Thursday, July 16, 2015

Year(s) Without a Summer

Fig.1. The end of Alaska’s Augustine eruption of 2006, which lasted several months (Credit: Cyrus Read, USGS via Flickr)

In 1816, the Northern Hemisphere experienced the coolest summer on record, triggering crop failures and famine throughout Europe. The “Year Without a Summer” was later attributed to the eruption of Mount Tambora in 1815, a volcano in modern day Indonesia that killed an estimated 92,000 people.

Few realized the global reach of Tambora in 1816, but today it is widely accepted that volcanic eruptions contribute to climate variability. Qualifying these contributions, however, is difficult due to inconsistencies of dates of eruptions in different types of environmental records, such as ice cores and tree rings. A recent study published in the journal Nature addresses these inconsistencies and expands upon the role of volcanoes and their climatic effects in human history.

When a volcano erupts, it injects sulfates high into the atmosphere. These compounds form a shield that protects Earth’s surface from incoming solar radiation, therefore briefly lowering the surface’s temperature. Historical volcanic eruptions can be seen in ice cores as spikes in sulfate concentrations when the chemicals are carried by wind to high latitudes and become trapped in ice.

Scientists at the Desert Research Institute in Nevada and an international team of collaborators analyzed 30 individual ice cores for sulfates to document the presence and size of eruptions from the past 2,500 years. They then used sophisticated statistical algorithms to match the eruptions with fluctuations in the climate, as shown by tree rings and written records.

The updated timescale shows that large volcanic eruptions caused 15 out of the 16 coldest summers between 500 BCE and 1,000 CE and were the dominant forces of climate variability over the past 2,500 years. This variability is not a permanent weather change nor does it herald a new ice age, but these temperature drops directly affect the welfare of human populations around the world.

Ice core collections and tree ring data sets were key to mapping the explosive phenomena to weather, then to human history. Such collections are often used to understand climate change over thousands or millions of years, but more recent studies prove that environmental records have novel applications. For example, scientists have used ice cores to trace the rise of human pollution over time and studied tree rings to exonerate the black rat of primary blame for the Black Death. From volcanoes to plagues, these records of frozen time explain the rise and fall of human civilizations over millennia.

Sigl, M. (2015). Timing and climate forcing of volcanic eruptions for the past 2,500 years. Nature. doi: 10.1038/nature14565

Hon, S., Candelone, J. P., Patterson, C., & Boutron, C. F. (1994). Greenland Ice Evidence of Hemispheric Lead Pollution Two Millennia Ago by Greek and Roman Civilizations. Science, 265 (5180): 1841-1843. doi: 10.1126/science.265.5180.1841


climate variability
Deviations of the climate in temperature or geography, which last for months, seasons, or over a year. The time frame is shorter than that of climate change which usually occurs over decades or longer.

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