Observations of a stratospheric aerosol &amp;lt;i&amp;gt;veil&amp;lt;/i&amp;gt; from a tropical volcanic eruption in December 1808: is this the &amp;lt;i&amp;gt;Unknown&amp;lt;/i&amp;gt;  ∼1809 eruption?

E. J. Hendy,K. V. Cashman,A. Guevara-Murua,A. C. Rust,C. A. Williams

doi:10.5194/cp-10-1707-2014

Abstract

Abstract. The Unknown eruption of 1808/1809 was the second most explosive SO2-rich volcanic eruption in the last two centuries, eclipsed only by the cataclysmic VEI 7 Tambora eruption in April 1815. However, no eyewitness accounts of the event, and therefore its location, or the atmospheric optical effects associated with its aerosols have been documented from historical records. Here we report on two meteorological observations dating from the end of 1808 that describe phenomena we attribute to volcanic-induced atmospheric effects caused by the Unknown eruption. The observations were made by two highly respected Latin American scientists. The first, Francisco José de Caldas, describes a stratospheric aerosol haze, a "transparent cloud that obstructs the sun's brilliance", that was visible over the city of Bogotá, Colombia, from 11 December 1808 to at least mid-February 1809. The second, made by physician José Hipólito Unanue in Lima, Peru, describes sunset after-glows (akin to well-documented examples known to be caused by stratospheric volcanic aerosols) from mid-December 1808 to February 1809. These two accounts provide direct evidence of a persistent stratospheric aerosol veil that spanned at least 2600 km into both Northern and Southern Hemispheres and establish that the source was a tropical volcano. Moreover, these observations confirm that the Unknown eruption, previously identified and tentatively assigned to February 1809 (±4 months) from analysis of ice core sulfate records, occurred in late November or early December 1808 (4 December 1808 ±7 days). This date has important implications for the associated hemispheric climate impacts and temporal pattern of aerosol dispersal.

Highlights

The 1810s were the coldest decade and longest sustained period of below-average hemispheric and tropical temperatures in the last 500 years (e.g. Jones et al, 1995; Briffa et al, 1998; D’Arrigo et al, 2009)
We argue that the atmospheric phenomena described by Caldas and Unanue provide evidence for a stratospheric aerosol cloud caused by the Unknown eruption of 1808–1809
We have presented evidence that a stratospheric aerosol cloud was present over Colombia from 11 December 1808 and extended into the Southern Hemisphere at least as far south as Lima, Peru, within the same week

Summary

Introduction

The 1810s were the coldest decade and longest sustained period of below-average hemispheric and tropical temperatures in the last 500 years (e.g. Jones et al, 1995; Briffa et al, 1998; D’Arrigo et al, 2009). Perhaps the most common atmospheric disturbances described following large explosive eruptions are vivid sunsets, and in particular a prolonged twilight after-glow due to the enhanced forward scattering caused by the volcanic aerosols in the stratosphere (Symons, 1888; Deirmendjian, 1973) This phenomenon has been observed worldwide after recent major volcanic eruptions and reported in contemporary documents following historic events such as Awu (Indonesia, 1641), Katla (Iceland, 1660), Laki (Iceland, 1783), Tambora (Indonesia, 1815) and Krakatau (Indonesia, 1883) We know that Roulin took part in a scientific mission to Colombia in the 1820s (Humboldt, 2011), and since his version is so similar to that of Caldas (in both content and the order in which the various anomalies are enumerated), we are confident that it was Caldas’ contribution to the 1809 Semanario that served as the source for the 1832 report, and that this is not an independent observation or verification

Results

Discussion

Conclusions