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@prefix dcterms: <http://purl.org/dc/terms/> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
@prefix gcis: <http://data.globalchange.gov/gcis.owl#> .
@prefix cito: <http://purl.org/spar/cito/> .
@prefix biro: <http://purl.org/spar/biro/> .

<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
   dcterms:identifier "key-finding-2-2";
   gcis:findingNumber "2.2"^^xsd:string;
   gcis:findingStatement "Aerosols caused by human activity play a profound and complex role in the climate system through radiative effects in the atmosphere and on snow and ice surfaces and through effects on cloud formation and properties. The combined forcing of aerosol–radiation and aerosol–cloud interactions is negative (cooling) over the industrial era (<em>high confidence</em>), offsetting a substantial part of greenhouse gas forcing, which is currently the predominant human contribution. The magnitude of this offset, globally averaged, has declined in recent decades, despite increasing trends in aerosol emissions or abundances in some regions. (<em>Medium to high confidence</em>)"^^xsd:string;
   gcis:isFindingOf <https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis>;
   gcis:isFindingOf <https://data.globalchange.gov/report/climate-science-special-report>;

## Properties of the finding:
   gcis:findingProcess "This key finding is consistent with the findings of IPCC AR5 that aerosols constitute a negative radiative forcing. While significant uncertainty remains in the quantification of aerosol ERF, we assess with <em>high confidence</em> that aerosols offset about half of the positive forcing by anthropogenic CO<sub>2</sub> and about a third of the forcing by all well-mixed anthropogenic GHGs. The fraction of GHG forcing that is offset by aerosols has been decreasing over recent decades, as aerosol forcing has leveled off while GHG forcing continues to increase."^^xsd:string;
   
   gcis:descriptionOfEvidenceBase "The Key Finding and supporting text summarize extensive evidence documented in the climate science literature, including in previous national (NCA3) and international assessments. Aerosols affect Earth’s albedo by directly interacting with solar radiation (scattering and absorbing sunlight) and by affecting cloud properties (albedo and lifetime). <br><br> Fundamental physical principles show how atmospheric aerosols scatter and absorb sunlight (aerosol–radiation interaction), and thereby directly reduce incoming solar radiation reaching the surface. Extensive in situ and remote sensing data are used to measure emission of aerosols and aerosol precursors from specific source types, the concentrations of aerosols in the atmosphere, aerosol microphysical and optical properties, and, via remote sensing, their direct impacts on radiative fluxes. Atmospheric models used to calculate aerosol forcings are constrained by these observations (see Key Finding 1). <br><br> In addition to their direct impact on radiative fluxes, aerosols also act as cloud condensation nuclei. Aerosol–cloud interactions are more complex, with a strong theoretical basis supported by observational evidence. Multiple observational and modeling studies have concluded that increasing the number of aerosols in the atmosphere increases cloud albedo and lifetime, adding to the negative forcing (aerosol–cloud microphysical interactions) (e.g., Twohy 2005; Lohmann and Feichter 2005; Quaas et al. 2009; Rosenfeld et al. 2014). Particles that absorb sunlight increase atmospheric heating; if they are sufficiently absorbing, the net effect of scattering plus absorption is a positive radiative forcing. Only a few source types (for example, from diesel engines) produce aerosols that are sufficiently absorbing that they have a positive radiative forcing. Modeling studies, combined with observational inputs, have investigated the thermodynamic response to aerosol absorption in the atmosphere. Averaging over aerosol locations relative to the clouds and other factors, the resulting changes in cloud properties represent a negative forcing, offsetting approximately 15% of the positive radiative forcing from heating by absorbing aerosols (specifically, black carbon). <br><br> Modeling and observational evidence both show that annually averaged global aerosol ERF increased until the 1980s and since then has flattened or slightly declined, driven by the introduction of stronger air quality regulations (Smith and Bond 2014; Fiore et al. 2015). In one recent study, global mean aerosol RF has become less negative since IPCC AR5, due to a combination of declining sulfur dioxide emissions (which produce negative RF) and increasing black carbon emissions (which produce positive RF). Within these global trends there are significant regional variations (e.g., Mao et al. 2014), driven by both changes in aerosol abundance and changes in the relative contributions of primarily light-scattering and light-absorbing aerosols. In Europe and North America, aerosol ERF has significantly declined (become less negative) since the 1980s. In contrast, observations show significant increases in aerosol abundances over India, and these increases are expected to continue into the near future. Several modeling and observational studies point to aerosol ERF for China peaking around 1990, though in some regions of China aerosol abundances and ERF have continued to increase. The suite of scenarios used for future climate projection (i.e., the scenarios shown in Ch. 1: Our Globally Changing Climate, Figure 1.4) includes emissions for aerosols and aerosol precursors. Across this range of scenarios, globally averaged ERF of aerosols is expected to decline (become less negative) in the coming decades, reducing the current aerosol offset to the increasing RF from GHGs."^^xsd:string;
   
   gcis:assessmentOfConfidenceBasedOnEvidence "There is <em>very high confidence</em> that aerosol radiative forcing is negative on a global, annually averaged basis, <em>medium confidence</em> in the magnitude of the aerosol RF, <em>high</em> confidence that aerosol ERF is also, on average, negative, and <em>low to medium confidence</em> in the magnitude of aerosol ERF. Lower confidence in the magnitude of aerosol ERF is due to large uncertainties in the effects of aerosols on clouds. Combined, we assess a <em>high level of confidence</em> that aerosol ERF is negative and sufficiently large to be substantially offsetting positive GHG forcing. Improvements in the quantification of emissions, in observations (from both surface-based networks and satellites), and in modeling capability give <em>medium</em> to <em>high confidence</em> in the finding that aerosol forcing trends are decreasing in recent decades."^^xsd:string;
   
   gcis:newInformationAndRemainingUncertainties "Aerosol–cloud interactions are the largest source of uncertainty in both aerosol and total anthropogenic radiative forcing. These include the microphysical effects of aerosols on clouds and changes in clouds that result from the rapid response to absorption of sunlight by aerosols. This finding, consistent across previous assessments (e.g., Forster et al. 2007; Myhre et al. 2013), is due to poor understanding of how both natural and anthropogenic aerosol emissions have changed and how changing aerosol concentrations and composition affect cloud properties (albedo and lifetime). From a theoretical standpoint, aerosol–cloud interactions are complex, and using observations to isolate the effects of aerosols on clouds is complicated by the fact that other factors (for example, the thermodynamic state of the atmosphere) also strongly influence cloud properties. Further, changes in aerosol properties and the atmospheric thermodynamic state are often correlated and interact in non-linear ways."^^xsd:string;

   a gcis:Finding .

## This finding cites the following entities:


<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
   cito:cites <https://data.globalchange.gov/article/10.1002/2014GL060349>;
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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   biro:references <https://data.globalchange.gov/reference/27a3f365-968e-437c-b605-22ce1bddc2c5>.

<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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<https://data.globalchange.gov/report/climate-science-special-report/chapter/scientific-basis/finding/key-finding-2-2>
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