Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 322

Eighty per cent of Australia's agricultural output by value comes from, and 70%
of its irrigated cropland and pasture is found in, the Murray-Darling basin that covers
much of New South Wales and Victoria. Much of New Zealand's agriculture is loc-
ated on or around its South Island Canterbury Plain. Both these areas are increasingly
seeing climate variability (specifically with respect to water supply), which is of cent-
ral concern regarding agricultural productivity. The impact of such variability can be
considerable. In New Zealand in a bad year agricultural output can decline by over a
fifth and knock the best part of a percentage point off the nation's annual GDP. Bearing
in mind that if a developed nation's GDP grows by 1-2% then its economy is con-
sidered to be doing well, losing a percentage point is a major sociopolitical concern.
The significance of climate variability and extreme events came home to Aus-
tralians in the State of Victoria on 'Black Saturday', 7th February 2010, and the sub-
sequent month, when there was a record-breaking heatwave that followed 2 months of
drought. Fires destroyed 2030 domestic dwellings and over a thousand other types of
building, and displaced more than 7500 people. Ecologically, other than the fires, the
most dramatic event was the grey-headed flying fox ( Pteropus poliocephalus ) 'mass-
death event'. This flying fox is a large fruit bat whose range in Australia has already
begun to move poleward. Being a flying creature, they have an active metabolism
which means that they often need to lose excess heat to their surroundings. However,
if the surrounding environment is too warm then the first law of thermodynamics
comes to the fore and the bat is unable to lose heat: 43 ◦ C is the critical temperature.
As the heatwave progressed the bats first unfurled their wings to maximise their sur-
face area and therefore heat loss. When this did not work they flew, with air passing
over their wings to cool them. However, when it is too hot even this strategy will not
work. On Black Saturday many flying foxes could be seen falling out of the sky, dying
from heat exhaustion. One of Victoria's largest grey-headed flying fox colonies, with
a population of around 10 000, saw 20% lost in the mass-death event. Prior to the
2010 heatwave, between 1994 and 2007, more than 24 500 grey-headed flying foxes
have been recorded as dying from extreme heat events. One of the largest losses of
foxes prior to 2010 was on 12 January 2002 when New South Wales temperatures
exceeded 42 ◦ C, which resulted in the loss of 3500 individuals in nine mixed-species
colonies (Welbergen et al., 2008). To put these losses into context, the species popu-
lation was thought to be around 300 000 at the turn of the millennium and it is listed
as vulnerable by the International Union for the Conservation of Nature (IUCN) Red
List.
The IPCC (2007a) anticipate that all of Australia and New Zealand is likely to get
warmer this century: somewhat more so than the surrounding oceans, but comparable
to the overall mean global warming. The concern is with climate variability and water
deficit. It is thought that precipitation over Australia will not change that much over
the 21st century but, factoring in warmth, moisture balance deficits are expected.
Given all of the above the very low stream flow in the River Murray, in Victoria,
Australia, for the 1998-2008 period is very rare - about a 1-in-1500 year event
(Climate Commission, 2011) - and of some concern. Equally of concern for New
Zealand agriculture is the anticipated increase in temperature and rainfall in the
eastern half of the South Island over the 21st century (IPCC, 2007b).