How does rising atmospheric CO2 affect marine organisms?

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Iceberg Lake, Alaska, USA
Loso, M.G. 2009. Summer temperatures during the Medieval Warm Period and Little Ice Age inferred from varved proglacial lake sediments in southern Alaska. Journal of Paleolimnology 41: 117-128.

Eleven outcrops of exposed lacustrine stratigraphy revealing continuous sediment deposition at southern Alaska's Iceberg Lake (6047'N, 14257'W) from AD 442 to 1998 were examined, photographed and sampled (for grain size, bulk density, organic-matter content, and for 14C and 137Cs dating), after which the authors developed a quantitative relationship between varve thickness and melt-season temperature that they used to construct a melt-season temperature history of the region. This work revealed a "strong steady pulse of medieval warming" between AD 1000 and 1100 that "overlaps with warming seen at other Alaskan sites," and it suggests that some of the later 20th-century temperature spikes were "slightly (0.1C) warmer than the warmest part of the Medieval Warm Period." It should be noted, however, that the most recent temperature of the record (1998) is actually about 0.1C cooler than the peak MWP temperature.