FERTILIZER RUNOFF CONTRIBUTION TO OCEANIC WARMING
Elliot B. Kennel
West Virginia University
Department of Chemical Engineering
PO Box 6102
Morgantown WV 26506-6102
ABSTRACT
It is suggested that agricultural fertilizer runoff may contribute to the surface warming of large bodies of water, such as the Gulf of Mexico, by increasing the proliferation of algae or other phytoplanktons in seawater, thus increasing the absorption coefficient of solar radiation. As a consequence, the surface of ocean waters may exhibit higher temperature. First order modeling shows that the temperature rise radiation could be on the order of a few degrees Celsius. Experimental methods are proposed to distinguish between this “greenhouse from below” effect, versus proposed CO2-induced “greenhouse from above” warming.
INTRODUCTION
A large body of evidence suggests that the average global temperature is rising, and that global warming is likely associated with human activities. In particular, the greenhouse effect has been suggested as a possible explanation for global warming, and a significant societal response has been initiated as governments, industries and individuals are making efforts of various kinds to reduce the emission of carbon dioxide. Yet, though it is qualitatively clear that the effect of carbon dioxide can be to amplify the greenhouse effect, its actual magnitude remains controversial. Hence it is probably not possible to conclude that global warming occurs solely do to the increase in carbon dioxide concentrations in the atmosphere. The possibility needs to be considered, then, whether there are other effects that may also contribute strongly to the observed global warming effect.
It is hypothesized that such effects may be exacerbated by the warming of the oceans surface due to increased algae concentration caused by agricultural fertilizer runoff. Increased algae concentration increases the absorption of sunlight in the near surface region of the oceans. Hence it is suggested that increases in agricultural fertilizer runoff may contribute to increasing the amount of phytoplankton (algae) present in near surface waters. In turn, the phytoplankton increases the absorption coefficient for sunlight, resulting in the oceans surface becoming warmer.
DISCUSSION
The extinction coefficient measures the dissipation of photons in a medium. The extinction coefficient is given by
k = 0.045 + 0.071S + 0.0012F m-1 , (1)
where S is the concentration of suspended solids in units of milliliters per cubic meter, F is the concentration of phytoplankton carbon in seawater, measured in units of grams per cubic meter of seawater.1 Thus, in the absence of suspended solids, the depth at which half the incident photon energy would be dissipated within is given by
D_1/2 ~ ln2/k = 15.4 meters (2)
Elliot B. Kennel
West Virginia University
Department of Chemical Engineering
PO Box 6102
Morgantown WV 26506-6102
ABSTRACT
It is suggested that agricultural fertilizer runoff may contribute to the surface warming of large bodies of water, such as the Gulf of Mexico, by increasing the proliferation of algae or other phytoplanktons in seawater, thus increasing the absorption coefficient of solar radiation. As a consequence, the surface of ocean waters may exhibit higher temperature. First order modeling shows that the temperature rise radiation could be on the order of a few degrees Celsius. Experimental methods are proposed to distinguish between this “greenhouse from below” effect, versus proposed CO2-induced “greenhouse from above” warming.
INTRODUCTION
A large body of evidence suggests that the average global temperature is rising, and that global warming is likely associated with human activities. In particular, the greenhouse effect has been suggested as a possible explanation for global warming, and a significant societal response has been initiated as governments, industries and individuals are making efforts of various kinds to reduce the emission of carbon dioxide. Yet, though it is qualitatively clear that the effect of carbon dioxide can be to amplify the greenhouse effect, its actual magnitude remains controversial. Hence it is probably not possible to conclude that global warming occurs solely do to the increase in carbon dioxide concentrations in the atmosphere. The possibility needs to be considered, then, whether there are other effects that may also contribute strongly to the observed global warming effect.
It is hypothesized that such effects may be exacerbated by the warming of the oceans surface due to increased algae concentration caused by agricultural fertilizer runoff. Increased algae concentration increases the absorption of sunlight in the near surface region of the oceans. Hence it is suggested that increases in agricultural fertilizer runoff may contribute to increasing the amount of phytoplankton (algae) present in near surface waters. In turn, the phytoplankton increases the absorption coefficient for sunlight, resulting in the oceans surface becoming warmer.
DISCUSSION
The extinction coefficient measures the dissipation of photons in a medium. The extinction coefficient is given by
k = 0.045 + 0.071S + 0.0012F m-1 , (1)
where S is the concentration of suspended solids in units of milliliters per cubic meter, F is the concentration of phytoplankton carbon in seawater, measured in units of grams per cubic meter of seawater.1 Thus, in the absence of suspended solids, the depth at which half the incident photon energy would be dissipated within is given by
D_1/2 ~ ln2/k = 15.4 meters (2)
Extending this to 99% photon energy dissipation results in 102m, which is comparable to measured values of approximately 120m.2 This critical depth, then, corresponds roughly to the depth at which solar energy can be absorbed in seawater. Thus, if phytoplanktons are present in sufficient concentrations, the deposition of photon energy will occur closer to the surface of the oceans. As a crude order of magnitude estimate, a critical value of phytoplankton carbon concentration is defined as that concentration which results in an extinction coefficient comparable to that of pristine seawater; i.e.,
Fcrit = 0.045/.0012 g/m3 ~ 0.04 g/m3. (3)
In the case of the Gulf of Mexico, the Mississippi River alone accounts for 1.6 million Tonnes of nitrogen (1.6 x 109 g) annually.3 Most of this nitrogen occurs near the lower density, warmer near-surface region. If this were to be distributed uniformly in the Gulf of Mexico, i.e., over an area of some 1.5 x 106 km2 the total volume of water would be equal to roughly 1.5 x 1014 m3.
Several studies suggest that the overall residence time for nitrogen is on the order of decades.4,5,6,7,8. Many of these estimates are made for other forms of nitrogen besides fertilizer-relevent ammonium. Nevertheless, an average residence time of ten years or more would thus correspond to a total volumetrically-averaged nitrogen concentration of some 1.1 x 10-2 g/m3.
From the Redfield ratio, carbon averages 6.6 times the molar ratio of nitrogen in marine phytoplankton. This suggests that, in the absence of other intervening effects, the mass of biomass in the Gulf of Mexico might be enhanced by as much as 0.07 g/m3; well above the threshold at which thermal effects are anticipated.
Stamnes and Jin calculated the peak temperature rise that might be expected if 1 gram of phytoplankton mass were added per cubic meter of seawater in the near-surface region at different latitudes on the summer solstice. The results are shown below in Figures 1 and 2. It is not possible to determine the value of the constants assumed by Stamnes and Jin versus those used by Cescon, but nevertheless a significant heating effect is predicted. The results show warming at depths of less than about 15 meters, and cooling at greater depths.
Thus it is plausible, though far from proven, that fertilizer runoff could result in increased proliferation of phytoplankton, leading to increased absorption of sunlight and, potentially, higher sea surface temperatures. In addition, there are other paths by which energy might be released, such as water vaporization, storms, current changes, etc.
Figure 1. Stamnes and Jin Estimate of Oceanic Heating Rate for a Clear Day at the
Summer Solstice for Clear Seawater.10
Summer Solstice for Clear Seawater.10
Figure 2. Stamnes and Jin Estimate of Oceanic Heating Rate for a Clear Day at the
Winter Solstice for Turbid (1 g/cm3) Seawater.10
IMPLICATIONS
The hypothesis presented here is supported only by first order calculations which carry very large uncertainties. A number of substantial refinements could be proposed to attempt to make a better estimate. In particular, there may be ways to resolve the issue via experimental observations and analysis of existing data bases. To that end, the following suggestions are proposed as means to distinguish between observed temperature fluctuations can be due to CO2-induced greenhouse warming, or to some sort of ocean warming effect such as the effect theorized here. Specifically,
a. One of the effects of greenhouse warming is to mitigate cooling at night. Allthough separating the effects of humidity and cloud cover is problematic, a study of the historical record might show significant night (air temperature) warming for recent years if greenhouse-type warming is the dominant effect.
b. If solar absorption in the ocean were being enhanced, this would show a larger
daily temperature rise under sunlit conditions, compared to the same measurement of decades past. The latter effect is likely to be on the order of a few percent of the total daily temperature change.
c. Comparison of sea surface temperature (SST) with marine air temperature
(MAT) should show that SST increase is a larger increase than MAT if the “greenhouse from below” ocean warming hypotheses is correct. Conversely, if CO2-induced greenhouse hypothesis is correct, the temperature increase should be observed primarily in the atmosphere, and the increase in MAT might be larger than the increase in SST.
d. SST should be correlated with regions of high algae growth. Moreover, fertilizer runoff is more strongly associated with the northern hemisphere than the southern hemisphere, so the global temperature rise might be more pronounced in the northern hemisphere than the southern hemisphere, as perhaps suggested by Figure 3.
e. Sampling of seawater at different locations and depths would allow direct experimental measurement of extinction coefficient as a function of phytoplankton concentration. This may be relevant to the claimed degradation of coral reefs, if the solar flux has diminished as a function of depth as hypothesized here.
f. The Gulf of Mexico is suggested as a region for study because it is smaller than the world oceans, and because it is fed by fertilizer runoff from a major agricultural region. Similar effects might be suspected for other bodies of water as well. First order effects such as the apparent disparity in observed temperature rise in the northern hemisphere versus southern hemisphere; and the changing size of the polar icecap may be reasons to suspect that oceanic effects may be important, and that they may be tied to human activity in some way.
CONCLUSIONS
A hypothesis is presented to suggest that fertilizer runoff might plausibly increase phytoplankton populations, thereby absorbing additional sunlight and creating warming of near-surface waters. The primitive first order models used to analyze the problem of increased solar heating of the near-surface waters of the Gulf of Mexico are plausible, but not sufficient for a definitive conclusion.
Predictions are suggested from the hypothesis that might allow climatologists to distinguish between atmospheric warming (“greenhouse from above”) and oceanic surface warming (“greenhouse from below”).
REFERNCES
Bruno Cescon et al., “Environmental Impact Study of Projects Affecting the Quality of Marine Ecosystems,” Croatica Chemica Acta CCACAA 71 (2) 361-389 (1998).
Andre Morel and David Antonine, “Heating Rate within the Upper Ocean in Relation to Its Bio-Optical State”, Journal of Physical Oceanography, 24(7), 1652-1665.
Donald A. Goolsby et al., Nitrogen Input to the Gulf of Mexico, Journal of Environmental Quality 30 (2): 329, 2001.
Rosswall, T. (1976) The internal nitrogen cycle between micro-organisms, vegetation and soil, in Svensson, B. H., and Söderlund, R. (eds) Nitrogen, Phosphorus and Sulphur—Global Cycles, SCOPE Report No. 7, Ecol.Bull. (Stockholm), 22, 157-167.
Söderlund, R., and Svensson, B. H. (1976) The global nitrogen cycle, in Svensson, B. H., and Söderlund, R. (eds) Nitrogen, Phosphorus and Sulphur—Global Cycles, SCOPE Report No. 7, Ecol.Bull. (Stockholm), 22, 23-73.
Söderlund, R., and Rosswall, T. (1982), “The Nitrogen Cycles,” in Hutzinger, O. (ed.) The Handbook of Environmental Chemistry Vol. 1 Part B. Berlin, Heidelberg, New York, Springer-Verlag 61-81.
Delwiche, C. C. (1970) The nitrogen cycle, Sci. Amer., 223(3), 137-146.
Burns, R. C., and Hardy, R. W. F. (1975) Nitrogen Fixation in Bacteria and Higher Plants, Berlin, Heidelberg, New York, Springer-Verlag.
A. C. Redfield, “The biological control of chemical factors in the environment”, American Science, 46, 205-222
K. Stamnes and J. Jin, “Solar Heating in the Upper Ocean,” Proceedings of the Fourth Atmospheric Radiation Measurement (ARM) Science Team Meeting, DOE CONF-940277, March 1994, Charleston, South Carolina.
Climatic Research Institute, http://www.cru.uea.ac.uk/cru/data/temperature/
