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   dcterms:identifier "key-message-19-4";
   gcis:findingNumber "19.4"^^xsd:string;
   gcis:findingStatement "The enhanced carbon uptake capacity of ocean and terrestrial systems in response to rising atmospheric CO<sub>2</sub> will likely diminish in the future. In the ocean, warmer and more CO<sub>2</sub>-enriched waters are expected to take up less additional CO<sub>2</sub>. On land, forest maturation, nutrient limitations, and decreased carbon residence time in soils will likely constrain terrestrial ecosystem response to rising CO<sub>2</sub> (<em>high confidence</em>)."^^xsd:string;
   gcis:isFindingOf <>;
   gcis:isFindingOf <>;

## Properties of the finding:
   gcis:descriptionOfEvidenceBase "<p>Although models tend to agree on the direction of the carbon uptake response to rising CO<sub>2</sub>, they show low agreement on the magnitude (i.e., size) of this response, particularly for terrestrial ecosystems (see Figure 19.5). However, some factors potentially important for limiting the CO<sub>2</sub> fertilization response of terrestrial ecosystems are not currently represented in models, including 1) the age distribution of forest trees, 2) nutrient limitation, and 3) soil carbon turnover rates.</p><p><strong>Forest Age.</strong> Ecosystem CO<sub>2</sub> enrichment experiments in North American forests tend to show that, in the short term (e.g., up to 10 years), CO<sub>2</sub> fertilization increases forest production by 20% to 25% (McCarthy et al., 2010; Norby et al., 2010; Talhelm et al., 2014). However, most of these forest experiments were conducted in young forests that also were accumulating biomass under ambient CO<sub>2</sub> concentrations. The few experiments that have been conducted on individual trees in more mature forests tend to show little or no growth response (Bader et al., 2013; Klein et al., 2016).</p> <p><strong>Nutrient Limitation.</strong> Nutrients will likely constrain land carbon cycle response to rising CO<sub>2</sub> (e.g., Norby et al., 2010). Many current models do not consider nutrient cycle processes (Ciais et al., 2013; Hoffman et al., 2014), contributing substantial uncertainty to the overall accuracy of CO<sub>2</sub>–carbon cycle feedback estimates. Even models that do consider nutrient cycling exhibit substantial uncertainty in responses of terrestrial ecosystems to increased atmospheric CO<sub>2</sub> (Walker et al., 2015; Zaehle and Dalmonech 2011).</p> <p><strong>Soil Carbon Turnover Rates.</strong> Results from some studies suggest that soil carbon storage may increase with rising atmospheric CO<sub>2</sub> (e.g., Iversen et al., 2012), even if the latter does not lead to increased carbon storage in forest biomass. However, soil carbon input may change microbial decomposition rates and the rate of soil carbon turnover, leading to less overall soil carbon storage (Hungate et al., 2013; van Groenigen et al., 2014).</p> <p>In the ocean, warmer and more CO<sub>2</sub>-enriched waters are expected to take up less additional CO<sub>2</sub> and be less resistant to changes in pH (Ciais et al., 2013). Several studies (Gattuso et al., 2015; Randerson et al., 2015; Bopp et al., 2013; Doney et al., 2009) have investigated in detail the impacts of contrasting emissions scenarios on ocean dynamics and marine and coastal ecosystems, including the goods and services that they provide. Alongside changes in ocean dynamics and a slowing of the ocean sink, these studies also highlight the fact that phytoplankton and zooplankton populations are likely to shift toward groups that favor higher temperature, greater physical stratification, and elevated CO<strong><sub>2</sub></strong> conditions, both in terms of trait diversity within groups (e.g., Dutkiewicz et al., 2013) and in some groups being favored over others (e.g., slow growing, CO<sub>2</sub>-limited nitrogen fixers; Hutchins et al., 2007).</p>"^^xsd:string;
   gcis:assessmentOfConfidenceBasedOnEvidence "Models tend to agree on the direction of land and ocean carbon uptake response to rising CO<sub>2</sub>, but they show less agreement on the magnitude of this response. However, multiple points of evidence suggest that the strength of net carbon uptake in response to rising CO<sub>2</sub> will decrease into the future. "^^xsd:string;
   gcis:newInformationAndRemainingUncertainties "See previous section describing the evidence base."^^xsd:string;

   a gcis:Finding .

## This finding cites the following entities:

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