<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
Oceanic ecosystem services critical for human survival, such as the provision of fish and seafood, carbon storage, coastal protection by reefs, and climate modulation, face significant risks from the combined effects of ocean acidification, warming, and sea level rise (Gattuso et al., 2015). Under the current rate of CO2 emissions, most marine organisms evaluated to date will face a very high risk of impacts by 2100, and some, including coral reefs (Hughes et al., 2017; Ainsworth et al., 2016; Hughes et al., 2018) and bivalve shellfish (Kroeker et al., 2013), already face moderate to high risk today (Gattuso et al., 2015; see Figure 17.5). For future scenarios without significant mitigation of CO2 emissions, predicted impacts to ocean ecosystem services are moderate for the early decades of this century but put all ecosystem services at high or very high risk by 2100 (Gattuso et al., 2015).
Figure 17.5: Ocean Impacts Projected by High and Low Carbon Dioxide (CO2) Emissions Scenarios
Rising CO2 will affect species differentially. Described here are the direct effects of rising CO2 rather than the impacts of warming, which are discussed comprehensively in CSSR (USGCRP 2017). Acidification by CO2 has been associated with a decline in shell-bearing benthic organisms (Hall-Spencer et al., 2008; Kroeker et al., 2011). Declines in oyster spat survival at a commercial hatchery in the U.S. Pacific Northwest that temporarily jeopardized the region’s oyster aquaculture industry have been definitively attributed to ocean acidification (Barton et al., 2015). Laboratory studies and meta-analyses have provided evidence for and against detrimental effects on marine biodiversity (Bijma et al., 2013; Dupont et al., 2010; Hendriks and Duarte 2010; Hendriks et al., 2010). Foundational organisms such as microbial populations, while not deeply studied, also demonstrate a range of positive to negative responses to ocean acidification (Bunse et al., 2016). The effects of ocean acidification on marine ecosystem structure are only now being identified. Models simulating ocean acidification’s impacts on bivalve shellfish have shown a restructuring of the entire California Current ecosystem by a combination of indirect predator-prey effects (Busch et al., 2013; Kaplan et al., 2010). Another model showed substantial restructuring of phytoplankton communities under ocean acidification and warming (Dutkiewicz et al., 2015), but studies still have not determined whether this restructuring would have significant effects on phytoplankton community function or food-web relationships.
On land, elevated atmospheric CO2 studies have demonstrated that seed yield can be increased (LaDeau and Clark 2001, 2006). In some crop species, increased seed production was accompanied by reduced quality (Ainsworth et al., 2002) but not in tree species (Way et al., 2010). Species show different growth responses to rising CO2 (Dawes et al., 2011), possibly giving dominant plants an advantage (McDonald et al., 2002; Moore et al., 2006) and leading to changes in forest structure. However, the impact on biodiversity will depend on ecological responses that will remain uncertain without long-term study of ecological responses to rising CO2 (Alin et al., 2015; Carey and Cottingham 2016; Elmendorf et al., 2016; Schimel et al., 2011).
17.6.2 Food and Fiber Provision
Ocean acidification is likely to have long-term effects on the population and diversity of fish and invertebrates, including economically and ecologically important shellfish (Pörtner et al., 2004). Although difficult to untangle, the combined effects of resource competition, pollution, overfishing, habitat modification, acidification, water temperature increases, and climate-driven changes on small-scale fisheries and aquaculture are likely to result in widespread changes in ocean ecosystems and in the fisheries themselves (HLPE 2014).
The impacts of ocean acidification on the food value, quality, and market value of marine species have yet to be conclusively determined. One preliminary study (Dupont et al., 2014) notes that the taste and texture of pink shrimp (Pandalus borealis) were poorer when the shrimp had been raised under more acidified conditions. Assuming that ocean acidification slows the growth of bivalve shellfish in the wild as it does in laboratory studies (Kroeker et al., 2013), harvest of the largest size class of sea scallop meat, which fetches a market price premium, is projected to decline under acidification (Cooley et al., 2015). The growth-retarding effects of acidification on king and Tanner crab as reported by Long et al. (2013a, 2013b) are projected to harm fishery revenues (Punt et al., 2016), but the implications of acidification for the market quality of Alaskan crabs (e.g., taste and texture) are not yet known. If the laboratory and model results reviewed above hold true in natural ecosystems, ocean acidification is likely to decrease the volume or quality of marine harvests beyond simply the impacts on oyster aquaculture observed to date. The larval production shortage in the mid-2000s experienced by the Pacific Northwest oyster aquaculture industry that was conclusively attributed to ocean acidification remains the bellwether example of impacts to fisheries from rising CO2 (Barton et al., 2015).
Terrestrial provisioning services (e.g., crops and livestock) also are responding to rising CO2. For example, crop production increased in response to experimentally elevated CO2 (Leakey et al., 2009), but increases in crop yield were accompanied by decreases in seed quality (Myers et al., 2014). Physiological changes also led to increased herbivory in some crops (DeLucia et al., 2012; Dermody et al., 2008). The effects of rising CO2 on crop yield are tempered by other global changes. Corresponding increases in ground ozone decreases productivity (Morgan et al., 2006), and increased drought may remove the positive effects of rising CO2 entirely (Gray et al., 2016). Carbon dioxide fertilization can have either direct or indirect consequences on agriculture. At higher levels of atmospheric warming and at low latitudes, model simulations show significant reductions in yields for all major crops, even with the positive benefits of CO2 fertilization (Challinor et al., 2014). Indirect effects of rising CO2 include the reduction in nutrient content and digestibility of pasture for livestock (Tubiello et al., 2007) and reductions in protein content by 10% to 14% in the edible portions of wheat, rice, barley, and potato and by 1.5% in soybeans (Müller et al., 2014; Taub et al., 2008).
Terrestrial food and fiber production over the next century may be more profoundly influenced by climate change than by rising CO2 itself. Climate changes could include heatwaves during growing seasons, droughts and lengthening of dry spells, and rising sea levels (Melillo et al., 2014; Nelson et al., 2014; Wiebe et al., 2015). The greater the greenhouse gas concentrations, the greater the change in the climate and climate-associated risks for agriculture and food security (Brown et al., 2015).
17.6.3 Carbon Storage in Vegetation and Soils
Vegetated coastal ecosystems store CO2 in seagrasses, marshes, kelp, and mangroves at rates comparable with those of forest ecosystems (McLeod et al., 2011). This “blue carbon” is believed to be an important sink for atmospheric CO2, but coastal habitats are under strong human-driven pressures worldwide including habitat destruction, rising ocean temperatures, sea level rise, and sediment starvation (see Ch. 15: Tidal Wetlands and Estuaries). For example, erosion of coastal wetlands or thawing of coastal Arctic permafrost exposes buried organic carbon, which can either be respired in situ to release CH4 or CO2, exacerbating atmospheric warming, or be released to nearshore waters and respired there, contributing to local acidification (Aufdenkampe et al., 2011; see Ch. 11: Arctic and Boreal Carbon). Seagrasses may help mitigate ocean acidification locally (Hendriks et al., 2014), underscoring the double benefit of protecting blue carbon habitats.
Carbon on land is stored in vegetation and soils. Forests account for approximately 66% of the land carbon sink (see Ch. 2: North American Carbon Budget and Ch. 9: Forests), a percentage which could increase if strategies were applied to minimize forest losses from deforestation. However, carbon sinks change with the age of forest regrowth—the rate of carbon accumulation is rapid in young forests but typically quite low in old-growth forests. Restoring the organic content of agricultural and natural soils also can increase soil carbon storage (Lal 2003). Historically, soils have lost vast amounts of carbon when transitioning from natural to human-modified landscapes (e.g., through urbanization and forest and agricultural management; see also Ch. 5: Agriculture and Ch. 12: Soils), but gauging the effect of land management on carbon storage is challenging. The land carbon sink is calculated using bookkeeping methods that sum together carbon into different respective ecosystem compartments (e.g., land, ocean, and atmosphere) at a variety of scales. The carbon sink is typically inferred by the existence of a residual (i.e., unaccounted) sink in the global carbon budget. Therefore, the effects of land management can be difficult to detect and attribute using carbon balance accounting methods (Erb et al., 2013).
17.6.4 Coastal Protection by Corals
In low-latitude areas around the world, coral reefs are particularly important for protecting coastlines, but the combined effects of rising temperature and ocean acidification slow the growth of stony coral reefs (Muehllehner et al., 2016; Wong et al., 2014), hindering their ability to grow or recover from damage (Hughes et al., 2017; Ainsworth et al., 2016; Hughes et al., 2018). Carbonate sediments also are being dissolved by ocean acidification, while sea level also rises; the net effect has accelerated the relative rate of sea level rise near Florida, Hawai‘i, and the U.S. Virgin Islands, exposing those coastal communities to heightened risk of flooding (Yates et al., 2017). Globally, the loss of the three-dimensional structure of the reef could expose 200 million people to greater effects of storms and tsunamis (Ferrario et al., 2014). People living in the low-elevation coastal zone (LECZ), below 10 m in elevation (Vafeidis et al., 2011), face a higher risk of coastal hazards such as flooding and sea level rise due to climate change (Lichter and Felsenstein 2012). In the United States, population in the LECZ is forecast to increase by 188% from 23 million in 2000 to 44 million in 2060 (Neumann et al., 2015), so losses of coral reefs that protect coastlines heighten overall coastal community risk.
17.6.5 Water Availability
Reduced transpiration due to increased plant water-use efficiency (Leakey et al., 2009; Norby and Zak 2011) may allow more water to pass through soils and enter freshwater ecosystems. As discussed in Ch. 13: Terrestrial Wetlands and Ch. 14: Inland Waters, inland waters act as hotspots for the degradation and outgassing of carbon originating from both terrestrial and aquatic sources. Increases in precipitation events, along with reductions in transpiration (Charney et al., 2016; van der Sleen et al., 2014), may facilitate the movement of materials from the landscape into water systems, altering ecosystem structure and function as seen extensively on Lake Erie (Smith et al., 2015). Conversely, the drying of systems that receive less precipitation will dramatically influence the timing of rainfed and snowmelt-driven ecosystems and municipalities reliant on surface waters for agriculture, fisheries, industry, and drinking water (Clow et al., 2010; Rao et al., 2004).
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