The last hundredyears 61with high pressure over the sub-tropical Atlantic and southern Europe
and mild winters over northwest Europe, has tended to be dominant since
the mid 1980s.
An interesting feature of the increasing temperature during the last
few decades has been that, in the daily cycle of temperature, minimum
temperatures over land have increased about twice as much as maximum
temperatures. A likely explanation for this, in addition to the effects of
enhanced greenhouse gases, is an increase in cloud cover which has
been observed in many of the areas with reduced temperature range. An
increase in cloud tends to obstruct daytime sunshine and tends also to
reduce the escape of terrestrial radiation at night.
As might be expected the increases in temperature have led on
average to increases in precipitation, although precipitation shows even
more variability in both space and time than temperature. The increases
have been particularly noticeable in the northern hemisphere in mid to
high latitudes, often appearing particularly as increases in heavy rainfall
events (see Table 4.1).
The broad features of these changes in temperatureand precipitation
are consistent with what is expected because of the influence of increased
greenhouse gases (see Chapter 5), although there is much variability in
the record that arises for reasons not associated with human activities.
For instance, the particular increase from 1910 to 1940 (Figure 4.1)
is too rapid to have been due to the rather small increase in greenhouse
gases during that period. The particular reasons for this will be discussed
in the next chapter where comparisons of observed temperatures with
simulations from climate models for the whole of the twentieth century
will be presented, not just as they concern the global mean but also
the regional patterns of change. We conclude there that, although the
expected signal is still emerging from the noise of natural variability,
most of the observed warming over the last fifty years is likely to have
been due to the increase in greenhouse gas concentrations.^3
Significant cooling of the lower stratosphere (the region at altitudes
between about 10 and 30 km) has been observed over the last two decades
(Figure 4.2). This is to be expected both because of the decrease in the
concentration of ozone (which absorbs solar radiation) and because of the
increased carbon dioxide concentration which leads to increased cooling
at these levels (see Chapter 2).
A further source of information regarding climate change comes
from measurements of change in sea level. Over the last hundred years sea
level has risen by between 10 and 20 cm. The best known contributions
to this rise are from the thermal expansion of ocean water because of
the global average temperature rise (estimated as up to about 7 cm)