ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - GREENHOUSE GASES EFFECTS ppsx

427 GREENHOUSE GASES EFFECTS INTRODUCTION The possibility that man-made emissions of carbon dioxide and other infra-red absorbing gases may enhance the natural greenhouse effect and lead to a warming of the atmosphere and attendant changes in other climate parameters such as precipitation, snow and ice cover, soil moisture and sea-level rise, constitutes perhaps the most complex and controversial of all environmental issues and one that is likely to remain high on both the scientific and political agenda for a decade or more. The issues have been obscured by a good deal of exaggeration and distortion by the media, and by some scientists, so that governments, the public, and scientists in other disciplines are confused and sceptical about the evidence for global warming and the credibility of the predictions for the future. Until very recently the atmospheric concentrations of carbon dioxide had been increasing and accelerating since regular measurements began in 1958. Only during the last few years has there been a levelling off, probably because of the world-wide recession, the run-down of industry in the former Soviet bloc, and the substitution of gas for coal. This pause is likely to be only temporary, and if the concentrations resume their upward trend, they will eventually lead to significant climate changes. The important questions concern the likely magnitude and timing of these events. Are they likely to be so large and imminent as to warrant immediate remedial action, or are they likely to be sufficiently small and delayed that we can live with them or adapt to them? Careful reconstruction of historical records of near-surface air temperatures and sea-surface temperatures has revealed that globally-averaged annual mean temperatures have risen about 0.6ЊC, since 1860 (see Figure 1). There is general con- sensus among climatolologists that this can now confidently be ascribed to enhanced greenhouse warming rather than to natural fluctuations. The last decade has been the warmest of this century and 9 of the 10 warmest years have occurred since 1990. Moreover, as described later, any temperature rise due to accumulated concentrations of greenhouse gases may have been masked by a concomitant increase in concentrations of aerosols and by delay in the oceans. ROLE OF CARBON DIOXIDE IN CLIMATE Carbon dioxide, together with water vapor, are the two main greenhouse gases which regulate the temperature of the Earth of its atmosphere. In the absence of these gases, the average surface temperature would be −19ЊC instead of the present value of +15ЊC, and the Earth would be a frozen, lifeless planet. The greenhouse gases act by absorbing much of the infrared radiation emitted by the Earth that would otherwise escape to outer space, and re-radiate it back to the Earth to keep it warm. This total net absorption over the whole globe is about 75 PW an average of 150 Wm −2 , roughly one-third by carbon dioxide and two-thirds by water vapour. There is now concern that atmospheric and surface temperatures will rise further, owing to the steadily increasing concentration of carbon dioxide resulting largely from the burning of fossil fuels. The concentration is now 367 ppmv, 31% higher than the 280 ppm which prevailed before the industrial revolution and, until very recently was increasing at 0.5% p.a. If this were to continue, it would double its pre- industrial value by 2085 AD and double its present value by 2135. However, if the world’s population continues to increase at the present rate, the concentration of carbon dioxide may well reach double the present value in the second half of this century. Future concentrations of atmospheric carbon dioxide will be determined not only by future rates of emissions, which can only be guessed at, but also by how the added CO 2 is partitioned between the atmosphere, ocean and biosphere. During the decade 1940–9, the rate of emission from the burning of fossil fuels and wood is estimated at 63 ± 0.5 GtC/yr (gigatonnes of carbon per year). The atmosphere retained 3.2 GtC (about half of that emitted), leaving 3.4 GtC/yr to be taken up by the oceans and terrestrial biosphere. Models of the ocean carbon balance suggest that it can take up only 1.7 ± 0.5 GtC/yr so that there is an apparent imbalance of 1.4 ± 0.7 GtC/yr. Some scientists believe that this difference can be accounted for by additional uptake by newly growing forests and by the soil, but this is doubtful, and the gap is a measure of the uncertainty in current understanding of the complete carbon cycle. Reliable quantitative estimates of the combined effects of the physical, chemical and biological processes involved, and hence of the magnitude and timing of enhanced greenhouse warming await further research. Nevertheless, very large and complex computer models of the climate system have been developed to simulate the present climate and to predict the likely effects of, say, doubling to atmospheric concentration of CO 2 , or of increasing it at an arbitrary rate. This approach bypasses the uncertainties © 2006 by Taylor & Francis Group, LLC 1840 1860 1880 1900 1920 1940 1960 1980 2000 –0.60 –0.50 –0.40 –0.30 –0.20 –0.10 0.00 0.10 0.20 0.30 0.40 °C COMBINED LAND, AIR AND SEA SURFACE TEMPERATURES RELATIVE TO 1951–80 AVERAGES FIGURE 1 Observed changes in the globally-averaged surface temperatures from 1860–1991 relative to the 30-year mean for 1951–1980. in future emissions and the natural regulation of atmospheric concentrations and is therefore unable to predict when the climate changes are likely to happen. MODEL SIMULATIONS AND PREDICTIONS OF CLIMATE CHANGE Introduction Since changes in global and regional climates due to anthropogenic emissions of greenhouse gases will be small, slow and difficult to detect above natural fluctuations during the next 10 to 20 years, we have to rely heavily on model predictions of changes in temperature, rainfall, soil moisture, ice cover, sea level, etc. Indeed, in the absence of convincing direct evidence, concern over an enhanced greenhouse effect is based almost entirely on model predictions, the credibility of which must be largely judged on the ability of the models to simulate the present observed climate and its variability on seasonal, inter-annual, decadal and longer time scales. Climate models, ranging from simple one-dimensional energy-balance models to enormously complex three- dimensional global models requiring years of scientific development and vast computing power, have been developed during the last 25 years, the most advanced at three centres in the USA and at the UK Meterological Office and, recently, at centers in Canada, France and Germany. Until very recently, effort was concentrated on develop- ing models (that evolved from weather prediction models) of the global atmosphere coupled to the oceans and cryosphere (sea and land ice) only through prescribing and up-dating surface parameters such as temperature and albedo, from observations. However, realistic predictions of long-term changes in climate, natural or man-made, must involve the atmosphere, ocean play, cryosphere and, eventually, the biosphere, treated as a single, strongly coupled and highly interactive system. The oceans play a major stabilizing role in global climate because of their inertia and heat storage capacity. Moreover, they transport nearly as much heat between the equator and the poles as does the atmosphere. The oceans absorb about half of the carbon dioxide emitted by fossil fuels and also absorb and transport a good deal of the associated additional heat flux and hence will delay warming of the atmosphere. During the 1980s the UK Meterological Office (UKMO) developed one of the most advanced models of the global atmosphere coupled to a shallow mixed-layer ocean and used this to simulate the present climate and to study the effects of nearly doubling the present level of carbon dioxide to 600 ppmv. A general description of the physical basis, structure and operation of the model, of its simulations and predictions may be found in Mason (1989). Simulation of the Present Climate Models of the type just mentioned, the most important computed variables of which are: E–W and N–S components of the wind Vertical motion Air temperatures and humidity 428 GREENHOUSE GASES EFFECTS © 2006 by Taylor & Francis Group, LLC GREENHOUSE GASES EFFECTS 429 Heights of the 11 specified pressure surfaces Short- and long-wave radiation fluxes Cloud amount, height and liquid-water content Precipitation/rain/snow Atmospheric pressure at Earth’s surface Land surface temperature Soil moisture content Snow cover and depth Sea-ice cover and depth Ice-surface temperature Sea-surface temperature are remarkably successful in simulating the main features of the present global climate—the distribution of temperature rainfall, winds, etc. and their seasonal and regional variations. They do, however, contain systematic errors, some different in different models, and some common to most. Identification of these errors and biases by compari- son with the observed climate is important since these must be taken into account when evaluating predictions. These may not appear to be too serious in making predictions of the effects of a prescribed (e.g., man-made) perturbation since these involve computation of the differences between a perturbed and a control (unperturbed) simulation in which the systematic errors may largely cancel. However this linear reasoning may not necessarily be valid for such complex non-linear systems even if the perturbations are small, and the predictions will carry greater credibility if the control runs realistically simulate the observed climate and its variability. FIGURE 2 Simulation of the global mean surface pressure field for June, July and August by the UKMO climate model compared with observation. © 2006 by Taylor & Francis Group, LLC 430 GREENHOUSE GASES EFFECTS FIGURE 3 Model simulation of the mean near-surface temperatures over land for June, July and August compared with observation. SUMMER SURFACE AIR TEMPERATURE (DEG C) SIMULATED OBSERVED 40 32 24 16 8 0 –8 –16 –85 40 32 24 16 8 0 –8 –16 –85 The main errors in model simulations of the present climate are discussed in IPCC (1992, 1996) and by Mason (2004). Simulations with the best models are close to reality despite the rather low model spatial resolution as illustrated by Figures 2 and 3. Model Simulations of Ocean Climate The role of the oceans in influencing climate and climate change is discussed in some detail in Mason (1993). Only the salient facts will be summarised here. The oceans influence climate change on seasonal, decadal and longer time scales in several important ways. The large- scale transports of heat and fresh water by ocean currents are important climate parameters and affect the overall magnitude, timing and the regional pattern of response of the climate system to external forcing. The circulation and thermal structure of the upper ocean control the penetration of heat into the deeper ocean and hence the time delay which the ocean imposes on the atmospheric response to increases of CO 2 and other greenhouse gases. The vertical and horizontal motions also control the uptake of CO 2 through the sea surface and thus influence the radiative forcing of the atmosphere. If ocean models are to play an effective role in the prediction of climate change, they must simulate realistically the present circulation and water mass distribution and temperature fields and their seasonal variability. Ocean modelling and validation are less advanced than atmospheric modelling, reflecting the greater difficulty of observing the interior of the ocean and of inadequate computer power. They suffer © 2006 by Taylor & Francis Group, LLC GREENHOUSE GASES EFFECTS 431 from inadequate spatial resolution, problems in parameter- izing sub-grid-scale motions, and in estimating the fluxes of heat, moisture and momentum across the air/sea interface. When forced with observed surface temperatures, salini- ties and wind stresses, ocean models have been moderately successful in simulating the observed large-scale circulation and mass distribution, but most models underestimate the meridional heat flux and make the thermocline too deep, dif- fuse and too warm. The deeper ocean is also driven, in part, by fluxes of radiant heat, momentum, and of fresh water derived from precipitation, river run-off and melting ice, but measurements of all these are difficult and very sparse at the present time. Different models show considerable differences in their simulations of the deep ocean circulation, but identification of systematic errors is hardly possible because of the paucity of observations. The distribution of temperature and salinity are the primary sources of information for checking model simulations, but it is very difficult to simulate the salinity field because the distribution of sources and sinks of fresh water at the surface is so complex. Perhaps the most effective way of checking ocean models on decadal time scales is to see how well they simulate the horizontal spread and vertical diffusion of transient tracers such as tritium/He 3 and C 14 produced in nuclear bomb tests. Current models simulate quite well their shallow penetration in the equatorial ocean and deep penetration in high latitudes but fail to reproduce the deep penetration at 30–50ЊN, probably because of inadequate resolution of the Gulf Stream and its interaction with the North Atlantic current. The computed poleward transport of heat and the transport across other designated vertical sections can be checked against hydrographic measurements being made from research ships as part of the World Ocean Circulation Experiment, as described in Mason (1993). Some detailed measurements are also being made on the seasonal variation in the depth of the mixed ocean layer and the thermocline that can be compared with the model simulations. Coupled Atmosphere—Deep Ocean Models The UKMO has developed a deep global ocean model coupled to its global atmospheric model to carry out long-period climate simulations and to make realistic predictions of climate changes produced by gradual increases of atmospheric CO 2 until it reaches double the present value. The results of the first of these enhanced CO 2 experiments, and of similar ones conducted elsewhere, are described in the following section. Here we summarise the structure and operation of the coupled model, its problems and deficiencies, and the research in progress to overcome them. A more detailed analysis of the first version is given by Murphy (1995). In the latest version, the model atmosphere is divided into 19 layers (20 pressure levels) between the surface and 50 km with 5 levels in the surface boundary layer (lowest 1 km) to allow calculation of the surface fluxes of heat, moisture and momentum. There are also four levels in the soil to calculate the heat flux and hence the surface temperature. The variables listed in the previous section Simulation of the Present Climate are calculated on a spherical grid with mesh 2.5Њ lat ϫ 3.75Њ long, about 7,000 points at each level. The incoming solar radiation is calculated as a function of latitude and season, and diurnal variations are included. Calculations of radiative fluxes at each model level use four wavebands in the solar radiation and six bands in the long-wave infra-red, allowing for absorbtion and emission by water vapour, carbon dioxide, ozone and clouds. Sub-grid-scale convection is represented by a simple cloud model that treats the compensat- ing subsidence and detrainment of air and the evaporation of precipitation. Precipitation is calculated in terms of the water and ice content of the cloud; cooling of the atmosphere by evaporation of precipitation is allowed for. Reduction in wind speed caused by the aerodynamic drag of mountains, oceans waves, and by the breaking of ororgraphically-induced gravity waves are computed. In calculating changes in the extent and thickness of sea ice, drifting of the ice by wind-driven ocean currents is taken into account. In the land surface model the different soil types and their differing albedos are specified, as are the different types of vegetation, their seasonal changes and their effects on evaporation, albedo, aerodynamic drag. The ocean model computes the current, potential temperature, salinity, density and the transports of heat and salt at 20 unequally-spaced levels (depths) in the ocean, eight of these being in the top 120 m in order to simulate better the physics and dynamics in the active, well-mixed layer, its seasonal variation, and the surface exchanges of heat, moisture and momentum with the atmosphere. The vertical veloc- ity at the sea floor is computed assuming flow parallel to the slope of the bottom topography specified on a 1Њϫ 1Њ data set. The horizontal grid, 2.5Њϫ 3.75Њ, the same as that of the atmospheric model, is too coarse to resolve oceanic meso-scale eddies of scale ෂ100 km which contain much of the total kinetic energy, but are crudely represented by sub- grid-scale turbulent diffusion and viscosity. The latter has to be kept artificially high to preserve computational stability with the penalty that the simulated currents, such as the Gulf Stream, are too weak. Lateral diffusion of heat and salt take place along ispycnal (constant density) surfaces using diffusion coefficients that decrease exponentially with increasing depth. The coefficients of vertical diffusion are specified as functions of the local Richardson number, which allows for increased mixing when the local current shear is large. Coupling with the atmosphere is accomplished in three stages. The atmospheric model, starting from an initial state based on observations, is run on its own until it reaches an equilibrium climate. The ocean model, starting from rest and uniform temperature and salinity is also run separately, driven by the wind stresses, heat and fresh-water fluxes provided by the atmospheric model. This spin-up phase of the ocean takes place over 150 years (restricted by available computer time) during which a steady state is achieved in the upper layers of the ocean as they come into equilibrium with the atmospheric forcing. Finally, the ocean is coupled to the atmosphere, sea-ice and land-surface components © 2006 by Taylor & Francis Group, LLC 432 GREENHOUSE GASES EFFECTS TABLE 1 Global mean changes in temperature and precipitation caused by doubling CO 2 in various models in “Equilibrium” Model T(°C) P(%) Remarks UKMO (1987)* 5.2 15 GDFL (1989) 4.0 8 GISS (1984)* 4.8 13 Very low (8° ϫ 10°) resolution SUNY (1991) 4.2 8 CSIRO (1991) 4.8 10 NCAR (1991) 4.5 5 Models with Computed Cloud Water/Ice UKMO (1989) 3.2 8 Fixed radiative properties 1.9 3 Variable radiative properties as function of water/ ice content GDFL Geophysical Fluid Dynamics Laboratory, Princeton, USA GISS Goddard Institute of Space Studies SUNY State University of New York SCRIO Commonwealth Scientific and Industrial Research Organization, Australia NCAR National Center for Atmospheric Research, Boulder, USA and run in tandem with two-way feedbacks between ocean and atmosphere transmitted at five-day intervals. Thus the atmospheric model is run separately for five days with unchanged sea-surface temperatures and sea-ice extents, accumulating relevant time-averaged surface fluxes, which are then used to drive the corresponding time step of the ocean model, following which the updated sea-surface temperatures and sea-ice cover are fed back to the atmosphere for the next iteration. When an internally consistent balance is obtained between all four main components of the climate system, the final state may be taken as the starting point for perturbation experiments such as the doubling of carbon dioxide. MODEL PREDICTIONS OF CLIMATE CHANGES CAUSED BY DOUBLING PRESENT CONCENTRATIONS OF CARBON DIOXIDE Introduction We recall that atmospheric concentrations of carbon dioxide are likely to double by the second half of this century and that simple radiative calculations, allowing only for feedback from the accompanying increases in water vapour, indicate that this might cause the globally and annually averaged surface air temperature to rise by about 1.5ЊC. Because, as discussed by Mason (1995), many other feedback processes, both positive and negative, operate within the complex climate system, and because their effects are likely to vary with season, latitude and geographical location, firmer estimates can come only from model experiments in which the climate simulated by a model perturbed by the doubling of CO 2 is compared with that from an unperturbed (control) model, the differences being attributed to the enhanced CO 2 . We now compare and discuss the results of two types of experiments, produced by different models. In one set, involving a global atmosphere coupled to only a shallow ocean, the CO 2 concentration is doubled in one step and the climatic effects are assessed after the system has reached a new equilibrium. In the second set, in which the atmosphere is coupled to a multi-layered deep ocean, the CO 2 is allowed to increase at 1% p.a. compound and so doubles after 70 years. Prediction of Global Mean Changes in the ‘Equilibrium’ Experiments All six models cited in Table 1 comprise a global atmosphere with 9–12 levels in the vertical, coupled to a shallow (50 m deep) ocean with prescribed heat transport. The input solar radiation to all models follows a seasonal cycle, but only those marked with an asterisk include a diurnal cycle. All the models have a rather low horizontal resolution and all the experiments were run for Ͻ50 years. Furthermore, all of them prescribe the cloud amount and height by empirical formulae that relate cloud to relative humidity and are based on satellite observations of cloud. The radiative properties of the clouds (classified into low, medium and high-level categories) are also prescribed and remain fixed during the model simulation. The predicted globally and annually-averaged increases in surface air temperature due to doubling of CO 2 are remarkably similar, ranging from 4.2ЊC to 5.2ЊC with an average of 4.6ЊC. This is probably because the sea-surface temperatures and sea-ice cover are constrained to be near observed values by adjusting the advective heat fluxes in the shallow ocean. The predicted increase in precipitation, © 2006 by Taylor & Francis Group, LLC GREENHOUSE GASES EFFECTS 433 not surprisingly, show a greater spread, from 5 to 15% with an average of 10%. These predictions were not much affected by doubling the horizontal resolution (having the grid spacing). However, they were much more sensitive to the formulation of physical processes, in particular the representation of clouds and their interactions with solar and terrestrial radiation. Model simulations in which the cloud water was computed from the model variables and their radiative properties (emissiv- ity, absorptivity and reflectivity) were allowed to vary with the liquid water and ice content produced significantly different results as summarized in Table 1. The UKMO model, using three progressively more sophisticated and realistic cloud/radiation schemes, has progressively reduced the predicted global warming from 5.2ЊK to 1.9ЊK and the corresponding precipitation increases from 15% to 3%. It is important to identify and understand the underlying physical reasons for these results which, if confirmed, are likely to have an important influence on the whole GHW debate. In the first version of the model, in which cloud cover was related empirically only to relative humidity and the radiative properties were fixed during the whole simulation, enhanced CO 2 produced unrealistic decreases in high-, medium- and low-level clouds, except at very high latitudes and, consequently, an exaggerated warming of the atmosphere. Decrease in cloud amount seems inconsistent with the predicted increase in precipitation and suggests that the empirically derived cloud cover was incompatible with the internal dynamics of the model. In the most sophisticated treatment, the cloud water is computed from the dynamical and physical equations; it is transformed progressively from liquid water to ice as the temperature falls below −15ЊC; rapidly growing ice crystals are allowed to fall out of the cloud; and the radiative properties are varied as a function of the cloud water path and the solar angle for the incoming solar radiation and as a function of the water/ice path for terrestrial long-wave radiation. In this case, enhanced CO 2 leads to a marked increase in the extent and optical depth of call clouds, and especially of low clouds in middle and high latitudes, which reflect more of the solar radiation to space and therefore reduce the GHW of the atmosphere to only 1.9ЊK. The small 3% increase in precipitation is consistent with a 2–3% increase in low cloud cover and a 2% increase in medium-level cloud in the Northern Hemisphere. A more detailed account is given by Senior and Mitchell (1993). Transient Experiments in Which CO 2 Increases at 1% p.a. The fact that we now have fully three-dimensional models of the global oceans coupled interactively to the atmosphere, land- surface and sea-ice components of the climate model, enabling FIGURE 4 Prediction of the UKMO coupled atmosphere—deep ocean model of global warming caused by increasing the concentration of atmospheric carbon dioxide by 1% p.a. compound after 75 years. COUPLED MODEL 10 YEAR ANNUAL MEAN (YEARS 66 TO 75) SURFACE AIR TEMPERATURE <–2 –2 to 0 C 0 to 1 C 1 to 2 C 2 to 4 C >4 C © 2006 by Taylor & Francis Group, LLC 434 GREENHOUSE GASES EFFECTS FIGURE 5 Predictions of globally—averaged warming caused by increasing the concentration of carbon dioxide by 1% p.a. compound over 75 years showing the year-to-year changes. The changes for the northern and southern hemispheres are shown separately. 051015 20253035 40 45 50556065 70 75 YEAR –1.0 –0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 TEMPERATURE DIFFERENCE (K) GLOBAL MEAN MEAN OVER S HEMISPHERE MEAN OVER N HEMISPHERE (c) (a) (b) more realistic simulations in which the carbon-dioxide, instead of being doubled in one step, is increased gradually at 1% p.a. compound to double after 70 years. On this time-scale, the atmospheric response will be influenced by changes occurring at depth in the oceans, and especially in the top 1 km. The first results of such an experiment were published by Manabe et al. (1990) from GDFL. The globally and annually averaged increase in surface air temperature was 2.3 K, lower than in earlier models with a shallow ocean. The reduced warming was especially marked in the Southern Hemisphere, which showed little amplification in the Antarctic compared with the Arctic. This is explained by the ocean circulation in the southern oceans having a downward branch at about 65ЊS, which carries much of the additional ‘greenhouse’ flux of heat from the surface to depth of Ͼ3 km, where it remains for many decades. Very similar results were produced with the earlier version of the UKMO model by Murphy (1990), Murphy and Mitchell (1995). The annually averaged response in global mean surface temperature to CO 2 increasing 1% p.a. over 75 years is shown in Figure 4, and also in Figure 5, which also shows the results for the hemispheres separately. Averaged over the years ’66 −’77, the global mean warming was 1.7ЊK. The corresponding increase for the Northern Hemisphere was 2.6ЊK, with warming of Ͼ4ЊK over large areas of the Arctic. The UKMO model, like the GDFL model, shows that the much smaller response of the Southern Hemisphere is due to the transport of heat from the surface to depth in a strong down-welling circulation near 60ЊS. A similar vertical circulation, caused by melting ice, and penetrating to about 1.5 km depth, occurs at about 60ЊN in the North Atlantic (see Figure 6). After a slow start, the enhanced global warming settles down at about 0.3 K/decade. Moreover, the model exhibits variability on inter-annual and decadal time-scales; the peak-to-peak variation on the decadal scale being about 0.3ЊK—of the same magnitude as the predicted signal due to ‘greenhouse’ warming. A similar long-term run with a coupled atmosphere— deep ocean model has been carried out at the Max Planck Institute in Hamburg by Cusbasch et al. (1992). CO 2 is allowed to increase rather more rapidly to double after 60 years and produces a global mean warming of 1.3ЊK, the lowest value so far reported. The transient responses to the doubling of CO 2 by all three models, ranging from 1.3 to 2.3 K, correspond to about 60% of the expected equilibrium response. This implies a lag of about 30 years due largely to the delaying effect of oceans. The predicted changes in precipitation, though small on average, are far from uniformly distributed. The UKMO model indicates increases in high latitudes of the Northern Hemisphere throughout the year, in middle latitudes © 2006 by Taylor & Francis Group, LLC GREENHOUSE GASES EFFECTS 435 in winter, and during the S.W. Asian monsoon. In the Southern Hemisphere precipitation increases in the middle- latitude storm tracks throughout the year. Soil moisture is enhanced over the middle latitude continents of the Northern Hemisphere in winter but, in summer, many areas show a deficit mainly because of the earlier retreat of the snow cover under the enhanced temperatures. Although the four models show broadly similar global patterns of response to double CO 2 concentrations, they show marked differences on regional and sub-regional scales, especially in precipitation and soil moisture. Predictions of globally-averaged changes in temperature, precipitation and soil moisture are of little value in assess- ing their political, economic and social impact. Although current global models with rather low spatial resolution cannot be expected to provide reliable scenarios in regional and sub-regional scales, the UKMO has been asked to make deductions from its ‘transient’ CO 2 experiment for Western Europe. The results, which should be treated with caution, may be summarized as follows. Summer temperatures rise throughout the 70-year experiment, stabilizing at about 0.3ЊK per decade after year twenty. There is a similar but less steady warming in winter, most pronounced over land. Winter precipitation increases rapidly during the first 30 years (possibly an artefact of an inadequate spin-up period) but thereafter remains rather steady at an average increase of about 0.3 mm/day, the main increases occurring over N. Europe and reductions in S. Europe and the Mediterranean. In summer the precipitation decreases by about 0.2 mm/day. The warmer, wetter winters and the slightly warmer drier summers are reflected in the changes of soil moisture. Since the decadal changes are comparable in magnitude to the decadal variability, the comparable in magnitude to the decadal variability, the confidence in these estimates is low, especially in respect of precipitation and soil moisture changes, which are only marginally significant relative to the variability of the ‘control’ model, for any single decade. THE EFFECT OF AEROSOLS Aerosol particles influence the Earth’s radiation balance directly by their scattering and absorption of solar radiation. FIGURE 6 Changes in the ocean temperatures averaged around latitude bands and shown as a function of depth after the carbon dioxide has doubled in the model experiment of Figures 4 and 5. These range from about 1°K near the surface to about 0.4 K at 3 km depth near 65°S. (See Color Plate VII) .5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 80 60 40 20 0 –20 –40 –60 –80 LATITUDE (DEG N) DEPTH (KM) COUPLED MODEL 10 YEAR ANNUAL MEAN TEMPERATURE (YEARS 66 TO 75) < –0.2 –0.2 to 0 C 0 to 0.2 C 0.2 to 0.4 C 0.4 to 0.8 C >0.8 C © 2006 by Taylor & Francis Group, LLC 436 GREENHOUSE GASES EFFECTS 1900 1950 2000 2050 –1 0 1 2 3 4 ∆T °K (a) (c) (b) FIGURE 7 Changes in the globally-averaged mean surface temperatures relative to the mean for 1850–1920; dotted curve—observed change since 1880; dashed curve—model computations of the effects of increasing greenhouse gases from 1850–1990 and extrapolated to 2050 AD; solid curve—model predictions of changes caused by both greenhouse gases and aerosols from 1850–2040. They also absorb and emit long-wave radiation but usually with small effect because their opacity decreases at longer wavelengths and they are most abundant in the lower troposphere where the air temperature, which governs emissions, is close to the surface temperature. Aerosols also serve as cloud condensation nuclei and therefore have the potential to alter the microphysical, optical and radiative properties of clouds. The larger aerosol particles of d Ͼ 0.1 ␮m, if produced in large quantities from local sources such as forest fires, volca- noes and desert storms, may significantly influence the radiation balance on local and regional scales, both by scattering and by absorption and emission, especially if they contain carbon particles. However, such particles are rapidly removed from the troposphere by precipitation and are not normally carried long distances. On the global scale, smaller particles of d Ͻ 0.1 ␮m are more important, their dominant effect being to cool the atmosphere by scattering solar radiation to space. Some recent calculations by Charlson et al. (1990) of the impact of anthropogenic sulphate particles on the short- wave radiation balance in cloud-free regions conclude that, at current levels, they reduce the radiative forcing over the Northern Hemisphere by about 1 W/m 2 with an uncertainty factor of two. A rather more sophisticated treatment by Kiehl and Briegel (1993) calculated the annually-averaged reductions in radiative forcing due to back-scattering of solar radiation by both natural and anthropogenic sulphate aerosols to be 0.72 W/m 2 in the N. Hemisphere, 0.38 W/m 2 in the southern hemisphere the global value of 0.54 W/m 2 being about half of that calculated by Charlson. However, the high aerosol concentrations over the heavily industrialised regions of the eastern USA, central Europe and South-East Asia produced reduction of Ͼ2 W/m 2 that are comparable to the cumulative increases produced by greenhouse gases emitted since the industrial revolution. In addition to the direct effect on climate, anthropogenic sulphate aerosols may exert an indirect influence by acting as an additional source of effective cloud condensation nuclei, thereby producing higher concentrations of smaller cloud droplets leading to increased reflectivity (albedo) of clouds, especially of low clouds, for solar radiation, which is sensitive to the ‘effective’ droplet radius rWN eff a( 13 ր ր ) where W is the liquid–water concentration of the cloud (in g/m 3 ) and N is the number concentration of the droplets. The first calculations of this indirect effect on climate have been made to the UKMO by Jones et al. (1994), using their climate model that predicts cloud liquid water and ice content and parameterizes r eff linking it to cloud type, water content and aerosol concentration. The concentration and size distribution of the aerosol, and its spatial distribution are calculated in the same manner as in Kiehl and Briegel but the particles are assumed to consist of ammonium sulphate as being characteristic of aerosol produced in industrially polluted air. The calculations indicate that the enhanced back-scatter of solar radiation, mainly from low-level clouds in the atmospheric boundary layer, produces an annually-averaged global cooling of 1.3 W/m 2 but that over the highly industrialized regions, where r eff may be reduced by as much as 3 ␮m, the cooling may exceed 3 W/m 2 . However, it must again be empha- sized that these calculations contain major uncertainties, probably even larger than those for the direct effect. Taking them at face value, the calculations of the direct and indirect effects combined, suggest an average global negative forcing of 1.5–2 W/m 2 that may have largely offset the positive forcing of 2.3 W/m 2 by greenhouse gases to-date, and this may be at least part of the reasons for failure to detect a strong greenhouse signal. The first results of introducing sulphate aerosols into a coupled atmosphere-ocean model come from the UKMO (Mitchell et al. 1995). The model, starting from an initial state determined by surface observations in 1860, was run forward to 1990 with no man-made greenhouse gases or aerosols as a control experiment. The model’s average global surface temperatures showed realistic inter-annual variations but no overall rise over this period. In the perturbation experiment greenhouse gases were gradually increased from 1860 to reach a 39% equivalent increase in CO 2 by 1990; this resulted in a temperature rise of 1ЊC compared with an observed rise of only 0.5ЊC, (Figure 7). The next step was to compute the effects of sulphate aerosols with best estimates of concentration and geographical distribution. The direct effects of increasing the back-scatter of solar radiation was to reduce the warming between 1860 and 1990 to only 0.5ЊC, very close to the observed, but over and downwind of the highly industrialized regions of North America, Europe and Southern Asia, the aerosols largely nullify the warming caused by the greenhouse gases. © 2006 by Taylor & Francis Group, LLC [...]... important consequence of greenhouse warming is the melting of sea-ice and ice sheets on land, only the latter resulting in a rise in sea level The sea level will also rise as the ocean waters expand in response to the additional warming Estimates of these consequences involve large uncertainties because of the lack of observations and understanding of the mass balance and dynamics of glaciers and ice sheets... UNCERTAINTIES IN MODEL PREDICTIONS In summarising the current state of knowledge and understanding of the likely magnitude, timing and impacts of enhanced greenhouse warming, it is virtually certain that the troposphere is warming very slowly in response to the continually increasing concentrations of carbon dioxide and other greenhouse gases, but the signal is as yet barely detectable above the large... temperature and salinity remain close to present-day climatology and that the control model climate does not drift during long runs Long-term drift in the climate of the Southern Hemisphere arises from an imbalance in the heat budget of the Antarctic leading to a spurious slow-melting of the ice This has now been corrected and changes in the pack-ice are now included Another important defect of current... defects in the treatment of atmosphere-ocean interactions There is also a need for an improved representation of atmospheric boundary layer Even if the various models agree quite well on the globally-averaged effects, they show larger differences on regional and sub-regional scales, which are politically and economically more relevant Further improvements in model 438 GREENHOUSE GASES EFFECTS development... atmosphere interface Representations of the radiative effects of clouds, of atmospheric convection and of the drag exerted by mountain-induced gravity waves have all been improved The model now remains stable when run for 1,000 years and shows no long-term drift in the global climate Changes are calculated at about one million grid points so that computation of one annual cycle of the global climate involves... very uncertain future global emissions of greenhouse gases and their retention in the atmosphere We must also realise that no existing climate model incorporates the carbon cycle in which exchanges of CO2 between the earth’s surface and the atmosphere are dominated by terrestrial and especially marine biology, man-made emissions being only about 3% of the natural two-way exchanges We are always faced with.. .GREENHOUSE GASES EFFECTS When the coupled model runs were carried forward from 1990 to 2050, increasing the CO2 by 1% p.a compound, the effect of aerosols was to reduce the global greenhouse warming from the 0.3ЊC/decade shown in Figure 5 to only 0.2ЊC/decade, and to largely offset it in highly polluted regions More reliable estimates of the effects of aerosols on climate must... might be provided by new and advanced technology, are discussed by Mason (1993) Despite these uncertainties and the fact that a doubling of CO2 will cause an increase of only ‫ %3ق‬in the downward flux of infra-red radiation from the greenhouse gases, future predictions of the resulting globally-averaged temperature rise are unlikely to lie outside the range 1ЊC to 2.5ЊC However, the models provide little... size, chemical composition and spatial distribution of both natural and anthropogenic aerosols, including strongly absorbing carbonaceous particles, and dusts, and on the difference between droplet concentrations and sizes in clean maritime and polluted continental clouds These data will be difficult and expensive to acquire; meanwhile, we are likely to have too many theories and computations chasing... model physics, much faster computers and, above, all, an adequate supply of global observations from both the atmosphere and the oceans, to feed and validate the models, and to monitor the actual changes in climate that may eventually become evident The need for observations from both the surface and the interior of the oceans, and how they might be provided by new and advanced technology, are discussed . 427 GREENHOUSE GASES EFFECTS INTRODUCTION The possibility that man-made emissions of carbon dioxide and other infra-red absorbing gases may enhance the natural greenhouse effect and lead. and humidity 428 GREENHOUSE GASES EFFECTS © 2006 by Taylor & Francis Group, LLC GREENHOUSE GASES EFFECTS 429 Heights of the 11 specified pressure surfaces Short- and long-wave radiation. fluxes of radiant heat, momentum, and of fresh water derived from precipitation, river run-off and melting ice, but measurements of all these are difficult and very sparse at the present

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