radiative impact of greenhouse gases in warming the Earth varies signi®cantly, depending on whether one considers the forcing at the tropopause or at the surface. Compared to the former, the surface forcing for some greenhouse gases is reduced by the interference of water vapour. Hence, we calculate alternative surface global warming potentials (SGWPs) that are derived from the surface forcing radiation of greenhouse gases for potential use in surface radiative energy balance models (SREBMs). For gases with a large water vapour overlap, the SGWPs are typically 30% smaller than current GWPs; for gases with relatively little overlap, the SGWPs are larger by more than 33%. These results may be used in conjunction with SREBMs as an additional means of calculating climate change, and may lead to an altered emissions c 2001 Royal Meteorological Society * budget compared to that outlined by the current Kyoto agreement. Keywords: GWP, radiative forcing, climate model, greenhouse gas, climate forcing, surface energy balance model.
1. INTRODUCTION Greenhouse gases such as water vapour, carbon dioxide, methane, nitrous oxide and the chloro¯uorocarbons are able to absorb the thermal energy emitted by the Earth's surface and atmosphere. This radiation is subsequently re-emitted in all directions with the downward component responsible for warming the planet's surface, and thus contributes to the phenomenon of global warming. The concept of a global warming potential (GWP) for each greenhouse gas was introduced about 10 years ago (IPCC, 1990) to help determine the relative ability that a particular gas may have towards forcing the Earth's climate. Speci®cally, the GWP of a greenhouse gas is the product of the radiative forcing of the gas, as determined at the tropopause, and its concentration, integrated over some time frame. The GWPs are usually referenced to the radiative impact of carbon dioxide. One of the main problems with this formulation is the dependence of the forcing radiation on overlapping absorption bands of other gases. The impact of the overlapping bands, particularly those of water vapour, on the GWP depends on whether one considers the radiative forcing at the tropopause, as currently accepted by IPCC, or whether one considers the surface forcing radiation, which is responsible for the actual warming of the Earth's surface. In general, these two quantities are only identical for an isothermal atmosphere containing well-mixed gases, a scenario that never occurs in nature. The interference effect of water vapour is particularly important since it has many absorption bands throughout the entire thermal infrared 1530-261X
c 2001 Royal Meteorological Society *
region that overlap to some extent with every other greenhouse gas. In addition, its relatively high concentration close to the Earth's surface results in a blanket of protection that effectively absorbs the downward greenhouse radiation from other gases higher in the atmosphere and reduces their warming impact on the Earth. The radiative trapping parameter is relatively unaffected by the presence of a low-lying water vapour layer, as will be discussed below. Hence, the GWP of gases that have a strong overlap with water vapour is expected to be less than the value that is currently reported by IPCC (1996), which is based on the radiative forcing determined at the tropopause.
2. DISCUSSION The contrast between the effect that the water vapour overlap has on the surface forcing and radiative trapping of nitrous oxide (N2O), for example, is illustrated in Figure 1. The surface forcing radiation (red line) and the radiative trapping (blue line), as calculated at the 12-kmhigh tropopause, were simulated with the FASCOD3 line-by-line radiation model (Clough et al., 1988) using the 1976 U.S. standard atmosphere to represent average global conditions of temperature and water vapour (Anderson et al., 1986). Also shown in Figure 1 is the absorption spectrum associated with atmospheric water vapour (orange line). The line transition parameters for these gases were taken
Figure 1. A comparison of the surface forcing and radiative trapping of N2O for the 1976 U.S. standard atmosphere. The comparison shows the impact that overlapping absorption bands of water vapour have in reducing the surface forcing radiation of N2O in the 500±1350-cm ÿ1 region (top panel) and the 1800±2500-cm ÿ1 region (bottom panel).
from the 1996 HITRAN database (Rothman et al., 1998). We have shown in the past through measurements of thermal emission spectra that FASCOD3 reliably predicts the forcing radiation of greenhouse gases (e.g. Evans and Puckrin, 2000). In Figure 1, the surface forcing of N2O in the 600 and 1300 cm ÿ1 bands, where the water vapour absorption is nearly unity, is reduced signi®cantly compared to the radiative trapping component (red vs blue lines). The N2O trapping at 600 cm ÿ1 is more than four times greater than the corresponding surface forcing; at 1300 cm ÿ1 it is about twice as large. Comparing the trapping and surface forcing radiation in the absence of water vapour (green and grey lines, respectively, in Figure 1) clearly shows the radiative trapping is relatively unaffected by the presence of the strong absorption bands of water vapour. By comparing the radiative trapping of N2O in Figure 1 with and without water vapour present, a discrepancy of 60% at most is introduced between the N2O bands. On the other hand, for the surface forcing scenario the 600 and 1300 cm ÿ1 bands are 12 and four times more intense, respectively, without water vapour present in the atmosphere. Furthermore, the comparison of the surface forcing with the radiative trapping in the absence of water vapour shows that the former is larger due to the thermal and pressure gradient of the atmosphere; however, this excess emission is more than compensated with the addition of water vapour, resulting in a reduction of the surface forcing bands relative to the trapped radiation. The N2O bands at 1200 and 2200 cm ÿ1 in Figure 1 are less affected by the presence of water vapour, which exhibits a smaller absorption of only about 30% in these regions. In general, gases with major radiation bands in the transparent window region of the atmosphere will not be affected signi®cantly by water vapour interference. In order to account for the interference effect that overlapping absorption bands of water vapour have on the warming capability of all greenhouse gases, we introduce the concept of a surface GWP (SGWP), which is based on the ratio of the surface radiative forcing to the radiative trapping (Table 1) at the tropopause. The concentrations of the gases used for the calculations in Table 1 were those reported in the last IPCC (1996) report. This scenario provides a global average picture of the water vapour interference impact, along with the effect of a thermal and pressure gradient, on the thermal radiation of greenhouse gases. Hence, we de®ne a FLUx forcing (FLUF) factor, which consists of the ratio of the surface forcing to the radiative trapping for all of the radiation bands of a gas. The FLUF factors for the ®ve most Table 1. Surface global warming potentials (GWPs) based on the surface forcing radiation of greenhouse gases for USS* conditions Greenhouse gas CO2 CH4 N2 O CFC11 CFC12
Direct GWP (100 yr) (IPCC, 1996) 1 21} 310 3800 8100
Trapped Flux{ (W/m2) 29.1 1.66 1.62 0.0803 0.197
Surface Flux{ (W/m2) 25.3 0.994 0.936 0.101 0.228
FLUF{ Factor
FLUF{ GWP
0.869 0.599 0.578 1.26 1.16
0.869 12.6 179 4800 9400
* USS represents the 1976 U.S. standard atmosphere, which is typical of average global conditions. { Simulated with the FASCOD3 line-by-line radiative transfer code. { FLUF factor is de®ned by the ratio of surface forcing to radiative trapping. } The surface GWPs are relative to the value for carbon dioxide. } Includes the direct and indirect radiative forcing effects.
SGWP} (100 yr) 1 14.5 206 5500 10 800
Change in GWP from IPCC value ± ÿ31% ÿ34% 45% 33%
important greenhouse gases are summarized in the ®fth column of Table 1. Multiplying the FLUF factor by the current GWP (IPCC, 1996) accounts for the reduction in warming due to the overlapping absorption bands of water vapour, as shown in the sixth column of Table 1. The new SGWPs, obtained by again referencing to CO2 , are presented in the seventh column of the table, and the percentage difference from the original GWPs are given in the last column. It should be noted that in the case of methane, both direct radiative and indirect chemical effects are included, with the assumption that the direct forcing of other gases perturbed by methane are moderated in a similar fashion. In Table 1, the FLUF factor is less than unity in regions where the water interference is the greatest, and thereby results in a reduced GWP for the corresponding greenhouse gas. The GWP of gases that exhibit a large overlap with water vapour have a typical reduction of about 30%. Gases with absorption bands in the transparent window region of the atmosphere between 800 and 1200 cm ÿ1 are not impacted much, since the water vapour absorption is reduced dramatically in this region. Therefore, gases such as the chloro¯uorocarbons ef®ciently warm the planet's surface and have FLUF factors greater than unity. That the factors exceed unity is attributed to the excess surface forcing that results from the temperature and pressure variation of the atmosphere. The excess forcing is only partially offset by the limited absorption of water vapour in this region.
3. CONCLUSION The formulation of the SGWPs provides an alternative representation to the GWP that is commensurate with use in surface radiative energy balance models (SREBMs). The absolute reduction of the GWPs for carbon dioxide, methane and nitrous oxide accompanied by a signi®cant increase for the chloro¯uorocarbons will potentially affect the calculation of equivalent carbon dioxide emissions. Such a calculation with a SREBM potentially could result in an altered emissions budget for carbon dioxide for each country as outlined in the Kyoto Protocol under the United Nations Framework Convention for Climate Change. For example, the greenhouse gas emissions budget may potentially be reduced by 15% based on the absolute value of the new SGWP for carbon dioxide, and up to 40% for other gases such as methane. In addition, the emissions burden would be shifted more to non-methane gases such as the chloro¯uorocarbons and other gases that have their dominant infrared bands situated between 800 and 1200 cm ÿ1. The use of SGWPs in SREBMs would have a direct application to cloudy sky scenarios. The greenhouse gas surface forcing below the cloud warms the surface while the greenhouse gas radiative trapping warms the atmosphere above the cloud. The radiative trapping con®guration results in the erroneous conclusion that greenhouse gases beneath cloud cover do not warm the planet's surface, whereas surface forcing directly addresses this warming effect. This is an extremely relevant point since the planet's surface is cloudy approximately 50% of the time. The radiative forcing under cloudy conditions could be evaluated much more accurately with SGWPs in a SREBM than is currently estimated with a GCM.
Acknowledgements We would like to thank AFRL (Hanscom AFB) for supplying the FASCOD3 radiation code and their expert instruction concerning its use. We gratefully acknowledge the ®nancial support of Enbridge Consumers Gas and a NSERC Industrially Oriented Research grant supporting this work.
REFERENCES Anderson, G. P., Clough, S. A., Kneizys, F. X., Chetwynd, J. H. and Shettle, E. P., 1986. AFGL atmospheric constituent pro®les (0±120 km). AFGL-TR86-0110, Optical Physics Div., Air Force Geophysics Laboratory, Hanscom AFB, MA, USA. Clough, S. A., Kneizys, F. X., Anderson, G. P., Shettle, E. P., Chetwynd, J. H., Abreu, L. W. and Hall, L. A., 1988. IRS '88: current problems in atmospheric radiation. In: Lenoble, J. and Geleyn, J. F., Eds. A. Deepak, 372±375. Evans, W. F. J. and Puckrin, E., 2000. The surface radiative forcing of nitric acid for northern mid-latitudes. Atmos. Environ., 35, 71±77. IPCC. 1990. Climate change 1990, the IPCC scienti®c assessment. Houghton JT, Jenkins GJ, Ephraums JJ (Eds). Cambridge: Cambridge University Press. IPCC. 1996. Climate change 1995, the science of climate change. Houghton JT, Meira Filho LG, Callander BA, Harris N, Kattenberg A, Maskell K (Eds). Cambridge: Cambridge University Press. Rothman, L. S., Rinsland, C. P. and Varanasi, P., 1998. The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric WorKStation): 1996. J. Quant. Spectrosc. Ra., 60, 665±710.
Global warming potentials modi®ed for surface radiative forcing for use in surface energy balance models W. F. J. Evans and E. Puckrin Environmental Resource Studies, Trent University, Peterborough, ON, Canada Abstract: The radiative impact of greenhouse gases in warming the Earth varies signi®cantly, depending on whether one considers the forcing at the tropopause or at the surface. Compared to the former, the surface forcing for some greenhouse gases is reduced by the interference of water vapour. Hence, we calculate alternative surface global warming potentials (SGWPs) that are derived from the surface forcing radiation of greenhouse gases for potential use in surface radiative energy balance models (SREBMs). For gases with a large water vapour overlap, the SGWPs are typically 30% smaller than current GWPs; for gases with relatively little overlap, the SGWPs are larger by more than 33%. These results may be used in conjunction with SREBMs as an additional means of calculating climate change, and may lead to an altered emissions c 2001 Royal Meteorological Society * budget compared to that outlined by the current Kyoto agreement. Keywords: GWP, radiative forcing, climate model, greenhouse gas, climate forcing, surface energy balance model.
1. INTRODUCTION Greenhouse gases such as water vapour, carbon dioxide, methane, nitrous oxide and the chloro¯uorocarbons are able to absorb the thermal energy emitted by the Earth's surface and atmosphere. This radiation is subsequently re-emitted in all directions with the downward component responsible for warming the planet's surface, and thus contributes to the phenomenon of global warming. The concept of a global warming potential (GWP) for each greenhouse gas was introduced about 10 years ago (IPCC, 1990) to help determine the relative ability that a particular gas may have towards forcing the Earth's climate. Speci®cally, the GWP of a greenhouse gas is the product of the radiative forcing of the gas, as determined at the tropopause, and its concentration, integrated over some time frame. The GWPs are usually referenced to the radiative impact of carbon dioxide. One of the main problems with this formulation is the dependence of the forcing radiation on overlapping absorption bands of other gases. The impact of the overlapping bands, particularly those of water vapour, on the GWP depends on whether one considers the radiative forcing at the tropopause, as currently accepted by IPCC, or whether one considers the surface forcing radiation, which is responsible for the actual warming of the Earth's surface. In general, these two quantities are only identical for an isothermal atmosphere containing well-mixed gases, a scenario that never occurs in nature. The interference effect of water vapour is particularly important since it has many absorption bands throughout the entire thermal infrared 1530-261X
c 2001 Royal Meteorological Society *
region that overlap to some extent with every other greenhouse gas. In addition, its relatively high concentration close to the Earth's surface results in a blanket of protection that effectively absorbs the downward greenhouse radiation from other gases higher in the atmosphere and reduces their warming impact on the Earth. The radiative trapping parameter is relatively unaffected by the presence of a low-lying water vapour layer, as will be discussed below. Hence, the GWP of gases that have a strong overlap with water vapour is expected to be less than the value that is currently reported by IPCC (1996), which is based on the radiative forcing determined at the tropopause.
2. DISCUSSION The contrast between the effect that the water vapour overlap has on the surface forcing and radiative trapping of nitrous oxide (N2O), for example, is illustrated in Figure 1. The surface forcing radiation (red line) and the radiative trapping (blue line), as calculated at the 12-kmhigh tropopause, were simulated with the FASCOD3 line-by-line radiation model (Clough et al., 1988) using the 1976 U.S. standard atmosphere to represent average global conditions of temperature and water vapour (Anderson et al., 1986). Also shown in Figure 1 is the absorption spectrum associated with atmospheric water vapour (orange line). The line transition parameters for these gases were taken
Figure 1. A comparison of the surface forcing and radiative trapping of N2O for the 1976 U.S. standard atmosphere. The comparison shows the impact that overlapping absorption bands of water vapour have in reducing the surface forcing radiation of N2O in the 500±1350-cm ÿ1 region (top panel) and the 1800±2500-cm ÿ1 region (bottom panel).
from the 1996 HITRAN database (Rothman et al., 1998). We have shown in the past through measurements of thermal emission spectra that FASCOD3 reliably predicts the forcing radiation of greenhouse gases (e.g. Evans and Puckrin, 2000). In Figure 1, the surface forcing of N2O in the 600 and 1300 cm ÿ1 bands, where the water vapour absorption is nearly unity, is reduced signi®cantly compared to the radiative trapping component (red vs blue lines). The N2O trapping at 600 cm ÿ1 is more than four times greater than the corresponding surface forcing; at 1300 cm ÿ1 it is about twice as large. Comparing the trapping and surface forcing radiation in the absence of water vapour (green and grey lines, respectively, in Figure 1) clearly shows the radiative trapping is relatively unaffected by the presence of the strong absorption bands of water vapour. By comparing the radiative trapping of N2O in Figure 1 with and without water vapour present, a discrepancy of 60% at most is introduced between the N2O bands. On the other hand, for the surface forcing scenario the 600 and 1300 cm ÿ1 bands are 12 and four times more intense, respectively, without water vapour present in the atmosphere. Furthermore, the comparison of the surface forcing with the radiative trapping in the absence of water vapour shows that the former is larger due to the thermal and pressure gradient of the atmosphere; however, this excess emission is more than compensated with the addition of water vapour, resulting in a reduction of the surface forcing bands relative to the trapped radiation. The N2O bands at 1200 and 2200 cm ÿ1 in Figure 1 are less affected by the presence of water vapour, which exhibits a smaller absorption of only about 30% in these regions. In general, gases with major radiation bands in the transparent window region of the atmosphere will not be affected signi®cantly by water vapour interference. In order to account for the interference effect that overlapping absorption bands of water vapour have on the warming capability of all greenhouse gases, we introduce the concept of a surface GWP (SGWP), which is based on the ratio of the surface radiative forcing to the radiative trapping (Table 1) at the tropopause. The concentrations of the gases used for the calculations in Table 1 were those reported in the last IPCC (1996) report. This scenario provides a global average picture of the water vapour interference impact, along with the effect of a thermal and pressure gradient, on the thermal radiation of greenhouse gases. Hence, we de®ne a FLUx forcing (FLUF) factor, which consists of the ratio of the surface forcing to the radiative trapping for all of the radiation bands of a gas. The FLUF factors for the ®ve most Table 1. Surface global warming potentials (GWPs) based on the surface forcing radiation of greenhouse gases for USS* conditions Greenhouse gas CO2 CH4 N2 O CFC11 CFC12
Direct GWP (100 yr) (IPCC, 1996) 1 21} 310 3800 8100
Trapped Flux{ (W/m2) 29.1 1.66 1.62 0.0803 0.197
Surface Flux{ (W/m2) 25.3 0.994 0.936 0.101 0.228
FLUF{ Factor
FLUF{ GWP
0.869 0.599 0.578 1.26 1.16
0.869 12.6 179 4800 9400
* USS represents the 1976 U.S. standard atmosphere, which is typical of average global conditions. { Simulated with the FASCOD3 line-by-line radiative transfer code. { FLUF factor is de®ned by the ratio of surface forcing to radiative trapping. } The surface GWPs are relative to the value for carbon dioxide. } Includes the direct and indirect radiative forcing effects.
SGWP} (100 yr) 1 14.5 206 5500 10 800
Change in GWP from IPCC value ± ÿ31% ÿ34% 45% 33%
important greenhouse gases are summarized in the ®fth column of Table 1. Multiplying the FLUF factor by the current GWP (IPCC, 1996) accounts for the reduction in warming due to the overlapping absorption bands of water vapour, as shown in the sixth column of Table 1. The new SGWPs, obtained by again referencing to CO2 , are presented in the seventh column of the table, and the percentage difference from the original GWPs are given in the last column. It should be noted that in the case of methane, both direct radiative and indirect chemical effects are included, with the assumption that the direct forcing of other gases perturbed by methane are moderated in a similar fashion. In Table 1, the FLUF factor is less than unity in regions where the water interference is the greatest, and thereby results in a reduced GWP for the corresponding greenhouse gas. The GWP of gases that exhibit a large overlap with water vapour have a typical reduction of about 30%. Gases with absorption bands in the transparent window region of the atmosphere between 800 and 1200 cm ÿ1 are not impacted much, since the water vapour absorption is reduced dramatically in this region. Therefore, gases such as the chloro¯uorocarbons ef®ciently warm the planet's surface and have FLUF factors greater than unity. That the factors exceed unity is attributed to the excess surface forcing that results from the temperature and pressure variation of the atmosphere. The excess forcing is only partially offset by the limited absorption of water vapour in this region.
3. CONCLUSION The formulation of the SGWPs provides an alternative representation to the GWP that is commensurate with use in surface radiative energy balance models (SREBMs). The absolute reduction of the GWPs for carbon dioxide, methane and nitrous oxide accompanied by a signi®cant increase for the chloro¯uorocarbons will potentially affect the calculation of equivalent carbon dioxide emissions. Such a calculation with a SREBM potentially could result in an altered emissions budget for carbon dioxide for each country as outlined in the Kyoto Protocol under the United Nations Framework Convention for Climate Change. For example, the greenhouse gas emissions budget may potentially be reduced by 15% based on the absolute value of the new SGWP for carbon dioxide, and up to 40% for other gases such as methane. In addition, the emissions burden would be shifted more to non-methane gases such as the chloro¯uorocarbons and other gases that have their dominant infrared bands situated between 800 and 1200 cm ÿ1. The use of SGWPs in SREBMs would have a direct application to cloudy sky scenarios. The greenhouse gas surface forcing below the cloud warms the surface while the greenhouse gas radiative trapping warms the atmosphere above the cloud. The radiative trapping con®guration results in the erroneous conclusion that greenhouse gases beneath cloud cover do not warm the planet's surface, whereas surface forcing directly addresses this warming effect. This is an extremely relevant point since the planet's surface is cloudy approximately 50% of the time. The radiative forcing under cloudy conditions could be evaluated much more accurately with SGWPs in a SREBM than is currently estimated with a GCM.
Acknowledgements We would like to thank AFRL (Hanscom AFB) for supplying the FASCOD3 radiation code and their expert instruction concerning its use. We gratefully acknowledge the ®nancial support of Enbridge Consumers Gas and a NSERC Industrially Oriented Research grant supporting this work.
REFERENCES Anderson, G. P., Clough, S. A., Kneizys, F. X., Chetwynd, J. H. and Shettle, E. P., 1986. AFGL atmospheric constituent pro®les (0±120 km). AFGL-TR86-0110, Optical Physics Div., Air Force Geophysics Laboratory, Hanscom AFB, MA, USA. Clough, S. A., Kneizys, F. X., Anderson, G. P., Shettle, E. P., Chetwynd, J. H., Abreu, L. W. and Hall, L. A., 1988. IRS '88: current problems in atmospheric radiation. In: Lenoble, J. and Geleyn, J. F., Eds. A. Deepak, 372±375. Evans, W. F. J. and Puckrin, E., 2000. The surface radiative forcing of nitric acid for northern mid-latitudes. Atmos. Environ., 35, 71±77. IPCC. 1990. Climate change 1990, the IPCC scienti®c assessment. Houghton JT, Jenkins GJ, Ephraums JJ (Eds). Cambridge: Cambridge University Press. IPCC. 1996. Climate change 1995, the science of climate change. Houghton JT, Meira Filho LG, Callander BA, Harris N, Kattenberg A, Maskell K (Eds). Cambridge: Cambridge University Press. Rothman, L. S., Rinsland, C. P. and Varanasi, P., 1998. The HITRAN molecular spectroscopic database and HAWKS (HITRAN Atmospheric WorKStation): 1996. J. Quant. Spectrosc. Ra., 60, 665±710.