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The AAPT meeting last week had Benjamin Santer (climate scientist who
has testified in Congress) as a plenary speaker. He had (at least)
two interesting points.
1. There is a unique ‘fingerprint’ to the current climate change that
points to humans: the lower atmosphere is warming while the upper
atmosphere is cooling. If warming were due to other influences (sun
activity for example) both lower and upper atmosphere would be
warming. The fact that climate models accurately predict this upper
cooling with lower warming indicate they are correctly modeling what
is happening and that it is caused by added greenhouse gasses from
human activity.
2. The apparent recent slowdown in warming is possibly due to
increased low level volcanic activity resulting in more sulfate
aerosols. Big volcanoes (eg. Penatubo) make obvious changes (cooling)
and are included in climate models but until recently lesser
volcanoes were not accurately accounted for. Including them indicates
that the model predictions may actually be too conservative (they
were only slightly higher than what actually happened during the
“hiatus” but should have been even higher).