Australia: The Land Where Time Began |
||||||||||||||
Anthropogenic Contributions to the Australian
Record Summer Temperatures of 2013
In this paper Lewis et
al. report investigations of anthropogenic contributions to the
record hot summer in Australia in 2013, the hottest summer in the
observational record, by the use of a suite of climate model
experiments. For the simulations with only natural forcings Australian
area-average summer temperatures were compared to simulations using
anthropogenic and natural forcings for the period 1976-2005 and the
RCP8.5 high-emission simulation of 2006-2020 from 9 Coupled Model
Intercomparison Project, phase 5, models. To compare the likelihood of
extreme Australian summer temperatures between the experiments, by using
fraction of attributable risk, it was very likely, with >90 %
confidence, that the odds of extreme heat due to human influences were
increased by at least a 2.5 times, by the use of simulations to 2005,
and when using the simulations for 2006-2020 there was a 5-fold risk
increase. Natural climate variations alone, which included
ENSO,
are considered by Lewis et al.
to be unlikely to explain the record temperatures, but the human
contribution to the increased odds for summer extremes in Australia like
2013 was substantial. In December 2012 to February 2013 the Australian
summer was the hottest on record, in which average conditions exceeded
the observed 1911-1940 mean by 1.34 K. On daily to seasonal timescales
summer temperature records were broken: with the hottest month on record
occurring as well as the hottest day for the entire Australian continent
(Bureau of Meteorology [BoM] 2013a). Sustained high temperatures were
also coincident with bushfires by late summer in southeastern Australia
in Victoria and South Australia, and in northeastern Australia in
Queensland and New South Wales (Bureau of Meteorology, 2013b) severe
flooding. It was dubbed the “angry summer” (Steffen, 2013), because of
the severe conditions. Over land, the
intensity, frequency and duration of heat waves increase that was
observed is widespread globally (Perkins et
al., 2012). On longer
timescales, monthly extremes are increasing at a rate that is relatively
faster than daily extremes (Coumou & Rahmstorf, 2012), such that many of
the large number of the record-breaking heat waves that have occurred
recently and summer extremes have been associated with anthropogenic
influences (Hansen et al.,
2012). Very large percentage changes in the occurrence of extremes
(Trenberth, 2012) can result from changes in climate means, the extreme
seasonal heat is considered in the context of average temperatures in
Australia having increased by 0.9oC since 1910 (BoM, 2012b).
The possible anthropogenic contribution to extreme seasonal temperatures
have not been considered before, while changes in average temperatures
in Australia have been attributed to anthropogenic climate change (Karoly
& Braganza, 2005; Stott et al.,
2010). According to Lewis et
al. the cause of a particular climatic event cannot be categorically
ascribed to anthropogenic climate change; however, the roles of various
factors that contribute to the change in odds of an event occurring can
be identified. A large ensemble of climate models is used to calculate
the probability of an event occurring and then compared to the
equivalent probability in a counterfactual experiment, in which only
climate forcings are imposed (Allen, 2003). Anthropogenic contributions
to specific climate events have been probabilistically estimated using
this conceptual framework (Stott et
al., 2004; Pall et
al., 2011; Christidis et
al., 2013). Event attribution
and climate prediction services are interrelated; attribution studies
are necessary in order to develop meaningful adaptive decisions (Stott
et al., 2012). This study
investigated the relative contributions of anthropogenic and natural
factors to the record temperatures in the summer of 2013 averaged across
Australia. Lewis et al. used a
suite of Coupled Model Intercomparison Project phase 5 (CMIP5) detection
and attribution experiments (Taylor et
al., 2012) to compare the
occurrence of extreme summer temperatures in a series of control
simulations and simulations with natural forcings only with those
occurring
in model experiments including both
natural and anthropogenic forcings. If anthropogenic influences
contribute to conditions that occurred in the record summer of 2013,
then if so, by how much.
Summary This study was
motivated by the recent 2012-2013 record temperatures in the Australian
summer of 2012-2013. Comparison of the distribution of Australian
area-averaged temperature anomalies from various CMIP5 experiments from
9 models shows a significant change in the probability of extreme warm
temperatures in summer in model experiments that were forced with
increasing human influences, compared with the equivalent naturally
forced experiments. Given the La Niña-neutral conditions that prevailed
during 2012-2013, and the strong increase in risk that was strongly
modelled, associated with this type of seasonal extreme in experiments
that were anthropogenically forced, on their own, natural climate
variations are not likely to have caused the record Australian “angry
summer” of
2013. A clear conclusion that climate
change had a substantial influence on the extreme heat over Australia in
summer, and that natural climate variations alone are not likely to
explain the record summer temperature is supported by these results.
It was conservatively
estimated by Lewis et al. that
the extreme summer heat was at least 2.5 times more likely (>90%
confidence) as a result of anthropogenic influences in the simulations
up to 2005. Using simulations centred on 2013, however, it is very
likely (>90% confidence) that human influences increased this risk by at
least 5-f0ld. The best estimate anthropogenic mean temperature change
for 2013 (0.84 K, that was determined from the RCP8.5 ensemble mean for
years 2006-2020) was applied to the observational record (for the period
1911-1940 of minimal anthropogenic warming), and the calculated FAR
value of 0.90 was found to be close to the equivalent RCP8.5 FAR value
of 0.87. I.e., it could be accurately estimated, for the increase in
risk for this type of event, simply from the observational record and
modelled mean temperature changes. An assessment of risk in this
instance could potentially occur in near-real time. It has also been
shown by previous studies that precomputing changes in the likelihood of
exceeding a temperature threshold of a suite of thresholds enables an
assessment that is near real time of the anthropogenic influences on the
observed temperatures for some regions (Christidis et
al., 2011). Also, there was a
substantial decrease in the return times (increase in the frequency) of
extreme summers between the historical (1976-2005) and RCP8.5
(2006-2020) model years. According to Lewis et
al. there is likely to be a
marked increase beyond the 2020 cutoff that was utilised in this study,
in the occurrence of extreme hot seasons under the RCP8.5 scenario. It
was shown by other studies that by 2080-2099 in the RCP8.5 simulations,
at least 65% of seasons are projected to be extremely hot over all land
areas, with frequent heat likely to have severe implications for human
and natural systems (Diffenbaugh & Giorgi, 2012). An increase in extreme
summer heat in the future is very likely. In eastern Australia, where
the highly populated areas that are bushfire prone, understanding the
risk of extreme summer temperatures and likely changes to the frequency
of summer extremes has implications for making adaptive decisions.
According to Lewis et al. it wold be useful to extend this analysis to particular heat
waves, such as those that occurred throughout the 2013 Australian
summer. There has been a general lengthening of summer-like conditions,
with spring and autumn being shortened in many places, beyond the
increase in seasonal temperatures that are anthropologically driven
(Hansen et al., 2012). It would also be valuable for understanding future
changes in heatwave intensity and frequency to investigate human
influences of out-of-season heatwaves, such as the record November 2012
and March 2013 heatwaves in southeastern Australia.
Lewis, Sophie C., and David J. Karoly. "Anthropogenic
Contributions to Australia's Record Summer Temperatures of 2013."
Geophysical Research Letters 40, no. 14 (2013): 3705-09.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |