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   dcterms:identifier "key-message-19-2";
   gcis:findingNumber "19.2"^^xsd:string;
   gcis:findingStatement "Land, ocean, coastal, and freshwater systems are currently net sinks of carbon from the atmosphere, taking up more carbon annually than they release. However, emerging understanding suggests that the future carbon uptake capacity of these systems may decline, depending on different emissions scenarios, with some reservoirs switching from a net sink to a net source of carbon to the atmosphere (<em>high confidence</em>)."^^xsd:string;
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## Properties of the finding:
   gcis:descriptionOfEvidenceBase "<p>Most work examining future carbon cycle changes and potential feedbacks with climate and rising atmospheric carbon dioxide (CO<sub>2</sub>) has been conducted at the global scale as part of coupled carbon-climate model intercomparison efforts including the Coupled Model Intercomparison Project Phase 5 (CMIP5; Friedlingstein 2015; Friedlingstein et al., 2014). As a result, published estimates of projections specific to both the land carbon sink and coastal ocean carbon uptake in North America are lacking.</p><p>To provide an estimate of future land carbon sink evolution in North America, this chapter relied on the globally gridded net biome productivity simulated by nine CMIP5 models (Ciais et al., 2013; Friedlingstein 2015). With the exception of CESM1-BGC, which was not available on the CMIP5 data download page, the models and set of simulations used here (and in Figures 19.3 and 19.4) are the same as those used in Ch. 6 of the <em>Intergovernmental Panel on Climate Change Fifth Assessment Report</em> (IPCC; Table 6.11): CanESM2, GFDL-ESM2G, GFDL-ESM2M, HadGEM2–ES, IPSL–CM5A-LR, MIROC-ESM, MPI–ESM–LR, NorESM1–ME, and INMCM4. The simulation output was placed into a consistent 0.5° grid and trimmed to North America (10° to 70°N and 50° to 170°E). Projected land sink estimates were evaluated for all four of the Representative Concentration Pathways (RCPs; van Vuuren et al., 2011) used in the latest IPCC report:</p><ol><li><p><strong>RCP8.5 High Emissions Scenario.</strong> Projects increasing CO<sub>2</sub> and methane (CH<sub>4</sub>) emissions over time due to increased energy intensity as a result of high population growth and lower rates of technology development leading to radiative forcing of 8.5 watts per square meter (W/m) by 2100. This scenario assumes an increase in cropland and grassland area driven by the demands of population growth.</p></li><li><p><strong>RCP6.0 Stabilization Scenario.</strong> Projects a range of technologies and strategies to reduce CO<sub>2</sub> emissions after the year 2080, coupled with fairly steady CH<sub>4</sub> emissions throughout the century to stabilize radiative forcing at 6 W/m in 2100. This scenario assumes an increase in cropland area, but a decline in pasture area due to aggressive implementation of intensive animal husbandry.</p></li><li><p><strong>RCP4.5 Stabilization Scenario.</strong> Projects a range of technologies and strategies to reduce CO<sub>2</sub> emissions after 2040, coupled with fairly steady CH<sub>4</sub> emissions throughout the century to stabilize radiative forcing at 4.5 W/m in 2100. This scenario assumes a decrease in cropland and grassland area due to climate policies that value carbon in natural vegetation.</p></li><li><p><strong>RCP2.6 Low Emissions Scenario.</strong> Projects an increased use of bioenergy and carbon capture and storage, which leads to substantial reduction in CO<sub>2</sub> emissions after 2020. This reduction coupled with declining CH<sub>4</sub> emissions from energy production, transportation, and livestock leads to a peak in radiative forcing of 3 W/m, followed by a decline to 2.6 W/m by 2100. Cropland area increases, but largely as a result of bioenergy production. Grassland area remains relatively constant as the increase in animal production is offset by more intensive animal husbandry.</p></li></ol><p>For the North American coastal ocean, this report used three CMIP5 models (GFDL-ESM2M [Dunne et al., 2013], HadGEM-ESM [Martin et al., 2011], and MIROC-ESM [Watanabe et al., 2011]) to estimate a range of historical (1870 to 1995) and future carbon uptake within the exclusive economic zones (EEZs) of North America (approximately 22.5 × 10 km). Since 1870, North American EEZs have taken up 2.6 to 3.4 petagrams of carbon (Pg C). These regions are projected to take up an additional 10 to 12 Pg C by 2050 and another 17 to 26 Pg C in the second half of this century (2050 to 2100). Global projections of ocean carbon uptake vary depending on emissions scenarios (Ciais et al., 2013). Under lower future emissions scenarios (e.g., RCP2.6 and RCP4.5), the strength of the ocean carbon sink starts to level off toward the end of the century. For the North American Pacific Coast, the combined effect of multiple factors (e.g., increasing atmospheric CO<sub>2</sub>, surface warming, less vertical mixing with greater vertical stratification, and increases in horizontal temperature gradients) may lead to greater and more persistent CO<sub>2</sub> outgassing nearshore and lower productivity offshore (see <a href=''>Ch. 16: Coastal Ocean and Continental Shelves</a>).</p>"^^xsd:string;
   gcis:assessmentOfConfidenceBasedOnEvidence "Land, ocean, coastal, and freshwater systems are currently net sinks of carbon from the atmosphere. Although projections vary depending on future climate and carbon emissions scenarios, it is likely that under some future climate and CO<sub>2</sub> emissions scenarios these systems will turn from a net sink to a net source of carbon."^^xsd:string;
   gcis:newInformationAndRemainingUncertainties "The balance between positive and negative influences of climate and atmospheric CO<sub>2</sub> on the global carbon cycle is not well constrained in models (see Figure 19.5; Ciais et al., 2013; Graven 2016). Although models tend to agree on the direction of the carbon uptake response to both climate warming and rising CO<sub>2</sub>, they show low agreement on the magnitude (size) of this response (Ciais et al., 2013). In land carbon cycling, many current models do not consider nutrient cycle processes or the coupling of the nitrogen and carbon cycles (Ciais et al., 2013). In addition, model response to climate warming is highly uncertain. Climate warming could lead to an increase or decrease in carbon uptake, depending on a number of factors that will vary by region and the species present within a given ecosystem (Graven 2016). Major sources of uncertainty in models are projected changes in permafrost and soil carbon storage (see Section 19.7.2). Many models do not explicitly account for permafrost dynamics and include outdated representations of soil carbon turnover that are inconsistent with emerging scientific understanding (Bradford et al., 2016)."^^xsd:string;

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## This finding cites the following entities:

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