Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmhg31a..05p&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #HG31A-05
Physics
1704 Atmospheric Sciences, 1749 Volcanology, Geochemistry, And Petrology, 8409 Atmospheric Effects (0370)
Scientific paper
Observations of the 1761 and 1769 transits of Venus were crucial to the early development of American geoscience. Accurate longitude measurements were needed for that, the Mason-Dixon survey, the 1804-1806 Lewis and Clark expedition to Oregon, and the westward expansion of the new republic [Woolf, "The Transits of Venus: A Study of 18th Century Science," Princeton, 1959]. As founder of the American Philosophical Society Benjamin Franklin promoted the transit observations, and procured a large telescope for the Philadelphia group. While serving as ambassador to France he observed that a "dry fog" from the 1783 Laki eruption in Iceland had obscured the Sun, and suggested that as a cause of the unseasonably cold weather of that summer and winter. Although the longitude, solar parallax and Sun-Earth distance measurements have long since been improved on, observations of the dark lunar eclipse just before the June 6, 1761 transit are still valuable for identifying a very large volcanic eruption that spring, using Benjamin Franklin's method. Many observers worldwide, while making a final check on their clock/longitude, found the May 18, 1761 totally eclipsed Moon very dark or even invisible, e.g., Wargentin (Stockholm Observatory) could not see the Moon for 38 minutes even with a 2-ft telescope [Phil. Trans. 52, 208, 1761-1762]. Whereas the totally eclipsed Moon is illuminated only by sunlight refracted by the Earth's atmosphere, I conclude that it was severely obscured, thus meeting Benjamin Franklin's first condition. Ice cores from Greenland and Antarctica show a large sulfuric acid peak at 1762 [Crowley, Geophys. Res. Lett. 20, 209, 1993; and Karlof, J. Geophys. Res. 105, D10, 12471, 2000], also satisfying Benjamin Franklin's second condition that the obscuration be due to a "dry fog" (sulfuric acid mist). The weather of 1761-1762 was indeed very cold, as recorded in chronicles, and frost-damaged rings of North American bristlecone pines [LaMarche and Hirschboeck, Nature 307, 121, 1984]. Annual weather reviews in imperial, provincial and county histories in China have been examined. Unseasonable cold are classified by their degree of severity: (1) Late (April-June) or early (July-Sept.) killing frosts; (2) Bitter cold/heavy snowfall; and (3) Heavy sustained snowfall, bitter cold with frozen wells, lakes and rivers. The latter cases were often widespread and multi-year, with the coast icebound also. The weather of 1761-1762 was a "3." Heavy sustained snow fell over many sites from the Tropic of Cancer to the Yellow River. In the north wells and rivers froze. Taihu (Great Lake near Shanghai) and nearby rivers froze over and were not navigable. Innumerable trees, birds and livestock perished, etc. Whereas all three of Benjamin Franklin's conditions have been met I conclude that a very large volcanic eruption early in 1761 had a major impact on the Earth's climate. Its location is unknown, but was probably low- or mid-latitude, as sulfuric acid from the volcanic cloud settled onto both poles. Finally Benjamin Franklin's criteria for a climate-altering volcanic eruption are still universally used (the appearance of brilliant red twilight displays have since been added). Moreover his legacy continues to inspire climate researchers. See, for example, "Climatic Impact of the mid-15th-Century Kuwae Caldera Formation...," Pang, Eos 74, No. 43, 106, 1993; and as cited in "Earth in Balance," Al Gore, p. 379, Penguin, 1993. See also "Constantinople's Volcanic Twilight," Lynn Simarski, Aramco World 47, No. 6, 8-13, 1996.
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