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California Dreamin': Some Remarks on a Recent Study of Grassland Responses to a Number of Global Environmental Changes and the Inverted Interpretation of the Study's Findings by Science Magazine and Many, Many Others
Volume 5, Number 51: 18 December 2002

Upon opening the 6 December 2002 online edition of Science magazine a short while ago, we were greeted by an ominous headline: "More CO2 Lowers Plant Productivity." Clicking on the link it represented, we were brought to a brief introductory paragraph describing the short Perspective piece of Morgan (2002) - which discusses the headline-provoking paper of Shaw et al. (2002) - where it was again stated that "increasing CO2 may inhibit plant growth."

What is a news reporter supposed to think when confronted with such matter-of-fact statements that clearly suggest atmospheric CO2 enrichment is bad for the biosphere? Is it any wonder members of the media decry the ongoing rise in the air's CO2 content and thereby persuade the public to prod their elected officials to reduce anthropogenic CO2 emissions at all costs? One would think that the staff of a magazine called Science would pay more careful attention to the principle of "truth in advertising," particularly when promoting the publication of a paper that deals with so politically-sensitive a subject as global environmental change, and especially when they know - or should know - that the paper's results do not imply what they suggest they do.

Perhaps we're being overly harsh in our criticism of Science, however, for the research voice of the study's sponsoring institution, the Stanford Report of 5 December 2002, says pretty much the same thing in an article entitled "High carbon dioxide levels can retard plant growth, study reveals," wherein it is further stated that "the results of the study may prompt researchers and policymakers to rethink one of the standard arguments against taking action to prevent global warming: that natural ecosystems will minimize the problem of fossil fuel emissions by transferring large amounts of carbon in the atmosphere to plants and soils."

So just what is the take-home message of the Shaw et al. study? Is it as gloomy as what nearly everyone is thinking and saying it is? Absolutely not. When the authors' data are considered in their entirety and with respect to global environmental changes that might realistically be expected to occur, we find a very positive result.

To begin, then, the scientists report observing substantial increases in the net primary production (NPP) of a moderately fertile California annual grassland in response to experimentally-provided increases in nitrogen deposition (7 g N m-2 year-1 above ambient), air temperature (80 W m-2 of thermal radiation above ambient), and precipitation (50% above ambient), applied either alone or in combination. However, they also report that the NPP increases they observed were significantly reduced by an approximate 300-ppm concomitant increase in the atmosphere's CO2 concentration in the final year of a three-year study. Although this latter observation does indeed have a negative ring to it, when it is considered in relation to the relative probabilities of the four global environmental changes Shaw et al. studied actually occurring in the real world, pessimism quickly turns to optimism.

As the starting point of this analysis, we note that the relating of any multi-factorial findings to the world of nature should logically begin with the consequences of the factor that is most likely to actually change in the world of nature; and in this case, that factor is the air's CO2 content, and the change it is likely to experience is an increase. Why? Because the atmospheric CO2 concentration has been trending upward ever since the inception of the Industrial Revolution, as a result of carbon dioxide's being one of the primary end-products of the combustion process; and with anthropogenic CO2 emissions poised to remain high for decades to come, it is just not in the cards for this trend to be reversing itself anytime soon. Hence, the most likely core consequence of the global environmental changes studied by Shaw et al., i.e., that resulting from an approximate 300-ppm increase in the air's CO2 concentration, would be, according to the results portrayed in Figure 3 of their paper, that the California grassland they studied will see its NPP increased by about 8%.

The second-most-likely-to-occur of the global environmental changes studied by Shaw et al. is the increase in nitrogen deposition they specified. Why? Because oxides of nitrogen are by-products of the combustion process, and they have risen hand-in-hand with the air's CO2 content for the past two centuries, making it "almost certain," as Shaw et al. say, that nitrogen deposition will continue to rise in tandem with the air's CO2 content in the decades ahead. Hence, the most likely first-order-adjusted consequence of the global environmental changes studied by Shaw et al., i.e., that resulting from an approximate 300-ppm increase in the air's CO2 concentration and a 7 g N m-2 year-1 increase in nitrogen deposition, would be, according to the results portrayed in Figure 3 of their paper, that the California grassland they studied will see its NPP increased by approximately 16%.

The third-most-likely-to-occur of the global environmental changes studied by Shaw et al. is the increase in temperature they specified. Why? Because the earth has been gradually warming since the beginning-of-the-end of the Little Ice Age, and it does not appear that it has yet reached the temperature level of the prior Medieval Warm Period (Esper et al., 2002) or the earlier Roman Warm Period (McDermott et al., 2001), which it seems only natural it should do. Hence, the most likely second-order-adjusted consequence of the global environmental changes studied by Shaw et al., i.e., that resulting from an approximate 300-ppm increase in the air's CO2 concentration and a 7 g N m-2 year-1 increase in nitrogen deposition and an 80 W m-2 increase in down-welling thermal radiation, would be, according to the results portrayed in Figure 3 of their paper, that the California grassland they studied will see its NPP increased by approximately 50%.

We come at last, then, to the least-most-likely-to-occur of the global environmental changes studied by Shaw et al., i.e., a 50% increase in precipitation, which we consider unlikely to occur at all. Why? Because this change, which is predicted to occur as a consequence of concurrent global warming solely on the basis of climate model simulations, is woefully wrong. Why? Because from approximately 1910 to the present - over which time the earth experienced a warming that Mann et al. (1999) and the IPCC consider to be unprecedented over the entire past millennium - mean global precipitation appears to have increased not one iota (New et al., 2001). Hence, the most likely third-order-adjusted consequence of the global environmental changes specified by Shaw et al. is exactly the same as the most likely second-order-adjusted consequence, i.e., a 50% increase in the NPP of the California grassland they studied. But if by some absolutely incredible one-in-a-million chance the mean global precipitation were to rise by 50%, the final result would still be a 40% increase in NPP.

So what's the problem, everyone? Is a 50% -- or even 40% -- increase in real-world grassland productivity not good enough for you? In truth, it is much greater - very much greater, in fact - than what we or anyone else would have ever predicted on the basis of an increase in atmospheric CO2 concentration alone. Hence, when viewed in this light, which is no more than a factual recounting of the results of Shaw et al.'s experiment, their findings can be appreciated to be truly phenomenal and something to be welcomed with open arms.

But the good news doesn't stop there. Shaw et al.'s results indicate that typically-dreaded global warming is actually good for the ecosystem they studied, both alone (an 18% increase in NPP) and in association with increases in nitrogen deposition (a 62% increase in NPP), precipitation (a 24% increase in NPP), and nitrogen deposition and precipitation together (a whopping 84% increase in NPP). Likewise, they indicate that typically-dreaded nitrogen deposition is also good, both alone (a 34% increase in NPP) and in association with increases in temperature (a 62% increase in NPP), precipitation (a 43% increase in NPP), and temperature and precipitation together (that whopping 84% increase in NPP). Last of all, they indicate that increases in precipitation are good as well, both alone (a 6% increase in NPP) and in association with increases in temperature (a 24% increase in NPP), nitrogen deposition (a 43% increase in NPP), and temperature and nitrogen deposition together (once again, the 84% increase in NPP). And all of these experimentally-observed increases in NPP might possibly be expected to manifest themselves over the current century or so, except for those related to increases in precipitation; for as we have noted, there is no real-world evidence that would suggest we shall ever see anything even remotely close to the 50% increase in global precipitation simulated in the experiment of Shaw et al.

So let those dreaded global environmental changes come. In fact, pray that they come; for we're going to need them to adequately sustain our ever-growing numbers, along with everything else that depends upon plants for food, and that includes, well, just about everything.

Hey, maybe California Dreamin's not so bad after all!

Sherwood, Keith and Craig Idso

References
Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.

McDermott, F., Mattey, D.P. and Hawkesworth, C. 2001. Centennial-scale Holocene climate variability revealed by a high-resolution speleothem 18 O record from SW Ireland. Science 294: 1328-1331.

Morgan, J.A. 2002. Looking beneath the surface. Science 298: 1903-1904.

New, M., Todd, M., Hulme, M. and Jones, P. 2001. Precipitation measurements and trends in the twentieth century. International Journal of Climatology 21: 1899-1922.

Shaw, M.R., Zavaleta, E.S., Chiariello, N.R., Cleland, E.E., Mooney, H.A. and Field, C.B. 2002. Grassland responses to global environmental changes suppressed by elevated CO2. Science 298: 1987-1990.