Global Warming

Feedbacks in theclimate system 93

Table 5.1Estimates of global average temperature changes under

different assumptions about changes in greenhouse gases and clouds

Change (in◦C) from current
average global surface
Greenhouse gases Clouds temperature of 15◦C

As now As now 0
None As now − 32
None None − 21
As now None 4
As now As now but+3% high
cloud

0.3

As now As now but+3% low
cloud

−1.0

Doubled CO 2
concentration
otherwise as now

As now (no additional

cloud feedback)

1.2

Doubled CO 2
concentration+
best estimate of
feedbacks

Cloud feedback

included

2.5

water content (how thick or thin it is) and on the average size of the cloud
particles. In general for low clouds the reflectivity effect wins so they
tend to cool the Earth-atmosphere system; for high clouds, by contrast,
the blanketing effect is dominant and they tend to warm the system. The
overall feedback effect of clouds, therefore, can be either positive or
negative (see box below).
Climate is very sensitive to possible changesin cloud amount or
structure, as can be seen from the results of models discussed in later
chapters. To illustrate this, Table 5.1 shows that the hypothetical effect
on the climate of a small percentage change in cloud cover is comparable
with the expected changes due to a doubling of the carbon dioxide con-
centration.

Ocean-circulation feedback

The oceans play a large part in determining the existing climate of the
Earth; they are likely therefore to have an important influence on climate
change due to human activities.