Extreme weather and climate change
Prof David Karoly
School of Earth Sciences, University of Melbourne
TC Larry, 2006
From Bureau of Meteorology
Are the recent extreme weather events due
to:
• Natural climate variability, or
• Greenhouse climate change ??
Maybe this is a silly question.
In practice, we need to quantify the different factors
affecting the likelihood of extreme weather and
climate events.
IPCC Fourth Assessment Report 2007
• ‘Warming of the climate system is unequivocal’
• ‘Most of the observed increase in global average
temperatures since the mid-20th century is very
likely due to the observed increase in
anthropogenic greenhouse gas concentrations’
• ‘It is very likely that hot extremes, heat waves, and
heavy precipitation events will continue to become
more frequent’
• ‘It is likely that future tropical cyclones will become
more intense, with larger peak wind speeds and
more heavy precipitation. There is less confidence
in projections of a global decrease in numbers of
tropical cyclones.’
Expected changes in temperature and rainfall
extremes in a warmer climate
Observed changes in Australian temperature extremes
From
Bureau
of Met
Observed changes in Australian rainfall extremes
From Bureau
of Met
A re a e x p e rie n c in g m u c h g re a te r th a n n o rm a l p ro
o f a n n u a l ra in fa ll fro m h e a v y ra in d a y s
From Gallant and
Karoly (2010)
Percent area of Australia
15
10
5
0
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
-5
-1 0
Attribution of observed changes in Australian extremes
from Alexander and Arblaster (2009)
Comparison of observed trend over 1957-99 with modelled
trends including increasing greenhouse gases from 9 models.
• Significant increase in warm nights agrees with models
• Increase in very heavy rain contribution is not significant
and is found in only some models
European summer
2003 heatwave
Likelihood of the summer heatwave
with (red) and without (green)
anthropogenic climate change
IPCC AR4 WGI Fig 9.13
Best estimate of the fraction of risk
shows a four-fold increase in the
likelihood of such a summer
heatwave due to climate change
Flooding in England and Wales in autumn 2000
(from Pall et al. 2011)
Significant reduction in return
period for severe daily runoff
due to climate change
Attributable risk of increase in
severe daily river runoff.
Histograms of the fraction of risk
attributable to anthropogenic
climate change. Best estimate is
more than a doubling of risk.
Severe tropical cyclones
TC Yasi Cat 5 TC on 2 Feb 2011
From
Callaghan and
Power 2010
The number of severe TCs making
landfall on the Qld coast has
decreased since 1870. Landfalling
TCs occur twice as often in La
Niña years as in El Niño years.
Sea surface temps
Australian region average
SSTs in December
SST deciles
Dec 2010
Summary
• Climate change expected to lead to increases in hot extremes
and reductions to cold extremes, as observed in Australia, and
increases in heavy rain days
• No observed increased in heavy rain days in Australia, but
observed increase in proportion of rain from heavy rain days
• Attribution of extreme climate events considers increase in risk
due to anthropogenic climate change, but no studies have been
undertaken for Australian events yet
• The strong La Niña is the main factor for the extreme events in
2010/11
• Climate change, leading to record high SST around Australia
and increased atmospheric water vapour, likely also a
contributing factor in the recent heavy rain and other extreme
events, but quantification not possible yet