THE GUARDIAN
Does the country's record-breaking heatwave have something to do with
climate change?
Bushfire smoke
obscures the sun in Tasmania. It is very unusual to have such widespread
extreme temperatures – and for them to persist for so long. Photograph: Kim
Foale/EPA
Australia
has started 2013 with a record-breaking heat wave that has lasted more than two
weeks across many parts of the country. Temperatures have regularly gone above
48°C, with the highest recorded maximum of 49.6°C at Moomba in South Australia.
The extreme conditions have been associated with a delayed onset of the
Australian monsoon, and slow moving weather systems over the continent.
Australia
has always experienced heat waves, and they are a normal part of most summers. However,
the current event affecting much of inland Australia has definitely not been
typical.
The
most significant thing about the recent heat has been its coverage across the
continent, and its persistence.
It
is very unusual to have such widespread extreme temperatures — and have them
persist for so long. On those two metrics alone, spatial extent and duration,
the last two weeks surpasses the only previous analogue in the historical
record (since 1910) – a two-week country-wide hot spell during the summer of
1972-1973.
A good measure of the spatial extent of the heat is the Australian-averaged maximum daily temperature. This
is the average of the highest daily temperature of the air just above the
surface of the Australian continent, including Tasmania. The national average
is calculated using a three-dimensional interpolation (including topography) of
over 700 observing sites each day.
On
Monday and Tuesday last week (January 7 and 8) that temperature rose to over
40°C. Monday's temperature of 40.33°C set a new record, beating the previous
highest Australian daily maximum of 40.17°C set in 1972. Tuesday's temperature
came in as the 3rd highest on record at 40.11°C.
The
accompanying map of temperatures shows just how much of the country experienced
extremely high temperatures, with over 70% of the continent recording
temperatures in excess of 42°C.
Highest
daily maximum temperature during the first two weeks of January. Source:
Australian Bureau of Meteorology
And it's not like these sorts of days occur that often. The records set
last week sit between two and three standard deviations above the long-term
January mean of 35°C.
Perhaps more unusually, the Australian mean temperature (representing
the average of the daytime maximum and night-time minimum) set record high
values on both days at 32.22 (January 7) and 32.32°C (January 8), that were
well above the previous high of 31.86°C, set in 1972.
However, it is really the duration of this extreme heat wave that makes
it so unusual, and so significant in terms of impacts.
While some towns in Australia are famous for their extended runs of hot
temperatures, the limited geographical nature of those events distinguish them
from this January's heat wave. Multiple days of extreme heat covering most of
the continent are both rare, and isolated.
It is not that common for the Australian-average temperature to exceed
39°C for even two days in a row. A run of three days above 39°C has occurred on
only three occasions, and a run of four days just once, in 1972.
The current heat wave has seen a sequence of Australian temperatures
above 39°C of seven days, and above 38°C of 11 days straight.
The sequence of Australian mean temperature has been just as impressive.
As things currently stand, the first two weeks of January 2013 now hold the
records for the hottest Australian day on record, the hottest two-day period on
record, the hottest three-day period, the hottest four-day period and, well,
every sequential-days record stretching from one to 14 days for daily mean
temperatures.
The
number of records that have tumbled for individual sites are now too numerous
to catalogue here, and the Bureau of Meteorology has prepared a Special Climate Statement with a detailed analysis the temperature records broken. The list of
records is limited to just those stations with at least 30 years of records.
So, does all this have something to do with climate change?
To put it in context, we need to look at the influence of background
changes in the climate system.
The
planet is warming, and so is Australia
Planet
Earth is warming up. Climate scientists use a range of different indicators to track global warming. These include ocean heat content, sea surface
temperatures, sea level, temperatures in the lower and middle troposphere, and
the rate of melting glaciers and ice sheets.
The surface of the earth, as measured by global mean temperature, has
warmed by about one degree Celsius during the past hundred years, and the
decade from 2001 to 2010 has been the warmest we have recorded.
This
warming has been strongly attributed to increasing greenhouse gases from human
activities. While there are a number of influences on the climate system, such
as changing solar radiation and changing atmospheric aerosols, it is very clear that warming has been dominated by increased carbon
dioxide levels.
The
globe doesn't warm uniformly everywhere, due mostly to natural regional
variations in climate. In Australia, land temperatures and the temperatures of the surrounding oceans have
warmed by approximately 1°C since 1910, fairly close to the global trends.
A
warmer planet means a warmer atmosphere for all our weather and climate
As the climate system warms due to increasing greenhouse gases, more
energy is retained in the lower atmosphere. That extra energy influences all
our weather and climate.
In essence, every weather system and
ocean current operates in a climate system that is now, on average, a degree
warmer than a century ago.
In
this way, the impact of global warming is clearly observed in a distribution
shift of daily weather, as well as shifts in monthly and seasonal climate, to
higher temperatures. As is now communicated by many climate scientists, the
warming planet is loading the climate dice in favour of warmer conditions.
So, while the "cause" of an individual weather event,
including heat waves, is always proximally linked to antecedent weather
conditions — it is possible to determine the influence of climate change on the
frequency of occurrence of such an event. This is expressed by the increased
likelihood that these extreme events will occur in comparison with the past, or
in comparison with climate modelling scenarios of an unchanging climate.
Even further, the antecedent weather conditions in the January heat wave
have themselves displayed the influence of a warming world.
The lead-in climate conditions for this event were four months of very
warm temperatures across Australia. September to December 2012 was the warmest
such period on record (since 1910) for daily maximum temperatures.
During November, a precursor of the January heat wave affected many
parts of the country for a prolonged period. It set the highest spring
temperature on record for Victoria (and NSW fell just short of its record; it
couldn't beat the extreme heat that occurred in 2009). In this context, the
recent heat wave is little more than an extension of a record hot four months
for Australia, made worse because it is mid-summer.
We're
seeing more record-breaking heat events than cold events
A relatively small change in the average temperature can easily double
the frequency of extreme heat events. Australia has warmed steadily since the
1940s, and the probability of extreme heat has now increased almost five-fold
compared with 50 years ago.
Within the past decade, the number of extreme heat records in Australia
has outnumbered extreme cold records by almost 3:1 for daytime maximum
temperatures and 5:1 for night-time minimum temperature.
The duration of heat waves has increased
in some parts, especially in the northern half of the continent. Put another
way, the frequency of abnormally hot days (above the 90th percentile) has
increased by 30% and the frequency of hot nights (above the 90th percentile) has
increased by 50%.
It
is worth noting the summer just gone in the US was the warmest on record, with extreme heat records broken at a rate never previously seen
before. Studies here and overseas are now showing that many of the recent
extreme summer heat events around the world — such as the European heat wave of
2003, the Russian heat wave of 2010, and US heat waves during 2011 and 2012 —
would have been very, very unlikely without the influence of global warming.
Global warming is not only warming summer but also broadening the
summer-like period of the year, creating the perfect set-up for record extreme
heat.
Of great concern in Australia is the substantial increasing trend in
severe fire weather — weather conducive to the spread and intensification of
bushfires and grass fires — in about half of the monitoring sites studied
around the country, with a concentrated increase in the southeast of the
continent. The fire season is now longer, reducing the time for preparation
such as fuel reduction.
Again this is not surprising, and has been predicted in advance — the
combined impact of warming and cool season drying is increasing the fire danger
in a region already highly fire prone.
We
expect extreme warm weather events will occur more often
Future warming of the climate due to greenhouse gas emissions will very
likely lead to further increases in the frequency of unusually hot days and
nights and continued declines in unusually cold days and nights.
These changes will result in weather events which are increasingly
beyond our prior experiences.
And it's not just temperature extremes. Climate model projections indicate
that the frequency of many different types of extreme weather will change as
the planet warms.
