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Abstract. For the radiative impact of individual climate forcings,most previous studies focused on the global mean values at the top of theatmosphere (TOA), and less attention has been paid to surface processes,especially for black carbon (BC) aerosols. In this study, the surface radiativeresponses to five different forcing agents were analyzed by using idealizedmodel simulations. Our analyses reveal that for greenhouse gases, solarirradiance, and scattering aerosols, the surface temperature changes aremainly dictated by the changes of surface radiative heating, but for BC,surface energy redistribution between different components plays a morecrucial role. Globally, when a unit BC forcing is imposed at TOA, the netshortwave radiation at the surface decreases by -5.87±0.67 W m−2 (W m−2)−1 (averaged over global land without Antarctica), which ispartially offset by increased downward longwave radiation (2.32±0.38 W m−2 (W m−2)−1 from the warmer atmosphere, causing a netdecrease in the incoming downward surface radiation of -3.56±0.60 W m−2 (W m−2)−1. Despite a reduction in the downward radiationenergy, the surface air temperature still increases by 0.25±0.08 Kbecause of less efficient energy dissipation, manifested by reduced surfacesensible (-2.88±0.43 W m−2 (W m−2)−1) and latent heat flux(-1.54±0.27 W m−2 (W m−2)−1), as well as a decrease inBowen ratio (-0.20±0.07 (W m−2)−1). Such reductions of turbulentfluxes can be largely explained by enhanced air stability (0.07±0.02 K (W m−2)−1), measured as the difference of the potential temperaturebetween 925 hPa and surface, and reduced surface wind speed (-0.05±0.01 m s−1 (W m−2)−1). The enhanced stability is due to the fasteratmospheric warming relative to the surface, whereas the reduced wind speedcan be partially explained by enhanced stability and reduced Equator-to-poleatmospheric temperature gradient. These rapid adjustments under BC forcingoccur in the lower atmosphere and propagate downward to influence thesurface energy redistribution and thus surface temperature response, whichis not observed under greenhouse gases or scattering aerosols. Our studyprovides new insights into the impact of absorbing aerosols on surfaceenergy balance and surface temperature response.
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Global Surface Temperature Response to 11-Yr Solar Cycle Forcing Consistent with General Circulation Model Results
ABSTRACT The 11-yr solar cycle is associated with a roughly 1 W m−2trough-to-peak variation in total solar irradiance and is expected to produce a global temperature response. The sensitivity of this response is, however, contentious. Empirical best estimates of global surface temperature sensitivity to solar forcing range from 0.08 to 0.18 K (W m−2)−1. In comparison, best estimates from general circulation models forced by solar variability range between 0.03 and 0.07 K (W m−2)−1, prompting speculation that physical mechanisms not included in general circulation models may amplify responses to solar variability. Using a lagged multiple linear regression method, we find a sensitivity of global-average surface temperature ranging between 0.02 and 0.09 K (W m−2)−1, depending on which predictor and temperature datasets are used. On the basis of likelihood maximization, we give a best estimate of the sensitivity to solar variability of 0.05 K (W m−2)−1(0.03–0.09 K; 95% confidence interval). Furthermore, through updating a widely used compositing approach to incorporate recent observations, we revise prior global temperature sensitivity best estimates of 0.12–0.18 K (W m−2)−1downward to 0.07–0.10 K (W m−2)−1. The finding of a most likely global temperature response of 0.05 K (W m−2)−1supports a relatively modest role for solar cycle variability in driving global surface temperature variations over the twentieth century and removes the need to invoke processes that amplify the response relative to that exhibited in general circulation models.
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- PAR ID:
- 10523680
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 34
- Issue:
- 8
- ISSN:
- 0894-8755
- Format(s):
- Medium: X Size: p. 2893-2903
- Size(s):
- p. 2893-2903
- Sponsoring Org:
- National Science Foundation
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