How does rising atmospheric CO2 affect marine organisms?

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Karakorum Mountains, Northern Pakistan
Treydte, K.S., Frank, D.C., Saurer, M., Helle, G., Schleser, G.H. and Esper, J. 2009. Impact of climate and CO2 on a millennium-long tree-ring carbon isotope record. Geochimica et Cosmochimica Acta 73: 4635-4647.

Esper, J., Frank, D.C., Wilson, R.J.S., Buntgen, U. and Treydte, K. 2007. Uniform growth trends among central Asian low- and high-elevation juniper tree sites. Trees 21: 141-150.

Treydte et al. (2009) developed "a millennium-long (AD 828-1998), annually resolved δ13C tree-ring chronology from high-elevation juniper trees in northern Pakistan [35.74-36.37N, 74.56-74.99W] together with three centennial-long (AD 1900-1998) δ13C chronologies from ecologically varying sites," in the process of which they defined "an 'optimum' correction factor that is best suited to remove non-climatic trends from [their] high-elevation trees in the Karakorum," in order to "provide new regional temperature reconstructions derived from tree-ring δ13C, and compare those records with existing regional evidence." This work revealed, in their words, that "the 1990s are substantially below MWP temperatures," and they state that their reconstruction "provides additional suggestions that High Asian temperatures during the MWP might have exceeded recent conditions," which finding, as they describe it, is also suggested by "ring-width data from living trees (Esper et al., 2007)." Consequently, as they state in their paper's abstract, they "find indications for warmth during the Medieval Warm Period," which indications imply summer temperatures "higher than today's mean summer temperature." Based on data presented in the optimum proxy temperature reconstruction (cf_opt) of the authors' figure 8b, we calculate that difference to be approximately 1C.