How does rising atmospheric CO2 affect marine organisms?

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Climate Variability of Central Asia
Mackay, A.W., Ryves, D.B., Battarbee, R.W., Flower, R.J., Jewson, D., Rioual, P. and Sturm, M.  2005.  1000 years of climate variability in central Asia: assessing the evidence using Lake Baikal (Russia) diatom assemblages and the application of a diatom-inferred model of snow cover on the lake.  Global and Planetary Change 46: 281-297.

What was done
The authors analyzed paleolimnological data obtained from a sediment core taken from the south basin of Lake Baikal, Russia, in an effort to reconstruct the climatic history of this area of central Asia over the past millennium.

What was learned
Cluster analysis identified three significant zones of variability in the sediment core that were coincident with the Medieval Warm Period (MWP: c. 880 AD - c. 1180 AD), the Little Ice Age (c. 1180 AD - 1840 AD) and the Current Warm Period.  Although it was not possible to obtain direct temperature estimates in this study, the authors say the diatom data support the idea "that the period known as the MWP in the Lake Baikal region was a relatively warm one."  Following the MWP, diatom species shifted toward taxa indicative of colder climates, implying maximum snow depth values during the Maunder Minimum (1645-1715 AD).  Thereafter, the diatom-derived snow accumulation data indicate a warming trend in the Lake Baikal region that began as early as c. 1750 AD.

What it means
The results of this study help to confirm the widespread existence of the Medieval Warm Period, which was likely as warm as, or even warmer than, the Current Warm Period.  They also demonstrate the existence of the natural millennial-scale climate oscillation that produced the Medieval Warm Period, the Little Ice Age and, as we would argue, the Current Warm Period.  Certainly, anyone can recognize that the first two of these warm and cool climatic states were completely natural in origin, as the atmosphere's CO2 concentration varied but little over their combined time periods.  Hence, the fact that the warming that brought the world the Current Warm Period began around 1750 AD, or nearly 100 years before the modern rise in atmospheric CO2 concentration, should be evidence enough to argue that the planet's current warmth is the result of nothing more than the most recent and expected upward swing of this natural climatic oscillation.

Reviewed 3 August 2005