Encyclopedia of Environmental Science and Engineering, Volume I and II

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 0.3
K 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

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

C007_002_r03.indd 435C007_002_r03.indd 435 11/18/2005 10:28:01 AM11/18/2005 10:28:01 AM