I had in mind an article that would start with some background on aerosols in the atmosphere natural sources, human sources, size distribution, nice electron micrographs of pollen, etc., and
then move on to the global observations, and some discussion, at an undergraduate level, of why
it's been so tough to nail down the magnitude of the aerosol cooling, and how important that is
for prediction climate sensitivity. Goals:
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Resource should be accessible to the general public with some college
Articles should be engaging, with a noticeable point of view, and a clear
purpose (start with questions)
Articles should focus on active research fields
Alignment with climate literacy goals of US & UK (& Europe? Australia?)
Endorsement and cross-linkage from AMS, RMS, EGU, AGU, AGS…?
Suggested title (from Editor but we should consider to change it):
The Aerosol Cooling Effect and the Problem of Climate Sensitivity
Article Preview /”Teaser” (max 165 words)
A major cause of the uncertainty in the human influence on climate is due to limited understanding
of the climate effect of atmospheric aerosols. Aerosols in the atmosphere have a complex and highly
variable chemical composition and sizes which arise from a variety of anthropogenic and natural
emissions. The aerosols influence the climate through a mixture of effects which are highly
dependent on the aerosol chemical composition and size. We describe the complexity associated
with aerosols from the emissions to the various climate effects, and illustrate the importance of the
climate effect of aerosols in the relation to the climate sensitivity. Despite still large uncertainty
associated with aerosols we describe recent progress in the understanding and further insight into
the complexity.
What is the source of the aerosols? (about 200-300 words)
Aerosols are suspension of liquid or solid particles in the atmosphere with a highly variable chemical
composition and size.
Emissions: natural mineral dust, sea salt, biomass burning , volcanic: Anthropogenic, so2, bc, oc,
nitrat, soa
Size
Table or figure
-discussion of chemical composition
-size distribution diameter (1 nm to 100 um)
-mixing and aging
Figure 1 b) Electron micrographs of particles from the marine atmosphere. (a) Seasalt particle that consists of a NaCl crystal, mixed-cation sulfate, and filamentous
organic material, presumably of primary marine origin (marked by arrows). (b) A
sulfate particle from the North Atlantic, with a carbonaceous filament. The mottled
texture results from decomposition of ammonium sulfate in the electron beam.
How is the distribution of aerosols?
Figure 2: MODIS aerosol optical depth (AOD) (column integrated aerosol extinction, where extinction
is the attenuation of solar light given in terms of length (m-1)) averaged over a 10 year period 20012010 (Remer et al., 2008).The contribution from different aerosol types to the total AOD in selected
regions as indicated oh the figure is estimated by use of a global aerosol model (Myhre et al., 2009)
and shown as pie charts. The aerosol types are Sul (sulphate), BC and OC from use of fossil fuel, Bio
(OC and BC from biomass burning), Nitrate, Sea (sea salt), Min (mineral dust). Water uptake of the
aerosols are taken into account and enhances the importance of some of the aerosol types
substantially e.g. sulphate aerosols.
Figure 2 shows the aerosol optical depth (AOD) from remote sensing from space by the MODIS
instrument The figure shows a highly inhomogeneous pattern in the AOD, with largest values in the
tropical regions in Africa and Asia. The pie charts show the contributions from different aerosol types
in selected regions. Although there are some similarities in the industrial regions, there is large
variability in the global aerosol composition.
The different aerosol species have varying impacts in the atmosphere, and the wide spread in both
sources and sizes further complicates a quantification of their effects. Remote sensing, both from
space and the surface, have substantially advanced the understanding of the total AOD, but there are
still large uncertainties in the chemical composition and the anthropogenic contribution to the AOD.
How do aerosols affect the climate?
All atmospheric aerosols scatter incoming solar radiation, and a few aerosol types also to some
extent absorb solar radiation. BC is the most important of the latter, but mineral dust and some OC
types are also solar absorbers. Aerosols which mainly scatter solar radiation have a cooling effect, by
enhancing the total reflected solar radiation from the Earth. Strongly absorbing aerosols have a
warming effect. In the atmosphere there is a mixture of scattering and absorbing aerosols, and the
degree of scattering is quite variable. It is quantified through the single scattering albedo =s/
where s is scattering AOD and  is total AOD. The net effect of the scattering and absorbing aerosols
is dependent on surface and cloud characteristics. Scattering aerosols with a dark surface and
absorbing aerosols with a bright surface are most efficient (see Figure 3a). Scattering aerosol with a
bright surface is of less efficiency since the solar radiation is reflected anyway, similarly for absorbing
aerosol and a dark surface the solar radiation will be to a large extend absorb. Since clouds are a
main contributor to the total reflection of solar radiation, absorbing aerosols are particularly efficient
when positioned above clouds.
Aerosols are vital for cloud formation, acting as sources of cloud condensation nuclei (CCN) and ice
nuclei (IN). An increased amount of aerosol may enhance the CCN number concentration, and lead to
more but smaller cloud droplets. This increases the albedo of the cloud, resulting in enhanced
reflection and hence a cooling effect, termed the cloud albedo or 1st indirect effect (Twomey, 1977)
(see Figure 3b). Smaller and more cloud droplets may also inhibit the precipitation process, since
many similarly sized cloud droplets reduce the growth to large cloud droplets necessary for initiation
of precipitation. This effect, called the cloud lifetime or 2nd indirect effect, may enhance the cloud
cover and also has a cooling effect (Albrecht, 1989).
Absorbing aerosols also have the potential for modification of clouds without directly acting as CCN
and IN, through i) reduced solar radiation reaching the ground diminishes the convection and thus
the potential for cloud formation, ii) changes in the temperature profile, which will generally lead to
reduced instability and weakened cloud formation, but certain layers in the atmosphere may reach
more instability, iii) increased temperature reduces the relative humidity, which inhibits cloud
formation and enhances evaporation of existing clouds. This is termed the semi-direct aerosol effect
(Ackerman et al., 2000; Hansen et al., 1997). The net effect of the semi-direct effect is uncertain and
depends highly on the vertical profile of BC (Johnson et al., 2004; Koch and Del Genio, 2010; Penner
et al., 2003). In addition BC (and other absorbing aerosols) may reduce the albedo of snow and ice
leading to reduced reflectance of solar radiation, and hence a heating effect (Hansen and Nazarenko,
2004).
Figure 3: a) An illustration of the direct aerosol effect for low surface albedo (in this example zero
surface albedo) and high surface albedo (in this example all solar radiation is reflected and surface
albedo equal one) for scattering and absorbing aerosols. A dark surface (i.e. a low surface albedo) will
absorb a large portion of the solar radiation (in this example all radiation that reach the surface), and
absorbing aerosols will thus have a small effect since solar radiation is absorbed anyway. On the
other hand, scattering aerosols may substantially amplify the total reflectance of solar radiation since
the solar radiation otherwise will absorb at the surface. Similarly over a bright surface (high surface
albedo), scattering aerosols have a small effect since the solar radiation is scattered anyway.
Absorbing aerosol may however substantially reduce the outgoing radiation under such conditions
and has a warming effect. b)Illustration of various indirect and semi-direct aerosol effects. A cloud
unperturbed by anthropogenic aerosols has relatively large cloud droplets. Anthropogenic aerosols
increase the CCN and reduce the size of the cloud droplets. This is the first indirect aerosol effect
(cloud albedo effect) and smaller and higher number of cloud droplets increases the reflection of solar
radiation from the cloud. The smaller size of the cloud droplets may also inhibit the precipitation
process and lead to longer lifetime of the clouds and larger cloud coverage. This is the second indirect
aerosol effect (cloud lifetime effect) which also increases the reflection of solar radiation. Absorbing
aerosols may increase the temperature in the atmosphere by absorption of solar radiation and lead
to reduced relative humidity and altered stability in the atmosphere. This effect termed the semidirect effect can alter the cloud cover, but the net effect on cloud cover and the radiation is uncertain.
Why is the uncertainty in the aerosols important for the predictions of the climate sensitivity?
The change in the temperature (T) as a result of radiative forcing (RF)(change in the Earth radiation
balance due to anthropogenic or natural origin), can be expressed in the following simple heat
balance equation:
c d(T)/dt = RF -T
(1)
Here c is the heat capacity and  is the climate sensitivity. In radiative equilibrium (d(T)/dt =0)
equation 1 reduces to T = RF/. However the Earth is not in radiative equilibrium, since less thermal
radiation is currently emitted to spaced compared to what is absorbed of solar radiation (Hansen et
al., 2005). This radiative imbalance is causing the Earth to gradually warm, with global warming as a
result (Trenberth, 2010). The simple equation above has two key uncertainties. The observed surface
temperature change is rather well determined, but the climate sensitivity and the total RF are both
highly uncertain. Quantifying the climate sensitivity , an essential parameter for prediction of future
climate change, has long been attempted by using global climate models or temperature records, but
it still has a wide range of reported values (IPCC, 2007; Knutti and Hegerl, 2008). The total RF through
the industrial era is also uncertain, mainly due to lack of quantification of the aerosol effects
discussed above. The implication of this uncertainty in the aerosol RF for the quantification of the
climate sensitivity can be illustrated as follows:
Figure 4a shows the climate sensitivity as a function of the total aerosol RF (shown both for the most
certain aerosol effects and a more comprehensive set of aerosol effects), where uncertainties in the
radiative imbalance are included. A similar figure has previously been presented in Andreae et al.
(2005).The figure shows that using the known industrial age warming of around 0.8K and the present
best knowledge on RF from non-aerosol components, radiative imbalance and the 90% confidence
interval of the total aerosol RF, we can get a climate sensitivity range from about 2 to 6?? K for a
doubling of CO2 Figure 4b breaks down the total aerosol forcing PDF into individual components,
indicating both the magnitudes and uncertainties of the effects.
a)
b)
Figure 4: a) Climate sensitivity for a doubling of CO2 as a function of the total aerosol RF. The curve is
shown for a best estimate of the radiative imbalance of 0.85 Wm-2 (Hansen et al., 2005) with grey
shading for imbalances between 0.7 and 1.0 Wm-2 representing the uncertainty. A thin dotted line for
radiative imbalance of zero (radiative equilibrium) is added. The observed temperature change is
taken as 0.8 K and radiative forcing of non-aerosol components is 2.9 Wm-2. The red line shows the
probability density function (PDF) for the total of the direct aerosol, cloud albedo effect and the BC on
snow and ice, whereas the dotted red line shows the total of all aerosol effects. b) PDFs of various
aerosol effects taken from Isaksen et al. (2009), except for ….. Monte carlo simulations are performed
to derive total aerosol RF shown with red curves in a) (Boucher and Haywood, 2001).
Has there been any progress in the understanding of the climate effect of aerosol?
There has been a tremendous improvement in the understanding of atmospheric aerosols and the
climate effect of aerosols over the last decades, with some important observational and modelling
breakthroughs. Long term measurements of aerosols (Malm et al., 2004; Putaud et al., 2004),
observational campaigns (Novakov et al., 1997; Quinn and Bates, 2005; Ramanathan et al., 2001) and
remote sensing from space and ground (Holben et al., 1998; Remer et al., 2008) have remarkably
increased the knowledge about the atmospheric aerosol composition and characteristics. However,
the understanding of the greater complexity of atmospheric aerosols has at the same time limited
more robust quantification of their climate effect. The first estimate of the direct aerosol effect in the
early 1990s was limited to sulphate aerosols (Charlson et al., 1991), with estimates for BC coming a
few years later (Haywood and Shine, 1995). Observations have shown that OC is an important
aerosol component (Novakov et al., 1997; Ramanathan et al., 2001) and substantial investigations
have later explored the complex composition and optical characteristics of this compound (Andreae
and Gelencser, 2006; Graber and Rudich, 2006; Kanakidou et al., 2005; Robinson et al., 2007). Global
aerosol models today provide RF estimates for a large set of aerosol components such as sulphate,
BC (from fossil fuel and biomass burning), OC (from fossil fuel and biomass burning), nitrate, SOA
(Jacobson, 2001; Koch et al., 2009; Liao and Seinfeld, 2005; Myhre et al., 2009). In addition multimodel studies are performed to understand and reduce uncertainties due to model differences
(Schulz et al., 2006).
An example of recent progress is the estimate of the total direct aerosol effect given by IPCC AR4
(Forster et al., 2007). This estimate was made possible by the advances that have occurred on both
the modelling and observational side, and was based on a combination of global aerosol models and
observational based methods. The latter mainly used remotely sensed data from space and ground.
The uncertainty range was however large due to stronger RF in the observational based methods
than from the models. Consistency between these two different approaches has later been reached,
and was found to arise from necessary and simplified assumptions of the pre-industrial aerosol
composition in the observational based method (Myhre, 2009). Although the uncertainty in the total
direct aerosol effect is reduced, the uncertainty for several of the aerosol components such as BC,
OC, nitrate is still large.
Similar to the early estimates of the direct aerosol effect, many of the first model estimates of the
aerosol indirect effect only accounted for the effect of sulfate particles acting as CCN (e.g.Boucher
and Lohmann, 1995; Jones et al., 1994; Kaufman and Chou, 1993). Furthermore, they only included
the influence of sulfate aerosols on cloud albedo, disregarding any effects on cloud lifetime and
extent. With the realization that also other aerosol species of anthropogenic origin could form cloud
droplets, and that effects on cloud lifetime and extent were also possible, global climate models
estimated the aerosol indirect effect to be stronger (e.g.Lohmann and Feichter, 1997; Menon et al.,
2002; Rotstayn, 1999)). Some even predicted this cooling effect to be comparable in magnitude to
the warming greenhouse effect. Recent publications have since pointed to oversimplifications in the
model representation of clouds and how their lifetimes are affected by aerosols (e.g.Stevens and
Feingold, 2009). It is now acknowledged that aerosol effects on cloud lifetime will vary with the cloud
type in question. For example, a few recent model studies have found that by forming ice in supercooled liquid clouds, aerosols may in fact shorten cloud lifetime, because of the more efficient
precipitation formation when cloud ice is present (e.g.Lohmann and Hoose, 2009; Storelvmo et al.,
2011). In summary, whether aerosols are acting as CCN, IN or are simply modifying atmospheric
stability by absorbing solar radiation, there is still much uncertainty associated with their effect on
cloud lifetime. In contrast, modelling and satellite estimates of the cloud albedo effect seem to
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