Encyclopedia of Environmental Science and Engineering, Volume I and II

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 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 5.2
K to 1.9
K 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 −15
C;

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

1.9
K. 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

C007_002_r03.indd 433C007_002_r03.indd 433 11/18/2005 10:28:00 AM11/18/2005 10:28:00 AM