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Volcanic and solar signals in 20th century simulations

PI: Casper Ammann
Institution: National Center for Atmospheric Research
Abstract:
I would like to compare the volcanic and solar signal in the IPCC experiments as they get available later this year. First, temporal structures of hemispheric/global response are a first order estimate of natural climate change, particularly if ensembles of 3 or more are available. Our wavelet extraction should be able to pick up the 11-, 22- and ~85-year signals resulting from most solar forcing series. The volcanic signatures will be much easier, but our automatic state-space model will quantify the climate response and deliver posterior probabilities. Second, spatio-temporal signal extraction and response of internal modes to the forcings will be another focus point. This will help to build a quantitative base for studies that 'recognize' a 'clear' solar signal in observed time series across the world, and thus contribute strongly towards a more solid reference for the role of natural forcing during the 20th century. I expect that we will recognize how this role changes over the course of the century.

Fundamentally, this work is crucial for the paleo chapter: as a calibration for the (few) transient simulations that have been run for pre-industrial conditions (NCAR, MPI, HadCenter, LMD,...). It will be a very strong message against the random criticism of massive underestimation of natural forcings if we can show that the identical approach as used in the 20th century of implementing natural forcings does actually result in acceptable climate under pre-industrial conditions. This way, as I often push for, the famous statement of "last 50 years" can be expanded to something more along the lines of "natural forcings explain well climate prior to large anthropogenic changes of atmospheric composition, but the last (3?) decades of the 20th century can only be explained when anthropogenic forcing is included".
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