<b>Cooley</b>, S. R., D. J. P. <b>Moore</b>, S. R. Alin, D. Butman, D. W. Clow, N. H. F. French, R. A. Feely, Z. I. Johnson, G. Keppel-Aleks, S. E. Lohrenz, I. B. Ocko, E. H. Shadwick, A. J. Sutton, C. S. Potter, Y. Takatsuka, A. P. Walker, and R. M. S. Yu, 2018: Chapter 17: Biogeochemical effects of rising atmospheric carbon dioxide. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 690-727, https://doi.org/10.7930/SOCCR2.2018.Ch17.
Biogeochemical Effects of Rising Atmospheric Carbon Dioxide
17.7.1 Current State of Knowledge
The rise of atmospheric CO2—attributable primarily to human-caused fossil fuel emissions and land-use change—has been dampened by carbon uptake by the ocean and terrestrial biosphere. Nevertheless, today’s atmospheric CO2 levels are higher than at any time in at least the past 800,000 years (Hönisch et al., 2012). Uptake of this fossil fuel CO2 has caused documented direct and indirect effects on terrestrial and oceanic systems and processes in different regions of North America and the rest of the planet. The capacity of these systems to continue to act as carbon sinks is not certain because the systems are dynamic and influenced by feedbacks related to CO2 levels (see Section 17.3). Another major set of consequences stems from the atmospheric warming caused by rising CO2; weather and climate changes affect nearly every terrestrial and oceanic process (see Section 17.3–17.5) and often lead to additional feedbacks. Although reviewed in detail in other reports, including the IPCC AR5 (IPCC 2013) and CSSR (USGCRP 2017), these consequences deserve mention here because of their combined effects with CO2 on systems and processes throughout the land and ocean domains.
17.7.2 Key Knowledge Gaps and Opportunities
Research has uncovered many of the direct and indirect responses of natural systems to rising CO2, but mechanisms often remain unclear. Since the SOCCR1 report, increasing computational power has enabled the development of complex models to examine the consequences of rising CO2 and a changing carbon cycle. Observational and modeling studies, such as the new generation of FACE experiments now underway, are being planned in concert to enable strategic data collection. Some of these approaches allow for limitations of multiple resources (e.g., nitrogen and phosphorus), which could lead to more realistic projections of the terrestrial carbon sink’s response to rising CO2. As Figure 17.1 illustrates, there are current FACE experiments in the Northwest, Northeast, Southern Plains, or any tropical ecosystem within the U.S. territories. While most experiments are in mesic (wet) or temperate ecosystems (see Figure 17.6), understanding the response of tropical forests or coniferous boreal forests is critical to account for carbon cycle feedbacks. Oceanic models are providing insight into ecosystem relationships and dynamics under global change and into the biophysical underpinnings of ocean-atmosphere interactions. Despite these insights, knowledge of how multiple global change factors affect modeled processes would greatly improve model forecast ability. In contrast, most experimental manipulations are single-factor experiments in which only one variable is manipulated.
Figure 17.6: Hypothesized Ecosystem Responses to Elevated Carbon Dioxide (CO2>) Relative to Nutrient and Water Availability
Disentangling the impacts of rising CO2 and other concurrent changes in climate, land use, nutrient cycles, and atmospheric chemistry across all ecosystems likely requires long-term, sustained carbon cycle observations and monitoring of ecosystem and socioeconomic consequences. Long-term observing networks are critical to managing ecosystems sustainably and adaptively (e.g., Schindler and Hilborn 2015), and a focus on data management and interoperability across data platforms would improve understanding of long-term responses to rising CO2 (Ciais et al., 2014). Few experiments on land or in the ocean extend to a decade in length, and therefore the long-term ecosystem responses are not clear.
Pörtner et al. (2014) conclude that there is medium to high agreement that ecosystem services will change. However, the effects of rising CO2 on biodiversity and vegetation changes after disturbance remain poorly understood and could result in altered ecosystem function and different ecosystem services. This lack of understanding also limits the ability to anticipate recovery from acute disturbances such as storms, fires, disease, or insect outbreaks.
As forecasts of future conditions improve, investigating past conditions on Earth is still important. Over short timescales, historical terrestrial work is limited to studies that involve reconstructions of plant growth (e.g., tree rings). Exploring historical conditions decades or centuries before via ice core analysis, seafloor sediment core studies, and geological research will continue to uncover aspects of prior ages that are analogous to today, aiding the anticipation of potential changes in the Earth system as global change continues.
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