Fundamental Limits to Volcanic Cooling and its Implications for Past Climate on Earth

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• David Wade, Cambridge University
• Monday 05 February 2018, 14:15-15:15
• Pfizer Lecture Theatre, Department of Chemistry.

If you have a question about this talk, please contact Paul Griffiths.

Volcanic eruptions are the dominant cause of short-term climatic cooling through their emission of aerosol precursor gases. This cooling response has been invoked to explain a number of climatic transitions: from the little ice age in Northern Europe to causing a completely ice-covered 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 Large Igneous Province (~700 Mya).

The eruption of Samalas resulted in the largest stratospheric injection of volatile gases in the Common Era. However, the cooling response modelled for the Past1000 experiment in the CMIP5 -PMIP3 model intercomparison experiment are overestimated compared to tree-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 simulations 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 understanding of the chemical and physical processes which determine the climate response. I will also highlight the crucial role of internal climate variability and the challenges this poses for interpreting the climate response directly from tree rings.

740 million years ago, Earth entered a prolonged period where glaciers reached the tropics, a so-called “Snowball Earth” episode. Recent work by Macdonald and Wordsworth [2] has suggested that annually-paced explosive eruptions from the Franklin Large Igneous Province 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 loadings and size distributions based on the volcanological reconstructions. These show that for size distributions consistent with such large eruptions, even a 25-times Pinatubo forcing is insufficient to cause a snowball Earth 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 cooling by stratospheric injections of aerosol precursors. Such strong cooling has also been invoked for the end Cretaceous bolide event – Brugger et al [3] simulate a 26 C cooling using sulfate emissions, which is entirely implausible given the known physical and chemical processes.

These results suggest previous modelling studies have overestimated the cooling response to large volcanic eruptions. This has important implications for our understanding of the role of volcanic forcing of past climate. Extreme caution 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.

[1] BL Otto-Bliesner et al, Climate Variability and Change since 850 C.E. : An Ensemble Approach with the Community Earth System Model (CESM), 2016, BAMS

[2] FA Macdonald and R Wordsworth, Initiation of Snowball Earth with volcanic sulfur aerosol emissions, 2017, GRL

[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

This talk is part of the Centre for Atmospheric Science seminars, Chemistry Dept. series.

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