Middle-latitude climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 110

and persistence), the location and strength of the centers of action, and seasonal
circulation variations are thus related to the seasonally changing geography of
the planetary energy budget (Harrington and Oliver 2003).
It is clear that the positions of the ridges and troughs are the keys to seasonal
weather and, more recently, these have been related to atmospheric teleconnec-
tions. Of particular note in this respect is the PNA. In an earlier discussion of this
feature (Section 2.4 ) the significance of location and extent of upper-air flows
were shown to be the key to the use of the PNA in explaining the climates of
North America.
4.5.2 Precipitation trends and patterns
Given the amount of published research relating to global warming, there is a
vast literature dealing with potential changes in circulation patterns and tem-
peratures in the NH. Research concerning precipitation patterns and trends,
while abundant, is less widely known, so this area of research is selected as
the topic for this section. Of interest in such research are the problems of data
reliability and the fact that a number of studies have related precipitation trends
and variability to teleconnections indices. There is also, of course, the question
of whether precipitation amounts have increased in recent decades and, if so,
whether the increase related to an increase in extreme events.
Among the best-known studies of future climates, including assessments of
precipitation, are those provided by the Intergovernmental Panel on Climate
Change (IPCC, www.ipcc.ch; Houghton et al. 1995 ). The work of the IPCC is
guided by the mandate given to it by its parent organizations the WMO and
UNEP. The IPCC has three working groups: Working Group I assesses scientific
aspects of climate systems and climate change; Working Group II studies the
vulnerability of human and natural systems to climate change and options for
adapting to them; Working Group III assesses options for limiting greenhouse
gas emissions and economic issues. Also, a task force studies national green-
house gas inventories.
Precipitation trends
In reporting future precipitation trends there is invariably a caution concerning
GCM and other simulations, for much uncertainty exists. Nonetheless, in
Europe, for example, the IPCC reports that most of the continent will experience
an increase in precipitation, especially in higher latitudes. The Mediterranean
regions and parts of Eastern Europe may experience declines. The European
Climate Assessment (ECA), which was initiated to analyze temperature and data
precipitation in Europe, provides a similar assessment (Klein Tank 2002 ).
A study considering global precipitation trends was completed by New et al.
( 2001 ). This research found that, over the land areas, from 1901 to the mid
1950s, precipitation remained below the century long mean. From 1950 until