How does rising atmospheric CO2 affect marine organisms?

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The Natural Variability of Climate
Hunt, B.G. 1998. Natural climate variability as an explanation for historical climate fluctuations. Climatic Change 38: 133-157.

What was done
A global climatic model was used to simulate climate over the past 500 years in an effort to determine if climatic events such as the Medieval Warm Period and the Little Ice Age could be expressions of natural (unforced) climate variability.

What was learned
The model simulation produced "substantial warming and cooling trends of global mean surface temperature with multi-decadal timescales." Global mean temperature anomalies over this period experienced a maximum range of 0.7C, while some regional temperature anomalies were in excess of 3C. Comparing the model output with the temperature history of the Northern Hemisphere over the last 500 years, the author noted that "most of the observed features in this climatic record can be reproduced by processes associated with internal mechanisms of the climatic system."

What it means
The author concludes that the results of his simulation suggest "much of the observed climatic variability over the past millennium may just be a reflection of natural climatic variability within the actual climate system, with occasional anomalies associated, for example, with volcanic eruptions, being superimposed on this background of natural variability."

Reviewed 15 January 1999