How does rising atmospheric CO2 affect marine organisms?

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Lomonosovfonna Ice Cap, Spitsbergen, Svalbard, Norway
Virkkunen, K., Moore, J.C., Isaksson, E., Pohjola, V., Peramaki, P., Grinsted, A. and Kekonen, T. 2007. Warm summers and ion concentrations in snow: comparison of present day with Medieval Warm Epoch from snow pits and an ice core from Lomonosovfonna, Svalbard. Journal of Glaciology 53: 623-634.

Virkkunen et al. compared various ion concentrations and ratios obtained from snow pits at the summit of Svalbard's Lomonosovfonna ice cap (7851'53"N, 1725'30"E) in 2001 and 2002 with similar data obtained from an ice core drilled in the same location in 1997 that extended back in time from AD 1990 to approximately AD 1130. Noting "there is controversy over how warm the Medieval Warm Epoch was compared with the present-day climate," they report that the deepest of the ice pits contained data "from the warmest (2001) and the longest (2000) summers observed in the instrumental temperature record in the period 1978-2001."

In looking for comparable data within the ice core, the Nordic researchers found the largest set of similar values within the core's bottom two meters, where concentrations of several ions trended steadily towards zero (indicative of increasing warmth) as the core bottom was approached, reaching levels that -- in their words -- were "even lower than seen in the autumn 2001 to autumn 1999 layer." Hence, they concluded that "Medieval Warm Epoch temperatures were likely to have been at least as warm as the summers of 2001 and 2000." But in light of their ion data tending to become even more extreme than they were in those two years as the start of their ion history was approached, it can safely be concluded that most of the latter portion of the Medieval Warm Period (~AD 1130-1200) was in all probability significantly warmer than it has been at any time during the Current Warm Period.