how/why our climate has changed neville nicholls, pandora hope, lynda chambers, & bertrand...
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How/why our climate has changed
Neville Nicholls, Pandora Hope,
Lynda Chambers, & Bertrand Timbal
Bureau of Meteorology Research Centre, Melbourne
Acknowledgements: Bryson Bates, Brian Ryan, Stephen Charles, Bryson Bates, Brian Ryan, Stephen Charles,
Eddy Campbell, Ian Smith, Wenju Cai, Eddy Campbell, Ian Smith, Wenju Cai, Jorgen FrederiksenJorgen Frederiksen
Wasyl Drosdowsky, Scott Power, Carsten Wasyl Drosdowsky, Scott Power, Carsten Frederiksen, David Jones, Dean Collins, Frederiksen, David Jones, Dean Collins, John CrambJohn Cramb
Brian Sadler & IOCIPBrian Sadler & IOCIP
http://www.bom.gov.au/silo/products/cli_chg/index.shtml
Sea surface temperature
Temperature over land
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.)
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 1950–99 and 1910–99. (Karoly & Braganza, 2005)
Why is the warming attributed to anthropogenic factors? ““Internal” (natural) climate variabilityInternal” (natural) climate variability
El Nino, Pacific Decadal Oscillation etc.El Nino, Pacific Decadal Oscillation etc. Would cause cooling in some areas…Would cause cooling in some areas… ……or cooling in oceans or solid earthor cooling in oceans or solid earth
Forcing from “external” natural factorsForcing from “external” natural factors Orbital, solar variability, volcanic aerosolsOrbital, solar variability, volcanic aerosols Should have caused cooling since mid-20Should have caused cooling since mid-20thth century century
Forcing from anthropogenic factorsForcing from anthropogenic factors TheoryTheory Model-based attributionModel-based attribution Warming unusually strongWarming unusually strong
Land-use changes or air pollution?Land-use changes or air pollution? Would not cause warming everywhereWould not cause warming everywhere
http://www.bom.gov.au/silo/products/cli_chg/index.shtml
May-July rainfall 1976-2003/1925-75 (%)
Pandora Hope
Perth Airport: rainfall spectrum
1949-1975 1976-2003
Perth June & July rainfall per year Total mm of Perth June & July rainfall per year
Decreased frequency of moderately-wet
days
Strong drop in rainfall from moderately-
wet days
Pandora Hope
Rottnest Island May-July rainfall
0
100
200
300
400
500
600
700
800
1880 1901 1921 1941 1961 1981
Year
Rai
nfa
ll (
mm
)
Self Organising Maps & associated rainfall anomalies
“Wet” types“Trough to west” types
Pandora Hope
Frequency of “wet” synoptic patterns (& rainfall)
Decrease in frequency of “wet” types accounts for ~50% of rainfall decreaseDecrease in rainfall associated with “troughs to west” types accounts for ~30%
Pandora Hope
May-July rain and Perth MSLP
0
50
100
150
200
250
300
350
400
450
500
1952 1957 1962 1967 1972 1977 1982 1987 1992 1997
Year
SW
WA
rai
nfa
ll (m
m)
1010
1012
1014
1016
1018
1020
1022
1024
1026
Per
th M
SL
P (
reve
rsed
)
(b)(a)
Storm tracks associated with cyclogenesis modes
Mean streamfunction of dominant cyclogenesis mode for 1949-68
1949-68 1975-94
Frederiksen & Frederiksen, 2005
Downscaled
“Full Forcing”
Downscaled
“Natural Forcing”
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)
Observed drying trend in Winter from 1958 to 1998
mm/year
B. Timbal Bertrand Timbal
-60
-40
-20
0
20
40
60
80
100
1960 1965 1970 1975 1980 1985 1990 1995
Observed; “Natural” range; “FF” range
No drying trend in Natural (solar & volcanoes).Drying trend in FF (+ greenhouse + ozone + aerosols) but weaker than observed.
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)
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
“Full Forcing” “FF + land clearance”
Is land clearance important?
Bertrand Timbal
June and July SWWA rainfall anomaly from 1901-2000 average
-200
-150
-100
-50
0
50
100
150
200
1900 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100
Year
NC
C (
mm
)
-60
-40
-20
0
20
40
60
A2
, B1
, C2
0 A
v o
f 7
m
od
els
(m
m)
NCC
NCC 1901-1975
NCC 1976-2000
C20 models
C20 1901-1975
C20 1976-2000
A2 models
A2 2001-2050
A2 2051-2100
B1 models
B1 2001-2050
B1 2050-2100
Pandora Hope
How/why has our climate changed SWWA (and Australia) has warmed ~1C over 20SWWA (and Australia) has warmed ~1C over 20thth century – century –
very likely anthropogenic.very likely anthropogenic. SWWA May-July rainfall decreased suddenly in mid-1970s.SWWA May-July rainfall decreased suddenly in mid-1970s. Declines in frequency of synoptic types associated with Declines in frequency of synoptic types associated with
rainfall (“troughs”) affecting SWWA.rainfall (“troughs”) affecting SWWA. Declined rainfall in synoptic types with off-shore trough. Declined rainfall in synoptic types with off-shore trough. Rainfall decrease associated with near-global pressure Rainfall decrease associated with near-global pressure
increase – so not just a local cause.increase – so not just a local cause. Models suggest that enhanced greenhouse effect contributed Models suggest that enhanced greenhouse effect contributed
to rainfall decline.to rainfall decline. Land use changes may also contribute.Land use changes may also contribute.