Aerosol radiative effects with MACv2

Abstract

Monthly global maps for aerosol properties of the Max Planck Aerosol Climatology version 2 (MACv2) are applied in an offline radiative transfer model to determine aerosol radiative effects. This model setup cannot address rapid adjustments by clouds, but current evidence suggests their contribution to be small when compared to the instantaneous radiative forcing. Global maps are presented to detail the regional and seasonal variability associated with (annual) global averages. Radiative effects caused by the aerosol presence (direct effects) and by aerosol modified clouds (indirect effects) are examined. Direct effects are determined for total aerosol, anthropogenic aerosol and extracted individual aerosol components. Indirect effects cover the impact of reduced cloud drop sizes by anthropogenic aerosol.Present-day global annual radiative effects for anthropogenic aerosol yield (1) a climate cooling of −1.0 W m −2 at the top of the atmosphere (TOA); (2) a surface net-flux reduction of −2.1 W m −2 ; and, by difference; (3) an atmospheric effect of +1.1 W m −2 . This atmospheric solar heating is almost entirely a direct effect. On a global basis, indirect effects (−0.65 W m −2 ) dominate direct effects (−0.35 W m −2 ) for the present-day climate response at the TOA, whereas the present-day surface radiative budget is more strongly reduced by direct effects (−1.45 W m −2 ) than by indirect effects (−0.65 W m −2 ). Natural aerosols are on average less absorbing and larger in size. However, their stronger solar TOA cooling efficiency is offset by a non-negligible infrared (IR) greenhouse warming efficiency. In the sum the global average annual direct forcing efficiencies (per unit AOD) for natural and anthropogenic aerosol are similar: −12 W m −2 per unit AOD for all-sky conditions and −24 W m −2 per unit AOD for clearsky conditions.The present-day direct TOA impact by all soot (BC) is +0.55 W m −2 , when globally and annually averaged. Be-tween +0.25 and +0.45 W m −2 of that can be attributed to anthropogenic sources, depending on assumptions for the preindustrial BC reference state. Similarly, the preindustrial fine-mode reference uncertainty has a strong influence not just on the direct effect but even more on the indirect effect. Present-day aerosol TOA forcing is estimated to stay within the −0.7 to −1.6 W m −2 range (with the best estimate at −1.0 W m −2 ).Calculations with scaled temporal changes to anthropogenic AOD from global modeling indicate that the global annual aerosol forcing has not changed much over the last decades, despite strong shifts in regional maxima for anthropogenic AOD. These regional shifts explain most solar insolation (brightening or dimming) trends that have been observed by ground-based radiation data.

Bounding Global Aerosol Radiative Forcing of Climate Change

“…But more recent studies challenged the need for scaling of models and the suitability of AERONET constraints, instead improving modeled BC by increasing the model horizontal resolution (X. Wang et al, ), reducing BC lifetime (Samset et al, ) to reduce overestimations of BC concentrations in remote areas (Kipling et al, ), or accounting for AERONET τ abs sampling errors (X. Wang et al, ) and possible high bias compared to in situ airborne absorption coefficients (Andrews et al, ). Compared to Kinne (), Bond et al () overestimated anthropogenic τ abs because they underestimated the contribution of mineral dust aerosol to τ abs and overestimated the anthropogenic fraction of BC aerosols. The revised calculation by Kinne () therefore motivates a downward revision of τ abs at 0.55 μm, with a range of 0.0025 to 0.005.…”

“…Compared to Kinne (), Bond et al () overestimated anthropogenic τ abs because they underestimated the contribution of mineral dust aerosol to τ abs and overestimated the anthropogenic fraction of BC aerosols. The revised calculation by Kinne () therefore motivates a downward revision of τ abs at 0.55 μm, with a range of 0.0025 to 0.005. Rapid adjustments due to anthropogenic absorption are discussed in section .…”

“…By determining the anthropogenic contribution to in the Monitoring Atmospheric Composition and Climate (MACC) Reanalysis (Benedetti et al, ); Bellouin, Quaas, et al () determine Δ τ a as 0.06. The Max Planck Institute Aerosol Climatology (MAC) (Kinne, ; Kinne et al, ) combines AERONET climatologies with aerosol properties from AeroCom models (Kinne et al, ). They report Δ τ a as 0.03, which is within the range of the GCM estimates.…”

“…The range in , 0.3 to 0.8, is taken from McCoy et al () and the range for Δ τ a is that spanned by Bellouin, Quaas, et al () and Kinne ().…”

“…Although the magnitude of calculated using this method is an underestimate (Penner et al, ), c N only depends on its spatial pattern. To obtain an uncertainty range in c N , alternative spatial distributions for are taken from McCoy et al () and for Δ τ a / τ a from Kinne (), yielding a range for c N of 0.19 to 0.29. The value of 0.1 used in Stevens () is therefore outside the 68% confidence interval obtained here, but he was likely referring to marine stratocumulus clouds, while the present range encompasses all liquid clouds.…”