64 Climates ofthe past
awareness of recent extremes in the forms of floods, droughts, tropical
cyclones and windstorms. It is therefore of great importance to know
whether there is evidence of an increase in the frequency or severity of
these and other extreme events. The available evidence regarding how
these and other relevant parameters have changed during the twentieth
century is summarised in Table 4.1 in terms of different indicators: con-
centrations of greenhouse gases; temperature, hydrological and storm-
related indicators and biological and physical indicators. To what extent
these changes are expected to continue or to intensify during the twenty-
first century will be addressed in Chapter 6.
Eventually, as greenhouse gases increase further, the amount of
warming is expected to become sufficiently large that it will swamp
the natural variations in climate. In the meantime, the global average
temperature may continue to increase or it could, because of natural
variability, show periods of decrease. Over the next few years, scientists
will be inspecting climate changes and climate events most carefully
as they occur to see how far actual events can be related to scientific
predictions especially those associated with increasinggreenhouse gas
emissions. Some details of these predictions will be discussed in later
chapters especially Chapter 6.The last thousand years
The detailed systematic record of weather parameters such as tem-
perature, rainfall, cloudiness and the like presented above for the last
140 years and which covers a good proportion of the globe is not avail-
able for earlier periods. Further back, the record is more sparse and doubt
arises over the consistency of the instruments used for observation. Most
thermometers in use 200 years ago were not well calibrated or carefully
exposed. However, many diarists and writers kept records at different
times; from a wide variety of sources weather and climate information
can be pieced together. Indirect sources, such as are provided by ice cores,
tree rings and records of lake levels, of glacier advance and retreat, and
of pollen distribution, can also yield information to assist in building
up the whole climatic story. From a variety of sources, for instance, it
has been possible to put together for China a systematic atlas of weather
patterns covering the last 500 years.
Similarly, from direct and indirect sources, it has been possible to
deduce the average temperature over the Northern Hemisphere for the
last millennium (Figure 4.3). Sufficient data are not available for the
same reconstruction to be carried out over the Southern Hemisphere.
In Figure 4.3 it is just possible to identify the ‘Medieval Warm Period’
associated with the eleventh to fourteenth centuries and a relatively cool