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SUMMARY:Fundamental Limits to Volcanic Cooling and its Implications for Pa
 st Climate on Earth - David Wade\, Cambridge University
DTSTART:20180205T141500Z
DTEND:20180205T151500Z
UID:TALK98077@talks.cam.ac.uk
CONTACT:Paul Griffiths
DESCRIPTION:Volcanic eruptions are the dominant cause of short-term climat
 ic cooling through their emission of aerosol precursor gases. This cooling
  response has been invoked to explain a number of climatic transitions: fr
 om the little ice age in Northern Europe to causing a completely ice-cover
 ed world. However\, there are physical limits to the strength of volcanic 
 cooling from a single eruption. I will present two case studies to support
  this: the eruption of Samalas (1257) and the eruption of the Franklin Lar
 ge Igneous Province (~700 Mya).\n\nThe eruption of Samalas resulted in the
  largest stratospheric injection of volatile gases in the Common Era. Howe
 ver\, the cooling response modelled for the Past1000 experiment in the CMI
 P5-PMIP3 model intercomparison experiment are overestimated compared to tr
 ee-ring proxy archives. Large ensemble simulations of the past 1000 years 
 have also been performed with CESM [1]. However\, these also overestimate 
 the cooling\, by around 2-3 times. I will present the results of simulatio
 ns using a novel configuration of the HadGEM-AO climate model\, validated 
 for the climate response to the Mount Pinatubo eruption in 1991\, to show 
 that the muted climate response is consistent with our current understandi
 ng of the chemical and physical processes which determine the climate resp
 onse. I will also highlight the crucial role of internal climate variabili
 ty and the challenges this poses for interpreting the climate response dir
 ectly from tree rings.\n\n740 million years ago\, Earth entered a prolonge
 d period where glaciers reached the tropics\, a so-called “Snowball Eart
 h” episode. Recent work by Macdonald and Wordsworth [2] has suggested th
 at annually-paced explosive eruptions from the Franklin Large Igneous Prov
 ince could have caused this snowball Earth. I will present the results of 
 simulations using HadCM3L\, a coupled atmosphere-ocean circulation model\,
  run under Neoproterozoic background conditions with plausible aerosol loa
 dings and size distributions based on the volcanological reconstructions. 
 These show that for size distributions consistent with such large eruption
 s\, even a 25-times Pinatubo forcing is insufficient to cause a snowball E
 arth state. Microphysical simulations with HadGEM-A show the peak cooling 
 due to annually-paced volcanic eruptions occurs in the 1-5 -times Pinatubo
  range\, suggesting an even smaller limit to the magnitude of volcanic coo
 ling by stratospheric injections of aerosol precursors. Such strong coolin
 g has also been invoked for the end Cretaceous bolide event - Brugger et a
 l [3] simulate a 26 C cooling using sulfate emissions\, which is entirely 
 implausible given the known physical and chemical processes.\n\nThese resu
 lts suggest previous modelling studies have overestimated the cooling resp
 onse to large volcanic eruptions. This has important implications for our 
 understanding of the role of volcanic forcing of past climate. Extreme cau
 tion should therefore be exercised before invoking volcanic forcing as the
  dominant cause of a climatic transition based on models with poor (or no)
  representations of aerosol microphysics or atmospheric dynamics.\n\n[1] B
 L Otto-Bliesner et al\, Climate Variability and Change since 850 C.E. : An
  Ensemble Approach with the Community Earth System Model (CESM)\, 2016\, B
 AMS\n\n[2] FA Macdonald and R Wordsworth\, Initiation of Snowball Earth wi
 th volcanic sulfur aerosol emissions\, 2017\, GRL\n\n[3] J Brugger et al\,
  Baby\, it's cold outside: Climate model simulations of the effects of the
  asteroid impact at the end of the Cretaceous\, 2017\, GRL
LOCATION:Pfizer Lecture Theatre\, Department of Chemistry
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