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Australia: The Land Where Time Began |
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Australian Heatwaves in the 21st
century – More frequent, Longer and Hotter
Since the mid-21st century extreme heat waves in summer and
warm spells in winter have had a significant impact of the Australian
climate, where there has been an increased frequency and duration of
these events. This study investigated projected changes in heat waves
and warm spells across Australia for 2 future emission scenarios with
the use of Coupled Model Intercomparison Project phase 5 (CMIP5) climate
models. For the historical period that encompasses the late 20th
century (1950-2005) an ensemble mean of 15 models is capable of
capturing the spatial distribution that is observed in the frequency and
duration of summer heat waves, in spite of overestimating these metrics
along coastal regions. A better comparison to observations is achieved
by the models in their simulation of temperature anomaly of the hottest
heat waves. The largest increase in frequency and duration to occur
across northern tropical regions of summer heat waves is projected by
model ensemble mean projects by the end of the 21st century,
while predicting an increase of ⁓3oC in the maximum
temperature of the hottest southern Australian heat waves. It is
suggested by model consensus that winter warm spell in winter will
increase in frequency and duration at a greater rate than summer heat
waves, and that the hottest events will be increasingly for both seasons
by the end of the century. Temperature increases of the hottest events
are projected for southern Australia. It is also suggested by results
also suggest that following a strong mitigation pathway in the future is
more effective in reducing the heat wave and warm spells frequency and
duration in the southern regions compared to the northern tropical
regions.
In many heavily populated regions of the world, which includes southern
and central Europe (e.g. Fischer & Schar, 2010; Kyselý, 2010; Carril et
al., 2008; Della-Marta et
al., 2007; Trigo et
al., 2005, North America
(e.g. Bumbaco et al., 2013;
Wu et al., 2012b; Mastrandrea
et al. 2011), China (Wu et
al., 2012a; Ding et al.,
2007), Russia (Trenberth & Fasullo, 2012; Barriopedro et
al., 2011), central and
southern Africa (Hao et al.,
2013; Lyon, 2009), and southern Australia (Bureau of Meteorology 2013;
Lewis & Karoly, 2013; Nairn & Fawcett, 2013; Tryhorn & Risbey, 2006).
Consecutive days above a threshold maximum temperature is often used to
characterise these types of events; they also incorporate a distinct
lack of night-time temperature relief with consecutive nights above a
minimum threshold (Pezza et al.,
2012; Nairn & Fawcett, 2013).
Heat waves occur in all seasons in Australia and in the austral winter
they are often referred to as warm spells. Across the central interior
and tropical regions where temperatures are high, often exceeding 35oC,
though the daily variability if low, based on the maximum temperature
definition of heat waves from (Perkins & Alexander, 2013), there are
about 2 heat wave events per extended summer, though such events tend to
be only ⁓2oC–4oC higher than the climatically
maximum temperature. In southern Australia, which is a region of great
agricultural importance, which is also where about ⅓ of the population
live, there are on annual average 1 or 2 heat waves per summer; such
events can, however, be up to 15oC warmer than the
climatological maximum (Perkins & Alexander, 2013). This results from a
high frequency of synoptic prefrontal weather systems (Tryhorn & Risbey,
2006) in the southern latitudes, which cause heat waves that are more
severe to develop due to the greater variability of summer temperatures
(Nairn & Fawcett, 2013). In the midlatitude regions of Australia
processes such as breaking Rossby waves and persistent anticyclones
(Pezza et al., 2012; Perkins
& Alexander, 2013; Marshall et al.,
2014; Parker et al., 2013)
often trigger heat waves, which allow advection of warm air over the
region that is affected. A low pressure trough or tropical depression
over the northwest of Australia often accompany anticyclones (Hudson et
al., 2011; Pezza et
al., 2012), which may
reinforce directly the anticyclone that is downstream (Parker et
al., 2013).
It is suggested by recent observations that the frequency, duration and
intensity of heat wave events are increasing over land regions around
the world (Coumou & Rahmstorf, 2012, and references therein), which
includes many regions of Australia (Pezza et
al., 2012; Perkins et
al., 2010; Perkins and
Alexander, 2013). In spite of various limitations in the simulation or
regional changes (Hao et al.,
2013), climate models are projecting that these trends will continue
with increasing emissions of greenhouse gases (Coumou & Robinson, 2013;
Meehl & Tebaldi, 2004). Also, there is considerable evidence suggesting
that anthropogenic forcings have contributed to recent extreme heat wave
events across the Northern Hemisphere (Hansen et
al., 2012) and to record warm
summer temperatures across Australia (Lewis &Karoly, 2103). The
likelihood of a contribution by humans to ⁓2.5-5 times greater than that
without greenhouse warming, according to Coupled Model Intercomparison
Project phase 5 (CMIP5) climate models. It is also projected by CMIP5
models that there will be a 7-fold increase in global monthly heat
extremes by the middle of the 21st century (Coumou &
Robinson, 2013). According to Cowan et
al. this follows on from
large increases (decreases) in the warm/dry (wet/cold) extremes across
many tropical and subtropical regions throughout the late 20th
century, which are captured well by the CMIP5 models (Hao et
al., 2013). A comprehensive,
detailed assessment of the way in which warming in the future will
impact heat waves across Australia by the use of CMIP5 climate model
simulations, however, has not yet been undertaken.
Therefore, Cowan et al.
investigated the changes in heat waves across Australia in the 21st
century that have been projected, comparable to the analysis of (Perkins
& Alexander, 2013), which examined trends that were observable. The
importance of understanding of how the trends in heat wave metrics will
change in a world that is warming is underscored by the impacts of these
extreme events on human health and mortality. Many studies have detailed
the increased risk of heat-related deaths that result from extreme heat
events in populated regions of Europe (Le Tertre et
al., 2006), North America
(Zanobetti & Schwartz, 2008), and Australia (e.g. Loughnan et
al., 2010; Tong et
al., 2010). According to
Cowan et al. this risk is
likely to increase severalfold by the late 21st century,
based on little or no acclimatisation (Gosling et
al., 2009). Therefore,
communities require greater certainty in understanding the risk of
increased heat waves in the future and a better understanding of whether
the likelihood of extreme heat conditions will be reduced by mitigation
(as in Coumou & Robinson, 2013).
For thus study, Cowan et al.
define Australian heat waves as events that only occur in summer, as
this is when they have the greatest impact on human health (e.g. Pezza
et al., 2012) as a result of
their intensity, as well as having an influence on other extreme events
that only happen in summer such as bushfires (Karoly, 2009). Winter warm
spells, however, will also be examined to investigate the seasonality of
the projected changes. The way in which heat wave and warm spell
frequency and hottest event anomalies across Australia will change under
2 emission scenarios over the 21st century, by use of CMIP5
data.
Discussion and conclusions
It is indicated by CMIP5 projections for Australia that by the end of
the 21st century hotter, more frequent and longer heat waves,
and warm spells in winter will occur, with more extreme conditions under
RCP8.5 compared to RCP4.5. The largest changes in the heat wave
frequency and duration are projected to occur across the northern
tropical regions of Australia, which is consistent with other studies
into heat extremes around the globe (Diffenbaugh & Scherer, 2011;
Stillmann et al., 2013),
while in southern Australia the maximum of heat waves increase more than
those in the north. In general, the CMIP5 models simulate adequately the
heat wave climatologies that are observed over the late 20th
century and the early 21st century, though they tend to
simulate an overly zonal climatologically spatial pattern for frequency
and duration, which means that there is a degree of uncertainty on the
changes that they project in the 21st century. There is
greater confidence in the simulated increases in the future, as the MMM
performs better when simulating the amplitude of heat waves that are
observed. The models simulate little increase in frequency for cities in
southern Australia over 1950-2005, which is consistent with heat waves
along southern Australia; however, they simulate an increase in
frequency of 6- to 20-fold in summer and a noticeable increase in summer
duration by 2100 for
RCP8.5.
It is suggested by the seasonality of the heat wave changes that are
projected that a larger increase in the frequency and duration of summer
heat waves and winter warm spells in the austral winter will occur
across the Australian populated regions, which includes the southern
Murray-Darling basin, when compared to any other season. Models project
the largest changes for the central and northern interior in spring and
summer heat waves. Conversely, changes in hottest seasonal heat waves
are largest in winter, with the exception of across southern regions,
where in spring the largest changes occur (South Australia) and summer
(southwest and southern Australia in RCP4.5). This seasonality mirrors
to a large extent the spatial variations in the climatology, which
exhibit heat waves of higher frequency and duration in summer across
northern Australia, though higher amplitude heat waves across southern
Australia. As such, it is suggested by the results that, as well as the
substantial increase in frequency, duration and maximum temperature of
heat waves during the 21st century, there will be enhanced
seasonality and spatial variation of heat waves. The greatest increase
in hottest heat waves occurs in spring for populated regions such as
southeast and southwest Australia, which is consistent with the trend
towards more events of the Indian Ocean Dipole that often lead to severe
bushfires in summer (Cai et al.,
2009).
The main cause of these heat wave trends in the future is the robust
global warming that results from anthropogenic greenhouse gas emissions
(e.g. Lewis & Karoly, 2013). The amplification of atmospheric
circulation patterns may also be responsible for forcing additional
changes in the intensity and frequency of heat waves, as has been
simulated for regions in Europe and North America (Meehl & Tebaldi,
2004). It is suggested by studies that used CMP3 models that a result of
global warming is that the intensity of the subtropical ridge over
Australia will increase over the 21st century (Timbal &
Drosdowsky, 2013), and the position of the ridge will shift polewards
(Kent at al., 2013), thereby
reducing rainfall in southeastern Australia. This model also projects
drier winters across southwest Australia in the 21st century
(Cai et al., 2011a), with
below average rainfall that is often associated with higher maximum
temperatures (Nicholls, 2004). Cowan et
al. suggest that this may
account for the largest increase in warm spells that are simulated
occurring during the austral winter, as well as the maximum temperatures
in winter having a smaller range than they do in summer, i.e., a
narrower probability distribution function. Atmospheric blocking is
expected, in spite of model biases, to increase in the summer months in
the Tasman Sea (Grose et al.,
2012) to the east of South Australia (Sadler et
al., 2012). However, future
warming of the Tasman Sea and a trend of positive southern annular mode
are also likely to promote an increase in summer rainfall over southeast
Australia (Shi et al., 2008),
which may account for the weakening in the summer HWF in Melbourne,
compared to the case in Perth and Sydney. The summer domain in the
northern tropics is not expected to change significantly in the future,
in spite of a general predicted increase in summer rainfall (Wang et
al., 2013), it is expected
that there will be delays and shorter durations in onset of the monsoon
(Zhang et al., 2012), which
Cowan et al. suggest may
partially explain the large increases in frequency and duration in this
region.
A robust result of this study is that there will be an increase in
frequency, duration and amplitude of winter warm spells and summer heat
waves across Australia, and this increase is strongly proportional to
the emission scenario, as well as the latitude of a particular region,
i.e., tropical versus extratropical. Also, the hot events will gradually
become hotter by the end of the 21st century over southern
Australia, especially during winter and for the RCP8.5 high emission
scenario, even if the heat waves are referenced to a mean state that is
warming. Humans can benefit from warm spells in winter, such as by the
reduced incidence of disease and fatalities (McMichael et
al., 2006), though there are
also likely to be adverse effects to agriculture that is temperature
dependent (e.g. Turner et al.,
2011). Also, there will be severe adverse effects on human health
related to increases in summer heat waves (Kyselý, 2010; Sherwood &
Huber, 2010), as well as ecosystems and agriculture (Coumou & Robinson,
2013, and references therein). While it has been established that
temporal evolution of heat waves occurs, it has not yet been understood
if a change in heat waves across is manifested by a men state
temperature change alone, or is in part due to changes in the
circulation of the atmosphere and ocean. A companion study will
investigate patterns of circulation and sea surface temperature prior to
and during summer heat waves, in order to gain a better insight into how
dynamics during summer heat waves across southern Australia will change
in a warming world.
Cowan, T., et al. (2014). "More Frequent, Longer, and Hotter Heat Waves
for Australia in the Twenty-First Century." Journal of Climate
27(15): 5851-5871.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |