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Increasing Export of Alkalinity by the Mississippi River: Evidence of Ever-Increasing Terrestrial Productivity
Volume 6, Number 52: 24 December 2003

What is alkalinity ? and why do we care about it?

Alkalinity, in the form of bicarbonate ions, is a product of the chemical weathering of carbonate and silicate minerals, which process requires the presence and participation of both water and carbon dioxide, the latter of which compounds may be obtained directly from the atmosphere or indirectly therefrom via the decomposition of plant-derived soil organic matter, the carbon of which was originally obtained from the atmosphere by means of plant photosynthesis.

The reason we care about alkalinity is that its transport from soils to rivers to oceans -- where it is used by marine plankton to build their tissues and skeletons, which ultimately settle to the bottom of the sea where their trapped carbon remains for an extraordinarily long time -- leads to the very-long-term sequestration of a large amount of the carbon in the CO2 that is continually being removed from the atmosphere by earth's vegetation, which phenomenon reduces the propensity for greenhouse gas-induced global warming.  Although many people may think this fact is reason enough to care about alkalinity, we like it even more for reasons that will become apparent as we continue.

So what's new on the alkalinity research front?  Based on a 48-year record derived from an average of 17 alkalinity measurements per year, Raymond and Cole (2003) recently demonstrated that the export of alkalinity from the USA's Mississippi River to the Gulf of Mexico has increased by approximately 60% since 1953.  "This increased export," in their words, "is in part the result of increased flow resulting from higher rainfall in the Mississippi basin," which has led to a 40% increase in annual Mississippi River discharge to the Gulf of Mexico over the same time period.  The remainder, however, must be due to increased rates of chemical weathering of soil minerals.

What factors might be responsible for this phenomenon?  Raymond and Cole note that potential mechanisms include "an increase in atmospheric CO2, an increase [in] rainwater throughput, or an increase in plant and microbial production of CO2 and organic acids in soils due to biological responses to increased rainfall and temperature."  Unfortunately, they forgot to mention the most important factor of all: the increase in terrestrial plant productivity that is produced by the increase in the aerial fertilization effect provided by the concomitant rise in the air's CO2 content, which also leads to "an increase in plant and microbial production of CO2 and organic acids in soils."

Over the years, we have discussed the results of a number of empirical studies dealing with this phenomenon that have demonstrated the reality of the ever-increasing prowess of the planet's plant life that is occurring in response to ever-increasing anthropogenic CO2 emissions (see Greening of the Earth in our Subject Index).  Of particular interest in this regard is the paper of Zhou et al. (2001), who used satellite-derived normalized difference vegetation index (NDVI) data to determine that the productivity of North America's plant life rose by about 8.4% over the period 1981-1999, during which time the air's CO2 concentration simultaneously rose by 28.6 ppm.  Based upon these numbers and the approximate 59-ppm increase in atmospheric CO2 concentration that occurred over the 48 years following 1953, we calculate there was a concomitant CO2-induced plant productivity increase of approximately 17% over this period.

The magnitude of this increase in vegetative productivity is very close to that of the simultaneous increase in the alkalinity of the Mississippi River's outflow at the Gulf of Mexico that was determined for this same time interval by Raymond and Cole, i.e., about 15%, as best we can determine from their graph of the relevant data.  Hence, we believe it to be highly likely that the CO2 increase was the ultimate cause of the alkalinity increase.

These results are very encouraging.  They suggest, first of all, that the planet's plants are responding in a positive manner to the ongoing rise in the air's CO2 content, at least in the United States, even in the face of the many environmental stresses -- both real and imagined -- we are said to be inflicting upon them.  Second, they argue for the reality of a very important negative feedback mechanism that tends to temper the propensity for warming that is produced by anthropogenic CO2 emissions.

Yes, the biosphere has not yet gone to hell in a hand basket.  In fact, the bulk of its vegetation is thriving, thanks in large part to the ongoing rise in the air's CO2 content, as ever more observations of real-world phenomena continue to demonstrate.

Sherwood, Keith and Craig Idso

Raymond, P.A. and Cole, J.J.  2003.  Increase in the export of alkalinity from North America's largest river.  Science 301: 88-91.

Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback, D., Shabanov, N.V. and Myneni, R.B.  2001.  Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999.  Journal of Geophysical Research 106: 20,069-20,083.