Underestimation of volcanic cooling in tree-ring-based reconstructions of hemispheric temperatures

Abstract

The largest eruption of a tropical volcano during the past millennium occurred in AD 1258-1259. Its estimated radiative forcing was several times larger than the 1991 Pinatubo eruption 1. Radiative forcing of that magnitude is expected to result in a climate cooling of about 2 °C (refs 2-5). This effect, however, is largely absent from tree-ring reconstructions of temperature 6-8, and is muted in reconstructions that employ a mix of tree-rings and other proxy data 9,10. This discrepancy has called into question the climate impact of the eruption 2,5,11. Here we use a tree-growth model driven by simulated temperature variations to show that the discrepancy between expected and reconstructed temperatures is probably an artefact caused by a reduced sensitivity to cooling in trees that grow near the treeline. This effect is compounded by the secondary effects of chronological errors due to missing growth rings and volcanically induced alterations of diffuse light. We support this conclusion with an assessment of synthetic proxy records created using the simulated temperature variations. Our findings suggest that the evidence from tree rings is consistent with a substantial climate impact 2-5 of volcanic eruptions in past centuries that is greater than that estimated by tree-ring-based temperature reconstructions. © 2012 Macmillan Publishers Limited. All rights reserved.