Biogeochemical Effects of Rising Atmospheric Carbon Dioxide

The most central planetary outcome of rising atmospheric carbon dioxide (CO₂), methane (CH₄), and black carbon is their warming effect on Earth’s atmosphere, which influences weather and climate (IPCC 2013). The Climate Science Special Report (CSSR; USGCRP 2017) concludes with high confidence that Earth’s observed temperature increase in the last century results from human influence, especially from emissions of greenhouse gases including CO₂ and CH₄ and particulates such as black carbon. Furthermore, CSSR (USGCRP 2017) demonstrates that the consequences of atmospheric warming are profound and diverse, significantly altering planetary surface temperatures and overall climate and thus also directly or indirectly altering countless oceanic and terrestrial processes.

Increased global temperatures lead to extremes in temperature and precipitation (IPCC 2013), causing heatwaves, droughts, floods, and changing storm system patterns (Melillo et al., 2014), with additional consequences for the carbon cycle. For instance, warming and changing weather melt polar ice cover and thaw Arctic permafrost, releasing CH₄ and CO₂ as stored organic matter is microbially respired (see Ch. 11: Arctic and Boreal Carbon). Melting glaciers and seawater expansion will raise sea levels, changing ecosystem boundaries and affecting net carbon fluxes (IPCC 2013; USGCRP 2017). Heating and ice melt will stratify the ocean, dampening the ability of vertical mixing to refresh surface waters with nutrients that support primary production (IPCC 2013). A warmer ocean will hold less carbon, because warmer ocean temperatures decrease the solubility of CO₂ in seawater (Zeebe and Wolf-Gladrow 2001). Both long-term increases in ocean temperature and short-term marine heatwaves may affect stocks of organic and inorganic carbon contained in marine ecosystems and sediments (see Ch. 16: Coastal Ocean and Continental Shelves). Changing snowpack dynamics will affect water availability significantly in riverine ecosystems. In midlatitudes, fire frequency and severity will change as a result of changes in temperature and precipitation. These shifts and feedbacks are very likely to have widespread, interacting effects on human and natural systems that elicit a variety of responses.

Upon this backdrop of accumulating, thermally driven planetary climate change that impacts the carbon cycle, rising atmospheric CO₂ is also affecting oceanic and terrestrial systems in nonthermal ways that have only begun to be understood since the First State of the Carbon Cycle Report (SOCCR1; CCSP 2007). The observed rise in atmospheric CO₂ since the 1950s is lower than the contributions from estimated emissions because both the ocean and land continue to take up a portion of the atmospheric CO₂ from anthropogenic (i.e., human) activities, indicating both systems are carbon sinks (Ballantyne et al., 2012). Ocean uptake prevents some degree of atmospheric warming but results in ocean acidification (see Ch. 16: Coastal Ocean and Continental Shelves), which drives a host of chemical and biological impacts, as reviewed below. The terrestrial “CO₂ fertilization effect” is the increased uptake of CO₂ per unit land area caused by rising CO₂, which is greater than could be expected from plant regrowth after land-use change and stimulation by increased nutrient availability. Global analysis suggests that CO₂ fertilization is responsible for up to 60% of the overall land sink (Schimel et al., 2015), but persistence of these benefits into the future is highly uncertain (Müller et al., 2014; Smith et al., 2016). Moreover, the thermal impacts of climate change will interact with, enhance, or in some cases overwhelm the nonthermal effects of rising atmospheric CO₂ on ecosystems; these different future scenarios are explored elsewhere in this report (see Ch. 19: Future of the North American Carbon Cycle). These findings have important implications; the current partitioning of anthropogenic CO₂ sinks among the ocean, atmosphere, and terrestrial biosphere, therefore, also will change in the future. Because CO₂ is involved in all aspects of growth in biological systems there also are important non-climate effects of increased atmospheric CO₂ concentration.

To better explain the non-climate effects of rising CO₂ on ecological systems, this chapter first reviews the historical context of rising CO₂ and then examines its impact on ocean and terrestrial systems (see Figure 17.1), including ocean acidification, productivity and ecosystem changes, interactions with other environmental changes, and carbon cycle feedbacks. Also examined are changes in ecosystem services (or benefits to humans) resulting from chemical changes in Earth system processes and how those intersect with thermally driven changes. This examination is followed by a review of outstanding research needs for gaining greater clarity on the effects of rising CO₂ on oceanic and terrestrial systems.

Such a comprehensive, collected examination of the effects of carbon cycle changes is new in the Second State of the Carbon Cycle Report (SOCCR2) and responds to the requirement in the Global Change Research Act that “analyzes the effect of global change on the natural environment, agriculture, energy production and use, land and water resources, transportation, human health and welfare, human social systems, and biological diversity” (Global Change Research Act 1990, Section 106). Since publication of SOCCR1 (CCSP 2007), many highly influential reports have assessed the consequences of carbon cycle changes on Earth systems, including the Third National Climate Assessment (Melillo et al., 2014), the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5; IPCC 2013), and the CSSR (USGCRP 2017). This chapter updates the conclusions of the reports cited above, with the most recent literature and with particular attention to North America. Those reports treat the direct and indirect effects of increasing CO₂ in greater detail than does this chapter, which focuses to a greater extent on the direct and non-climatic effects of increased atmospheric CO₂ concentrations.