2. History drought situation and future projection
As a normal part of climate variability, drought has occurred
many times over the past 1000 years across many parts of the world
[20]. Since the 20th century, however, drought has increased on
both the regional and global scales [20]. The global area impacted
by drought has increased sharply since the 1970s, a rise attributed,
in part, to global warming [19,28]. Some recent research proposes
that the increase in global drought conditions has been overestimated
due to the inherent deficiencies of the drought index and
that little change in global drought has occurred over the past
60 years [51]. However, an analysis of global drought trends using
the SPEI index develop by Vicente-Serrano [60] suggests that areas
impacted by severe and extreme drought have increased dramatically.
Although the effects of this drought may have been overestimated,
the severity of the drought has actually intensified.
The drought induced temperature extremes had been reported
in several studies [26,37], and the basic mechanism behind this
phenomenon is soil moisture variations affecting the partitioning
of sensible and latent heat [37]. This issue can be traced back to
the relationship between precipitation and temperature. In early
studies, more stresses were play on the impacts of temperature
on precipitation. For example Isaac and Stuart [27] computed temperature-
precipitation relationships for Canadian stations using
daily data. For the east and west coasts and northern Canada, more
precipitation accompanies warm conditions in winter and cool
conditions in summer, thereby reversing the correlation with seasons.
Therefore, we think that it is still hard to determine the causal
relationship between temperature and water deficiency. This
also implies the great importance for the further study of the relationship
between drought and temperature, which is crucial for
attributes analysis for drought event and the reduction of the
uncertainty of future drought projection.
Global warming and regional droughts are predicted to
intensify and become more frequent during this century due to