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Growth Response to CO2 with Other Variables (Multiple) -- Summary
Well over a decade ago, readers of the 6 December 2002 online edition of Science magazine were greeted by an ominous headline: "More CO2 Lowers Plant Productivity." Clicking on the link it represented brought one to a brief introductory paragraph describing the short Perspective piece of Morgan (2002) - which discussed 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 many 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 have paid more attention to the principle of "truth in advertising," particularly when promoting the publication of a paper that dealt with so politically-sensitive a subject as global environmental change, especially when they should have known that the paper's results did not imply what they suggested they did.

Perhaps this is being overly harsh with Science, however, for the research voice of the study's sponsoring institution, the Stanford Report of 5 December 2002, said pretty much the same thing in an article entitled "High carbon dioxide levels can retard plant growth, study reveals," wherein it was 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 what was the true take-home message of Shaw et al.'s study? Was it as gloomy as what nearly everyone was thinking and saying it was? Absolutely not! When the authors' data were considered in their entirety and with respect to environmental changes that might realistically be expected to occur, a very positive response was evident.

For starters, the scientists reported 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 (provided by 80 W m-2 of thermal radiation above ambient), and precipitation (50% above ambient), applied either alone or in combination. But they also reported that the NPP increases they observed were significantly reduced by an approximate 300-ppm increase in the air's CO2 concentration in the final year of their three-year study. Although this latter observation was indeed negative, if it is considered in light of 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, it should be noted 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 was the air's CO2 content, and the change it was likely to experience was an increase. Why? Because the atmospheric CO2 concentration had been trending upwards 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 was just not in the cards for this trend to be reversing itself anytime soon. And, therefore, 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 atmosphere's CO2 concentration - would be, according to the results portrayed in Figure 3 of their paper, that the California grassland they studied would see its NPP increased by about 8%.

The second-most-likely-to-occur of the global environmental changes studied by Shaw et al. was the increase in nitrogen deposition they specified. Why? Because oxides of nitrogen are by-products of the combustion process, and they had risen hand-in-hand with the air's CO2 content for the prior two centuries, making it "almost certain," as Shaw et al. indicated, that nitrogen deposition would continue to rise in tandem with the atmosphere'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 would see its NPP increased by approximately 16%.

The third-most-likely-to-occur of the global environmental changes studied by Shaw et al. was the increase in temperature they specified. Why? Because the earth had been gradually warming since the beginning-of-the-end of the Little Ice Age; and it did not appear that it had 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 that it should sometime reach. 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 would 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 is unlikely to occur at all. And why? Because this change, which was 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). Thus, 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 that the mean global precipitation were to rise by 50%, the final result would still be a 40% increase in NPP. So 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 considered to be truly 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, increases in precipitation appear to be 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 noted above, 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.

Four years later, Qaderi et al. (2006) grew well-fertilized seven-day-old canola (Brassica napus) plants for an additional eleven days in controlled-environment chambers maintained at either ambient (370 ppm) or elevated (740 ppm) atmospheric CO2 concentrations in a 2:1:1:0.25 mix of peat moss, Perlite, Vermiculite and Terragreen that was either well-watered (to field capacity) or drought-stressed (at the wilting point) at either low temperatures (22°C day/18°C night, which are optimal for canola growth under environmental conditions experienced at Calgary, Canada) or higher more stressful temperatures (28°C day/24°C night). And it was under these conditions that they measured a number of plant physiological parameters and properties, after which they determined the final dry weights of the plants' primary organs.

These protocols and measurements revealed, in the words of the three researchers, that "drought-stressed plants grown under higher temperature at ambient CO2 had decreased stem height and diameter, leaf number and area, dry matter, leaf area ratio, shoot/root weight ratio, net CO2 assimilation and chlorophyll fluorescence" compared to well-watered plants grown at optimum temperatures in ambient-CO2 air, but that elevated CO2 "partially reversed the inhibitory effects of higher temperature and drought on leaf dry weight accumulation." More specifically, and in terms of whole-plant biomass, in the drought-stressed plants grown at higher temperatures in ambient-CO2 air, final dry matter accumulation was reduced by approximately 70% compared to that of the well-watered plants grown at optimum temperatures at the ambient CO2 concentration (as best as can be determined from Qaderi et al.'s graphical representations of their results). But when the drought-stressed plants were grown at higher temperatures in elevated-CO2 air, final dry matter accumulation was reduced by a much less drastic 25%, which is really quite a feat, considering the soil moisture content of the drought-stressed plants was continuously maintained at the wilting point for one of the worst-case scenarios of CO2-induced warming ever imagined.

In another multi-stress-factor study, Su et al. (2007) used a process-based model (BIOME-BGC) "to investigate the response of Picea schrenkiana forest to future climate changes and atmospheric carbon dioxide concentration increases in the Tianshan Mountains of northwestern China," which they "validated by comparing simulated net primary productivity (NPP) under current climatic conditions with independent field-measured data." The specific climate change scenario employed in this endeavor was a double-CO2-induced temperature increase of 2.6°C and a precipitation increase of 25%.

So what was learned? When the predicted precipitation increase was considered by itself, the NPP of the P. schrenkiana forest increased by 14.5%; while the predicted temperature increase by itself increased forest NPP by 6.4%, and the CO2 increase by itself boosted NPP by only 2.7%. When the predicted increases in precipitation and temperature occurred together, forest NPP increased by a larger 18.6%, which is just slightly less than the sum of the two individual effects. But when the CO2 concentration increase was added to the mix and all three factors increased together, the Chinese researchers report that forest NPP "increased dramatically, with an average increase of about 30.4%."

Su et al. thus noted that comparison of the results derived from the various scenarios of their study indicated that "the effects of precipitation and temperature change were simply additive, but that the synergy between the effects of climate change and doubled CO2 was important," as it made the whole response much larger than the sum of its separate responses, due to the fact that "feedback loops associated with the water and nitrogen cycles [which may be influenced significantly by atmospheric CO2 enrichment] ultimately influenced the carbon assimilation response."

Moving along, Pan et al. (2009) examined "how changes in atmospheric composition (CO2, O3 and N deposition), climate and land-use affected carbon dynamics and sequestration in Mid-Atlantic temperate forests during the 20th century," by modifying and applying "a well-established process-based ecosystem model with a strong foundation of ecosystem knowledge from experimental studies," which they validated "using the U.S. Forest Inventory and Analysis (FIA) data." This work revealed that for previously harvested and currently regrowing forests, the calibrated model produced the following percentage changes in net ecosystem productivity (NEP) due to observed changes in N deposition (+32%), CO2 (+90%), O3 (-40%), CO2 + O3 (+60%), CO2 + N deposition (+184%), and CO2 + N deposition + O3 (+138%), while corresponding changes in NEP for undisturbed forests were +18%, +180%, -75%, +78%, +290%, +208%. In addition, the results of Pan et al. revealed that "the 'fertilization' effect of N deposition mainly stimulates carbon allocation to short-lived tissues such as foliage and fine roots," but that "the 'fertilization' effect by elevated CO2 likely enhances more sustainable carbon storage such as woody biomass (including coarse roots)."

In discussing the future implications of their findings, the four USDA Forest Service scientists said they indicate that "the change in atmospheric composition, particularly elevated CO2, will gradually account for more of the carbon sink of temperate forests in the Mid-Atlantic region," and they opined that "such a significant 'fertilization effect' on the forest carbon sequestration could eventually result in a 'greener world' after a long period of chronic change in atmospheric composition and cumulative impact."

One year later, Lazzarotto et al. (2010) wrote that "white clover (Trifolium repens L.) is the most important pasture legume grown in temperate climates in association with a variety of grasses, notably perennial ryegrass (Lolium perenne L.)," adding by way of explanation that "white clover improves the nutritional quality and digestibility of the herbage," and that it "contributes substantially to the nitrogen status of the sward through biological nitrogen fixation." They noted, however, that there was some concern that future drought, such as is typically predicted by climate alarmists to occur in tandem with CO2-induced global warming, will hurt clover more than the grass with which it is intermingled, thereby degrading the nutritional quality and digestibility of pasture swards.

In light of this mix of facts and presumptions, Lazzarotto et al. planned and conducted a study wherein, as they describe it, "mechanisms controlling transient responses to elevated CO2 concentration and climate change in an unfertilized grassland on the Swiss Plateau were examined in light of simulations with PROGRASS," a process-based model of grass-clover interactions developed by Lazzarotto et al. (2009), where "daily weather for a series of transient climate scenarios spanning the 21st century were developed for the study site with the help of the LARS-WG weather generator," which is described by Semenov and Barrow (1997) and Semenov et al. (1998), and where "changes in the length of dry and wet spells, temperature, precipitation and solar radiation defining the scenarios were obtained from regional climate simulations carried out in the framework of the PRUDENCE project," which is described by Christensen and Christensen (2007).

These efforts revealed that "compared to 1961-1990," in the words of the four Swiss and UK scientists, the climate scenarios they developed for a CO2 concentration increase from 370 to 860 ppm "indicated that for 2071-2100 there would be a noticeable increase in temperature (roughly 3°C in winter and 5°C in summer), a significant drop in summer precipitation (of the order of -30%) and a nearly 2-fold increase in the length of dry spells." So how strongly were these significant changes in climate calculated to affect the grass-clover swards?

Lazzarotto et al. report that "clover abundance did not decline even in the absence of CO2 stimulation." And when the atmospheric CO2 concentration was programmed to gradually rise from an initial value of 370 ppm to a final value of 860 ppm, they discovered that "clover development benefited from the overall positive effects of CO2 on nitrogen acquisition," which they said was also "the reason for increasing productivity of the [entire] sward." For Swiss grass-clover swards, therefore, it would appear that the rather large increases in temperature and decreases in precipitation that are predicted for the remainder of the 21st century, even if they come to pass, will not have much of an effect on them, but that the concomitant increase in the air's CO2 content will benefit them considerably. In addition, Lazzarotto et al. say that it is likely that "technical progress in the management of grasslands and pastures," which will surely occur, will help such pastures even more. All things considered, therefore, the future of Switzerland's (and many other countries') clover-grass associations would appear to be bright and promising.

Last of all, Rampino et al. (2012) have indicated that "plants are challenged by various abiotic stresses such as cold, high temperatures, high salinity, drought, etc.," which they say are "responsible for the extensive curtailing of crop productivity worldwide," citing Munns and Tester (2008) and Ahuja et al. (2010). And they add that "high temperatures and water deficits are two of the main environmental factors causing severe yield loss," citing Wahid et al. (2007) and Fleury et al. (2010). Therefore, in an effort designed to determine the potential for breeding crop varieties that are best equipped to cope with the combination of high temperature and a scarcity of water - which compounded catastrophic scenario is predicted by climate alarmists to become ever more common as the air's CO2 content continues to rise - Rampino et al. studied the modulation of gene expression in durum wheat (Triticum turgidum subsp. durum) plants that were classified as being able to acquire thermotolerance and drought tolerance, which investigation they conducted by means of cDNA-AFLP performed on RNAs extracted from flag leaves of plants sown in pots and grown in controlled environment chambers.

This work of the researchers allowed them to identify genes specifically involved in wheat response to combined stresses, which finding clearly showed, as they describe it, that "the effect of combined stress on wheat plants is different from that of heat or drought stress applied individually," in accordance with (1) "data already reported by other authors on different plant species (Mittler, 2006)," and indicative of the fact that (2) "combined stress also activates a pool of genes that are not induced by each single stress."

Rampino et al. thus concluded their paper by saying that their findings "not only add new information to the broad picture of plant stress activated genes, but can also be considered the starting point for future analysis of gene expression in crop plants subjected to stresses," while adding, most importantly, that "the functional characterization of these genes will provide new data that could help the developing of breeding strategies aimed at improving durum wheat tolerance to field stress," especially stress of the type that is predicted by the world's climate alarmists to soon be at our doorstep.

All things considered, therefore, and even accounting for the inhospitable climate that many people are predicting for the planet's future, the various studies described above paint a more positive view of things, wherein earth's vegetation - including mankind's major crops - will likely fare even better in the future than they do nowadays.

Ahuja, R.C.H., de Vos, A.M., Bones, R.D. and Hall, R.D. 2010. Plant molecular stress responses face climate change. Trends in Plant Science 15: 664-674.

Christensen, J.H. and Christensen, O.B. 2007. A summary of the PRUDENCE model projections of changes in European climate by the end of this century. Climatic Change 81: 7-30.

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.

Fleury, D., Jefferies, S., Kuchel, H. and Langridge, P. 2010. Genetic and genomic tools to improve drought tolerance in wheat. Journal of Experimental Botany 61: 3211-3222.

Lazzarotto, P., Calanca, P. and Fuhrer, J. 2009. Dynamics of grass-clover mixtures -- an analysis of the response to management with the PROductive GRASsland Simulator (PROGRASS). Ecological Modeling 220: 703-724.

Lazzarotto, P., Calanca, P., Semenov, M. and Fuhrer, J. 2010. Transient responses to increasing CO2 and climate change in an unfertilized grass-clover sward. Climate Research 41: 221-232.

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 18O record from SW Ireland. Science 294: 1328-1331.

Mittler, R. 2006. Abiotic stress, the field environment and stress combination. Trends in Plant Science 11: 15-19.

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

Munns, R. and Tester, R.M. 2008. Mechanisms of salinity tolerance. Annual Review of Plant Biology 59: 651-681.

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.

Pan, Y., Birdsey, R., Hom, J. and McCullough, K. 2009. Separating effects of changes in atmospheric composition, climate and land-use on carbon sequestration of U.S. Mid-Atlantic temperate forests. Forest Ecology and Management 259: 151-164.

Qaderi, M.M., Kurepin, L.V. and Reid, D.M. 2006. Growth and physiological responses of canola (Brassica napus) to three components of global climate change: temperature, carbon dioxide and drought. Physiologia Plantarum 128: 710-721.

Rampino, P., Mita, G., Fasano, P., Borrelli, G.M., Aprile, A., Dalessandro, G., De Bellis, L. and Perrotta, C. 2012. Novel durum wheat genes up-regulated in response to a combination of heat and drought stress. Plant Physiology and Biochemistry 56: 72-78.

Semenov, M.A. and Barrow, E.M. 1997. Use of a stochastic weather generator in the development of climate change scenarios. Climatic Change 35: 397-414.

Semenov, M.A., Books, R.J., Barrow, E.M. and Richardson, C.W. 1998. Comparison of the WGEN and LARS-WG stochastic weather generators for diverse climates. Climate Research 10: 95-107.

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.

Su, H., Sang, W., Wang, Y. and Ma, K. 2007. Simulating Picea schrenkiana forest productivity under climatic changes and atmospheric CO2 increase in Tianshan Mountains, Xinjiang Autonomous Region, China. Forest Ecology and Management 246: 273-284.

Wahid, A., Gelani, S., Ashraf, M. and Foolad, M.R. 2007. Heat tolerance in plants: an overview. Environmental and Experimental Botany 61: 199-223.

Last updated 2 October 2013