Anthropogenic aerosol forcing – insights from multiple estimates from aerosol-climate models with reduced complexity
<p>This study assesses the change in anthropogenic aerosol forcing from the mid-1970s to the mid-2000s. Both decades had similar global-mean anthropogenic aerosol optical depths but substantially different global distributions. For both years, we quantify (i) the forcing spread due to model-in...
| Main Authors: | , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-05-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/19/6821/2019/acp-19-6821-2019.pdf |
| Summary: | <p>This study assesses the change in anthropogenic aerosol forcing from the
mid-1970s to the mid-2000s. Both decades had similar global-mean
anthropogenic aerosol optical depths but substantially different global
distributions. For both years, we quantify (i) the forcing spread due to
model-internal variability and (ii) the forcing spread among models. Our
assessment is based on new ensembles of atmosphere-only simulations with five
state-of-the-art Earth system models. Four of these models will be used in
the sixth Coupled Model Intercomparison Project
(CMIP6; <span class="cit" id="xref_altparen.1"><a href="#bib1.bibx14">Eyring et al.</a>, <a href="#bib1.bibx14">2016</a></span>). Here, the complexity of the anthropogenic
aerosol has been reduced in the participating models. In all our simulations,
we prescribe the same patterns of the anthropogenic aerosol optical
properties and associated effects on the cloud droplet number concentration.
We calculate the instantaneous radiative forcing (RF) and the effective
radiative forcing (ERF). Their difference defines the net contribution from
rapid adjustments. Our simulations show a model spread in ERF from <span class="inline-formula">−0.4</span> to
<span class="inline-formula">−0.9</span> W m<span class="inline-formula"><sup>−2</sup></span>. The standard deviation in annual ERF is 0.3 W m<span class="inline-formula"><sup>−2</sup></span>, based
on 180 individual estimates from each participating model. This result
implies that identifying the model spread in ERF due to systematic
differences requires averaging over a sufficiently large number of years.
Moreover, we find almost identical ERFs for the mid-1970s and mid-2000s for
individual models, although there are major model differences in natural
aerosols and clouds. The model-ensemble mean ERF is <span class="inline-formula">−0.54</span> W m<span class="inline-formula"><sup>−2</sup></span> for the
pre-industrial era to the mid-1970s and <span class="inline-formula">−0.59</span> W m<span class="inline-formula"><sup>−2</sup></span> for the pre-industrial
era to the mid-2000s. Our result suggests that comparing ERF changes between two
observable periods rather than absolute magnitudes relative to a poorly
constrained pre-industrial state might provide a better test for a model's
ability to represent transient climate changes.</p> |
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| ISSN: | 1680-7316 1680-7324 |