finding 2.1 : key-finding-2-1

Human activities continue to significantly affect Earth’s climate by altering factors that change its radiative balance. These factors, known as radiative forcings, include changes in greenhouse gases, small airborne particles (aerosols), and the reflectivity of Earth’s surface. In the industrial era, human activities have been, and are increasingly, the dominant cause of climate warming. The increase in radiative forcing due to these activities has far exceeded the relatively small net increase due to natural factors, which include changes in energy from the sun and the cooling effect of volcanic eruptions. (Very high confidence)



This finding is from chapter 2 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: This key finding is consistent with that in the IPCC Fourth Assessment Report (AR4)c54b9473-cdc3-4f22-97a8-4df5253f9682 and Fifth Assessment Report (AR5);f03117be-ccfe-4f88-b70a-ffd4351b8190 namely, anthropogenic radiative forcing is positive (climate warming) and substantially larger than natural forcing from variations in solar input and volcanic emissions. Confidence in this finding has increased from AR4 to AR5, as anthropogenic GHG forcings have continued to increase, whereas solar forcing remains small and volcanic forcing near-zero over decadal time scales.

Description of evidence base: The Key Finding and supporting text summarizes extensive evidence documented in the climate science literature, including in previous national (NCA3)dd5b893d-4462-4bb3-9205-67b532919566 and internationalf03117be-ccfe-4f88-b70a-ffd4351b8190 assessments. The assertion that Earth’s climate is controlled by its radiative balance is a well-established physical property of the planet. Quantification of the changes in Earth’s radiative balance come from a combination of observations and calculations. Satellite data are used directly to observe changes in Earth’s outgoing visible and infrared radiation. Since 2002, observations of incoming sunlight include both total solar irradiance and solar spectral irradiance.f53dfa03-617e-46b5-9379-d41c3560117c Extensive in situ and remote sensing data are used to quantify atmospheric concentrations of radiative forcing agents (greenhouse gases [e.g., Ciais et al. 2013;7316c70f-6b67-4a68-a23a-c9d9f604c003 Le Quéré et al. 2016e10cd7fc-e821-474a-bbde-74426a52b1d0] and aerosols [e.g., Bond et al. 2013;c024a923-aedd-4e72-8555-3a37ccc41e14 Boucher et al. 2013;9e2542c2-865e-4863-98d1-242b11016592 Myhre et al. 2013;6c7c285c-8606-41fe-bf93-100d80f1d17a Jiao et al. 2014;492282f1-6e4b-4b21-af10-3035fe9c0b4e Tsigaridis et al. 2014;f7c1592e-bd28-4c85-8295-1dccf12194d3 Koffi et al. 201683d045d5-f415-41b8-857e-460edcdaaa3b]) and changes in land cover,4aef4bed-c87a-4b31-8ba5-8897bfa09084 bceda009-3449-4f65-8bd7-4036cef210eb d0ac221f-fcde-44a5-ac76-ffd20f8f2560 as well as the relevant properties of these agents (for example, aerosol microphysical and optical properties). Climate models are constrained by these observed concentrations and properties. Concentrations of long-lived greenhouse gases in particular are well-quantified with observations because of their relatively high spatial homogeneity. Climate model calculations of radiative forcing by greenhouse gases and aerosols are supported by observations of radiative fluxes from the surface, from airborne research platforms, and from satellites. Both direct observations and modeling studies show large, explosive eruptions affect climate parameters for years to decades.3c5a360c-5c2c-403a-8127-81e5055be81f 03b1fb81-03ea-4e6c-ac28-56b1bec6a3bd Over the industrial era, radiative forcing by volcanoes has been episodic and currently does not contribute significantly to forcing trends. Observations indicate a positive but small increase in solar input over the industrial era.6c7c285c-8606-41fe-bf93-100d80f1d17a 7b930f16-cff7-43de-81f9-ce5a2c39a33b fe286f33-366f-4f8c-b2c0-69805ae52c78 Relatively higher variations in solar input at shorter (UV) wavelengthsac23e428-6630-428f-a570-51e1f4d9e0fa may be leading to indirect changes in Earth’s radiative balance through their impact on ozone concentrations that are larger than the radiative impact of changes in total solar irradiance,819cc52a-0533-4a48-a341-a5fbca035f25 f53dfa03-617e-46b5-9379-d41c3560117c 79a55fb7-4d6c-4b6e-b46a-86c5aff48293 6296f916-3d89-440c-8c80-bd58c0020a84 162cf736-f352-4dfe-bd54-9f64f0a13b96 but these changes are also small in comparison to anthropogenic greenhouse gas and aerosol forcing.6c7c285c-8606-41fe-bf93-100d80f1d17a The finding of an increasingly strong positive forcing over the industrial era is supported by observed increases in atmospheric temperatures (see Ch. 1: Our Globally Changing Climate) and by observed increases in ocean temperatures (Ch. 1: Our Globally Changing Climate and Ch. 13: Ocean Changes). The attribution of climate change to human activities is supported by climate models, which are able to reproduce observed temperature trends when RF from human activities is included and considerably deviate from observed trends when only natural forcings are included (Ch. 3: Detection and Attribution, Figure 3.1).

New information and remaining uncertainties: The largest source of uncertainty in radiative forcing (both natural and anthropogenic) over the industrial era is quantifying forcing by aerosols. This finding is consistent across previous assessments (e.g., IPCC 2007;c54b9473-cdc3-4f22-97a8-4df5253f9682 IPCC 2013f03117be-ccfe-4f88-b70a-ffd4351b8190). The major uncertainties associated with aerosol forcing is discussed below in the Traceable Accounts for Key Finding 2.

Recent work has highlighted the potentially larger role of variations in UV solar irradiance, versus total solar irradiance, in solar forcing. However, this increase in solar forcing uncertainty is not sufficiently large to reduce confidence that anthropogenic activities dominate industrial-era forcing.

Assessment of confidence based on evidence: There is very high confidence that anthropogenic radiative forcing exceeds natural forcing over the industrial era based on quantitative assessments of known radiative forcing components. Assessments of the natural forcings of solar irradiance changes and volcanic activity show with very high confidence that both forcings are small over the industrial era relative to total anthropogenic forcing. Total anthropogenic forcing is assessed to have become larger and more positive during the industrial era, while natural forcings show no similar trend.

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