K.P. SHINE, Y. FOUQUART, V. RAMASWAMY, S. SOLOMON, J. SRINIVASAN Contributors:
M.O. Andreae, J. Angell, G. Brasseur, C. Brtihl, RJ. Charlson, M.D. Chou,
J.R. Christy, T. Dunkerton, E. Button, BA. Fomin, C. Granier, H. Grassl, J. Hansen, Harshvardhan, D. Hauglustaine, P. Hobbs, D.J. Hoffman, L. Hood, N. Husson,
I. Karol, YJ. Kaufman, J. Kiehl, S. Kinne, M.K. W. Ko, K. Labitzke, H Le Treut,
A. McCulloch, AJ. Miller, M. Molina, E. Nesme-Ribes, A.H. Oort, J.E. Penner,
S. Pinnock, V. Ramanathan, A. Robock, E. Roeckner, M.E. Schlesinger, K. Sassen,
G.-Y Shi, A.N. Trotsenko, W.-C. Wang.
Summary 167 4.1 Introduction 169
4.1.1 Definitions of Radiative Forcing 169
4.2 Greenhouse Gases 171 4.2.1 Spectroscopy 171 4.2.2 Calculating the Radiative Forcing 171
43 Radiative Forcing due to Changes in Ozone 173
4.3.1 Introduction 173 4.3.2 Radiative Forcing due to Changes in
Stratospheric Ozone 173 4.3.3 Significance of the Forcing due to Stratospheric
Ozone Loss 177 4.3.4 Temperature Changes in the Lower Stratosphere 177
4.3.5 Sensitivity of Surface-Troposphere Climate 179 4.3.6 Radiative Forcing due to Changes in
Tropospheric Ozone 179 4.3.7 Radiative Forcing due to Total Atmospheric
Ozone Change 180 4.3.8 Outstanding Issues 180 4.4 Effects of Tropospheric and Stratospheric Aerosols 181
4.4.1 Tropospheric Aerosols 181 4.4.1.1 The direct aerosol effect 181 4.4.1.2 Indirect effect of anthropogenic aerosols
on cloud albedo 183 4.4.1.3 Summary of anthropogenic aerosol
effects 185 4.4.2 Radiative Forcing due to Stratospheric Aerosol 186
4.4.2.1 Introduction 186
4.4.2.2 Stratospheric aerosol radiative effects 186 4.4.2.3 Observations and simulations of climatic
effects 187 4.4.2.4 Summary 189 4.5 Solar Variability 189
4.5.1 Observations of Variability in Solar Irradiance
since 1978 189 4.5.2 Inferences of Variability in Solar Irradiance on
Longer Time-scales 190 4.5.3 Correlations between Climate and Solar Variability 191
4.5.4 Summary 192 4.6 Other Forcings 192 4.7 Estimates of Total Forcing 192
4.7.1 Relative Confidence in Estimates of Radiative
Forcing 192 4.7.2 Best Estimates of Forcings since Pre-industrial
Times 193 4.7.3 Estimates of Forcing from Observed
Temperature Records 195 4.7.4 Future Forcing 196 4.8 Forcing-Response Relationships 197
4.8.1 Background 197 4.8.2 Forcing-Response Relationships for
Well-mixed Greenhouse Gases 197 4.8.3 Forcing-response Relationship for Other Forcings 198
4.8.4 Summary 199
References 199
The concept of radiative forcing
• Global-mean radiative forcing is a valuable concept for giving at least a first-order estimate of the potential climatic importance of various forcing mechanisms. However, there could be limits to its utility because the relationship between global-mean radiative forcing and global-mean surface temperature change may not be as simple as previously thought. For example, its general applicability has recently been questioned in the cases of forcing due to ozone and tropospheric aerosols, where changes in concentration are highly variable horizontally and/or vertically.
Greenhouse gases
• The direct global-mean radiative forcing due to changes in concentrations of the greenhouse gases (carbon dioxide (C02), methane (CH4), nitrous oxide (N20) and the halocarbons) since pre-industrial times is 2.4 Wm"2 and is believed to be accurate to within 15%. This value is essentially unchanged from previous IPCC assessments.
• The global-mean radiative forcing due to increases in tropospheric ozone since pre-industrial times is positive and is estimated to be a few tenths of a Wm2. Since the late 1970s, decreases in stratospheric ozone have led to a global-mean radiative forcing estimated to be about -0.1 Wm"2. The accuracy of these estimates is presently limited by incomplete knowledge of the temporal, horizontal and vertical changes in ozone.
Tropospheric aerosols
• The direct global-mean radiative forcing due to increases in sulphate aerosol amounts since pre-industrial times is negative and may lie in the range -0.25 to -0.9 W m2, but there are substantial uncertainties due to the lack of detailed knowledge of the amount of sulphate aerosols and their radiative properties. The effect of aerosols emitted as a result
of biomass burning has received much less attention and is even more uncertain; the direct global-mean radiative forcing since pre-industrial times may lie in the range -0.05 to -0.6 W m2. Soot carbon in tropospheric aerosols has the potential to reduce the magnitude of the aerosol forcing; however global estimates are not yet available.
• Estimates exist for the indirect effect of aerosols on cloud droplet sizes. The radiative forcing is believed to be negative and may be of a similar magnitude to the direct radiative forcing due to aerosol changes but the uncertainty is much larger. The possible effects of aerosols on cloud liquid water content and cloud cover are just beginning to be investigated.
Volcanic aerosols
• The volcanic eruption of Mt. Pinatubo in June 1991 resulted in a large enhancement to the stratospheric aerosol layer; this caused a transient but large global-mean negative forcing that peaked at about 4 Wm"2 and exceeded 2 Wm"2 for about one year. Calculated radiative forcings are broadly consistent with values deduced from satellite observations.
• General Circulation Model (GCM) simulations of the effect of the Mt. Pinatubo eruption on surface and lower stratospheric temperatures show encouraging agreement with observations. This suggests that the GCM's temperature response to a transient large-scale radiative forcing of a large magnitude is reasonable.
Solar variability
• Extension of the current understanding of the relationship between observed changes in solar output and other indicators of solar variability suggests that long-term increases in solar irradiance since the 17th century Maunder Minimum might have been climatically significant. A global-mean radiative forcing of a few tenths of a Wm'2 since 1850 has been suggested, but uncertainties are large.
/6A Radiative Forcing Problem in combining global-mean radiative forcing
from different mechanisms
• An estimate of the net global mean radiative forcing due to all human activity is not presented because the usefulness of combining estimates of global-mean radiative forcing of differing signs, and resulting from different spatial patterns, is not currently
understood. For example, if, by coincidence, the global-mean radiative forcing were to be zero, due to the cancellation of positive and negative forcings from different mechanisms, this cannot be taken to imply the absence of regional-scale or possibly even global climate change.
4.1 Introduction
This chapter considers recent advances in our knowledge of the factors which, by perturbing the planetary radiation budget, might result in climate change. Such a perturbation is referred to here as "radiative forcing" - some authors use the term "climate forcing" for the same concept.
The concept of radiative forcing (e.g., IPCC 1990 and 1992; and see Sections 4.1.1 and 4.8) is based on earlier climate model calculations which show that there is an approximately linear relationship between the global-mean radiative forcing at the tropopause and the equilibrium global mean surface temperature change. Importantly, model calculations have shown that, for a number of forcing mechanisms, the relationship is relatively unaffected by the nature of the forcing (e.g., whether it be due to a change in greenhouse gas concentration or solar output), provided it is appropriately defined (see Section 4.1.1). If the global mean radiative forcing is given by AF (Wm~2) and the global mean surface temperature response is ATS (in K) then
Ars = ^AF (4.1)
where X is a climate sensitivity parameter. A. is determined by a number of processes such as water vapour feedback, cloud feedbacks and ice-albedo feedback and its computed value is found to vary greatly amongst different GCMs (over a range of at least 0.3 to 1.4 K/(Wm"2)), mostly because of uncertainties in the calculation of cloud feedbacks (see e.g., IPCC 1990 and 1992). While knowledge of the climate feedbacks is of crucial importance for climate prediction, consideration of recent work in these areas is beyond the scope of this chapter, but will be discussed in more detail in the next full IPCC assessment in 1995.
Section 4.2 to 4.6 will concentrate on forcing due to changes in:
(a) greenhouse gas concentrations, including ozone;
(b) aerosols, both in the troposphere (mainly as a result of human activity) and in the stratosphere (mainly as a result of volcanic activity);
(c) solar output.
In the cases of forcing due to changes in lower stratospheric ozone and volcanic aerosols, changes in surface and lower stratospheric temperatures add useful supplementary information; since these responses are not considered elsewhere in this report, they will be briefly considered in this chapter. Section 4.7 provides an overall summary of our knowledge of radiative forcing.
The radiative forcing concept encapsulated in Equation (4.1) has served the climate community well over the past two decades. Recently its general applicability has been questioned for forcing due to changes in both ozone and sulphate aerosols where changes in concentration are
highly variable horizontally and/or vertically. Section 4.8 reviews studies regarding this topic which are, as yet, too preliminary to allow firm conclusions to be drawn.
However, they do suggest that the climate sensitivity (k in Equation (4.1)) may be significantly different for the forcing due to spatially inhomogenous changes in tropospheric aerosols and in ozone than it is for the well-mixed greenhouse gases. In particular, the work suggests that the degree of cancellation, in the global mean, between the positive greenhouse forcing and the negative tropospheric aerosol forcing may be a poor guide to eventual climate response. For example, if, by coincidence, the global-mean radiative forcing were to be zero, due to an exact cancellation of positive and negative forcings with different spatial characteristics, this could not be taken to imply the absence of regional-scale or possibly even global climate change. We continue to compare global mean radiative forcings in this chapter, but we do so with caution. The resolution of this issue will be an important topic for future IPCC reports.
It should also be noted that if a climate change mechanism leads to altered conditions in the stratosphere, the dynamics of the troposphere can also be affected (e.g., Rind el al., 1990, 1992); thus there is the potential for surface/tropospheric climate change to he induced by mechanisms other than by directly perturbing the radiation budget.
Despite these possible problems, global-mean radiative forcing remains a useful concept for comparing the potential climatic effects of changes in different greenhouse gases, with the possible exception of ozone. It is also a useful single number for comparing different estimates of tropospheric aerosol forcing or ozone forcing.
4.1.1 Definitions of Radiative Forcing
In IPCC (1990) and IPCC (1992) a specific definition of radiative forcing was adopted:
The radiative forcing of the surface-troposphere system (due to a change, for example, in greenhouse gas concentration) is the change in net irradiance (in Wm"2) at the tropopause AFTER allowing for stratospheric temperatures to re-adjust to radiative equilibrium, but with surface and tropospheric temperatures held fixed at their unperturbed values.
This follows earlier work (e.g., Ramanathan el al., 1985, Hansen el al., 1981 and references therein). The tropopause is chosen because, in simple models at least, it is considered that in a global and annual mean sense the surface and troposphere are so closely coupled that they behave as a single thermodynamic system. The adjustment of stratospheric temperatures could he considered as a feedback; however, it is counted as part of the forcing
170 Radiative Forcing because the timescale of the adjustment is a few months,
compared with the decadal time-scale required for the surface-atmosphere system to adjust to the forcing, due to the large thermal inertia of the oceans. One additional feature of allowing for stratospheric adjustment is that the change in the net irradiance is then the same at the tropopause as at the top of the atmosphere; this is not the case when stratospheric temperatures are unadjusted (see Hansen et ai, 1981).
The utility of radiative forcing will be the subject of Section 4.8, but it is useful here to illustrate the potential importance of the adjustment process using 1-dimensional radiative-convective model results reported by Rind and Lacis (1993) (see Table 4.1).
The surface temperature response to an imposed radiative forcing, AT0, is the so-called no-feedback case, in which clouds, water vapour and surface albedo are held fixed. For this case, the equivalent form of Equation (4.1)
where XQ is the climate sensitivity in the absence of feedbacks and AF is the adjusted forcing and is the same as that in Equation (4.1). This form will now be shown to be more useful than an alternative approach using the instantaneous forcing such that:
Ar, = \ A f \
where the subscript "i" refers to the instantaneous forcing.
Table 4.1 shows values of AF;, AF, \ and A.Q for a number of different forcing mechanisms. The difference between Xs and A.Q shows the strength and sign of the adjustment process. The sign of the adjustment process
depends on whether the change in forcing leads to a heating or a cooling of the stratosphere with a consequent increase or decrease in the thermal infrared emission from the lower stratosphere to the troposphere. For a doubling of C02, the stratosphere cools so that the adjusted forcing is about 6% less than the instantaneous. For an increase in CFC-11 from 0 to 1 ppbv the lower stratosphere warms and this makes the adjusted forcing 7% higher than the instantaneous. For changes in stratospheric ozone, the sign of X0 reverses between using instantaneous and adjusted forcing. For other forcing mechanisms, such as changes in methane, or solar output, the adjustment process is less important.
The most important conclusion from Table 4.1 is that k calculated using adjusted forcing is much less dependent on the forcing mechanism than that computed using the instantaneous forcing.
Cess et al. (1985) also reported similar experiments using a 1-dimensional radiative-convective model, which had no stratosphere; these showed that forcing due to changes in solar output, C 02 and tropospheric aerosols result in a similar climate sensitivity, although heavily-absorbing soot aerosols were found to have a markedly different sensitivity.
In preparing this review some difficulty has been experienced in intercomparing work performed by different authors because some have applied the term
"radiative forcing" to the instantaneous change in tropopause irradiance, not allowing for any change in stratospheric temperature. In other publications it is not clear which definition of radiative forcing has been adopted. The forcing should be calculated as a global mean using appropriate temperature, humidity and cloud Table 4.1: Surface temperature changes using a 1-D radiative-convective model (assuming no feedbacks) for a range of forcing mechanisms, together with the radiative forcing at the tropopause (AF) and the climate sensitivity parameter
(KJ. The subscript "o" refers to the no-feedback case. The subscript "i" refers to the instantaneous forcing with no feedbacks. These results are from Rind and Lacis (1993) and Lacis (pers. comm.). (Note that surface temperature
changes would be about 1 to 4 times larger if climate feedbacks were included - see IPCC (1992)). The changes in radiative properties used here are meant to be illustrative and do not necessarily represent actual or projected changes.
Forcing Mechanism
0 , 50% reduction at all altitudes Solar Const +2% at all wavelengths Stratospheric Aerosol T = 0.15
(K)
conditions - again, it is not always clear, in published estimates, what conditions are being used for calculations.
Since the adjustment process is generally most important for changes in greenhouse gases, we suggest that, in future, the greenhouse gas radiative forcing be referred to as either:
(i) instantaneous radiative forcing if no change in stratospheric temperature is accounted for, or (ii) adjusted radiative forcing if the stratospheric
temperature has been allowed to re-adjust to the instantaneous forcing.
Assumptions concerning the vertical profiles of temperature and all radiatively active constituents (including cloudiness) should be stated.
4.2 Greenhouse Gases
Our understanding of the enhanced greenhouse effect is dependent on two broad areas. First we need to understand the fundamental radiative properties ("spectroscopy") of the gases involved. Next, these spectroscopic data need to be included in radiative transfer models to calculate the radiative forcing due to changes in gas concentration, for a given atmospheric profile of temperature, water vapour and other trace gases, and cloudiness.
In this section the discussion will concentrate on the radiative forcing of greenhouse gases as a result of direct emissions of that gas. This is referred to as the direct greenhouse forcing. Greenhouse gas concentrations can change not only as a result of emissions of that gas, but when emissions of other gases lead to chemical reactions which alter the concentration of that gas; this is termed the indirect greenhouse forcing. Changes in ozone appear to be the most important indirect effect; these will be considered in Section 4.3.
4.2.1 Spectroscopy
In the thermal infrared (approximately 4-500 /<m) molecules absorb and emit radiation by changing the energy with which they vibrate and/or rotate; the wavelengths of the vibration/rotation transitions occur over narrow spectral intervals. Laboratory and theoretical studies are required to determine the wavelengths, strengths and widths of these transitions; in the case of heavy molecules, such as the halocarbons, the individual transitions are so closely-packed, that they are generally not resolved in laboratory observations.
The main databases of spectral parameters of atmospheric gases are subject to periodic update; the two main catalogues, HITRAN (Rothman et a!., 1992) and GEISA (Husson et al., 1992) have both been substantially revised since IPCC (1992). These revisions are based on
improvements to both laboratory measurements and theoretical techniques. Of the gases of most direct importance to radiative forcing, ozone and methane have undergone the most substantial revision, and many previously neglected weak spectral lines have been added.
A detailed assessment of the effect of these revisions has not yet been reported; however, Fomin et al. (1993) report that the net irradiance at the tropopause, evaluated for a mid-latitude clear-sky atmosphere, changes by less than 1% between using the 1986 and 1992 versions of HITRAN. It seems unlikely that the effect of these revisions on radiative forcing will be greater than 5% but there is a need to assess both the effect of the changes, and the potential impact of remaining uncertainties.
Continuum absorption, especially by water vapour, is also of importance in calculating radiative forcing. In spite of recent progress in describing the effect, further theoretical and laboratory investigations are required to resolve remaining uncertainties.
Further work on the infrared absorption cross-sections of halocarbons has been reported; this is particularly important for some of the HCFCs (hydrochlorofluoro-carbons) and MFCs (hydrofluoro(hydrochlorofluoro-carbons) as some of the data used in IPCC (1990) were from a single source.
Comparisons of strengths of many CFCs (chloro-fluorocarbons) and HCFC-22 are presented in McDaniel et al. (1991), Cappellani and Restelli (1992) and Clerbaux et al. (1993). For newer HCFCs, MFCs and periluorocarbons, measurements are more limited; these are tabulated in WMO(1994).
As examples, for HCFC-123, HCFC-141b, and HFC-142b, the spread of results is more than 25% of the mean cross-section; for HFC-134a the spread is about 10%.
Detailed descriptions, including temperatures and pressures of the measurements, are not available for all the datasets, so it is difficult to comment on the discrepancies;
only Cappellani and Restelli (1992) and Clerbaux et al.
(1993) have published cross-sections for a number of HFC/HCFCs over a range of temperatures.
4.2.2 Calculating the Radiative Forcing
In Section 4.7.2 updated calculations of radiative forcing due to changes in greenhouse gas concentration will be presented; in this section the basis on which these calculations are made will be assessed.
A whole hierarchy of different radiative transfer schemes are available to compute the radiative forcing, ranging from line-by-line models through to so-called wide-band models (Ellingson et al., 1991). In addition, the results from such models can he represented by relatively simple empirical formulae, such as those presented in IPCC (1990) and Shi (1992), to allow rapid, and reasonably accurate, computation of forcing.
172 Radiative Forcing Many details of the radiation schemes (such as methods
of handling clouds, spectral overlap of gases, treatment of water vapour continuum) affect the radiative forcing and are not handled in the same way by all schemes (see Ellingson et al., 1991).
The ultimate test of such models is their ability to reproduce observed irradiances given the observed state of the atmosphere. Until recently, the quality of observations was generally inadequate to assess the models; now high quality experimental data (see e.g., Ellingson et al., 1992) are becoming available and should provide valuable checks on the realism of radiative models.
The radiative forcing due to changes in concentrations of CO,, CH4, N20, CFC-11, CFC-12 presented in IPCC
The radiative forcing due to changes in concentrations of CO,, CH4, N20, CFC-11, CFC-12 presented in IPCC