I started writing this last week but didn't get time to finish. I was
interested in how much of the Figbird's invasion of NSW/Victoria could be
explain by climate change so I did some back-of-the-envelope arithmetic.
The IPCC reckons the global mean temperature climbed by 0.74C in the
hundred years up to 2005, with most of this increase occurring in the
last 30 years. And if you look at BOM's temperature record for SE
Australia for SE Australia it shows roughly the same rise.
Now as you come south along the NSW coast the mean temperature declines.
I couldn't find a serious calulation so I did a quick estimate I quickly
googled mean temps for 4 places (Gympie,Grafton, Sydney, Batemans Bay)
and found a linear regression fitted well with mean temperature falling
by 0.44C for every degree of latitude (110km) you came south.
So the 0.74C temperature rise over the last 100 years is equivalent
to moving 1.7 degrees of latitude (190km) north. This could explain
the figbird's southern limit moving from Bellingen River south to
about Forster. The figbird range has actually increased by 7 degrees
of latitude (to Mallacoota) over the last 100 years so mean temperature
change can explain about 25% of its invasion.
But biologically response to climate change may be slow taking decades or
even centuries - which is why rapid climate change is such a conservation
concern. So climate change might not be responsible for any of the
invasion and any figbird response to the past 0.7C increase might be
still be years away - good news if you want to see figbirds in Melbourne.
Climate change hasn't stopped of course - the IPCC projects a global
temperature increase of about 0.02C/year for the next couple of decades
which, if I have my arithmetic right, and if bird ranges were determined
by mean temp only, would see bird ranges in coastal E Australia moving
5km further south every year. 5km/year doesn't sound much but it adds up
over decades or centuries.
Of course the real world is much more complicated, e.g. other climatic
variables like rainfall may be more biologicaly significant and some
species's range will not be determined by climate at all. Also
climate change at a regional level is harder to predict and may follow
quite different patterns.
Andrew
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