The Little Ice Age: An Account from Southern Alaska

Caption: 'The Frozen Thames'', 1677. Original painting in the collection of the Museum of London. By Abraham Hondius, digital image courtesy of wikipedia commons.

The Little Ice Age (LIA) occurred from approximately 1400-1850 AD, although timing varies by location. It was a period of cool climate for the Northern Hemisphere and possibly the globe. The event inspired several famous paintings, one seen above, and is particularly well documented in Europe. In Alaska, although it has been demonstrated through records of glacial advance and reduced tree ring width (due to a shorter growing season) to be a period of colder climate as well, the moisture history is relatively unconstrained. The goal of this study was to examine if atmospheric circulation patterns were different during the LIA, and what impact this may have had on moisture input to the region.

Caption: In the summer of 2007 two sediment cores were retrieved from Kepler Lake, Alaska

Through analysis of the sediments physical and chemical composition, we were able to generate an 800 year (2007 to 1200 AD) record of climate and environmental change predominantly by examining changes in the ratio of the ¹⁸O and ¹⁶O isotopes of oxygen. These two isotopes, due to their different masses (¹⁸O has two more neutrons than ¹⁶O) behave differently. In the Kepler Lake system, there are two dominant controls of the oxygen isotope composition of lake water: precipitation/evaporation budget, and atmospheric circulation (which controls the water coming into the lake).

Under conditions of evaporation, the lighter ¹⁶O isotopes escape more readily to the vapor phase, much like the separation between the lightest, fittest runners and the rest of the field in a race. The main pack that has been left behind represents the remaining lake water, which now has a higher ratio of ¹⁸O (slower runners) to ¹⁶O (faster runners). Based on these principles we examine how the oxygen isotope composition has changed over the last 800 years. From this record we can determine when the climate was wetter (lower ¹⁸O ratio) or drier (higher ¹⁸O ratio, many of the fast running ¹⁶O isotopes have left the pack). To keep up with our analogy, atmospheric circulation works in a similar way: it controls what runners showed up to the race to begin with! Depending on where the storms passing over the region are coming from, they will have different isotopic signatures.

During the LIA at Kepler Lake, the oxygen isotope record displays a significant positive shift. This suggests one or two possible explanations: there was a change in atmospheric circulation patterns, and the storm trajectories brought in moisture containing a higher ¹⁸O ratio, and/or, the LIA was a drier period. More than likely these two interpretations are not at odds with one another, as changing atmospheric circulation patterns would also affect the amount of moisture delivered to the region. Beyond just understanding the conditions that occurred during the LIA, this study has implications for understanding how future climate change will affect Southern Alaska. In order to provide accurate predictions of climate change under different human-induced warming scenarios, it is necessary to understand how climate varies naturally, and how different regions respond to changes in climate.

More details on this study are forthcoming in a paper which is currently in review with the Journal of Paleoliminology.

Gonyo, A., Yu, Z., Bebout, G. (In Review) An An 800-Year Multiple-Proxy Record of Atmospheric Circulation and Climate Change from Kepler Lake, South-Central Alaska. Journal of Paleoliminology.