How does rising atmospheric CO2 affect marine organisms?

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Elevated CO2 vs. Warming and Drying
Dermody, O., Weltzin, J.F., Engel, E.C., Allen, P. and Norby, R.J. 2007. How do elevated [CO2], warming, and reduced precipitation interact to affect soil moisture and LAI in an old field ecosystem? Plant and Soil 301: 255-266.

What was done
Using open-top chambers, the authors studied the interactive effects of atmospheric CO2 enrichment (ambient and ambient + 300 ppm), warming (ambient and ambient + 3C) and drying (25 vs. 2 mm of irrigation water applied each week) on soil volumetric water content over the growing seasons (March-October) of two different years (2004 and 2005) at the Old-Field Community Climate and Atmospheric Manipulation experiment that was conducted at the Oak Ridge National Laboratory in eastern Tennessee (USA).

What was learned
Dermody et al. report that "warming and drying reduced soil moisture, and interactions between these factors exacerbated their individual effects." However, they found that "declines in soil moisture in warm and dry conditions were mitigated by elevated CO2," and that in 2004 "elevated CO2 more than compensated [our italics] for the effects of warming and drying when these factors were combined." In addition, they report that "the response of leaf area index to warming and elevated CO2 closely corresponded to changes in soil moisture content."

What it means
Global warming, if it continues, will -- by definition -- lead to an increase in temperature in most places, some of which increases (those in cooler locations) will enhance plant growth, while others (those in warmer locations) may do the opposite. Simultaneously, continued global warming will lead to both increases and decreases in precipitation, dependent upon location, some of which changes (both increases and decreases) will be beneficial (more water in dry locations and less water in wet locations), while others may again be detrimental (less water in dry locations and more water in wet locations). In what is arguably the most stressful set of circumstances (warming and drying in warmer and drier locations), however, the results of this study suggest that a concomitant increase in the air's CO2 content has the potential to greatly reduce the negative impacts of these environmental changes on soil moisture content; and in some of these cases, the increase in the atmosphere's CO2 concentration can even more than compensate for the negative effects, by actually increasing plant-available soil moisture and thereby helping to keep the biosphere strong and productive.

Reviewed 3 September 2008