Radiative forcing of climate change

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1 Radiative forcing of climate change Joanna D. Haigh Imperial College of Science, Technology and Medicine, London Radiative forcing concept, definition and applications On a global and annual average, and if no climate change is taking place, the earth is in radiative equilibrium. This means that the amount of solar radiation it absorbs is exactly balanced by the amount of infrared radiation it emits, with the balance achieved by the adjustment of the surface and atmospheric temperatures. If some factor, such as atmospheric composition, changes then the radiation balance is likely to be disturbed and the temperatures will adjust until a new equilibrium is established. The simplest definition of radiative forcing (RF) is the change in the value of the net radiative flux (i.e. the incoming flux minus the outgoing flux) at the top of the atmosphere in response to some perturbation. The perturbing factors might include changes in solar irradiance or planetary albedo as well as the concentrations of radiatively active gases, aerosols or cloud. The RF value is estimated assuming an instantaneous response in the radiation field and thus that no other changes (specifically in temperature) have occurred. If the RF is positive then there is an increase in energy entering the system, or equivalently a decrease in energy leaving the system, and it will tend to warm until the outgoing energy matches the incoming and the net flux is again zero. Conversely, a negative RF, meaning a decrease in energy entering or an increase in energy leaving, will result in global cooling. 278 The concept of RF has been found to be a useful tool in analysing and predicting the response of surface temperature to imposed radiative perturbations. This is because experiments with general circulation models (GCMs) of the coupled atmosphere± ocean system have found that the change in globally averaged equilibrium surface temperature, T g, is approximately linearly related to the radiative forcing: DT g = lrf where l is the `climate sensitivity parameter which, although uncertain (see Collins and Senior, this issue), has been found to be fairly insensitive to the nature of the RF perturbation in individual GCMs. Thus a calculation of the RF due to a particular perturbation gives a first-order indication of the potential magnitude of its effect on surface temperature without the need for costly GCM runs. The following represents a summary of the main findings of Chapter 6 of the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC TAR) ``Radiative forcing of climate change. Instantaneous and adjusted RF It has been found that the value of l, and thus the usefulness of the RF concept, is more robust if, instead of using the instantaneous change in net flux at the top of the atmosphere, RF is defined at the tropopause with the stratosphere first allowed to adjust to the imposed

2 changes. Thus, a formal definition of RF, as used by the IPCC, is the change in net flux at the tropopause after allowing stratospheric temperatures to adjust to radiative equilibrium, but with surface and tropospheric temperatures held fixed. The effects of the stratospheric adjustment are complex as can be illustrated by the case of changes in stratospheric ozone. An increase in ozone masks the lower atmosphere from solar ultraviolet, i.e. reducing net flux and thus RF; however, the presence of ozone in the lower stratosphere increases the downward infrared emission (and RF) both directly, because of the increase in its concentration, and indirectly through the increase in stratospheric temperatures which it produces. Whether the net effect is positive or negative depends on whether the short-wave or long-wave effect dominates and this is determined by the vertical distribution of the ozone change; for perturbations to lower stratospheric ozone RF is positively correlated with ozone concentration. Greenhouse gases such as CO 2 and CH 4 tend to cool the stratosphere so that for them the adjusted RF values are slightly lower than the instantaneous values. Feedbacks In an equilibrium state, water vapour and clouds form part of the overall radiation balance. However, a factor which induces a change in humidity or in cloud cover, drop size or altitude will introduce an RF. If the changes are brought about by another forcing factor then their effects might be viewed as a feedback on the initial forcing. For example, an increase in greenhouse gases might cause a surface warming and this might induce enhanced evaporation or convection. Any increase in humidity would probably increase this warming, whereas an increase in thick convective cloud would reduce it. However, such feedback effects are included in the GCMs used to assess the viability of the RF concept and are thus implicitly included in the values of the climate forcing parameter derived from GCM runs. Therefore, the humidity or cloud produced by a dynamical response to other forcings cannot be viewed as an additional forcing component. Only if changes to cloud properties are induced in situ by chemical or microphysical processes can they produce an RF, in the climate change sense, rather than a feedback. Such circumstances are discussed below in the section on the indirect effects of aerosols. Uncertainties and approximations in the representation of cloud formation and cloud radiative properties remain a major cause of uncertainty in current climate prediction models. This uncertainty is reflected in the wide range of estimates for l which is given in the IPCC TAR as 0.3 <l<1.0 K (W m -2 ) -1. RF since 1750 Figure 1 (p. 289) (which is Fig. 6.6 of the IPCC TAR) shows the RF values deduced for the period 1750 to 2000 for a range of different factors. The height of the bar indicates a best estimate and the vertical lines between Xs indicate a range of the uncertainty based on published estimates. The vertical lines between Os, with no bar, give a range of values for components for which large uncertainties preclude the presentation of a best estimate. For each forcing a `level of scientific understanding (LOSU) is assigned (H, M, L, VL corresponding to high, medium, low and very low respectively). This is a subjective assessment of the reliability of the RF estimates based on knowledge of the assumptions made in calculating them and of the physical/chemical mechanisms involved in producing them. Well-mixed greenhouse gases The largest contribution to RF since pre-industrial times comes from the greenhouse gases whose lifetimes are sufficiently long that the gases are well mixed in the troposphere. These comprise carbon dioxide, which has contributed +1.46Wm -2, methane (+0.48), nitrous oxide (+0.15) and the halocarbons (+0.34). The uncertainty on the total of +2.43Wm -2 is estimated at 10%. Ozone Stratospheric ozone: The destruction of stratospheric ozone by halocarbons has caused a global average RF of ± 0.15Wm -2 (±0.1 Wm -2 ). 279

3 This has been concentrated in high latitudes and particularly the Antarctic where ozone depletion has been most marked. The evolution of the ozone column amounts is quite well known but the large uncertainty in RF arises mainly from the problems in assessing the lower stratospheric temperature change (as discussed above). As the ozone layer recovers in response to the banning of harmful halocarbons this negative component will reduce and thus provide less of an offset to the effects of the well-mixed greenhouse gases. Tropospheric ozone: Ozone is transported into the troposphere from the stratosphere but is also produced in situ during biomass burning, in urban pollution and due to aircraft emissions. The lifetime of ozone in the troposphere is of the order of weeks so its distribution is highly variable in both space and time. This is a key uncertainty in calculating its RF, as is a lack of global information on its pre-industrial distribution. The RF for tropospheric ozone since 1750 is estimated as +0.35Wm -2 (±0.15). Aerosols Direct effects: Human activity has introduced into the atmosphere a range of particulate material which may impact both the solar and infrared radiation streams. To assess the radiative effects requires information on both the distributions and the radiative properties of the particles. Knowledge of the distributions is based on limited observations and the results of chemical transport models which, given prescribed production rates, have been used to estimate the chemical transformation and transport of aerosols. Sulphate aerosol is mainly found over, and downwind of, industrial regions. The optical properties of sulphates mixed with water are well known, but the growth of the aerosols, and thus their size and scattering properties, depends on the local humidity. This leads to significant uncertainties in the calculation of RF values. Their absorption of infrared radiation is very small compared with their scattering of solar radiation giving a negative forcing, a best estimate of which is ± 0.4 Wm -2 with a factor of 2 uncertainty. 280 Other anthropogenic aerosols include black carbon and organic carbon emitted during the combustion of fossil fuel and biomass burning. The first of these absorbs solar radiation, giving a positive RF, while the latter scatters solar radiation, giving a negative RF. The RF of fossil fuel black carbon is estimated as +0.20Wm -2, again with a range of a factor of 2. The uncertainties here are in the distribution (particularly in the vertical and whether it lies above or below cloud), particle size and the degree to which these particles mix with the sulphate droplets. The organic carbon consists of complex chemical compounds; the radiative forcing due to fossil fuel organic carbon is estimated as ± 0.10Wm -2 within a range of a factor of 3. Uncertainties are due to a lack of knowledge of distribution, optical properties, humidity and cloud chemistry effects. The combined effects of biomass burning, black and organic carbon are estimated to produce a radiative forcing of ± 0.20Wm -2 within a range of a factor of 3. The other direct aerosol component included in Fig. 1 is mineral dust released into the atmosphere by human activities, mainly in changing land use. Even the sign of this effect is uncertain but its RF is assessed to lie in the range ± 0.60 to +0.40Wm -2. The effect on surface albedo of changes in land use is estimated to have produced a radiative forcing of ± 0.20Wm -2 but with a100%uncertainty range. Indirect effects: Atmospheric aerosols act as condensation nuclei for cloud droplets. Thus changes in the concentration of aerosol have the potential to affect cloud properties and hence the radiation budget. This chain of events is referred to as producing an indirect RF due to the aerosol. Two mechanisms for aerosol indirect effects have been proposed. In the first, an increase in aerosol concentration results in an increase in cloud droplet concentration and a decrease in droplet size. A cloud composed of smaller droplets generally has a higher albedo so the effect of the aerosol is to produce a negative RF. This mechanism has been observed to take place in satellite observations of ship tracks where the impact of a ship s exhaust can be seen as a brighter band within an existing cloud. However, calculations of the magnitude of such processes globally have

4 been based on GCMs using a range of assumptions which are difficult to assess quantitatively from observations. These include the models representations of cloud and aerosol distributions, of aerosol± cloud interactions and of the optical properties of the mixtures. The IPCC TAR was unable to present a best value for the aerosol indirect effect (1st type) but presented a range of ± 2.0 to 0.0 Wm -2 for all aerosol types. In the second indirect effect, clouds produced with smaller droplets may have a reduced precipitation efficiency, resulting in a higher water content and longer lifetime. Large (several Wm -2 ) negative values have been estimated by some authors for this effect but quantifying it is even more difficult than for the first indirect effect as it requires comparison of the results of two GCM runs (with and without added aerosol) and assessment of whether any simulated differences are statistically significant, given large natural variability. The IPCC TAR was unable to present with confidence even a range of values for the second indirect effect. Time evolution of forcings and natural forcings The RF values discussed above give the total RF due to different components from the preindustrial period to the present. However, it is useful to know how the values have evolved during that period as it indicates the importance of different components over different periods and provides input for climate models. Figs. 2(a) and (b) (from Fig. 6.8 of the IPCC TAR) show an assessment of these for the greenhouse gases, stratospheric and tropospheric ozone and direct aerosol effects. (Note the different scales on the ordinates of the four panels in Fig. 2.) Care should be taken in interpreting these figures in terms of temperature change ± the relationship between DT g and RF presented above applies to the equilibrium surface temperature response so that an instantaneous value of RF cannot be related to an instantaneous value of DT g. Solar variability: The solar contribution to RF over the past 250 years is assessed to be 0.3±0.2 Wm -2. The year 1750 was chosen to represent the pre-industrial atmosphere but for a naturally varying factor like the sun this date is somewhat arbitrary. As shown in Fig. 2(c), a choice of early in the nineteenth century would have given a significantly larger value. Note, also from this figure, that estimations of solar irradiance variations vary widely, which contributes to the uncertainty range given in the bar chart. The IPCC gives the assignation `very poor to the LOSU associated with solar RF, thereby acknowledging that there may be factors as yet unknown, or not fully understood, which may act to amplify (or even diminish) its effects. These may include additional effects due to solar-induced ozone changes or possibly a direct influence of variations in galactic cosmic rays on cloud cover. Volcanoes: Large volcanic eruptions inject significant loadings of sulphate aerosol into the stratosphere where they produce local heating but mask the troposphere from solar radiation thus producing a negative RF. Because of the episodic nature of volcanic eruptions the IPCC did not include a value for its RF since However, as shown in Fig. 2(d), volcanoes can have a significant effect with global RF values exceeding 1 Wm -2 for periods of months or longer. It is interesting to note that over the latter part of the twentieth century the negative volcanic RFs are larger than the positive solar RFs so that, if these estimates are correct, natural RF processes are unlikely to be responsible for the warming over that period. However, care should be taken in summing RF values as discussed below. Spatial distributions and summing forcings The RF values discussed above all relate to global averages. However, some of the agents have very marked geographical distributions, e.g. sulphate aerosol loading is much higher in the Northern Hemisphere, biomass burning aerosol only occurs downwind of fires, stratospheric ozone depletion is mainly in high latitudes. No attempt should be made to deduce regional temperature changes from local RF values using the global average climate sensitivity parameter because the response in any particular region will depend sensitively on the local situation (topography, albedo, land± sea 281

5 Fig. 2 Time evolution of global radiative forcing components. (a) Well-mixed greenhouse gases, tropospheric and stratospheric ozone, (b) aerosol direct effects, (c) solar variability using two different published estimates of solar irradiance and (d) volcanic aerosol in the stratosphere using two different published estimates of stratospheric aerosol loading. (References are from Chapter 6 of the IPCC TAR.) distribution, etc.) and is also quite likely to be non-local. Another potential misuse of RF values is to sum different contributions to find a net effect. This would not be a problem if each agent acted independently but this is not always the case. For example, any change in cloud cover or properties would affect the RF values of most of the other agents; mixtures of gases can result in chemical changes or overlap of spectral absorption bands, both of which will produce nonlinear effects. This argument applies equally to local RF contributions or to the global average values in Fig. 1. Conclusions RF remains a useful tool for providing a firstorder assessment of the relative contributions of different agents to global surface temperature change. However, care must be taken in applying the RF concept, or RF values, to situations other than a global annual mean and also in combining the effects of different agents. Uncertainties in the magnitudes of some forcings remain large and, in particular, the potential of anthropogenic aerosol to affect cloud distribution and properties is not well understood or quantified. 282

6 Acknowledgements The material in this article is largely drawn from Chapter 6 of the IPCC TAR; I am happy to acknowledge my co-authors on that chapter: V. Ramaswamy, O. Boucher, D. Hauglustaine, J. Haywood, G. Myhre, T. Nakajima, G. Y. Shi and S. Solomon. Correspondence to: Prof J. D. Haigh, Space and Atmospheric Physics, Blackett Laboratory, Imperial College of Science, Technology and Medicine, London SW7 2BW. j.haigh@ic.ac.uk # Royal Meteorological Society, Projections of future climate change 283