Epilogue - Natural Hazards - page 289

stream. The Earth passes through the concentrated
head of this stream every 2520 years with another
minor concurrence every 1100 years. The Earth's last
encounter occurred around 1500 AD. This is when the
last mega-tsunami affected the New South Wales coast-
line. Australian Aboriginal and Maori legends describe
a comet shower that can be dated to this time. We have
also found a recent impact crater, generated by a comet
about 1 km in diameter, off the south coast of Stewart
Island, New Zealand. Currently, the Earth is not
affected by large or plentiful objects in the Taurids. As
a result, Australia - together with much of the globe
- exists presently within a quiescent epoch where
tsunami are generated mainly by earthquakes or
volcanoes. In Australia, the dichotomy is that field
evidence for mega-tsunami does not match a limited
historical record devoid of such events. The dichotomy
can be resolved knowing that tsunami are very much
characteristic of a changing hazard regime.
Other climatic and geological hazard regimes have
also changed. For example, since 1700 the Earth has
experienced more and larger volcanic eruptions than at
any time during the last 10 000 years. We are also
witnessing the termination of the Little Ice Age,
which appears to follow a 1500-year climate cycle.
During cold phases, storms are more frequent and
intense in northern Europe (Figure 3.12), and temper-
atures more extreme. For example, in the 1420s, the
Burgundy region of France - during one of the periods
of the Middle Ages when temperatures were cooling
rapidly - witnessed six of the coldest years in
1000 years. However, embedded within this cooling
was the warmest summer experienced for a millen-
nium. This summer was not just slightly above normal,
but exceptionally so. The grape harvest in that year
occurred at the beginning of August instead of during
the second or third week of September.
Finally, the extreme 1 in 100 year rainfalls that many
regions of the world have been experiencing may
simply represent the occurrence in time of part of a
much more intense rainfall regime. Wollongong,
Australia, mentioned in this text, is a region where
extreme rainfalls of 200-400 mm in 24 hours are
common. Embedded in rainfall records for this region
are rainfalls of 840 mm in 9 hours, and 1000 mm within
one month. These rainfalls do not plot on the straight
line in probability of exceedence diagrams. They
represent a different climate regime conducive to
extreme floods. This aspect is not unique to Wollongong.

Across Australia, there is geomorphic evidence for
floods that are greater than any that could be produced
by maximum probable rainfalls. Nor is Australia
unique in the world. The increase in flooding globally
(Figure 6.10) is evidence that rainfall (or drought)
regimes are changeable.
The effects of global warming, whether caused by
the termination of the Little Ice Age or by enhanced
Greenhouse gas concentrations, can also be accommo-
dated within the concept of a changing hazard regime.
Climatologists acknowledge that the Earth's climate is
not consistent over time. At the height of the Last
Ice Age 22 000 years ago, temperatures fluctuated
dramatically, rising and falling 4-6°C over the space of
a few years. These 'flickerings' are probably subdued
under our present climate regime, but they appear
when climate gets colder. The fluctuations in the 1420s
in France were probably evidence of this fundamental
feature of Pleistocene climate.
The warming that is occurring presently and is
hypothesized to increase represents a change in hazard
regime. The potential effect of changes in carbon
dioxide induced by human activity has been the subject
of scientific enquiry for the past 155 years. In 1861,
John Tyndall suggested that carbon dioxide changes
in the atmosphere might be responsible for changes in
climate. In 1896, Svante Arrhenius proposed that a
doubling of carbon dioxide might warm the atmosphere
by 5°C. In 1938, Callender showed how carbon dioxide
and water vapor absorb long wave radiation at different
wavelengths. He suggested that anthropogenic produc-
tion of carbon dioxide would alter the natural balance
between incoming solar radiation and outgoing long
wave radiation, and cause global warming that would
exceed natural variations in historical times. The
seminal conference that catalyzed world attention on
the significant ramifications of enhanced 'Greenhouse'
gas concentrations in the atmosphere was the WMO-
sponsored conference of 9-15 October 1985 in Villach,
Austria. A policy statement released by this conference
pointed out that most current planning and policy
decisions assumed a constant climate - an immutable
hazard regime - when, in reality, increases in 'Green-
house' gases by the 2030s would warm the globe
between 1.5 and 4.5°C, leading to a sea level rise of
20-140 cm. The only thing that has changed since then
is the realization that numerous gases - CO 2 , CH 4 ,
N 2 O, CO, O 3 and CFCs - are implicated in these
predictions. While present temperatures have been