How/why our climate has changed

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How/why our climate has changed

• Neville Nicholls, Pandora Hope,
• Lynda Chambers, Bertrand Timbal
• Bureau of Meteorology Research Centre, Melbourne

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Acknowledgements

• Bryson Bates, Brian Ryan, Stephen Charles, Eddy
  Campbell, Ian Smith, Wenju Cai, Jorgen
  Frederiksen
• Wasyl Drosdowsky, Scott Power, Carsten
  Frederiksen, David Jones, Dean Collins, John
  Cramb
• Brian Sadler IOCIP

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http//www.bom.gov.au/silo/products/cli_chg/index.
shtml
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Temperature over land
Sea surface temperature
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Comparison of the reconstructed multi-proxy
temperature history for the southern hemisphere
(wavy line) compiled by Mann and Jones (2003)
with ground surface temperature histories (bold
line) for four boreholes in southwest Australia.
(Appleyard, 2005, Aust. J. Earth Sci.)
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Karoly2
Trends in Australian-average mean temperature
from anthropogenically forced (GS, open symbols)
and natural externally forced (NAT, solid
symbols) model simulations and observations
during 195099 and 191099. (Karoly Braganza,
2005)
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Why is the warming attributed to anthropogenic
factors?

• Internal (natural) climate variability
• El Nino, Pacific Decadal Oscillation etc.
• Would cause cooling in some areas
• or cooling in oceans or solid earth
• Forcing from external natural factors
• Orbital, solar variability, volcanic aerosols
• Should have caused cooling since mid-20th century
• Forcing from anthropogenic factors
• Theory
• Model-based attribution
• Warming unusually strong
• Land-use changes or air pollution?
• Would not cause warming everywhere

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http//www.bom.gov.au/silo/products/cli_chg/index.
shtml
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May-July rainfall 1976-2003/1925-75 ()
Pandora Hope
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Perth Airport rainfall spectrum
Strong drop in rainfall from moderately-wet days
Decreased frequency of moderately-wet days
Perth June July rainfall per year
Total mm of Perth June July rainfall per year
1949-1975
1976-2003
Pandora Hope
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Self Organising Maps associated rainfall
anomalies
Wet types
Trough to west types
Pandora Hope
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Frequency of wet synoptic patterns ( rainfall)
Decrease in frequency of wet types accounts for
50 of rainfall decrease Decrease in rainfall
associated with troughs to west types accounts
for 30
Pandora Hope
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Mean streamfunction of dominant cyclogenesis mode
for 1949-68
1949-68
1975-94
Storm tracks associated with cyclogenesis modes
Frederiksen Frederiksen, 2005
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Observed drying trend in Winter from 1958 to 1998
mm/year

• Rainfall trends from the NCAR PCM ensembles
• No drying trend in the Natural Forcing ensemble
  (solar volcanoes)
• Drying trend in the Full Forcing ensemble
  (Natural greenhouse
  ozone aerosols)

Bertrand Timbal
B. Timbal
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Observed Natural range FF range
No drying trend in Natural (solar
volcanoes).Drying trend in FF ( greenhouse
ozone aerosols) but weaker than observed.
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• Pitman et al., 2004
• Percent change in model rainfall with land cover
  changes
• We find strong evidence that observed changes in
  rainfall and temperature over SWWA can be
  partially explained by the large-scale LCC.
• The relative role of a large-scale atmospheric
  reorganisation, increasing greenhouse gases and
  LCC in explaining the observed changes over SWWA
  remains unknown.

Land cover change (trees to grass or crops
between 1788 and 1988)
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Is land clearance important?
Full Forcing
FF land clearance

• Land clearance effect
• Rainfall decline twice as large with land
  clearance included
• Not due to atmospheric pressure response
• Enhanced response of the model large-scale
  rainfall

Bertrand Timbal
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Pandora Hope
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How/why has our climate changed

• SWWA (and Australia) has warmed 1C over 20th
  century very likely anthropogenic.
• SWWA May-July rainfall decreased suddenly in
  mid-1970s.
• Declines in frequency of synoptic types
  associated with rainfall (troughs) affecting
  SWWA.
• Declined rainfall in synoptic types with
  off-shore trough.
• Rainfall decrease associated with near-global
  pressure increase so not just a local cause.
• Models suggest that enhanced greenhouse effect
  contributed to rainfall decline.
• Land use changes may also contribute.