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Radiative forcing by carbon dioxide depends on the difference between surface and stratospheric temperature scaled by the logarithm of its concentration (Wilson and Gea-Banacloche 2012; Jeevanjee et al. 2021). This relationship arises due to the cooling-to-space theory or theτ= 1 law, where all emission of infrared radiation originates from the atmospheric pressure level where the gas reaches sufficient optical thickness (in the case of CO2, in the stratosphere). Here we develop theoretical understanding of forcing by other well-mixed greenhouse gases including methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs). Radiative forcing by an optically thin absorber (e.g., CFC-12) is governed by emission throughout the troposphere and scaled by the total change in gas concentration, such that a linear increase in gas abundance yields a linear increase in forcing. We examine the factors that control the magnitude of radiative forcing, demonstrating analytically that CFC-12 is a stronger per-molecule absorber than CO2due to its larger average cross-section, rather than its band width or spectral position. Application in idealized atmospheres with simplified lapse rates illustrates how radiative forcing by optically thin gases depends almost linearly on lapse rate. Finally, gases that are both optically thin and optically thick across their absorption spectrum, such as N2O and CH4, can be understood as a combination of the two regimes, yielding a super-logarithmic relationship to concentration. Our theory is in excellent agreement with full-physics line-by-line calculations in atmospheres with and without spectral overlap by water vapor
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Drivers of Global Clear Sky Surface Downwelling Longwave Irradiance Trends From 1984 to 2017
Abstract Radiation changes at the Earth's surface alter climate, however, the causes of observed surface radiation changes are not precisely quantified globally. With complete global coverage by ERA‐Interim, the drivers of the clear sky surface downwelling longwave irradiance (SDLI) trends from 1984 to 2017 are quantifiable everywhere. Trends in atmospheric temperature and water vapor contributed significantly (∼90%) to clear sky SDLI trends, including trends consistent with Arctic warming and Southern Ocean cooling. CO2contributed ∼10% and other greenhouse gases (CH4, N2O, CFC‐11, and CFC‐12) ∼1% to the SDLI trends. These observation‐based results are consistent with early CO2‐doubling climate model calculations wherein temperature and water vapor changes drove ∼90% of the SDLI change. The well‐mixed greenhouse gases drive location‐dependent SDLI trends that are strongest over regions with climatologically high temperatures and low water vapor amounts.
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- Award ID(s):
- 1822015
- PAR ID:
- 10366698
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 48
- Issue:
- 22
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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