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Were the Little Ice Age's Coldest Intervals Caused by Low Levels of Solar Activity?
Reference
Mauquoy, D., van Geel, B., Blaauw, M. and van der Plicht, J. 2002. Evidence from northwest European bogs shows 'Little Ice Age' climatic changes driven by variations in solar activity. The Holocene 12: 1-6.

Background
Rainfed peat bogs or ombrotrophic mires are closely coupled to atmospheric conditions, receiving all their water via precipitation. As a result, the plants they support are highly sensitive to changes in the difference between precipitation and evapotranspiration. This effective precipitation parameter rises with both increases in precipitation and decreases in temperature, the latter of which act to reduce evapotranspiration, thereby producing wet-shifts that lead to cool, moist conditions conducive to increased representation of Sphagnum tenellum and Sphagnum cuspidatum in the ecosystem, the subfossil leaves of which plants thus become proxies for those climatic conditions. The authors also note that these biological materials have high accumulation rates and are exceptionally well preserved, which allows high-resolution reconstructions of Holocene palaeoenvironments to be made and compared with the Holocene history of atmospheric 14C (an excellent proxy for solar activity) contained in tree-ring records.

What was done
The authors extracted peat monoliths of 1-m depth from ombrotrophic mires at Lille Vildmose (LVM), Denmark (5650'N, 1015'E) and Walton Moss (WLM), UK (5459'N, 0246'W), which sites, being separated by about 800 km, as the authors note, "offer the possibility of detecting supraregional changes in climate." From these monoliths, vegetative macrofossils were extracted at contiguous 1-cm intervals and examined using light microscopy. Where increases in the abundances of S. tenellum and S. cuspidatum were found, a closely spaced series of 14C AMS-dated samples immediately preceding and following each of the wet-shifts was used to "wiggle-match" date them (van Geel and Mook, 1989), thereby enabling comparison of the wet-shifts with the history of 14C production during the Holocene.

What was learned
The VLM and WLM data reveal the existence of wet-shift climatic deteriorations that began in the mid-1400s and early 1600s, while the WLM data additionally reveal a wet-shift that started about 1215. These three climatic deteriorations mark the beginnings of periods of inferred cool, wet conditions that correspond fairly closely in time with the Wolf, Sporer and Maunder Minima of solar activity, as manifest in contemporary delta14C data. The authors further report that "these time intervals correspond to periods of peak cooling in 1000-year Northern Hemisphere climate records."

What it means
The authors conclude that their work adds to the "increasing body of evidence" that "variations in solar activity may well have been an important factor driving Holocene climate change," to which we add that this hypothesis is beginning to look more and more convincing with each passing day [see Little Ice Age (Solar Influence) in our Subject Index].

Reference
Van Geel, B. and Mook, W.G. 1989. High resolution 14C dating of organic deposits using natural atmospheric 14C variations. Radiocarbon 31: 151-155.


Reviewed 18 December 2002