Biogeochemical Effects of Rising Atmospheric Carbon Dioxide

Climate and rising atmospheric CO₂ can alter the amount of carbon taken up or released by ecosystems and the ocean. Rising temperatures influence the response of the carbon cycle to rising CO₂ in diverse and complicated ways, yielding both positive and negative feedbacks (Deryng et al., 2016; Dieleman et al., 2012; Holding et al., 2015). Positive feedbacks tend to be additive of the original effect, negative feedbacks tend to counteract the original effect. Overall, rising temperatures tend to release more land and ocean carbon into the atmosphere, while rising CO₂ is projected to increase land and ocean uptake (Friedlingstein et al., 2006). However, the importance of this positive feedback is variable according to different locations and time frames. Earth System Model assessments that incorporate carbon cycle feedbacks to projected climate change show that the combined effects of climate change result in an overall larger increase in CO₂ concentrations, thus contributing to additional climate warming (a positive feedback). However, this feedback is highly uncertain due to its dependence on various factors, so different studies may report large ranges in predicted CO₂ concentrations (Blok et al., 2010; Elberling et al., 2013; Hodgkins et al., 2014; McCalley et al., 2014; Schneider von Deimling et al., 2012; Schuur et al., 2009). Temperature also indirectly influences radiative CO₂ effects. For example, increased evaporation from the ocean in a warmer world yields higher atmospheric water vapor concentrations that further amplify the impact of CO₂ on climate warming (Myhre et al., 2013). Another chapter in this report presents a broader discussion of the impacts of multiple environmental changes (see Ch. 19: Future of the North American Carbon Cycle).

On land, the direct effect of rising CO₂ on plant photosynthesis and growth interacts with rising temperature (Gray et al., 2016; Zhu et al., 2016). Rising CO₂ increases the photosynthetic temperature optimum (Long 1991) because of the decreasing relative solubility of CO₂ versus oxygen at higher temperatures (Jordan and Ogren 1984). While photosynthesis, respiration, and decomposition sensitivities to temperature act on short timescales of decades, chemical weathering sensitivities act over several hundred thousand years and are largely responsible for moderating CO₂ levels throughout the geological record. Rising temperatures affect biogeochemical processes through enhanced NPP, faster microbial decomposition of organic matter and increased emissions of CO₂ from microbial respiration in soils, and increased rates of chemical weathering (Galloway et al., 2014). However, interactions between rising CO₂ and temperature are complicated by nonuniform warming patterns, and research shows that climate warming can either stimulate or suppress plant productivity depending on the season and region (Xia et al., 2014). In the cryosphere, higher temperatures thaw permafrost and melt ice, processes which release stored CO₂ and CH₄ back into the atmosphere (Schneider von Deimling et al., 2012).

Chemical weathering of minerals, which consumes CO₂ from the atmosphere, provides an important feedback mechanism for CO₂ in the carbon cycle (Berner 1992; Colbourn et al., 2015; Kump et al., 2000; see Ch. 12: Soils). Carbon dioxide is found in soils and surficial deposits because of plant and microbial respiration as well as chemical weathering of minerals. Carbonic acid, which is formed naturally when CO₂ becomes dissolved into infiltrating rainwater, can dissolve primary minerals, a process that consumes CO₂. Also, CaCO₃ may precipitate in soils and surficial deposits if concentrations are high enough, a process that may be enhanced by low soil moisture and in semiarid and arid climates (Berner 1992). The rates of mineral reactions depend on several factors, including temperature, pressure, and mineral saturation state, all of which are influenced by climate. As temperatures rise, weathering rates of most minerals increase, leading to greater CO₂ consumption (Brady and Carroll 1994; Velbel 1993). Precipitation (e.g., rain and snowmelt) flushes solutes away, lowering the saturation state for primary minerals in solution, thereby promoting higher mineral weathering rates (Clow and Mast 2010; Kump et al., 2000). Thus, greater precipitation would lead to lower mineral saturation states, higher weathering rates, and greater CO₂ consumption (Clow and Mast 2010). These feedback mechanisms have the potential to help mitigate the effects of rising atmospheric CO₂ concentrations, but their effects will vary spatially and temporally in concert with changes in temperature and precipitation. For example, while the northeastern United States may see relatively strong increases in weathering rates because of increasing temperature and precipitation (IPCC 2013), the Southwest might experience more mixed impacts because of increasing temperature but decreasing precipitation (IPCC 2013).