Volcano Weather

A photo of the 1991 Mt. Pinatubo eruption, which resulted in worldwide cooling the following year. (From: USGS).
Volcanoes and Climate
One of the possible reasons for climate change on scales of centuries and millennia, like the Little Ice Age, is increased volcanic activity. Great volcanic eruptions release huge amounts of gases and aerosol particles which impact global climate by reducing the amount of solar radiation reaching the Earth's surface, lowering temperatures, and changing atmospheric circulation patterns. The result is a cool summers and severe winters for the period that follows. Large-scale volcanic activity may last only a few days, although its influence on climate patterns lasts 5 years or so. Multiple eruptions (not necessarily from the same volcano) may be assumed for long-lasting spells of unusually low temperatures.

Tree-Rings Records of Volcanic Activity

Graphs of average temperatures 5 years before and 5 years after the listed various volcanic eruptions. Note that anomalously low temperatures always occur after an eruption (occurring at Month #0). Graph e. is a composite of the graphs a.d. Graph f. is the response to the most recent Mt. Pinatubo eruption. (From: Climate Research Unit).
We can study the details of the effects of volcanic eruptions on climate by using information from tree-rings over the last several centuries. It turns out that the density of wood is a good indicator of summer warmth: high density = warm temperatures, and vice versa. A recent article by Briffa et al. (1998) summarized the occurrence of volcanic eruptions and data from average tree-ring density around the northern hemisphere. This compilation revealed that the timing of lowest tree-ring densities closely follows major eruptions in almost all cases. It appears from the compilation of Briffa et al. (1998) that volcanic activity was an important factor in the cold spells of the Little Ice Age (1350 A.D. to 1850 A.D.). Presumably such spells were the most severe, since they were superimposed on the background of a generally cooler climate. The largest of the eruptions, in Peru in 1601, is thought to have caused such severe economic damage in Peru and its neighbors that it took 150 years to recover (according to Silva and Zielinski, 1998). The second largest eruption, Tambora in 1815, is known to have emitted enormous amounts of particles into the stratosphere (according to Lamb, 1972), including sulfurous aerosol which is the most effective at blocking radiation. Severe winters, late frost, and cool summers tend to follow such eruptions, with the consequences of poor harvests stimulated emigration. For example, the Tambora eruptions in the early 19th century resulted in much economic stress, with the French historian Ladurie claiming that all of Europe spent the summer around the fireplace in 1816, with frost in July, followed by famine.

A list of years characterized by low tree-ring densities, with the name of the volcano eruption immediately preceding the tree-ring event indicated in parentheses (n.d. = not determined).