Geoscience Reference
In-Depth Information
Some regional climate extremes - such as the south-east Australian droughts
from 1997-2009 - may be the result of an accumulation of weather or climate
conditions that are not extreme when considered independently. While all
extreme weather events can be said to result from intrinsic climate variability,
such variability is now influenced and altered by the effect of human-induced
warming of the climate system (IPCC, 2012).
The most robust observed evidence of changes in extreme events comes from
changes in the frequency of their occurrence, and particularly the increase in
extreme heat events. The Intergovernmental Panel on Climate Change report
Managing the Risks of Extreme Events and Disasters to Advance Climate Change
Adaptation
(IPCC, 2012) concluded that it is very likely that global warming has
caused changes in the frequency of some extreme weather events observed since
1950. It is very likely that there has been an overall decrease in the number of
cold days and nights, and an overall increase in the number of warm days and
nights, at the global scale (IPCC, 2012). It is also very likely that these changes
have occurred at the continental scale in North America, Europe and Australia
(IPCC, 2012). There have been statistically significant increases in the number
of heavy precipitation events in most regions (IPCC, 2012).
Determining the extent to which an
individual
extreme event is influenced
by a specific cause, such as increasing greenhouse gases, remains difficult. This
is due to a number of factors. Specifically, extreme events themselves are rare,
thereby providing small sample sizes for studying and determining change over
time. Additionally, and equally importantly, extreme events have a very large
range even in an unchanging climate, which also contributes to difficulties in
determining significant changes (IPCC 2007: FAQ 9.1). Many types of extreme
weather events are complex, with many different antecedent drivers in the
weather and climate system. Hence, for instance, larger changes in global mean
temperature are likely required before changes in the magnitude of individual
extremes are clearly distinguishable from background variability.
Nevertheless, advances are being made in determining the 'fraction of attrib-
utable risk', which links a particular extreme event to specific causal relationships
(Stott et al., 2004). These techniques have allowed us to determine that the
occurrence of some recent individual extreme events, notably the 2003 European
heatwave, 2010 Russian heatwave, and the extreme Australian summer of
2012-13, would be very much less likely without global warming (Stott et al.,
2004; Rahmstorf and Coumou, 2011; Otto et al., 2012; Lewis and Karoly, 2013).
According to the IPCC (2012):
●
it is likely that human influences have already led to warming of extreme daily
minimum and maximum temperatures at the global scale;
●
there is medium confidence that human influences have contributed to the
intensification of extreme precipitation at the global scale;
●
it is likely that there has been a human influence on increasing extreme
coastal high water due to an increase in mean sea level;
