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Climate engineering through artificial enhancement of natural forcings: Magnitudes and implied consequences

Author(s): Ammann CM, Washington WM, Meehl GA, Buja L, Teng H

Published: November, 2010

Publisher: Journal of Geophysical Research

DOI: 10.1029/2009JD012878

Tags: Stratospheric Aerosols, Volcanism, Climate Modelling

URL: http://www.agu.org/pubs/crossref/2010/2009JD012878.shtml

Abstract: Explosive volcanism and solar activity changes have modulated the Earth's temperature over short and century time scales. Associated with these external forcings were systematic changes in circulation. Here, we explore the effect of similar but artificially induced forcings that mimic natural radiative perturbations in order to stabilize surface climate. Injection of sulfate aerosols into the stratosphere, not unlike the effects from large volcanic eruptions, and a direct reduction of insolation, similar to total solar irradiance changes, are tested in their effectiveness to offset global mean temperature rise resulting from a business-as-usual scenario, thereby reducing surface temperatures to conditions associated with committed warming of a year 2000 stabilization scenario. This study uses a coupled Atmosphere-Ocean General Circulation Model to illustrate the character of resulting climate and circulation anomalies when both enhanced greenhouse (A2 scenario) and opposing geoengineering perturbations are considered. First we quantify the magnitude of the required perturbation and compare these artificial perturbations to the natural range of the respective forcing. Then, we test the effectiveness of the “correction” by looking at the regional climate response to the combined forcing. It is shown that widespread warming could be reduced, but overcompensation in the tropics is necessary because sea ice loss in high latitudes cannot be reversed effectively to overcome higher ocean heat content and enhanced zonal winter circulation as well as the continuous IR forcing. The magnitude of new, greenhouse gas-countering anthropogenic forcing would have to be much larger than what natural forcing from volcanoes and solar irradiance variability commonly provide.


Effect of climate sensitivity on the response to volcanic forcing

Author(s): Wigley TML, Ammann CM, Santer BD, Raper SCB

Published: May, 2005

Publisher: Journal of Geophysical Research

DOI: 10.1029/2004JD005557

Tags: Volcanism, Climate Science, Climate Modelling

URL: http://www.agu.org/journals/ABS/2005/2004JD005557.shtml

Abstract: The results from 16 coupled atmosphere/ocean general circulation model (AOGCM) simulations are used to reduce internally generated noise and to obtain an improved estimate of the underlying response of 20th century global mean temperature to volcanic forcing. An upwelling diffusion energy balance model (UD EBM) with the same forcing and the same climate sensitivity as the AOGCM is then used to emulate the AOGCM results. The UD EBM and AOGCM results are in very close agreement, justifying the use of the UD EBM to determine the volcanic response for different climate sensitivities. The maximum cooling for any given eruption is shown to depend approximately on the climate sensitivity raised to power 0.37. After the maximum cooling for low-latitude eruptions the temperature relaxes back toward the initial state with an e-folding time of 29–43 months for sensitivities of 1–4°C equilibrium warming for CO2 doubling. Comparisons of observed and modeled coolings after the eruptions of Agung, El Chichón, and Pinatubo give implied climate sensitivities that are consistent with the Intergovernmental Panel on Climate Change (IPCC) range of 1.5–4.5°C. The cooling associated with Pinatubo appears to require a sensitivity above the IPCC lower bound of 1.5°C, and none of the observed eruption responses rules out a sensitivity above 4.5°C.


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