Biogeochemical Effects of Rising Atmospheric Carbon Dioxide

Key Finding 2

While atmospheric CO₂ rises at approximately the same rate all over the globe, its non-climate effects on land vary depending on climate and dominant species. In terrestrial ecosystems, rising atmospheric CO₂ concentrations are expected to increase plant photosynthesis, growth, and water-use efficiency, though these effects are reduced when nutrients, drought, or other factors limit plant growth (very high confidence, very likely). Rising CO₂ would likely change carbon storage and influence terrestrial hydrology and biogeochemical cycling, but concomitant effects on vegetation composition and nutrient feedbacks are challenging to predict, making decadal forecasts uncertain.

Description of evidence base

Research definitively shows that the bodies of marine and terrestrial organisms have incorporated CO₂ released from the burning of fossil fuels, based on the change in isotope ratios within their biological material (Fraile et al., 2016; Hilton et al., 2006; Suess 1955).

On land, the historical record of the impact of rising CO₂ is more complex. Physiological theory suggests that, as CO₂ rises, photosynthesis should increase. Using preserved plant specimens, isotopomer analysis appears to support this physiological prediction (Ehlers et al., 2015), though this is a novel technique. The effects of rising CO₂ on tree biomass over multiple decades may be inferred from tree-ring records, but they provide mixed results (Andreu-Hayles et al., 2011; Cole et al., 2009; Knapp and Soulé 2011; Koutavas 2013). Studies from a wide range of forest types across broad geographic regions have observed changes in the ratio of the ¹³C isotope to the ¹²C isotope (δ¹³C), observations which imply trees have experienced increased water-use efficiency as CO₂ has risen over the last two centuries, but growth was not clearly stimulated by rising CO₂ (Peñuelas et al., 2011).

Rising CO₂ tends to make plants close their stomata and thus use water more efficiently. The primary enzyme responsible for CO₂ uptake, ribulose-1,5-bisphosphate carboxylase-oxygenase (RUBISCO), accounts for a substantial portion of every plant’s nitrogen requirement. As CO₂ rises, less RUBISCO is required for the same carbon gain, so plants become more efficient in nutrient use. These physiological effects play out differently in various types of plants and under diverse environmental conditions. Plants that lack a CO₂ concentration mechanism and pass a 3-carbon sugar molecule into the Benson-Calvin cycle (C₃ plants) are more likely to show an instantaneous photosynthetic response than plants with a CO₂ concentration mechanism like C₄ plants (that pass a 4-carbon sugar molecule to the Benson-Calvin cycle) or those that use crassulacean acid metabolism (CAM).

Twenty years of CO₂ enrichment experiments have shown that elevated CO₂ enhances photosynthetic carbon gain over the long term for certain ecosystem types but only over the short term for others (Leakey et al., 2009; Leuzinger et al., 2011; Norby and Zak 2011). Plant communities dominated by trees and grasses generally have shown greater stimulation of photosynthetic carbon uptake compared to that of legumes, shrubs, and nonleguminous C₃ crops (Ainsworth and Rogers 2007).

Net primary production (NPP) is calculated as either the balance between carbon gained through photosynthesis and lost through respiration or the sum of all growth over a year. NPP is enhanced by ~23% across a broad range of early successional forests in response to elevated CO₂ (Norby et al., 2005). These results are likely not indicative of all forests, and smaller responses have been observed in the limited number of studies carried out in old-growth temperate, boreal, and tropical forests (Hickler et al., 2008; Körner et al., 2005). Also clear is that the temporal pattern of NPP responses to elevated CO₂ differs among forests. For example, McCarthy et al. (2010) reported that NPP in coniferous forests was enhanced by 22% to 30% and sustained over 10 years of exposure to 550 parts per million (ppm) of CO₂. In contrast, Norby et al. (2010) found that NPP was significantly enhanced for 6 years in hardwood forest plots exposed to 550 ppm CO₂ (compared with plots under current ambient CO₂), after which time the enhancement of NPP under elevated CO₂ declined from 24% to 9%.

Plants balance carbon gain and water loss. Stomatal conductance is depressed at elevated CO₂, so plants may reduce water loss without reducing carbon gain. This physiological effect has been observed at the leaf and canopy scales (Keenan et al., 2013; Leakey et al., 2009; Peñuelas et al., 2011) and represents the major mechanism leading to observations of decreased canopy evapotranspiration under elevated CO₂. For the hydrological cycle, this mechanism results in increased soil moisture. Even plants with CO₂ concentration mechanisms (i.e., C₄ and CAM plants) may experience increased water-use efficiency without any direct stimulation in photosynthesis (Leakey et al., 2009). Under drought conditions, elevated CO₂ may not directly stimulate photosynthesis in C₄ plants but can indirectly increase carbon gain by increasing water-use efficiency.

Physiological theory and experimental evidence indicate that 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). These results imply that biomes that experience high temperatures may experience disproportionately enhanced photosynthesis and growth. Interannual variation in the increased growth of Lobolly pine trees was disproportionately enhanced by experimentally elevated CO₂ in warmer years (Moore et al., 2006).

Plant growth is not limited by CO₂ alone (Körner 2015). If, for example, another environmental factor limits growth, then experimentally increasing CO₂ has reduced effects on photosynthesis and growth (Ainsworth and Rogers 2007). This outcome is called “sink limitation.” Research suggests that nitrogen limitation may be one mechanism leading to declining NPP responses to elevated CO₂ in some ecosystems (Norby et al., 2010).

Nitrogen is sequestered in long-lived biomass and soil pools and may not be readily available to plants under some conditions. In this case, nitrogen limitation inhibits increases in plant production associated with elevated CO₂, an effect which is referred to as a negative feedback. In systems where nitrogen supply was sufficient, CO₂ fertilization effects on NPP persisted (Drake et al., 2011; Finzi et al., 2006). Nevertheless, elevated CO₂ also increases photosynthetic nitrogen-use efficiency, defined as the net amount of CO₂ assimilated per unit of leaf nitrogen (Ainsworth and Rogers 2007; Bader et al., 2010; Leakey et al., 2009).

Elevated atmospheric CO₂ experiments 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 this was not observed in tree species (Way et al., 2010). Species show different growth responses to rising CO₂ (Dawes et al., 2011), and dominant plants may have an advantage with rising CO₂ (McDonald et al., 2002; Moore et al., 2006), leading to changes in forest structure.

Major uncertainties

Unclear is whether rising CO₂ will lead to larger standing biomass and carbon storage or simply faster cycling of carbon (Norby and Zak 2011). While instantaneous and annual fluxes of carbon are well studied in the Free-Air CO₂ Enrichment (FACE) literature, the allocation of carbon to different pools varies between experiments (DeLucia et al., 2005), and enhancement of multidecadal carbon stocks (e.g., woody biomass and soil organic matter) is not well studied (Leuzinger and Hattenschwiler 2013; Norby and Zak 2011). Plant growth is increased by CO₂, but gross plant respiration is also stimulated (Leakey et al., 2009). Root growth and the incorporation of organic material below ground are observed in response to elevated CO₂ but so too is enhanced soil respiration fueled by releases of carbon from root systems (Drake et al., 2011; Hoosbeek et al., 2007; Jackson et al., 2009; Lagomarsino et al., 2013; Selsted et al., 2012). Increased carbon supply from plants can lead to enhanced activity of soil fauna and more rapid cycling of carbon, rather than increased carbon storage in soils (Phillips et al., 2012; van Groenigen et al., 2011, 2014). Observed changes in soil carbon were small over the timescale of the FACE studies (3 to 16 years), and thus firm conclusions remain elusive (Luo et al., 2011). In general, large effects of rising CO₂ on carbon storage in soils are not expected (Schlesinger and Lichter 2001).

The long-term effects of rising CO₂ are uncertain because there is only one whole-ecosystem study (i.e., of a salt marsh) that extends to 20 years. Instantaneous physiological responses to CO₂ (Farquhar et al., 1980) typically are modified by feedbacks in system-level studies (Leakey et al., 2009; Norby and Zak 2011). Long-term records from tree-ring analyses are limited to reconstructions of aboveground growth. These studies rarely account for changes in carbon allocation strategies (DeLucia et al., 2005; Norby et al., 2010) caused by rising CO₂ or changes in nutrient limitation (Finzi et al., 2006; McCarthy et al., 2010; Zhu et al., 2016) or belowground carbon storage (Drake et al., 2011; Phillips et al., 2012; van Groenigen et al., 2014).

Summary sentence or paragraph that integrates the above information

While CO₂ is rising globally, there is high confidence that its effects on terrestrial ecosystems will vary across spatial scales because the effects of CO₂ on plants vary by species and may be altered by nutrient and water availability. The long-term impacts of rising CO₂ on carbon storage in terrestrial ecosystems are uncertain.

Key Finding 3

Consequences of rising atmospheric CO₂ are expected to include difficult-to-predict changes in the ecosystem services that terrestrial and oceanic systems provide to humans. For instance, ocean acidification resulting from rising CO₂ has decreased the supply of larvae that sustains commercial shellfish production in the northwestern United States. In addition, CO₂ fertilization (increases) plus warming (decreases) are changing terrestrial crop yields (high confidence, likely).

Description of evidence base

Commercial oyster larvae in the U.S. Pacific Northwest were significantly damaged by ocean acidification, which caused much higher than usual larval mortality for several years in the mid-2000s (Barton et al., 2015). Harmful impacts on oysters by ocean acidification were well documented (e.g., Kroeker et al., 2013, and references therein). Crop production increased in response to experimentally elevated CO₂ (Leakey et al., 2009), accompanied by decreases in seed quality. Decreased protein content has been documented in wheat, barley, rice, potatoes, and soybeans grown at high CO₂ (Myers et al., 2014; Taub et al., 2008). Physiological changes also led to increased herbivory in some crops (DeLucia et al., 2012; Dermody et al., 2008). Additional effects are expected for human populations via changes in ocean services, as reviewed in Pörtner et al. (2014). Gattuso et al. (2015) completed a literature review, plus expert judgement assessment, to determine the risk that ocean ecosystem services face from the combined effects of ocean acidification and warming.

Major uncertainties

Uncertainty is related to how rising CO₂ may have affected an array of marine and terrestrial harvests and how they may be affected in the future. Evaluating ecosystem services is difficult, and forecasting changes to these services is even more challenging.

Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement

Very high confidence in the existence and attribution of impacts to increased atmospheric CO₂; medium confidence about future projected impacts on ecosystem services.

Estimated likelihood of impact or consequence, including short description of basis of estimate

Studies have already documented impacts to marine and terrestrial harvests. Whether rising CO₂ will affect all marine and terrestrial harvests is uncertain.

Summary sentence or paragraph that integrates the above information

Rising CO₂ has affected commercial shellfish stocks (very high confidence) and changed crop production yields (very high confidence). Additional consequences expected for human populations include more changes to ecosystem services or changes to benefits that terrestrial and oceanic systems provide to humans (medium confidence). Uncertainty centers around the difficulty of evaluating all exploited species and all ecosystem services and projecting potential future impacts on all of them.

Key Finding 4

Continued persistence of uptake of carbon by the land and ocean is uncertain. Climate and environmental changes create complex feedbacks to the carbon cycle; how these feedbacks modulate future effects of rising CO₂ on carbon sinks is unclear. There are several mechanisms that would reduce the ability of land and ocean sinks to continue taking up a large proportion of rising CO₂ (very high confidence).

Description of evidence base

Acidification varies depending on latitude because CO₂ solubility depends on temperature, with lower-temperature waters holding more CO₂. Polar ecosystems may become undersaturated with calcium carbonate (Ca₃O^2–) minerals in the near future (Orr et al., 2005; Steinacher et al., 2010) because of the large amount of CO₂ already dissolved in cold high-latitude ocean areas. Even though low-latitude ocean areas will not become corrosive to Ca₃O^2– minerals in the future, conditions will soon surpass the bounds of natural variability (see Figure 17.4). In some places, conditions have already done so (Sutton et al., 2016), exposing low-latitude organisms, such as warm-water coral reefs, to chemical conditions that are considered suboptimal in regard to growth and calcification (Fabricius et al., 2011).

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). Although the sensitivities of photosynthesis, respiration, and decomposition 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. Higher temperatures affect biogeochemical processes through 1) enhanced NPP; 2) faster microbial decomposition of organic matter involving increased emissions of CO₂ from microbial respiration in soils; and 3) increased rates of chemical weathering, which consumes CO₂ from the atmosphere (Galloway et al., 2014). However, interactions between rising CO₂ and temperatures are complicated by nonuniform climate warming patterns, and research shows that this warming can either stimulate or suppress productivity depending on the season and region (Xia et al., 2014). Higher temperatures and drought have been implicated in widespread tree mortality (Breshears et al., 2009; Allen et al., 2010, 2015), and increased aridity in recent years has had a substantially negative effect on forest growth (Allen et al., 2015); these effects are expected to continue (Ficklin and Novick 2017). While some amelioration of physiological stress might be caused by rising CO₂ (Ainsworth and Rogers 2007; Blum 2009; Morison et al., 2008), extreme droughts may reduce or eliminate these benefits (Gray et al., 2016). There are very few experiments on tree mortality, but no evidence was found that elevated CO₂ reduced drought mortality (Duan et al., 2014).

In the ocean, higher temperatures affect the carbon cycle by decreasing CO₂ solubility in seawater (Zeebe and Wolf-Gladrow 2001); a warmer ocean will hold less carbon. Also, increased surface ocean stratification from the warmer water will prevent CO₂ absorbed by the surface ocean from penetrating into deeper water masses by reducing deep mixing, thereby decreasing overall oceanic carbon uptake and storage (IPCC 2013). In the cryosphere, higher temperatures thaw permafrost and melt ice, processes which release CO₂ and methane (CH₄) from microbial respiration back into the atmosphere (Schneider von Deimling et al., 2012).

Rising temperatures thus influence the response of the carbon cycle to rising CO₂ in diverse and complicated ways, yielding both positive and negative feedbacks to atmospheric CO₂ (Deryng et al., 2016; Dieleman et al., 2012; Holding et al., 2015). Overall, higher temperatures tend to release land and ocean carbon into the atmosphere, while rising CO₂ is projected to increase land and ocean uptake (Friedlingstein et al., 2006), but magnitudes are variable and uncertain. Earth System Model assessments that include carbon cycle feedbacks to climate change show that the combined effects of environmental change yield an overall increase in CO₂ concentrations and thus would likely contribute to more climate warming. The multimodel average CO₂ concentration in 2100 is 985 ± 97 ppm, compared to a concentration of 936 ppm in models lacking carbon cycle feedbacks (Collins et al., 2013). This feedback is highly uncertain because of its dependence on a variety of factors, and thus studies arrive at large ranges in responses (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 CO₂ radiative effects. For example, enhanced 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).

Major uncertainties

The source or sink status of coastal zones has been difficult to determine, but evidence points to weakening CO₂ release from low-latitude coastal zones and strengthening CO₂ uptake from mid- and high-latitude systems, leading to greater release of dissolved inorganic carbon to the ocean (Cai 2011).

The effect of rising CO₂ on succession and biodiversity remains poorly understood and quantified and could result in changed 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.

Disentangling the impacts of rising CO₂ and other concurrent changes in climate, land use, nutrient cycles, and atmospheric chemistry across all ecosystems probably will require 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, and the balance of conclusions from observational studies is not settled.

Summary sentence or paragraph that integrates the above information

Both oceanic and terrestrial ecosystems are influenced by CO₂ and a variety of environmental controls, including temperature. The effects of climate and CO₂ are likely to interact with each other (i.e., the effect of changing CO₂ depends on the climatic conditions). These interactions likely will cause complex feedbacks to climate.