Comments%20on%20Discussion%20paper%20

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Comments on Discussion paper Detecting,
understanding, and attributing climate change

• David Karoly
• School of Meteorology
• University of Oklahoma

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Why do detection and attribution?

• To identify the causes of recent observed climate
  variations
• To evaluate the performance of climate models in
  simulating the observed climate variations over
  the last century
• To constrain the projections of future climate
  change

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IPCC Third Assessment (2001)

• The global average surface temperature has
  increased over the 20th century by about 0.6C
• Most of the observed warming over the last 50
  years is likely to have been due to the increase
  in greenhouse gas concentrations
• Key uncertainties include relating regional
  trends to anthropogenic climate change
• Surface temperature changes are detectable only
  on scales greater than 5,000 km

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Detection of regional warming

• Calculate observed linear trend in each grid-box
  and test for 95 significance (marked with )
  using model control simulations to provide
  estimate of natural variability of trends (Karoly
  and Wu, 2005). Similar results found by Knutson
  et al. (2006)

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New approach to detection of anthropogenic
temperature changes

• Reducing the noise associated with natural
  climate variations will increase the likelihood
  of detecting any anthropogenic climate change
• Optimal fingerprint method rotates the signal
  pattern away from the pattern of natural climate
  variability
• A large fraction of the interannual variability
  of surface temperatures is associated with
  rainfall variations (dry years are hot in
  Australia)
• Removing the rainfall-related temperature
  variations will reduce the noise and enhance the
  detection of any anthropogenic signal in the
  residual temperature variations

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Interannual temperature variations

• Scatterplot of interannual variations of Tmax and
  precip for the southern Aust region
• Strong out-of-phase relationship in both obs and
  model
• Shift of this relationship to warmer temperatures
  during 1976-2003 in both obs and GS-forced model
  simulations (from Karoly and Braganza, 2005)

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Trends over last 50 years

• Compare observed trends over last 50 years with
  model estimates of natural variability of trends
  in each gridbox. Maps show probability of trend
  significant different from zero for Tmax (left)
  and residual Tmax after removing rainfall
  variations (right). From Karoly and Braganza
  (2005)

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Continental-scale temperature projections
Uncertainty plume for changes relative to 1990s
in Australian area-mean temperature using
scalings based on continental-scale attribution.
Probabilities are represented by the depth of
shading. From Stott et al. (2006)
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New references

• Karoly, D.J., and Q. Wu (2005) Detection of
  regional surface temperature trends. J. Climate,
  18, 43374343.
• Karoly, D.J., and K. Braganza (2005) A new
  approach to detection of anthropogenic
  temperature changes in the Australian region.
  Meteor. Atmos. Phys., 89, 57-67.
• P. A. Stott, J. A. Kettleborough, and M. R. Allen
  (2006) Uncertainty in continental-scale
  temperature predictions GRL, 33, L02708,
  doi10.1029/2005GL024423
• T. R. Knutson et al. (2006) Assessment of
  Twentieth-Century Regional Surface Temperature
  Trends Using the GFDL CM2 Coupled Models. J
  Clim., 19, 1624-51.