How does rising atmospheric CO2 affect marine organisms?

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The State of Earth's Terrestrial Biosphere:
How is it Responding to Rising Atmospheric CO2 and Warmer Temperatures?

Continental-Scale Analyses of Terrestrial Productivity: North America - Western USA

Moving on to the western United States, we begin with a discussion of two studies taking a model-based approach to the issue.

In an attempt to evaluate the consequences of a doubling of the air's CO2 content and 2 to 4°C increases in air temperature on ecosystem performance in a high-elevation Rocky Mountain watershed, Baron et al. (2000) utilized an empirically-based hydro-ecological simulation model. Their results indicated that "both photosynthesis and transpiration were highly responsive to doubled CO2." It was also determined that the positive effects of the 4°C temperature increase "were additive, so a warmer and carbon-rich environment increased plant growth by 30%." Hence, the authors concluded that "forests will expand at the expense of tundra in a warmer, wetter, and enriched CO2 world," and that observed increases in tree height and density in recent decades illustrate "the rapidity with which vegetation can respond to climate change."

Because urban environments are affected by urban heat islands, carbon dioxide domes, and high-level nitrogen deposition, Shen et al. (2008) say that "to some extent they portend the future of the global ecosystem," and that they "provide a unique 'natural laboratory' to study potential ecosystem responses to anthropogenic environmental changes." Against this backdrop, therefore, the team of four authors used a version of the Patch Arid Land Simulator-Functional Types (PALS-FT) process-based ecosystem model -- originally developed for the Chihuahuan Desert but modified to represent the Larrea tridentata-dominated ecosystem characteristic of the Sonoran Desert within which Phoenix is located -- to investigate the impacts of previously documented city-to-desert gradients of atmospheric CO2 concentration, air temperature (TA), and nitrogen deposition (Ndep) on aboveground net primary productivity (ANPP) and soil organic matter (SOM).

In response to the mean maximum rural-to-urban increases in CO2 (160 ppm), Ndep (24 kg per ha/year) and TA (4.0°C) characteristic of Phoenix, mean ANPP changes of +52.5, +42.7 and -7.8 g dry matter (DM) per m2/year were obtained, respectively, from the 76.3 g DM per m2/year characteristic of desert conditions, when each of the three factors was increased individually. And when all three parameters were increased together, the net increase in ANPP was found to be even greater than the sum of the three individual results: 108 vs. 87.4 g DM per m2/year, which numbers translate to respective percentage increases of 142% vs. 115%. In the case of SOM, increases of 18.5, 12.3 and 1.2 g C per m2/year were obtained for mean maximum individual increases in CO2, Ndep and TA, respectively, while the combined increase was 30.9 g C per m2/year. Given such findings, it should be abundantly clear that even in a desert region as hot as Phoenix, the types of CO2, temperature and nitrogen deposition increases predicted for the years ahead portend huge increases in indigenous ecosystem productivity and soil organic matter buildup.

Transitioning from model- to empirical-based studies, Zavaleta and Kettley (2006) examined patterns of production, standing biomass, carbon (C) and nitrogen (N) storage, community composition, and soil moisture along a 25-year chronosequence of sites within an annual, exotic-dominated grassland at Stanford University's Jasper Ridge Biological Preserve in the interior foothills of the central coast range south of San Francisco, California (USA), various parts of which had been invaded at a number of different times over the past quarter-century by Baccharis pilularis shrubs.

According to the two researchers, increasing above- and below-ground biomass along the chronosequence "drove increases in ecosystem N sequestration of ~700% and in C storage of over 125%," including a 32% increase in total soil C over the 25-year period. What is more, they found that "increases in carbon storage also did not appear to be saturating at 25 years after shrub establishment in any pool, suggesting the potential for additional carbon gains beyond 25 years." In addition, they found that Baccharis shrubs began to decline in prominence after about 20 years, as native oaks "with life spans of centuries" and the potential to drive even larger ecosystem changes began to grow in the shrub-dominated areas.

Of further interest, the two researchers say they "initially hypothesized that Baccharis-invaded sites would experience increasing N limitation as N was immobilized in biomass and litter." However, as they continue, they found that "total soil N increased rapidly with shrub age" and that "the magnitude of increase in total soil nitrogen was much larger than the increase in nitrogen immobilization in biomass and litter over time."

Zavaleta and Kettley conclude their analysis by saying that their findings "illustrate the potential for important vegetation-mediated ecosystem responses and feedbacks to atmospheric CO2 and climate change," as indeed they do. In particular, they highlight the great potential for a CO2-induced range expansion of trees; and they pretty much lay to rest the claim (Hungate et al., 2003) that the availability of nitrogen, in forms usable by plants, will probably be too low for large future increases in carbon storage driven by CO2-induced increases in plant growth and development.

In a laboratory-based study, Peterson and Neofotis (2004) grew velvet mesquite (Prosopis velutina Woot.) seedlings for six weeks from their time of planting (as seeds) in small pots within environmentally controlled growth chambers that were maintained at atmospheric CO2 concentrations of 380 and 760 ppm and two levels of water availability (high and low). Although they did not see a significant CO2-induced increase in plant growth, the authors say that by the end of their six-week study, they observed a highly significant reduction of approximately 41% in the volume of water transpired by P. velutina in response to the experimental doubling of the air's CO2 content. "This large reduction in whole-plant water use," as they describe it, "occurred because the reduction in transpiration per unit leaf area at elevated CO2 was not offset by a proportional increase in total leaf area."

The pair of scientists from the Biosphere 2 Center near Oracle, Arizona, USA, say their findings suggest that "under a future [high-CO2] scenario, seedlings may deplete soil moisture at a slower rate than they do currently," and that "this could facilitate seedling survival between intermittent rain events," noting that their work "corroborates the conclusions of Polley et al. (1994, 1999, 2003) that increasing levels of atmospheric CO2 may facilitate the establishment of mesquite seedlings through a reduction in soil water depletion." That such has indeed occurred is suggested by the fact, again quoting Peterson and Neofotis, that "mesquites and other woody species in the semiarid southwestern United States have shown substantial increases in population density and geographic range since Anglo-American settlement of the region approximately 120 years ago," in support of which statement they cite the studies of Van Auken and Bush (1990), Gibbens et al. (1992), Bahre and Shelton (1993), Archer (1995), Boutton et al. (1999), Van Auken (2000), Ansley et al. (2001), Wilson et al. (2001) and Biggs et al. (2002).

Among such studies could also be listed the work of Feng (1999), who derived variations in plant intrinsic water-use efficiency over the preceding two centuries from 23 carbon isotope tree-ring chronologies. These results were nearly identical to the historical trend in the air's CO2 content, with plant intrinsic water-use efficiency rising by 10 to 25% from 1750 to 1970, during which time the air's CO2 concentration rose by approximately 16%. Feng thus concluded that "in arid environments where moisture limits the tree growth, biomass may have increased with increasing transpiration efficiency," noting that the enhanced growth of trees in arid environments may "have operated as a carbon sink for the anthropogenic CO2" emitted over that time span.

Finally, working at eight different sites within the Pacific Northwest of the United States, Soule and Knapp (2006) studied ponderosa pine trees to see how they may have responded to the increase in the atmosphere's CO2 concentration that occurred after 1950. The two geographers say the sites they chose "fit several criteria designed to limit potential confounding influences associated with anthropogenic disturbance." In addition, they selected locations with "a variety of climatic and topo-edaphic conditions, ranging from extremely water-limiting environments ... to areas where soil moisture should be a limiting factor for growth only during extreme drought years." They also say that all sites were located in areas "where ozone concentrations and nitrogen deposition are typically low."

At all eight of the sites that met all of these criteria, Soule and Knapp obtained core samples from about 40 mature trees that included "the potentially oldest trees on each site," so that their results would indicate, as they put it, "the response of mature, naturally occurring ponderosa pine trees that germinated before anthropogenically elevated CO2 levels, but where growth, particularly post-1950, has occurred under increasing and substantially higher atmospheric CO2 concentrations." Utilizing meteorological evaluations of the Palmer Drought Severity Index, they thus compared ponderosa pine radial growth rates during matched wet and dry years pre- and post-1950.

So what did they find? Overall, the two researchers discovered a post-1950 radial growth enhancement that was "more pronounced during drought years compared with wet years, and the greatest response occurred at the most stressed site" (see Figure 25). As for the magnitude of the response, they determined that "the relative change in growth [was] upward at seven of our [eight] sites, ranging from 11 to 133%".

Figure 25. Relative and absolute radial growth increases of Ponderosa pine (Pinus ponderosa) in the Pacific Northwest, USA, post-1950 compared with pre-1950 during years falling into various Palmer Drought Severity Index (PDSI) categories of drought (moist) severity. Adapted from Soule and Knapp (2006).

With respect to the significance of their observations, Soule and Knapp say their results show that "radial growth has increased in the post-1950s period ... while climatic conditions have generally been unchanged," which further suggests that "nonclimatic driving forces are operative." In addition, they say the "radial growth responses are generally consistent with what has been shown in long-term open-top chamber (Idso and Kimball, 2001) and FACE studies (Ainsworth and Long, 2005)." Hence, they conclude that their findings "suggest that elevated levels of atmospheric CO2 are acting as a driving force for increased radial growth of ponderosa pine, but that the overall influence of this effect may be enhanced, reduced or obviated by site-specific conditions."

Summarizing their findings, Soule and Knapp recount how they had "hypothesized that ponderosa pine ... would respond to gradual increases in atmospheric CO2 over the past 50 years, and that these effects would be most apparent during drought stress and on environmentally harsh sites," and in the following sentence of their paper they say their results "support these hypotheses." Hence, they conclude by stating that "an atmospheric CO2-driven growth-enhancement effect exists for ponderosa pine growing under specific natural conditions within the interior Pacific Northwest," providing yet another important example of the ongoing CO2-induced greening of the Earth.

It would thus appear, given the findings of all studies presented in this section above, that the late 20th-century rise in temperature and CO2 has improved the productivity of plant communities in the western region of the U.S., notwithstanding climate-alarmist concerns of unprecedented ecological disaster due to rising temperatures and drought.

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