Volcanic eruptions as proxies to investigate rapid climate change

Article: Yost, C. L., Jackson, L. J., Stone, J. R., & Cohen, A. S. (2018).

https://doi.org/10.1016/j.jhevol.2017.11.005

 

‘The bright sun was extinguished’

 

The famous line from Lord Byron’s poem “Darkness” depicts a gloomy picture of Western Europe in 1816 caused by the super eruption of Mount Tambora in Indonesia. This eruption event was also known as a year without a summer. The series of volcanic eruptions added sulfate aerosols (by reflecting back the incoming solar radiation, Lelieveld and Heintzenberg, 1992) into the upper atmosphere, combined with a period of low solar activity, which led to crop failure in Europe that ended in food riots and famine (Oppenheimer, 2003). The eruption event not only affected farmers but also captured the imagination of artists such as the English poet, Lord Byron and painter, JMW Turner. These artists tried to encapsulate the suddenly changing world (Figure 1) which now forms a crucial historical record of volcanic super eruptions.

 

Figure 1: “Chichester Canal,” by J.M.W. Turner took inspiration from the profound aftermath of Mount Tambora’s powerful eruption to paint overcast skies with orange hues.

 

A complicated history

 

Not all super eruptions lead to a global cooling trend. The eruption of Mount Toba around 74 thousand years ago, which was an order or two magnitude greater than Tambora (present-day Indonesia), is an example. The impact of the super eruption has led to a healthy discussion regarding the near extinction of humans thousands of years ago (Williams et al., 2012). A recently published paper in Journal of Human Evolution by Yost and co-authors tested the hypothesis that the Toba super eruption at ~74 thousand years caused an environmental catastrophe in east Africa capable of severely reducing the human population. These researchers performed numerous analytical techniques on two lake sediment cores from Lake Malawi, East Africa (Figure 2) to investigate the effect of the Toba super eruption on a subdecadal time resolution.

Figure 2: The red box depicts Lake Malawi in East Africa where the authors studied two lake sediment cores to study the variability of vegetation through time. The stars signify archaeological sites closest to Lake Malawi.

The authors used phytoliths (Figure 3), pollen data combined with charcoal records, to observe any changes in vegetation ~100 years before and >200 years after the Toba super eruption. Changes in vegetation could indicate a dieback, meaning less food and a potential famine. However, Yost and co-authors found that the proxy data did not indicate changes in vegetation, thus the Toba catastrophe hypothesis suggesting a reduction in human population could not be supported. Furthermore, the authors found that vegetation at higher elevations was more sensitive to the effects of the Toba eruption. The forests in the highlands may have partially died-off, been subjected to wildfire burning, and replaced by grasses.

Figure3: Phytoliths are microscopic silica fillings and casts of plant cells. Phytoliths are similar in size to pollen grains, but unlike pollen, can be produced in different locations throughout a plant. The white box in the bottom right corner represents 10 μm.

The abrupt climate changes associated with the Mount Toba super eruption did not initiate a massive global cooling event as perceived in the literature. The authors suggest that the lack of a global cooling event may lie in the overestimation of atmospheric sulfur dioxide aerosols simulated in climate models. The authors conclude that a modelled ~1.3°C drop, using a more accurate sulfate simulation in the model is a more conservative estimate compared to the previously modelled 4.5°C drop in temperature in east Africa. Yost et al., present a new study that suggests that the super eruption of Mount Toba did not lead to a catastrophic decline in our human ancestors around 74 thousand years ago.

 

Work Cited:

 

Lelieveld, J., & Heintzenberg, J. (1992). Sulfate cooling effect on climate through in-cloud oxidation of anthropogenic SO2. Science258(5079), 117-120.

Oppenheimer, C. (2003). Climatic, environmental and human consequences of the largest known historic eruption: Tambora volcano (Indonesia) 1815. Progress in physical geography27(2), 230-259.

Williams, M. (2012). The∼ 73 ka Toba super-eruption and its impact: History of a debate. Quaternary International258, 19-29.

Yost, C. L., Jackson, L. J., Stone, J. R., & Cohen, A. S. (2018). Subdecadal phytolith and charcoal records from Lake Malawi, East Africa imply minimal effects on human evolution from the∼ 74 ka Toba supereruption. Journal of human evolution116, 75-94.

 

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Natasha Sekhon

I'm a PhD Candidate at in the Jackson School of Geosciences at UT Austin and study paleoclimatology, stable isotope geochemistry, and climate dynamics.

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