Executive Summary

Changes to the carbon cycle can affect North Americans in a wide variety of ways. For example, the ocean provides multiple benefits or “services,” including the provision of fish, carbon storage, coastal protection by reefs, and climate modulation. These services face significant risks from the combined effects of ocean acidification, warming ocean waters, and sea level rise (see Ch. 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide). Rising atmospheric CO₂ has decreased seawater pH, leading to ocean acidification as evidenced from measurements at long-term observing stations around North America (see Ch. 16: Coastal Ocean and Continental Shelves and Ch. 17). This decrease in pH, mainly due to oceanic uptake of CO₂, also is affected by other factors including circulation and eutrophication (i.e., nutrient enrichment of water that can lead to increased primary production and, subsequently, poorer water quality). Ocean acidification also enhances corrosive conditions and can inhibit the formation of calcium carbonate shells essential to marine life. Compared to many other coastal waters, Arctic and North Pacific coastal waters are already more acidic, and therefore small changes in pH due to CO₂ uptake have affected marine life in these waters more significantly (see Ch. 16). In addition to impacts on marine species, ocean acidification has altered fundamental ecosystem processes, with further effects likely in the future.

In terrestrial ecosystems, rising atmospheric CO₂ enhances photosynthesis and growth and increases water-use efficiency (see Ch. 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide). These carbon cycle–induced increases in plant growth and efficiency are referred to as “CO₂ fertilization.” For example, crops exposed to higher atmospheric CO₂ often show increased yield. However, the CO₂ fertilization effect is not observed consistently in all ecosystems because of nutrient limitations or other factors. Furthermore, CO₂ fertilization typically is associated with increased leaf fall and root production, which can enhance microbial decomposition of organic materials in soils, thereby increasing net CO₂ emissions to the atmosphere (see Ch. 12: Soils). All these changes have altered and will continue to alter vegetation composition (e.g., species distribution, biodiversity, and invasive species), carbon distribution and storage, terrestrial hydrology, and other ecosystem properties. Current and future changes to climate that are driven by altered carbon cycling also will affect ecosystems and their services, as well as interact with effects such as ocean acidification and CO₂ fertilization.

Overall, alterations to the North American carbon cycle will continue to affect the benefits that terrestrial and ocean systems provide to humans. The effects of rising atmospheric CO₂ concentrations interact with climate, sea level rise, and other global changes as described in SOCCR2 companion reports such as the Third National Climate Assessment (Melillo et al., 2014) and Climate Science Special Report (USGCRP 2017). For example, the frequency and intensity of disturbances such as fire, insect and pathogen outbreaks, storms, and heatwaves are expected to increase with higher temperatures and climate variability. Moreover, ecosystem responses to and interactions with such effects are often unpredictable and depend on ecosystem type, disturbance frequency, and magnitude of events (see Ch. 17).