AUSTRALIA has long been a land of droughts
and flooding rains, and the past two years have been a good example
of that well-known climatic pattern.
When the dams are full, the paddocks are green and
the dry soil has turned soggy, we tend to forget about drought and
its risks for our health and well-being.
However, it has always been wise to take a
long-term perspective and to acknowledge that a few wet years don't
mean we'll never see a severe drought again. Furthermore, with
climate change now in the mix, it is even more important to take a
broader, long-term perspective.
Climate change is influencing more than just
droughts, as the recent CSIRO-Bureau of Meteorology State of the
Climate 2012 report clearly outlines.
Temperatures over land and in the oceans continue
to increase rapidly, sea levels are rising and extremely hot days
have become more common. But it is the recent period of very wet,
cool weather bringing floods to many parts of Australia that has
grabbed the most attention in the past few months.
The Climate Commission's report on the science
behind southeast Australia's wet, cool summer provides the broader,
long-term perspective needed to understand the significance of the
big wet.
The 2010-2011 period was the wettest two-year
period on record in Australia. The exceptionally high rainfall was
driven by back-to-back La Nina events, the phase of natural climate
variability that periodically brings heavy rainfall to eastern
Australia.
La Nina events are associated with
higher-than-average sea surface temperatures across Australia, which
lead to higher rates of evaporation from the surface ocean and more
water in the air for rainstorms. In fact, the sea surface
temperatures to the north of Australia during the spring and early
summer of 2010-2011 were the highest on record, very likely
contributing to the amount and intensity of the rainfall.
The very high sea surface temperatures were also,
in part, a result of the underlying global warming trend. We are not
only seeing increasing temperatures over land, but the surface waters
of the ocean are also heating up. For this reason, many scientists
are concerned global warming may have contributed to the strength of
the La Nina event and thus to the heavy rainfall and flooding.
In the longer term, it is likely that the wet
years of 2010 and 2011 were only a short interruption in the drier
conditions we've observed in southeast Australia during the past 40
years.
The bulk of the rain in the past two years fell in
spring and summer, not in the normal autumn-winter period of wetter
conditions that typifies the southeast's climate. Changes in the
timing of rainfall, not just the amount, have important consequences
for farming and water resources.
The drop in autumn-winter rainfall, primarily
since the mid-1990s, is associated with a southwards shift of the
rain systems from the Southern Ocean that normally provided the rain
for this season. This same shift is largely responsible for the
reduction in autumn-winter rainfall in southwest Western Australia.
This emerging pattern of long-term drying across
southern Australia, exacerbated by hot days and weeks and
periodically interrupted by very intense rainfall and flooding, comes
as no surprise to climate scientists. It is entirely consistent with
what we expect from a changing climate.
How will droughts and floods change in the future?
Again, a long-term perspective is essential.
Extended dry periods are expected to increase in southwest and
southeast Australia by the end of this century, increasing the risk
of drought.
On the other hand, it is more likely than not that
heavy rainfall events will also become more frequent across much of
Australia. So when long dry periods are interrupted by welcome
periods of wet weather, the rain is more likely to fall as heavy
downpours than as extended drizzle.
It is virtually certain the global average
temperature will continue to rise through the 21st century. This is
likely to increase the number, length and intensity of heat waves
across many regions of Australia. Very hot days coupled with extended
dry periods create significant stress for plants and animals, and
pose serious risks for human health and well-being.
The quintessential Australian climatic pattern of
intense droughts and flooding rains will still be with us in the
future. But the added risks associated with climate change make it
even more important that we plan and act on a careful analysis of the
risks that climate variability and climate change together bring.
The magnitude of these risks ultimately depends on
the effectiveness of global emission reduction efforts, including by
Australia. The transition to a clean energy economy, which is
gathering speed in many parts of the world, gives us great hope that
we can minimise these potential risks.
Will Steffen is Climate Commissioner and
executive director of the Australian National University's Climate
Change Institute.
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