Looking at past global climate helps scientists to gain an
overall picture of climate variability; studying past events aids in
discovering whether present day climate is becoming more unpredictable, or
whether it is part of a longer trend. This study carried out by Zheng et al. (2006) aims to reconstruct the climate of eastern China over the past 1500 years using
a combination of different proxies. In order to do this, a new dataset was
constructed out of 3 sets of historical data: this is not ideal as different sourcing
techniques were used and so the dataset could contain error or inconsistences
(it is noted that there are many missing records before 1470).
In order to overcome this problem, Zheng et al. produced a
Dry-Wet Index which analyses the long term change in precipitation instead of
analysing individual records. This would help to decrease the error within the
dataset and to focus on climate variability trends. The study area was divided
into 3 regions based on their climatic differences, so that the natural
occurrence of precipitation did not impact too much on the study; the aim was
to find the variation in precipitation so it had to be accounted for that some
regions were naturally wetter than others.
Of course, it was found that the dry-wet cycle within
eastern China varied considerably over the past 1500 years (please refer to the
article). But what about what we’re really interested in here, extreme weather?
The results from the study indicated that 18 flood events
occurred within eastern China between 501-2000, with a 4 year flooding event
occurring 1422-1425. Three major wet epochs were identified 880s-990s,
1240s-1420s and 1540s-1910s. Of course, there was variation in the climate
within these times too. The occurrence of flooding has continued into the last
300 years, although the number of extreme droughts has decreased. Perhaps this
is a sign of the changing climate? It has been said that the monsoon cycle is
intensifying, could it perhaps be possible that these records are reflecting
this?
By looking at the figure above, taken from Zheng et al., it
can clearly be seen that the frequency and severity of droughts (indicated by
the red bar) is decreasing as the records near the present day. The flood
occurrences (blue), however, appear to becoming more frequent and are just as
severe (indicated by the bar width) as those occurring in the past 1500 years.
Although there has to be some allowances made for the dataset used, with better
records obviously available towards modern day, the general trend does infer
that eastern China is generally becoming more vulnerable to extreme flood
events.
In my opinion, these results concur with recent research
that the hydrological cycle is intensifying in light of global warming (Duracket al. (2012)). This will likely increase the intensity
of the monsoon (Ruosteenoja 2003) as with a warmer atmosphere which has a
greater ability to absorb moisture, and higher evaporation rates occurring from
warmer ocean surface waters (Cubasch et al. 2001) there is greater potential
for precipitation. Currently the monsoonal rains occur from June to September,
but in a warmer world this window could widen. Add to this the fact that sea
levels are rising due to increased melt water and thermal expansion (Cubasch et
al. 2001) and it is easy to see how the likelihood of extreme flood events
seems to be increasing. Of course, it is extremely difficult to predict how our
climate will alter in the future, but at the moment, the trends are all heading
in the same direction…
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