How/why our climate has changed Neville Nicholls, Pandora Hope, Lynda Chambers, & Bertrand Timbal Bureau of Meteorology Research Centre, Melbourne.

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Presentation transcript:

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, Eddy Campbell, Ian Smith, Wenju Cai, Jorgen Frederiksen 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 Wasyl Drosdowsky, Scott Power, Carsten Frederiksen, David Jones, Dean Collins, John Cramb Brian Sadler & IOCIP Brian Sadler & IOCIP

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 variability “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 Forcing from “external” natural factors  Orbital, solar variability, volcanic aerosols  Should have caused cooling since mid-20 th century Forcing from anthropogenic factors Forcing from anthropogenic factors  Theory  Model-based attribution  Warming unusually strong Land-use changes or air pollution? Land-use changes or air pollution?  Would not cause warming everywhere

May-July rainfall / (%) Pandora Hope

Perth Airport: rainfall spectrum Perth June & July rainfall per yearTotal mm of Perth June & July rainfall per year Decreased frequency of moderately-wet days Strong drop in rainfall from moderately- wet days Pandora Hope

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 decrease Decrease in rainfall associated with “troughs to west” types accounts for ~30% Pandora Hope

(b)(a) Storm tracks associated with cyclogenesis modes Mean streamfunction of dominant cyclogenesis mode for 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

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

Pandora Hope

How/why has our climate changed SWWA (and Australia) has warmed ~1C over 20 th century – very likely anthropogenic. SWWA (and Australia) has warmed ~1C over 20 th century – 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 rainfall (“troughs”) affecting SWWA. Declines in frequency of synoptic types associated with 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 increase – so not just a local cause. Rainfall decrease associated with near-global pressure increase – so not just a local cause. Models suggest that enhanced greenhouse effect contributed to rainfall decline. Models suggest that enhanced greenhouse effect contributed to rainfall decline. Land use changes may also contribute. Land use changes may also contribute.