ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - GREENHOUSE GASES EFFECTS ppsx

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 atmosph

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

pre-cipitation, 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

sig-nificant 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 0.6C, 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

concen-trations 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 −19C instead of the present value of +15C, 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 tem-peratures 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 diox-ide 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 bio-sphere 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 atmo-sphere retained 3.2 GtC (about half of that emitted), leaving 3.4 GtC/yr to be taken up by the oceans and terrestrial bio-sphere 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 under-standing of the complete carbon cycle Reliable quantitative estimates of the combined effects of the physical, chemical and biological processes involved, and hence of the mag-nitude 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, dou-bling to atmospheric concentration of CO 2 , or of increasing it

at an arbitrary rate This approach bypasses the uncertainties

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

cli-mate changes are likely to happen

MODEL SIMULATIONS AND PREDICTIONS OF

CLIMATE CHANGE

Introduction

Since changes in global and regional climates due to

anthro-pogenic 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

predic-tions 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

devel-oped 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 sur-face parameters such as temperature and albedo, from obser-vations 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 com-puted variables of which are:

E–W and N–S components of the wind Vertical motion

Air temperatures and humidity

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

tempera-ture 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.

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 struc-ture 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

predic-tion of climate change, they must simulate realistically the present circulation and water mass distribution and tempera-ture fields and their seasonal variability Ocean modelling and validation are less advanced than atmospheric model-ling, reflecting the greater difficulty of observing the interior

of the ocean and of inadequate computer power They suffer

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

measure-ments of all these are difficult and very sparse at the

pres-ent time Differpres-ent models show considerable differences in

their simulations of the deep ocean circulation, but

identifi-cation 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

check-ing 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

simu-late 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

shal-low penetration in the equatorial ocean and deep penetration

in high latitudes but fail to reproduce the deep penetration

at 30–50N, 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

cli-mate simulations and to make realistic predictions of clicli-mate

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

cou-pled 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 2.5 lat  3.75 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 temper-ature, 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 sea-sonal 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 1  1 data set The horizontal grid, 2.5  3.75, the same as that

of the atmospheric model, is too coarse to resolve oceanic meso-scale eddies of scale 苲100 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 diffu-sion 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 pro-vided 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

TABLE 1 Global mean changes in temperature and precipitation caused by doubling

Models with Computed Cloud Water/Ice

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

atmo-spheric 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,

follow-ing 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

experi-ments 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

feed-back from the accompanying increases in water vapour,

indi-cate that this might cause the globally and annually averaged

surface air temperature to rise by about 1.5C Because, as

discussed by Mason (1995), many other feedback processes,

both positive and negative, operate within the complex

cli-mate 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 atmo-sphere 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
50 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 4.2C to 5.2C with an average of 4.6C 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,

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

physi-cal 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

dif-ferent results as summarized in Table 1

The UKMO model, using three progressively more

sophis-ticated and realistic cloud/radiation schemes, has progressively

reduced the predicted global warming from 5.2K to 1.9K 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

simula-tion, 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

atmo-sphere 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 −15C; 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 1.9K 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)

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

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.

YEAR –1.0

–0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

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

gradu-ally 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

65S, which carries much of the additional ‘greenhouse’ flux

of heat from the surface to depth of 3 km, where it remains

for many decades

Very similar results were produced with the earlier

ver-sion 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 1.7K

The corresponding increase for the Northern Hemisphere

was 2.6K, with warming of 4K 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 60S A similar vertical circulation, caused by melting ice, and penetrating to about 1.5 km depth, occurs at about 60N in the North Atlantic (see Figure 6) After a slow start, the enhanced global warm-ing 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 0.3K—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 1.3K, 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

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 0.3K 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

precipi-tation 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

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2.0

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LATITUDE (DEG N)

COUPLED MODEL

10 YEAR ANNUAL MEAN TEMPERATURE (YEARS 66 TO 75)

1900 1950 2000 2050 –1

0 1 2 3

4

∆T

°K

(a)

(c)

(b)

FIGURE 7 Changes in the globally-averaged mean surface temper-atures 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

tropo-sphere 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  0.1 m, if produced in

large quantities from local sources such as forest fires,

volca-noes and desert storms, may significantly influence the

radia-tion 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
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

aero-sols 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 2 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

r eff a( W N 1 3

)

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 atmo-spheric 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, prob-ably 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 varia-tions 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 1C compared with an observed rise of only 0.5C, (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 0.5C, 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