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Carbon Dioxide and Earth's Future: Pursuing the Prudent Path

8. Declining Vegetative Productivity


The claim: Rising temperatures and increased weather extremes will decimate the productivity of critical earth ecosystems.

Climate-model projections have long suggested that the "twin evils" of the world's radical environmentalist movement (atmospheric CO2 enrichment and global warming) will wreck havoc with earth's natural and agro-ecosystems. However, a vast body of real-world research indicates that these two phenomena will likely do just the opposite.

Carbon dioxide is one of the two chief constituents of life on earth, the other being water; and the combining of the two of them via the process of photosynthesis is the very beginning of the planet's many "food chains," be they aquatic or terrestrial. Fortunately, it is a simple matter to assess the effect of an increase in the air's CO2 content on this phenomenon as it operates in terrestrial plants, for it can be accomplished by merely increasing the CO2 concentration of the air surrounding the plants in question and measuring the CO2 exchange between the air and the plants (in the case of photosynthesis) or the production of biomass (in the case of growth). And there have been literally thousands of such experiments performed in both the laboratory and the field, throughout most of the inhabited parts of the planet.

The world's largest repository of the results of such studies is located at the website of the Center for the Study of Carbon Dioxide and Global Change in two huge and ever-expanding databases (http://www.co2science.org/data/plant_growth/plantgrowth.php) to which new results are added weekly: one for photosynthesis and one for biomass or plant dry weight production. In the former category, one could find, at the end of 2010, the results of 71 individual experiments conducted on rice (an average increase in the rate of photosynthesis of 48.5% in response to a 300-ppm increase in the air's CO2 concentration), while in the case of biomass production, one could find the results of 178 individual experiments (an average dry weight increase of 34.5% in response to a 300-ppm increase in the air's CO2 concentration). Likewise, in the case of wheat, there were 91 individual determinations of the increase in photosynthesis caused by a 300-ppm increase in atmospheric CO2 (an average increase of 62.7%) and 235 individual determinations of the increase in dry weight production (an average increase of 32.1%).

A complete summary listing of such results for all plants in the CO2 Science database may be found in Appendix 2 (for dry weight or biomass) and Appendix 3 (for photosynthesis) of Climate Change Reconsidered (Idso and Singer, 2009), as things stood as of 23 March 2009. In addition, Idso and Idso (2000), in analyzing how things stood about a decade earlier, had determined the mean percentage yield increases in response to a 300-ppm increase in the atmosphere's CO2 concentration to be approximately 15% for CAM plants, 49% for C3 cereals, 20% for C4 cereals, 25% for fruits and melons, 44% for legumes, 48% for roots and tubers, 36% for vegetables, and 51% for woody crop plants.

In light of this vast array of real-world research, it can be appreciated that the aerial fertilization effect of enriching earth's atmosphere with CO2 is a very beneficent phenomenon. As Sylvan Wittwer stated in his 1995 book (Food, Climate, and Carbon Dioxide: The Global Environment and World Food Production):

"The rising level of atmospheric CO2 could be the one global natural resource that is progressively increasing food production and total biological output, in a world of otherwise diminishing natural resources of land, water, energy, minerals, and fertilizer. It is a means of inadvertently increasing the productivity of farming systems and other photosynthetically active ecosystems. The effects know no boundaries and both developing and developed countries are, and will be, sharing equally."

A second major benefit that earth's plants experience as a result of the ongoing rise in the air's CO2 content is enhanced water use efficiency. As mentioned above, when the atmosphere's CO2 concentration is increased, nearly all plants exhibit increased rates of photosynthesis and biomass production, while simultaneously, on a per-unit-leaf-area basis, they often lose less water via transpiration, as they tend to reduce their stomatal apertures and thereby decrease the rate of water loss from their leaves. Thus, the amount of biomass produced per unit of water lost -- or plant water use efficiency -- typically rises significantly as the air's CO2 content rises, which means that plants can produce more biomass while letting less water escape to the air, a phenomenon which, like the aerial fertilization effect of CO2, has also been observed in a plethora of agricultural crops in numerous experiments conducted under laboratory conditions (Malmstrom and Field, 1997; De Luis et al., 1999; Zhu et al., 1999; Gavito et al., 2000; Kyei-Boahen et al., 2003; Kim et al., 2006; Fleisher et al., 2008), as well as in greenhouses (Ceusters et al., 2008; Sanchez-Guerrero et al., 2009) and in the field (Garcia et al., 1998; Hakala et al., 1999; Hunsaker et al., 2000; Conley et al., 2001; Olivo et al., 2002; Dong-Xiu et al., 2002; Leavitt et al., 2003; Triggs et al., 2004; Yoshimoto et al., 2005).

Much the same has been observed in several species of young trees that have been similarly studied (Anderson and Tomlinson, 1998; Beerling et al., 1998; Egli et al., 1998; Rey and Jarvis, 1998; Tjoelker et al., 1998; Wayne et al., 1998; Centritto et al., 1999; Runion et al., 1999; Bucher-Wallin et al., 2000; Lodge et al., 2001; Tognetti et al., 2001; Wullschleger and Norby, 2001; Centritto, 2002; Centritto et al., 2002; Gunderson et al., 2002; Greenep et al., 2003), as well as in many species of older trees that have lived through the historical increase in the air's CO2 content of the past century or so, and whose temporal water use efficiency histories have been determined from dated tree-ring cellulose δ13C measurements (Bert et al., 1997; Duquesnay et al., 1998; Feng, 1999; Arneth et al., 2002; Saurer et al., 2004; Hietz et al., 2005; Liu et al., 2007; Silva et al., 2009), plus a few studies of trees where some of them had spent their entire lifetimes growing within CO2-enriched air close to CO2-emitting springs or vents, while others had grown further away from the springs/vents in normal ambient-CO2 air (Fernandez et al., 1998; Tognetti et al., 1998; Bartak et al., 1999; Blaschke et al., 2001).

Grasslands also exhibit the same increased water use efficiency response to atmospheric CO2 enrichment that trees and agricultural crops do, as evidenced by the findings of LeCain and Morgan (1998), Seneweera et al. (1998), Szente et al. (1998), Clark et al. (1999), Leymarie et al. (1999), Adams et al. (2000), Roumet et al. (2000), Grunzweig and Korner (2001), Engloner et al. (2003) and Moore and Field (2006). And all of these findings for all types of vegetation imply that the moisture contained in soils upon which plants are growing can be maintained at increasingly higher levels for longer periods of time as the air's CO2 content continues its upward climb, as has been shown to be the case in numerous field studies (Owensby et al., 1993; Ham et al., 1995; Bremer et al., 1996; Freden et al., 1997; Niklaus et al., 1998; Owensby et al., 1999; Sindhoj et al., 2000; Volk et al., 2000; Bunce, 2001; Morgan et al., 2001; Reich et al., 2001; Higgins et al., 2002; Hungate et al., 2002; Ferretti et al., 2003; Obrist et al., 2003; Zavaleta et al., 2003; Eguchi et al., 2004; Nelson et al., 2004; Niklaus and Korner, 2004). And this phenomenon reduces the severity and length of time that droughts can negatively affect both crops and natural vegetation, which secondary phenomenon leads to still greater season-long plant productivity, which phenomenon has also been observed in desert vegetation (Hamerlynck et al., 2002; Housman et al., 2006).

Last of all, it should be noted that this "water conservation effect" of atmospheric CO2 enrichment appears to operate even in the face of rising temperatures, as was found to be the case in the experimental studies of Dermody et al. (2007) and Saleska et al. (2007). And in an informative review of the direct and indirect effects of rising air temperature and atmospheric CO2 concentration on plant behavior, Kirschbaum (2004) makes a number of pertinent and revealing observations, the primary ones of which we here briefly summarize.

With respect to rising temperatures and their effect on photosynthesis, Kirschbaum states that "all plants appear to be capable of a degree of adaptation to growth conditions," noting that "photosynthesis in some species can function adequately up to 50°C." In fact, he says that "photosynthesis can acclimate considerably to actual growth conditions," noting that "optimum temperatures for photosynthesis acclimate by about 0.5°C per 1.0°C change in effective growth temperature (Berry and Bjorkman, 1980; Battaglia et al., 1996)." This response, wherein plants adjust the workings of their photosynthetic apparatus to perform better at higher temperatures as temperatures rise, would appear to be especially beneficial in a warming world.

With respect to rising CO2 concentrations and their effect on photosynthesis, Kirschbaum notes that CO2 assimilation rates generally rise as the air's CO2 content rises: by 25-75% in C3 plants in response to a doubling of the air's CO2 content, and by something on the order of 25% in C4 grasses, according to the major review of Wand et al. (1999). This response, wherein plants adjust the workings of their photosynthetic apparatus to perform better at higher atmospheric CO2 concentrations as atmospheric CO2 concentrations rise, would also appear to be especially beneficial in a CO2-acreting atmosphere.

With respect to the synergistic effect of simultaneous increases in both atmospheric CO2 concentration and temperature on photosynthesis, Kirschbaum notes that plant growth responses to increasing CO2 are usually much more pronounced for plants grown at higher temperatures," presenting a graph that suggests an approximate six-fold amplification of the aerial fertilization effect of atmospheric CO2 enrichment at an air temperature of 35°C compared to one of 5°C. Consequently, in a world where both air temperature and CO2 concentration are rising, this response would appear to be hugely beneficial.

Nevertheless, according to Robock et al. (2005), "most global climate model simulations of the future, when forced with increasing greenhouse gases and anthropogenic aerosols, predict summer desiccation in the midlatitudes of the Northern Hemisphere," and they state that "this predicted soil moisture reduction, the product of increased evaporative demand with higher temperatures overwhelming any increased precipitation, is one of the gravest threats of global warming, potentially having large impacts on our food supply." But inquisitive enough to want to know for themselves what actually happens in the real world, they went on to analyze 45 years of gravimetrically-measured plant-available soil moisture in the top one meter of soil for 141 stations from fields with either winter or spring cereals in the Ukraine over the period 1958-2002, finding, in their words, "a positive soil moisture trend for the entire period of observation." And they emphasized that "even though for the entire period there is a small upward trend in temperature and a downward trend in summer precipitation, the soil moisture still has an upward trend for both winter and summer cereals."

Two years later, Li et al. (2007) compared soil moisture simulations derived from the IPCC's Fourth Assessment climate models (which were driven by observed climate forcings) for the period 1958-1999 with actual measurements of soil moisture made at over 140 stations or districts in the mid-latitudes of the Northern Hemisphere, which were averaged in such a way as to yield six regional results: one each for the Ukraine, Russia, Mongolia, Northern China, Central China and Illinois (USA). And in doing so, they found that the models showed realistic seasonal cycles for the Ukraine, Russia and Illinois but "generally poor seasonal cycles for Mongolia and China." In addition, they said that the Ukraine and Russia experienced soil moisture increases in summer "that were larger than most trends in the model simulations." In fact, they reported that "only two out of 25 model realizations show trends comparable to those observations," and they noted that the two realistic model-derived trends were "due to internal model variability rather than a result of external forcing," which means that the two reasonable matches were actually accidental.

Noting further that "changes in precipitation and temperature cannot fully explain soil moisture increases for [the] Ukraine and Russia," Li et al. noted that in response to elevated atmospheric CO2 concentrations, "many plant species reduce their stomatal openings, leading to a reduction in evaporation to the atmosphere," so that "more water is likely to be stored in the soil or [diverted to] runoff," correctly reporting that this phenomenon had recently been detected in continental river runoff data by Gedney et al. (2006). In addition, in a free-air CO2-enrichment study conducted in a pasture on the North Island of New Zealand, Newton et al. (2003) found there was a significant reduction in the water repellency of the soil in the elevated CO2 treatment, where they describe water repellency as "a soil property that prevents free water from entering the pores of dry soil," as per Tillman et al. (1989). In fact, they wrote that "at field moisture content the repellence of the ambient soil was severe and significantly greater than that of the elevated [CO2] soil," suggesting that the reduction in the repellency of the soil provided by atmospheric CO2 enrichment would allow more water to enter and remain in the soil.

As time goes on, therefore, the multifaceted "water conservation effect" of atmospheric CO2 enrichment is becoming ever more important, as ever more land and water resources are being taken from "wild nature," in order to support the planet's growing human population, with the problem being that there's not much pristine land and water left for us to take. However, this problem, as we have noted, is being significantly mitigated by the continued strengthening of the very positive effect of atmospheric CO2 enrichment, in that the yearly upward trend in the air's CO2 content enables plants to yearly grow bigger and better -- and more successfully reproduce - especially where it was previously too dry for them to do so. And there is a large and growing body of real-world empirical evidence that suggests that this phenomenon has been occurring for some time now from arid to moist areas throughout the world, in a gradual transformation of the planet's terrestrial landscape that is frequently referred to as the greening of planet earth.

Evaluating this phenomena from a global perspective, Cao et al. (2004), in what they called "the first attempt to quantify interannual variations in NPP [net primary production] at the global scale," found that over the last two decades of the 20th century, when the heat was on, "there was an increasing trend toward enhanced terrestrial NPP," which they say was "caused mainly by increases in atmospheric carbon dioxide and precipitation." And one year later, Cao et al. (2005) calculated -- from real-world data -- that global net ecosystem production increased "from 0.25 Pg C yr-1 in the 1980s to 1.36 Pg C yr-1 in the 1990s."

In another study, Xiao and Moody (2005) found that the most intense recent greening of the globe was observed in high northern latitudes, portions of the tropics, southeastern North America and eastern China, in harmony with the increases in global vegetative productivity over the latter part of the 20th century that had been detected by Kawabata et al. (2001), Ichii et al. (2002) and Nemani et al. (2003). Working with satellite-derived NDVI data for the period 1982-1999, Young and Harris (2005) obtained similar results, determining that "globally more than 30% of land pixels increased in annual average NDVI greater than 4% and more than 16% persistently increased greater than 4%," while "during the same period less than 2% of land pixels declined in NDVI and less than 1% persistently declined," so that "between 1982 and 1999 the general trend of vegetation change throughout the world has been one of increasing photosynthesis."

In a study of tropical forests in the Amazon, Africa and Asia over the period 1982-1999, Ichii et al. (2005) reported that "recent changes in atmospheric CO2 and climate promoted terrestrial GPP [gross primary productivity] increases with a significant linear trend in all three tropical regions," such that in the Amazonian region, the rate of GPP increase was 0.67 PgC year-1 per decade, while in Africa and Asia it was about 0.3 PgC year-1 per decade; and they state that "CO2 fertilization effects strongly increased recent net primary productivity trends in regional totals."

In another study, Lewis, et al. (2009a), in a thorough review of the scientific literature, noted that both theory and experiments suggest that over the past several decades "plant photosynthesis should have increased in response to increasing CO2 concentrations, causing increased plant growth and forest biomass," and they did indeed find that "long-term plot data collectively indicate an increase in carbon storage, as well as significant increases in tree growth, mortality, recruitment, and forest dynamism," that satellite measurements "indicate increases in productivity and forest dynamism," and that five Dynamic Global Vegetation Models, incorporating plant physiology, competition, and dynamics, all predict increasing gross primary productivity, net primary productivity, and carbon storage when forced using late-twentieth century climate and atmospheric CO2 concentration data," while noting that "the predicted increases in carbon storage via the differing methods are all of similar magnitude (0.2% to 0.5% per year)." And so they concluded that "these results point toward a widespread shift in the ecology of tropical forests, characterized by increased tree growth and accelerating forest dynamism, with forests, on average, getting bigger (increasing biomass and carbon storage)."

Contemporaneously, Tans (2009) employed measurements of atmospheric and oceanic carbon contents, along with reasonably constrained estimates of global anthropogenic CO2 emissions, to calculate the residual fluxes of carbon (in the form of CO2) from the terrestrial biosphere to the atmosphere (+ values) or from the atmosphere to the terrestrial biosphere (- values), obtaining the results depicted in the following figure.


Five-year smoothed rates of carbon transfer from land to air (+) or from air to land (-) vs. time. Adapted from Tans (2009).

As can be seen from this figure, earth's land surfaces were a net source of CO2-carbon to the atmosphere until about 1940, primarily due to the felling of forests and the plowing of grasslands to make way for expanded agricultural activities. From 1940 onward, however, the terrestrial biosphere has become, in the mean, an increasingly greater sink for CO2-carbon; and it has done so even in the face of massive global deforestation, for which it has more than compensated. And in light of these findings, plus the fact that they do "not depend on models" but "only on the observed atmospheric increase and estimates of fossil fuel emissions," Tans concluded that "suggestions that the carbon cycle is becoming less effective in removing CO2 from the atmosphere (e.g., LeQuere et al., 2007; Canadell et al., 2007) can perhaps be true locally, but they do not apply globally, not over the 50-year atmospheric record, and not in recent years." In fact, he goes on to say that "to the contrary" and "despite global fossil fuel emissions increasing from 6.57 GtC in 1999 to 8.23 in 2006, the five-year smoothed global atmospheric growth rate has not increased during that time, which requires more effective uptake [of CO2] either by the ocean or by the terrestrial biosphere, or both, to satisfy atmospheric observations." And the results portrayed in the figure we have adapted from Tans' paper clearly indicate that this "more effective uptake" of CO2-carbon has occurred primarily over land.

The story reported on a continent by continent basis is much the same as it is for the globe as a whole. Consequently, and in order to curtail the size of this burgeoning treatise, in what follows we provide only references, grouped by continent, of studies where real-world data have documented the greening of the earth phenomena.

For Africa, Prince et al. (1998), Eklundh and Olsson (2003), Anyamba and Tucker (2005), Olsson et al. (2005), Seaquist et al. (2006), Ciais et al. (2009) and Lewis et al. (2009b). For Asia,Fang et al. (2003), Piao et al. (2005a), Brogaard et al. (2005), Piao et al. (2005b), Lapenis et al. (2005), Piao et al. (2006a), Schimel et al. (2001), Kharuk et al. (2006), Piao et al. (2006b), Tan et al. (2007), Piao et al. (2007), Zhou et al. (2007), Zhu et al. (2007), Mu et al. (2008), Mao et al. (2009), Zhuang et al. (2010) and Forbes et al. (2010). For Australia, Harrington and Sanderson (1994), Russell-Smith et al. (2004) and Banfai and Bowman (2006). For Europe, Osborne et al. (2000), Lopatin et al. (2006), Martinez-Vilalta et al. (2008), Alcaraz-Segura et al. (2008) and Hallinger et al. (2010). For North America, Hicke et al. (2002), Westfall and Amateis (2003), Lim et al. (2004), Peterson and Neofotis (2004), Xiao and Moody (2004), Soule and Knapp (2006), Tape et al. (2006), Wang et al. (2006), Voelker et al. (2006), Piao et al. (2006c), Hudson and Henry (2009), Springsteen et al. (2010), Pan et al. (2010) and Cole et al. (2010). And for South America, Beerling and Mayle (2006) and Silva et al. (2009).

Given the above findings, our assessment of the future of earth's natural and agro-ecosystems is indeed bright. Crop yields will increase by a goodly amount over the course of this century, and tree and shrub growth will surge even more, as the air's CO2 content continues to promote a great greening of the earth.

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