Geoscience Reference
In-Depth Information
America, the most widely used index to assess drought severity is that devised by
Palmer (
1965
), which is known appropriately as the Palmer Drought Severity Index
(PDSI) and was discussed in detail in
Chapter 23
. The PDSI provides a rapid means
of assessing soil moisture deficits in different regions across the United States and
hence of allowing relative drought severity to be compared. During the 1700-1978
period, based on analysis of 425 tree-ring chronologies, the 'Dust Bowl' drought of
the 1930s proved to have been the most severe (Cook et al.,
1999
). Cook et al. (
2007
)
have expanded on their earlier work and have developed a very useful North American
Drought Atlas.
Using a similar methodology based on the PDSI and a network of 327 tree-ring
chronologies, Cook et al. (
2010
) produced the Monsoon Asia Drought Atlas. The
results showed four intervals of severe andwidespread drought during the last thousand
years, centred on 1638-1641, 1756-1768, 1790-1796 and 1876-1878. It is interesting
that these were all times when the volcanic dust veil index compiled by H. H. Lamb
showed peak values (Lamb,
1970
;Lamb,
1972
;Lamb,
1977
), and they were therefore
related to cooler-than-average weather in the Northern Hemisphere mid-latitudes.
It thus seems that where there is a detailed tree-ring chronology, the PDSI can
serve as a useful tool for comparing the relative severity of different historic droughts.
Whether the PDSI can be used to check recent trends in drought magnitude and
frequency, however, is a matter for debate. Sheffield et al. (
2012
) have critically
analysed the methodology used to construct the PDSI. They pointed out that the PDSI
uses temperature data to calculate potential evaporation (E
pot
) and ignores other key
controlling variables, such as near-surface radiation, wind speed and humidity (see
Chapter 23
for details). If global temperatures are increasing, as observations indicate
to be the case, a formula based on temperature will necessarily show an increase
in drought frequency. Sheffield et al. (
2012
) used a physically based equation to
determine potential evaporation and found very little evidence of any change in global
drought frequency over the past sixty years, which is also consistent with assessments
of droughts in North America (Karl and Heim,
1990
; Idso and Balling,
1992
; Soule,
1993
). These results flatly contradict the IPCC (
2007a
) assertion (based on evidence
obtained from using the PDSI) that: 'More intense and longer droughts have been
observed over wider areas since the 1970s, particularly in the tropics and subtropics.
Increased drying linked with higher temperatures and decreased precipitation has
contributed to changes in drought'. The 2012 IPCC report on extreme events displays
commendable caution in discussing drought trends, commenting on the earlier undue
reliance by the IPCC on the PDSI resulting in possible overestimation of the increase
in regional and global droughts (Seneviratne et al.,
2012
).
The crux of the problem in predicting likely future climatic impacts revolves around
the degree to which human activities have altered and are likely to alter global and
regional climates, leading us into hitherto uncharted waters. However, we can draw
on the geological insights of James Hutton (
1795
) and Charles Lyell (
1830
-
1833
)
