We examine the sensitivity of a climate model to a wide range of radiative forcings, including changes of solar irradiance, atmospheric CO2, O3, CFCs, clouds, aerosols, surface albedo, and a “ghost” forcing introduced at arbitrary heights, latitudes, longitudes, seasons, and times of day. We show that, in general, the climate response, specifically the global mean temperature change, is sensitive to the altitude, latitude, and nature of the forcing; that is, the response to a given forcing can vary by 50% or more depending upon characteristics of the forcing other than its magnitude measured in watts per square meter. The consistency of the response among different forcings is higher, within 20% or better, for most of the globally distributed forcings suspected of influencing global mean temperature in the past century, but exceptions occur for certain changes of ozone or absorbing aerosols, for which the climate response is less well behaved. In all cases the physical basis for the variations of the response can be understood. The principal mechanisms involve alterations of lapse rate and decrease (increase) of large-scale cloud cover in layers that are preferentially heated (cooled). Although the magnitude of these effects must be model-dependent, the existence and sense of the mechanisms appear to be reasonable. Overall, we reaffirm the value of the radiative forcing concept for predicting climate response and for comparative studies of different forcings; indeed, the present results can help improve the accuracy of such analyses and define error estimates. Our results also emphasize the need for measurements having the specificity and precision needed to define poorly known forcings such as absorbing aerosols and ozone change. Available data on aerosol single scatter albedo imply that anthropogenic aerosols cause less cooling than has commonly been assumed. However, negative forcing due to the net ozone change since 1979 appears to have counterbalanced 30–50% of the positive forcing due to the increase of well-mixed greenhouse gases in the same period. As the net ozone change includes halogen-driven ozone depletion with negative radiative forcing and a tropospheric ozone increase with positive radiative forcing, it is possible that the halogen-driven ozone depletion has counterbalanced more than half of the radiative forcing due to well-mixed greenhouse gases since 1979.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/96JD03436

Radiative forcing and climate response

Atomic charges were obtained from ab initio molecular orbital calculations using a variety of procedures to compare them and assess their utility Two procedures based on the molecular orbitals were examined, the Mulliken population analysis and the Weinhold–Reed Natural Population Analysis Two procedures using the charge density distribution were included; the Hirshfeld procedure and Bader's Atoms in Molecules method Charges also were derived by fitting the electrostatic potential (CHELPG) and making use of the atomic polar tensors (GAPT) The procedures were first examined for basis set independence, and then applied to a group of hydrocarbons The dipole moments for these molecules were computed from the various atomic charges and compared to the total SCF dipole moments This was followed by an examination of a series of substituted methanes, simple hydrides, and a group of typical organic compounds such as carbonyl derivatives, nitriles, and nitro compounds In some cases, the ability of the charges to reproduce electrostatic potentials was examined © John Wiley & Sons, Inc

Comparison of atomic charges derived via different procedures

The infrared absorption cross sections of CFC-11 and 16 other halogenated compounds have been measured. These spectra were used in detailed line-by-line calculations to derive radiative forcing values. The radiative forcing values for 14 of these gases have not, to our knowledge, been previously reported in the literature. The accuracy of a computationally inexpensive narrowband scheme, which included the effect of clouds and stratospheric adjustment, was investigated. Global warming potentials are presented where atmospheric lifetimes are available. In light of the substantial disagreement in values for the forcing due to CFC-11 reported in the literature and its use as a standard to which other halogenated gases are often compared, we examined the sensitivity of CFC-11 forcing to a number of assumptions. We find that the uncertainties in the calculated value of the radiative forcing caused by neglect of the temperature and pressure dependence of the IR absorption spectrum are much smaller than those resulting from uncertainties in the absolute absorption cross sections or the vertical profile of CFC-11. Our best estimate is 0.285 W m−2 ppbv−1, which is 30% higher than the value adopted by the Intergovernmental Panel on Climate Change and is believed to be accurate to within about 10%. For the other gases represented here the lack of detailed knowledge of the likely vertical and horizontal distribution probably represents the most significant uncertainty in evaluating their radiative forcing.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JD01137

Radiative forcing of climate change by CFC-11 and possible CFC replacements

Raman spectra of liquid 1-ethyl-3-methylimidazolium (EMI+) salts, EMI+BF4-, EMI+PF6-, EMI+CF3SO3-, and EMI+N(CF3SO2)2-, were measured over the frequency range 200−1600 cm-1. In the range 200−500 cm-1, we found five bands originating from the EMI+ ion at 241, 297, 387, 430, and 448 cm-1. However, the 448 cm-1 band could hardly be reproduced by theoretical calculations in terms of a given EMI+ conformer, implying that the band originates from another conformer. This is expected because the EMI+ involves an ethyl group bound to the N atom of the imidazolium ring, and the ethyl group can rotate along the C−N bond to yield conformers. The torsion energy for the rotation was then theoretically calculated. Two local minima with an energy difference of ca. 2 kJ mol-1 were found, suggesting that two conformers are present in equilibrium. Full geometry optimizations followed by normal frequency analyses indicate that the two conformers are those with planar and nonplanar ethyl groups against the imidazolium ring pl...

Evidence of Conformational Equilibrium of 1-Ethyl-3-methylimidazolium in Its Ionic Liquid Salts: Raman Spectroscopic Study and Quantum Chemical Calculations

We derive expressions for molecular gradients and hessians for the case when the energy is evaluated using density functional theory. Although derivative expressions have been proposed previously, our derivation is based on the unitary exponential parameterization of the wavefunction, and our expressions are valid for local and non–local potentials. Density functional theory, although similar in implementation to standard SCF theory, differs in that it introduces an exchange–correlation term which is density dependent. The presence of such a quantity introduces additional derivative terms which are not present in standard approaches of electronic structure theory. Expressions are derived for both the exact Coulombic repulsion, as well as the case where the density is expressed as a fitted quantity. Given these choices our final equations offer a computationally tractable expression with particular emphasis on conditions which ensure that the computed quantities are numerically correct. We show that althou...

Molecular gradients and hessians implemented in density functional theory

The analysis of infrared intensity data to obtain atomic ``polar tensors,'' introduced first by Biarge, Herranz, and Morcillo (1961), is reviewed and slightly reformulated to make it more consistent with nomenclature currently in use for normal coordinate calculations. Some properties of these polar tensors are examined, including their relation to the ``effective charges'' introduced by King, Mast, and Blanchette (1972). The procedure is illustrated for formaldehyde and for ethylene to obtain the polar tensors and effective charges for the H, C, and O atoms. A revised set of normal coordinates is used for the latter molecule. The results are compared and discussed.

Dipole moment derivatives and infrared intensities. I. Polar tensors

Atomic polar tensors have been determined for H, C, and X atoms of the methyl halide molecules (CH3X), using infrared intensity data reported by Russell, Needham, and Overend. The signs for the ∂p/∂Qi’s have been chosen by considering Coriolis coupling constants, agreement between intensity parameter values for CH3X and CD3X, and by considering the values of the atomic polar tensors calculated by CNDO methods for CH3F and CH3Cl. The resulting atomic polar tensors show systematic trends for the CH3X molecules as X change from F to I. The possibility that atomic polar tensors can be transferred from one molecule to another (with some allowance for expected trends) cannot be ignored. Atomic effective charges for each atom have been calculated, and ξH is found to be nearly constant for the four molecules. Additional properties of the atomic polar tensor are examined, including the anisotropy. A ''modified bond moment hypothesis'' appears to be reasonably successful in modeling the infrared intensities of the methyl halides.

Dipole moment derivatives and infrared intensities. II. Polar tensors in methyl halide molecules

Absolute integrated infrared absorption intensities of trichlorofluoromethane and trifluoroiodomethane fundamentals in the gas phase. Intensity sum rule

The atomic polar tensors for H and F atoms derived from the experimental values for the integrated infrared molar absorption coefficients for the fundamental vibrations of CH3F are used to predict the intensities of the fundamental vibrations of CF4, CF3H, and CH2F2. These predictions are compared with the values found by experimental measurements. In general, the agreement is quite good (the predicted values agree with the experimental values within a factor of 2, and often are much better), encouraging further investigation of these parameters for prediction of infrared intensities.

James H. Newton

Papers

Dipole moment derivatives and infrared intensities. I. Polar tensors

Dipole moment derivatives and infrared intensities. II. Polar tensors in methyl halide molecules

Absolute integrated infrared absorption intensities of trichlorofluoromethane and trifluoroiodomethane fundamentals in the gas phase. Intensity sum rule

Dipole moment derivatives and infrared intensities. III. Transferable polar tensors for fluorinated methanes

Dipole moment derivatives and infrared intensities. 5. Atomic polar tensors from methane