To investigate this question, Zhao et al. grew well watered and fertilized cotton plants in sunlit controlled environment chambers maintained at atmospheric CO2 concentrations of 360 or 720 ppm from emergence until three weeks past first-flower stage under three levels of UV-B radiation (0, 8 and 16 kJ m^-2 d^-1); and on five dates between 21 and 62 days after emergence, they measured a number of plant physiological processes and parameters. Over the course of the experiment, the mean net photosynthetic rate of the upper-canopy leaves in the CO2-enriched chambers was increased - relative to that in the ambient-air chambers - by 38.3% in the low UV-B treatment (from 30.3 to 41.9 m m^-2 s^-1), 41.1% in the medium UV-B treatment (from 28.7 to 40.5 m m^-2 s^-1), and 51.5% in the high UV-B treatment (from 17.1 to 25.9 m m^-2 s^-1). In the medium UV-B treatment, the growth stimulation from the elevated CO2 was sufficient to raise net photosynthesis rates 33.7% above the rates experienced in the ambient air and no UV-B treatment (from 30.3 to 40.5 m m^-2 s^-1); but in the high UV-B treatment the radiation damage was so great that even with the help of the 51.5% increase in net photosynthesis provided by the doubled-CO2 air, the mean net photosynthesis rate of the cotton leaves was 14.5% less than that experienced in the ambient air and no UV-B treatment (dropping from 30.3 to 25.9 m m^-2 s^-1).

It should be noted, however, that the medium UV-B treatment of this study was chosen to represent the intensity of UV-B radiation presently received on a clear summer day in the major cotton production region of Mississippi, USA, under current stratospheric ozone conditions, while the high UV-B treatment was chosen to represent what might be expected there following a 30% depletion of the ozone layer, which has been predicted to double the region's reception of UV-B radiation from 8 to 16 kJ m^-2 d^-1. Consequently, a doubling of the current CO2 concentration and the current UV-B radiation level would reduce the net photosynthetic rate of cotton leaves by just under 10% (from 28.7 to 25.9 m m^-2 s^-1), whereas in the absence of a doubling of the air's CO2 content, a doubling of the UV-B radiation level would reduce cotton net photosynthesis by just over 40% (from 28.7 to 17.1 m m^-2 s^-1).

Viewed in this light, it can be seen that a doubling the current atmospheric CO2 concentration would compensate for over three-fourths of the loss of cotton photosynthetic capacity caused by a doubling of the current UV-B radiation intensity; and it may possibly do even better than that, for in the study of Zhao et al. (2003), it was reported that both Adamse and Britz (1992) and Rozema et al. (1997) found that doubled CO2 totally compensated for the negative effects of equally high UV-B radiation.

In another noteworthy study, Deckmyn et al. (2001) grew white clover plants for four months in four small greenhouses, two of which allowed 88% of the incoming UV-B radiation to pass through their roofs and walls and two of which allowed 82% to pass through, while one of the two greenhouses in each of the UV-B treatments was maintained at ambient CO2 (371 ppm) and the other at elevated CO2 (521 ppm). At the mid-season point of their study, they found that the 40% increase in atmospheric CO2 concentration stimulated the production of flowers in the low UV-B treatment by 22% and in the slightly higher UV-B treatment by 43%; while at the end of the season, the extra CO2 was determined to have provided no stimulation of biomass production in the low UV-B treatment, but it significantly stimulated biomass production by 16% in the high UV-B treatment.

The results of this study indicate that the positive effects of atmospheric CO2 enrichment on flower and biomass production in white clover are greater at more realistic or natural values of UV-B radiation than those found in many greenhouses. As a result, Deckmyn et al. say their results "clearly indicate the importance of using UV-B transmittant greenhouses or open-top chambers when conducting CO2 studies," for if this is not done, their work suggests that the results obtained could significantly underestimate the magnitude of the benefits that are being continuously accrued by Earth's vegetation as a result of the ongoing rise in the air's CO2 content.

Quaderi and Reid (2005) grew well watered and fertilized canola plants (Brassica napus) from seed to maturity in pots within controlled environment chambers maintained at either 370 or 740 ppm CO2 with and without a daily dose of UV-B radiation in the amount of 4.2 kJ m^-2, while measuring a number of plant parameters at various times throughout the growing season. With respect to the bottom-line result of final seed yield, this parameter was determined to be 0.98 g/plant in the control treatment (ambient CO2, with UV-B). Doubling the CO2 concentration increased yield by 25.5% to 1.23 g/plant, while removing the UV-B radiation flux increased yield by 91.8% to 1.88 g/plant. Doing both (doubling the CO2 concentration while simultaneously removing the UV-B flux) increased final seed yield most of all, by 175.5% to 2.7 g/plant. Viewed from a different perspective, doubling the air's CO2 concentration in the presence of the UV-B radiation flux enhanced final seed yield by 25.5%, while doubling CO2 in the absence of the UV-B radiation flux increased seed yield by 43.6%. Given these findings, Qaderi and Reid concluded that "elevated CO2 may have a positive effect on plants by mitigating the detrimental effects caused by UV-B radiation."

In a follow-up paper, Qaderi et al. (2007) examined the effects of elevated CO2 and UVB radiation on the photosynthetic rates and water use efficiency of the maturing husks or siliquas that surround the canola plant's seeds. For the plants exposed to 4.2 kJ m^-2 d^-1 of UVB radiation, the experimental doubling of the air's CO2 concentration led to a 29% increase in siliqua net photosynthesis, an 18% decrease in siliqua transpiration, and a 58% increase in siliqua water use efficiency; while for the plants exposed to no UVB radiation, siliqua net photosynthesis was increased by a larger 38%, transpiration was decreased by a larger 22% and water use efficiency was increased by a larger 87% in the CO2-enriched air.

Also working with canola, Tohidimoghadam et al. (2011) grew two varieties (Okapi and Talaye) out-of-doors over the 2008 and 2009 growing seasons beneath rigid frames covered with polyethylene plastic film in air maintained at ambient and elevated atmospheric CO2 concentrations of 400 and 900 ppm, at ambient and elevated levels of UV radiation, and under well-watered and deficit-watered conditions, during and after which periods they measured numerous plant properties. Results indicated that "water stress significantly decreased yield and yield components, oil yield, protein percentage, height, specific leaf area and the number of branches," but that elevated CO2 "increased the final yield, 1000-seed weight, oil percentage, oil yield, height, specific leaf area and number of branches," while UV radiation "decreased the yield, yield components, oil and protein percentages and growth parameters." They also note that "the highest seed weight was obtained from the 'Talaye' cultivar treated with compete irrigation and elevated CO2 and grown under sunlight radiation," while "the seed weights of both cultivars visibly decreased due to UV-B, UV-C and water stress under an ambient CO2 concentration." Given such findings, the three Iranian researchers who conducted the study state that "an increase in UV exposure deceases plant growth and development," but that "elevated CO2 ameliorate(s) the adverse effects of UV radiation in the final yield, seed weight, oil percentage, oil yield, plant height, specific leaf area and number of branches per plant," concluding that an increase in the atmosphere's CO2 concentration "could improve yield, yield components and growth parameters for plants subjected to elevated levels of UV radiation."

In a study of UV-B and CO2 effects on a natural ecosystem, which was conducted at the Abisko Scientific Research Station in Swedish Lapland, Johnson et al. (2002) studied plots of subarctic heath composed of open canopies of downy birch and dense dwarf-shrub layers containing scattered herbs and grasses. For a period of five years, they exposed the plots to factorial combinations of UV-B radiation (ambient and that expected to result from a 15% stratospheric ozone depletion) and atmospheric CO2 concentration (ambient, around 365 ppm, and enriched, around 600 ppm) after which they determined the amounts of microbial carbon (Cmic) and nitrogen (Nmic) in the soils of the plots.

When the plots were exposed to the enhanced UV-B radiation, the amount of Cmic in the soil was reduced to only 37% of what it was at the ambient UV-B level when the air's CO2 content was maintained at the ambient concentration. When the UV-B increase was accompanied by the CO2 increase, however, not only was there not a decrease in Cmic, there was an actual increase of 37%. The story with respect to Nmic was both similar and different at one and the same time. In this case, when the plots were exposed to the enhanced level of UV-B radiation, the amount of Nmic in the soil experienced a 69% increase when the air's CO2 content was maintained at the ambient concentration; and when the UV-B increase was accompanied by the CO2 increase, Nmic rose even more, by a whopping 138%.

These findings, in the words of Johnson et al., "may have far-reaching implications ... because the productivity of many semi-natural ecosystems is limited by N (Ellenberg, 1988)." Hence, the 138% increase in soil microbial N observed in this study to accompany a 15% reduction in stratospheric ozone and a 64% increase in atmospheric CO2 concentration (experienced in going from 365 ppm to 600 ppm) should significantly enhance the input of plant litter to the soils of these ecosystems, which phenomenon represents the first half of the carbon sequestration process, i.e., the carbon input stage. With respect to the second stage of keeping as much of that carbon as possible in the soil, Johnson et al. note that "the capacity for subarctic semi-natural heaths to act as major sinks for fossil fuel-derived carbon dioxide is [also] likely to be critically dependent on the supply of N," as is indeed indicated to be the case in the literature review of Berg and Matzner (1997), who report that with more nitrogen in the soil, the long-term storage of carbon is significantly enhanced, as more litter is chemically transformed into humic substances when nitrogen is more readily available, and these more recalcitrant carbon compounds can be successfully stored in the soil for many millennia.

In a slightly more complicated experiment, Koti et al. (2007) investigated the interactive effects of elevated atmospheric CO2 (720 vs. 360 ppm), UV-B radiation levels (0 vs. 10 kJ/m²/day), and temperature (38/30°C vs. 30/22°C day/night) on the growth and development of six well watered and fertilized soybean (Glycine max L.) genotypes. Their results indicated that "elevated CO2 partially compensated [for] the damaging effects on vegetative growth and physiology caused by negative stressors such as high temperatures and enhanced UV-B radiation levels in soybean," with the authors specifically noting, in this regard, CO2's positive influence on the physiological parameters of plant height, leaf area, total biomass, net photosynthesis, total chlorophyll content, phenolic content and wax content, as well as relative plant injury. Thus, with respect to almost all of the ways in which high air temperatures and high UV-B radiation levels negatively impact the growth and development of soybeans, elevated atmospheric CO2 concentrations appear to be able to provide significant ameliorative relief.

Lastly, in a study that did not include UV-B radiation as an experimental parameter, Estiarte et al. (1999) grew spring wheat in FACE plots in Arizona, USA, at atmospheric CO2 concentrations of 370 and 550 ppm and two levels of soil moisture (50 and 100% of potential evapotranspiration). They found that leaves of plants grown in elevated CO2 had 14% higher total flavonoid concentrations than those of plants grown in ambient air, and that soil water content did not affect the relationship. An important aspect of this finding is that one of the functions of flavonoids in plant leaves is to protect them against UV-B radiation. More studies of this nature should thus be conducted to see how general this beneficial response may be throughout the plant world.

In light of these several findings, it would appear that the ongoing rise in the air's CO2 content is a powerful antidote for the deleterious biological impacts that might possibly be caused by an increase in the flux of UV-B radiation at the surface of the earth due to any further depletion of the planet's stratospheric ozone layer that may occur in the future.

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