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					Int. J. Global Warming, Vol. 4, Nos. 3/4, 2012                                                  219


The aerosol-cloud-climate conundrum

         Jost Heintzenberg
         Leibniz-Institute for Tropospheric Research,
         Permoserstr 15, Leipzig 04318, Germany
         E-mail: jost@tropos.de

         Abstract: The complexity of the atmospheric aerosol and its connection with
         clouds and climate are illustrated with a host of examples against the background
         of our present limited state of understanding. A discussion of related feedbacks
         demonstrates the difficulties of resolving all respective research issues. The
         key role of aerosols and clouds in anthropogenic climate change make the high
         uncertainties related to them ever more painful. Nevertheless, there are suggestions
         to manipulate aerosols and clouds by climate engineering to counteract global
         warming. Before considering such remedies the aerosol-cloud-climate conundrum
         needs to be reduced to a level of uncertainty that is comparable to those related
         to anthropogenic greenhouse gases. Considering the complexity of the aerosol/
         cloud system the challenge will be to identify the necessary essential knowledge
         and differentiate that from marginal details and focus research efforts on these
         essentials in order to simplify the complex aerosol-cloud system without loosing
         indispensable features.

         Keywords: aerosol; cloud; climate change; anthropogenic climate change;
         geoengineering.

         Reference to this paper should be made as follows: Heintzenberg, J. (2012) ‘The
         aerosol-cloud-climate conundrum’, Int. J. Global Warming, Vol. 4, Nos. 3/4,
         pp.219–241.

         Biographical note: 1963–1974 Academic training in meteorology with PhD in
         natural sciences at University of Mainz; 1974–1975 visiting scholar at University of
         Washington, Seattle; 1977–1993 Guest researcher and Prof. in Physical and Chemical
         Meteorology; 1993–2009 Director of the Leibniz-Institute for Tropospheric Research
         and chair in Atmospheric Physics at the University of Leipzig.




1   Introduction
What is a conundrum? The free merriam-webster dictionary offers several explanations. We
discard the first one of “a riddle whose answer is or involves a pun” because the underlying
subject of anthropogenic climate change is too serious to address with puns. Both the other
two definitions of conundrum apply very well to the aerosol-cloud-climate issue, which is
the topic of this review: “A question or problem having only conjectural answer”, i.e., ‘based
on incomplete information’, which is the normal situation in characterising the atmosphere
or just ‘an intricate and difficult problem’.
   Let us start our look into this conundrum with the energy balance of the Earth. At present
and for some time in the past, energy input from the sun and output in the form of thermal

Copyright © 2012 Inderscience Enterprises Ltd.
220        J. Heintzenberg

radiation are not and have not been in balance (Hansen et al., 2005; Trenberth and Fasullo,
2010) because of rapidly rising concentrations of greenhouse gases in the atmosphere.
Nearly one Watt per square meter more solar radiation is being absorbed by Earth than is
radiated back to space. Consequently, it must get warmer until the ensuing higher thermal
radiation causes energy input and output to be balanced again. But how much warmer,
when and where? These are the central questions of climate research. At present the biggest
stumbling blocks on the road to answering these questions are aerosols and clouds and their
interaction with climate.
    Presently there are about 1040 molecules of the main anthropogenic greenhouse gas CO2
in the atmosphere. Aside from isotopic differences the CO2 molecules all behave the same,
absorbing and emitting radiation, mainly in the infrared region. The particular counterparts
to CO2, i.e., aerosols and clouds comprise only very small impurities in the atmosphere.
Combining the results of global marine surface aerosol measurements (Heintzenberg et al.,
2000, 2004) with aerosol studies of the free and upper troposphere (Clarke and Kapustin,
2002; Heintzenberg et al., 2011) we can estimate the global average number concentrations
of aerosol particles to 100°cm–3 to derive an estimate of the total number of aerosol particles
in the atmosphere of some 1026. A coarse estimate of the total number of cloud particles in
the atmosphere (drops and ice crystals) we can derive from global averages of liquid water
(O’Dell et al., 2006) and ice water paths (Eliasson et al., 2011) leading also to roughly
1026. So, why worry about these relatively few particles compared to so many more CO2
molecules? As Earth’s albedo is strongly controlled by clouds there is no doubt that they
matter in the energy balance e.g., (Ramanathan et al., 1989). But even the minute amount of
aerosols does matter in the energy balance as demonstrated by comparing modeled global
energy balances with and without aerosols to the energy balance derived from the ERBE1
satellite experiment (Haywood et al., 1999).
    Compared to the similarity of CO2 molecules and their rather even distribution in the
atmosphere aerosol and cloud particles exhibit an enormous variability. Their size range and
their lifetimes in the atmosphere cover about six orders of magnitude; their concentrations
some ten orders (Jaenicke, 1988; Heintzenberg, 2003). As illustrated in Figure 1 their
shapes are unlimited, reaching from simple spherical drops to intricate crystals and complex
organisms (or parts of the latter two). Their color ranges from transparent drops and salt
crystals via reddish dust to black soot particles. In terms of particle number most of the
atmospheric aerosol derives from the condensation of natural and anthropogenic vapors.
The shapes of these particles are less complex (cf. sulfuric acid and ammonium sulfate in
Figure 1) than those, which often control particulate mass (cf. dust, sea salt and biomass
smoke in Figure 1). As illustrated in Figure 1 soot particles derived from high temperature
combustion exhibit highly complex and variable structures and shapes, which makes it
particularly difficult to quantify their crucial role in the atmospheric energy transfer (Chylek
and Wong, 1995) as well as its life cycle in the atmosphere (Ogren and Charlson, 1983).
The ubiquitous uncontrolled low temperature biomass burning combustion sources produce
complex particles such as ‘tar balls’ (Pósfai et al., 2004). At the low upper temperatures
of the upper troposphere glassy particles are suspected to form from inorganic or organic
constituents (Zobrist et al., 2008), the physical properties of which are yet to be understood.
    Traditionally, clouds and aerosols have been treated as discrete, clearly separated entities
in the atmosphere. However, increased understanding of the aerosol change with relative
humidity (Swietlicki et al., 2008) and observational evidence (Koren et al., 2007; Charlson
et al., 2007) have led to theoretical questioning of this concept (Stevens and Schwartz, 2011).
       The aerosol–cloud–climate conundrum                                                         221

Figure 1   Variability of aerosol and cloud particle shapes (modified from Caroline Leck, private
           communication)




Individually and as complicated functions of their size, shape, orientation and composition,
aerosol and cloud particles scatter, absorb and emit radiation over both visible and infrared
spectral regions (Pósfai and Buseck, 2010). They interact with each other and with important
Earth system processes and compartments. All life on the continents is dependent on them
through aerosol dependent clouds supplying the necessary water. Particles with substantial
water content influence atmospheric dynamics through their exchange of latent heat in
condensation, freezing and evaporation on cloud scales (van den Heever and Cotton, 2007),
possibly even on larger scales (Graf et al., 2003). On global average the release of latent heat
provides 30% of the thermal energy that drives Earth’s general circulation (Chahine, 1992)
and the potentially strong aerosol influence on this important term of the energy budget has
led to the suggestion of a cloud-mediated thermodynamic climate forcing of the atmospheric
aerosol (Rosenfeld et al., 2008). Dust particles provide nutrients to marine life (Jickells
et al., 2005), rain forests (Koren et al., 2006; Boy and Wilcke, 2008) and other terrestrial
ecosystems (Garstang et al., 1998) through their mineral content. Viable particles spread life
and diseases over the globe.
    In the following sections the complexities of the atmospheric aerosol/cloud system will
be illustrated before reviewing how this system is considered in global assessments of its
climate relevance by the Intergovernmental Panel on Climate Change (IPCC). The present
imbalance of Earth’s climate system and its envisioned anthropogenic negative development
in the near future has revived ideas about climate engineering as emergency solution to the
climate problem. In the light of the complexities of the aerosol-cloud issue related suggestions
of climate engineering will be discussed. In the conclusions the most serious caveats against
aerosol and cloud related climate engineering are summarised with an outlook into the future
of climate related aerosol/cloud research.

2   Complexities of the atmospheric aerosol/cloud system
2.1 Aerosol-cloud-atmospheric chemistry
The complexities of processes connecting the atmospheric aerosol, clouds and climate will
be illustrated with modeling, conceptual and experimental examples. To begin with volatile
222        J. Heintzenberg

aerosol components, i.e., substances with strongly temperature-dependent gas-particulate
exchange pose major challenges to both, experimental quantification and modeling. If we
take the major aerosol component nitrate as an inorganic example of such materials the
global model of Metzger et al. (2002) shows that in the morning most nitrate resides in the
particle phase, scattering solar radiation whereas towards the sunset it may be all gone from
the particle phase with no scattering effect remaining. To a lesser degree volatile organic
particle components pose similar problems.
    To an air chemist volatile particle components are a strong hint of a link between aerosol
particles and chemical processes in the atmosphere. Indeed, aerosols and clouds (and
climate) are intimately connected with atmospheric chemistry. Condensible vapors forming
new particles from the gas phase are formed by photochemical reactions from inorganic
and organic precursor gases (Kulmala, 2003). However, not even the simple case of particle
nucleation from sulfuric acid is completely understood (Laaksonen et al., 2008; Berndt
et al., 2008). The formation and atmospheric transformation of organic particle components
presents presently the biggest challenge to atmospheric chemistry because of multitude of
available organic reactants, possible reactions and product (Jimenez et al., 2009). Present
estimates of their source strength vary over more than an order of magnitude (Henze et al.,
2008; Goldstein and Galbally, 2007) and a ‘missing source’ is still attracting much research
attention e.g., (Heald et al., 2010).
    Important chemical reactions take place on airborne particle surfaces that affect
greenhouse gas concentrations e.g., (Liao and Seinfeld, 2004; Liao et al., 2009). Cloud water
is an essential chemical reactor producing on a global scale e.g., most of the particulate
sulfate through liquid-phase reactions (Lelieveld and Crutzen, 199). A host of other reactions
involving inorganic and organic aerosol components and dissolved gases are possible in
cloud droplets e.g., (Herrmann et al., 2000) the importance of which for the climate issue is
under investigation.

2.2 The microscale aerosol-cloud connection
Without aerosol particles serving as condensation nuclei there would be no cloud in Earth’s
atmosphere because a homogenous nucleation of water molecules would require water
vapor supersaturations that cannot be attained by natural atmospheric processes. Size,
wettability, soluble and surface-active components of a particle determine at which water
vapor supersaturation it will grow to a cloud particle (Pruppacher and Klett, 1997). In a
recent report (Dusek et al., 2006) present experimental evidence for particle size being the
most important parameter controlling the uptake of aerosol particles in cloud drops as can
be expected from Köhler theory of hygroscopic particle growth (Köhler, 1923) and more
recent theoretical papers e.g., (Wex and Stratmann, 2008). Single parameter representations
of their hygroscopic growth and cloud droplet condensation have been formulated (Petters
and Kreidenweis, 2008; Petters and Kreidenweis; 2007; Rissler et al., 2006; Wex et al.,
2007). Does that imply that the connection between aerosol particles and cloud elements
is sufficiently well understood or does it only mean that we can describe the hygroscopic
growth and droplet formation in artificial clouds that are formed on scales of decimeters to
meters in cloud chambers? Even in this simple setting of warm clouds (no ice involved) there
are important open mechanistic questions. A parameter as basic as the mass accommodation
coefficient of water vapor onto growing particles is still uncertain (Ruehl et al., 2008;
Davidovits et al., 2006; Marek and Straub, 2001; Voigtländer et al., 2007). The growth and
       The aerosol–cloud–climate conundrum                                                 223

incorporation into cloud droplets of certain aerosol components such as soot (Conant et al.,
2002), slightly soluble substances and soluble gases (Kulmala et al., 1997) pose substantial
problems in understanding atmospheric clouds. As pointed out in Heintzenberg and Covert
(Heintzenberg and Covert, 1990) the state of mixture in the atmospheric aerosol still is a
stumbling block on the path to a general description of cloud droplet formation e.g., (Deng
et al., 2011; Medina et al., 2007; Ervens et al., 2010). For example, (Wex et al., 2010) found
that in aerosols with external mixtures with hygroscopicities derived from bulk aerosol
composition, only the hygroscopicity of more soluble aerosol particles is captured. Bulk
or even size resolved composition data will be insufficient to predict cloud condensation
nuclei under many conditions unless independent information about particle mixing state is
available.
    In warm clouds there is a yet unresolved old controversy about the role of very large salt
particles on the formation of precipitation, beginning with the studies of Woodcock (1953)
and Woodcock and Blanchard (1955). Even with advanced experimental tools the results are
not clear. (Johnson, 1982; Blyth et al., 2003; Gerber et al., 2008) maintained that the number
concentration of very large soluble particles is sufficient to explain the formation of drizzle
drops. Also Rudich et al. (2002) deduced this effect based on satellite data from the Aral
Sea where salt-dust interacts with clouds. Other researchers, however, did not find a clear
relationship between large aerosol particles and precipitation (Colon-Robles et al., 2006;
Hudson and Mishra, 2007; Knight et al., 2008).
    In the more complicated cases of ice clouds and mixed phase clouds (concurrent ice
particles and liquid droplets) our mechanistic understanding is much more limited than in the
case of warm clouds (Chuang et al., 2009). In the cold environment of the former clouds even
the basic environmental parameters such as water vapor mixing ratio and supersaturation are
not well constrained by measurements. We have no specific instrumentation to observe the
many possible ice nucleation processes in the atmosphere. In particular our understanding
of the role of soot (Kärcher et al., 2007) and biological particles as ice nuclei remains poor.
In the latter case we have substantial discrepancies in between experimental results (Pratt
et al., 2009; Kamphus, 2010) and uncertainties about their global importance (Hoose et al.,
2010). These deficiencies in the study of cold clouds are particularly worrisome because
there are reports of their being affected by anthropogenic emissions (Ström and Ohlsson,
1998; Kristensson et al., 2000).

2.3 Aerosol-cloud-interactions on larger scales
Interactions and feedbacks on cloud scales and larger involving aerosols and clouds
have been hypothesised for some time beginning with the seminal CLAW hypothesis on
climate regulation by sulfur emissions from oceanic phytoplankton affecting low clouds
and thus ocean temperatures (Charlson et al., 1987). Whereas the CLAW hypothesis was
seen as a subsystem of the Gaia hypothesis (Lovelock, 1989; Shaw et al., 1998) proposed a
similar hypothesis involving deep convection that does not require the active participation
of ocean biota. To date it has not been possible to validate the full feedback loop from
biogenic emissions to ocean temperature. Also, it has proven difficult to find hemispheric
differences in cloud properties that should arise from the strong hemispheric differences in
anthropogenic sulfur emissions (Schwartz, 1988; Anderson et al., 2009). However, there
is some experimental evidence concerning the first parts of the loop (Ayers et al., 1991)
and a connection between phytoplankton blooms and cloud properties (Meskhidze and
224          J. Heintzenberg

Nenes, 2006). An extension of the hypothesis by Raes et al. (1993), who tried to resolve the
problem of rare evidence of new particle formation in the marine boundary layer, involves
new particle formation in the upper troposphere.
    There are important feedbacks between the aerosol/cloud system, atmospheric radiation
and dynamics. The interaction between aerosol and cloud particles and dynamics starts at
the millimeter scale of turbulence in clouds. “First, turbulence influences droplet growth
via the thermodynamic process of condensation in a supersaturated environment. Second,
turbulence influences droplet growth via the dynamical interaction of droplets in the
collision-coalescence process. Understanding the nature of these processes and separating
the two in actual measurements, is a significant challenge that remains” (Shaw, 2003). A
recent review by Stratmann et al. (2009) confirms droplet-turbulence interactions as posing
a major problem in cloud physics.
    Feedbacks between the aerosols, clouds, atmospheric radiation and dynamics are
illustrated on the mesoscale, beginning in the Sahara, the World’s largest natural dust
source by mass. Wind is the driving force for dust generation. However, the wind-lifted
dust strongly affects the energy balance of its carrier air through the scattering of solar and
the absorption of solar and thermal radiation. The resulting stabilisation of the planetary
boundary layer in turn affects surface winds and thus feeds back onto the dust generation
process. Heinold et al. (2008, 2011) included this feedback in a mesoscale weather and
dust model and demonstrated substantial differences in temperature (cf. Figure 2) and wind
(not shown in Figure 2) distributions when simulating these processes. On larger scales
the effects of dust on atmospheric dynamics had been simulated earlier e.g., (Karyampudi
and Carlson, 1988; Prospero and Carlson, 1972; Dunion and Velden, 2004) with the dust
feedback even affecting the Asian summer monsoon in climate models (Miller et al., 2004;
Perlwitz et al., 2001).

Figure 2    Maps of temperature difference in the SAMUM 2 working region. Shown are differences
            between a mesoscale model including aerosol radiative forcing and model results without
            including feedback between dust generation, energy budget and atmospheric dynamics,
            averaged over the period from 25 January to 7 February 2008. Top panels: 500 hPa level,
            bottom panels: 950 hPa level. Left panels: 12:00 UTC, center panels: 24:00 UTC and
            right panels: daily mean




           Source:   Heinold et al., 2011
       The aerosol–cloud–climate conundrum                                                          225

Several aerosol-dynamics feedbacks concern the major anthropogenic aerosol component
soot, which is the strongest particulate absorber of solar radiation in the atmosphere. In the
heavily polluted Pearl River delta in Southern China, particulate mass concentrations up to
300 µg m–3 with light absorption coefficients in the visible of about 6 10–5 m–1 were measured
during sunlit days. The concurrently measured diurnal evolution of the height of the planetary
boundary layer could only be simulated with a columnar model when including the heating
and thus atmospheric stabilising effect of particulate (soot) absorption (Wendisch et al., 2008).
Consequently, this aerosol feedback on boundary layer dynamics affects the ventilation of air
pollution to the free atmosphere. This type of feedbacks has been detected over many highly
polluted regions (Podgorny et al., 2000; Ramanathan and Ramana, 2005; Ramanathan et al.,
2001; Krishnan and Ramanathan, 2002; Menon et al., 2002; Jacobson, 1998). An ensuing
reduction in wind speed has been hypothesised by Jacobson and Kaufman (2006). Even
changes in the atmospheric general circulation with implications on monsoon, ENSO2 and
Arctic Oscillation have been simulated with models that include an aerosol-absorption-climate
feedback (Chung and Ramanathan, 2003; Chung et al., 2002). The soot-dynamics feedback
can be extended to the cloudy atmosphere where model simulations indicate a dessication of
clouds (Ackerman et al., 2000; Hansen et al., 1997).
    Another type of feedback can be envisioned in aerosol-cloud interactions when non-
precipitating clouds develop successively in a given air mass carrying water-soluble gases
that can be oxidised in the cloud water. As visualised in Figure 3 each cloud would release
more sub-micrometer particulate matter as a consequence of irreversible liquid-phase
reactions such as S(IV) (sulfur dioxide) to S(VI) (sulfuric acid or sulfate) reactions leading
to ever more aerosol in the size range that most efficiently scatters solar radiation. The
structure of a large body of marine particle size distributions has been explained by Hoppel
et al., (1986) with this type of aerosol processing. For individual clouds this effect has been
simulated by Lelieveld and Heintzenberg (1992). Related first experimental evidence has
been reported from cap-cloud experiments (Birmili et al., 1999; Yuskiewicz et al., 1999).
Feedback studies in consecutive clouds in the same air mass, however, are lacking to date.

Figure 3   Conceptual picture of consecutive non-precipitating clouds developing in an air mass
           with soluble gases and irreversible liquid-phase reactions in the clouds yielding more
           particulate mass after each cloud cycle




The same type of irreversible liquid-phase reactions as well as physical rearrangement of
aerosol particles consisting of loose aggregates e.g., (Krämer et al., 2000; Weingartner et al.,
1995) may make cloud-processed aerosol more hygroscopic or more hydrophilic. Thus,
consecutive clouds might develop differently (with different ensuing aerosol processing).
226          J. Heintzenberg

   There is an open discussion on possible aerosol influence on deep convection and tropical
storms, the so called ‘cloud invigoration’. This invigoration of deep convection by aerosol
pollution has been seen in several field studies e.g., (Freud et al., 2008; Koren et al., 2005;
Rosenfeld and Woodley, 2003; Andreae et al., 2004) and cloud statistics (Bell et al., 2008).
The cloud resolving simulation of Khain et al. (2005, 2008) confirm the substantial aerosol
effects of the field studies. Other modeling of deep convection (Morrison and Grabowski,
2011; Seifert and Beheng, 2006; Rosenfeld et al., 2011) however does not, neither does a
recent study based on active satellite data (Massie et al., 2011). In particular, the role of
desert dust remains unclear.


3     The atmospheric aerosol/cloud system in global climate assessments
Climate effects of aerosols and clouds are discussed by the IPCC primarily in terms of
anthropogenic radiative forcings (W m-2) and climate response in terms of global average
surface temperature changes (K). In this discussion the definitions of radiative forcings are
getting ever more complicated from a simple instantaneous disturbance of the radiative
balance at some reference level of the atmosphere to radiative flux perturbations (Haywood
et al., 2009; Lohmann et al., 2010), for which sea surface temperature and sea ice are fixed.
Figure 4 compares the summarised forcings of long-lived greenhouse gases and aerosols
between 1750 and 2005 according to IPCC (2007) yielding on average a positive net
forcing that supports the anthropogenic warming simulated by climate models. It should be
noted that of the many conceivable aerosol/cloud interactions influenced by anthropogenic
emissions e.g., IPCC (2007) only quantifies the so-called cloud albedo effect whereas the
whole complex of anthropogenic effects in mixed and cold clouds is not treated.


Figure 4    Radiative forcings according to IPCC 2007 for the time period 1750 to 2005. Box with
            black top: Sum of long-lived greenhouse gases. Box with black bottom: Sum of accounted
            for aerosol forcings. Grey box: Net forcing (sum of greenhouse gas + aerosol forcings).
            The ranges given in IPCC 2007 have been interpreted as 90% confidence limits




           Source: IPCC, Climate change 2007
       The aerosol–cloud–climate conundrum                                                  227

Adding up the forcings of different agents in the climate system has been justified with
climate models for some time even though the instantaneous flux disturbances by e.g.,
aerosol cooling and CO2 warming coincide neither in time nor in space with aerosol
cooling occurring mainly in highly polluted sunlit regions such as India and Southern China
whereas the CO2 warming is strongest in regions of the atmosphere where low water vapor
concentrations and simultaneous high surface temperatures allow for additional greenhouse
effects and that are independent of the time of day. Recent studies of the spatial scales of
climate response to inhomogeneous radiative forcing with four climate models have limited
the range of meridional forcing influence to some 3500 km Shindell et al., 2010).
    The uncertainties in Figure 4 are taken from to IPCC (2007) where they are given as 90%
confidence ranges, which they are not. Instead they have constructed in different ways from
the span of results of a small number of climate models. Boucher and Haywood, (2001)
explore with a Monte-Carlo approach the range of model results and arrive at non-Gaussian
probability distribution functions with a net anthropogenic forcing having even a small
probability of being negative, similar to the range derived by simple error propagation in
Figure 4.
    The range of possible climate responses to the anthropogenic forcings for the time
period 1990 to 2100 has been reported by last three IPCC reports to 0.8 – 3.5 K in 1996,
1.4 – 5.8 K in 2001 and 1.1 – 6.4 K in 2007. In the most recent IPCC a 90% confidence
limit of a factor of two is shown for the climate response in 2100. Schwartz et al. (2007)
have asked why this range is much smaller than the factor of four range of 90% confidence
given for the anthropogenic forcings in the same report with an ensuing discussion reported
in Forster et al. (2007). It may be suspected that the range of climate response will increase
further in the next IPCC report if more system processes (with their inherent uncertainties)
are included in the climate models, in particular concerning the many processes related to
aerosols and clouds.
    Are there any major questions left concerning the aerosol-cloud-climate conundrum?
Figure 1 in Lohmann et al. (2010) shows model, satellite and inverse estimates of the aerosol
indirect effects over the last two decades indicating two things: One, ever more climate
modellers simulate indirect aerosol effects and two, the estimated forcings appear to converge
towards a value of –1 Wm–2. Does that mean that the estimates are approaching reality? The
fact that the estimates range from near zero to –3 Wm–2 have stimulated the comment by
Stevens and Schwartz (20): “In such a situation it would seem that the modelling is going far
beyond the understanding”. One particularly grave uncertainty came out of the discussions
in Siebesma et al. (2009) and is illustrated in Figure 5. Cloud processes and cloud entities
in Earths atmosphere cover hierarchical and connected spatial scales from nanometer to
megameter and time scales from milliseconds to weeks. To date there is neither a theoretical
understanding nor a complete chain of models to cover these scales. Consequently, we do
not know to which extent any anthropogenic aerosol forcing of clouds, which takes place at
the left end of the scales in Figure 5 propagates towards the right end of these scales at which
climate modelling is simulating atmospheric processes. Nevertheless, observational evidence
is accumulating that indicates significant influence of anthropogenic aerosols on clouds well
beyond microphysical scales. As an example (Rosenfeld, 2000) reports ‘pollution tracks’ in
mesoscale stratiform cloud systems downwind of urban aerosol sources. Puzzling weekly
cycles in clouds and precipitation that can be explained by anthropogenic aerosol influences
have been found by several studies over the North American continent (Bell et al., 2008;
Rosenfeld and Bell, 2011).
228          J. Heintzenberg

Figure 5    Space and time scales of aerosol and cloud related processes and atmospheric entities.
            The scale regions of anthropogenic forcing of clouds and of climate models are marked
            with block arrows




           Source:   modified after Figure 12.1 in Heintzenberg and Charlson (2009)

Figure 6    Rate of increase of CO2 (ppm year-1) as a function of time measured on Mauna Loa,
            Hawaii (data courtesy of the US Department of Commerce National Oceanic &
            Atmospheric Administration NOAA Research). The low values in the beginning of the
            1990ies are affected by the eruption of the volcano Pinatubo. The dashed line indicates
            the year in which the Kyoto protocol to limit CO2 emissions was established




           Source: (Angert et al., 2004)


4     Aerosols and clouds in climate engineering
The rates of anthropogenic greenhouse gas emissions are increasing despite international
mitigation efforts such as the Kyoto Protocol (cf. Fig. 6). Concurrently, the consumption of
oil as the main fossil fuel with CO2 emissions increases, in particular in emerging economies,
       The aerosol–cloud–climate conundrum                                                 229

despite mounting evidence of ‘Peak Oil’ having been reached e.g., (IEA, 2008; Aleklett
et al., 2010). Consequently, voices are getting more frequent and louder advocating some
climate engineering in order to compensate ongoing and future greenhouse gas warming.
The components of Earth’s energy budget are Fs = S(1-Ap) with Fs being the solar energy
input, S being the extraterrestrial solar constant ≈ 1370 W m–2 and Ap being the planetary
albedo and the output Fl in the form of thermal radiation. In a balanced budget the solar input
Fs must be equal to the output Fl, with Fl = T4. This energy budget prescribes the engineering
possibilities to reduce a warming: We would need to reduce S or to increase Ap or Fl. It
should be noted right here, however, that climate engineering to increase Ap or Fl can have
substantially different net effects on the climate system. Following Bala et al., (2008) for
the same surface temperature change, changes in Ap result in relatively larger changes in net
radiative fluxes at the surface. These are compensated by larger changes in the sum of fluxes
of water vapor sensible heat. Hence, the hydrological cycle is more sensitive to temperature
adjustment by changes in Ap than by changes in Fl. This implies according to Bala et al.
(2008) that a reduction in solar input might offset temperature changes or hydrological
changes from greenhouse warming, but could not cancel both at once.
    Whereas weather and climate modification has been speculated about for more than
a century the recent paper by Crutzen (2006) reviving an idea of Budyko (1982) greatly
stimulated the discussion of climate engineering. Commentaries, new proposals, model
calculations and several reviews (http://royalsociety.org/Geoengineering-the-climate/,
(MacCracken et al., 2010)) have been formulated since. Besides carbon dioxide removal
and extraterrestrial engineering proposals to reduce the solar input most approaches suggest
some manipulation of aerosols and/or clouds to achieve some cooling through the increase
of Ap. It should be noted that no climate engineering directed at increasing Ap would reduce
the problem of ocean acidification (Doney et al., 2009) due to the CO2 increase in the
atmosphere.
    Additional reflecting aerosols in the stratosphere appear to provide a plausible engineering
approach because of the long lifetime of particles in the middle atmosphere (Jaenicke,
1988). Rockets, planes, canons and tethered balloons have been suggested as means to
inject aerosols or particle forming material in the stratosphere. A host of publications has
considered effectiveness, optimisations and side effects of this approach. Concerning the
latter the comparison with the largest volcanic eruption of the 20th century, Pinatubo, is
impressive. This volcano injected some 20 Mt of SO2 into the stratosphere (Guo et al., 2004;
Bluth et al., 1992) forming a large number of sulfuric acid particles similar to the proposed
climate engineering. Hegerl and Solomo (2009) showed that this injection led to substantial
reductions in global precipitation and continental runoff, which should be seen as grave
warning against similar deliberate injections.
    In the troposphere, the entities most favored for potential climate engineering are marine
boundary layer clouds. They cover large areas over otherwise dark and highly solar absorbing
ocean surfaces. Because of their small drop number these clouds are susceptible to increases
in this parameter, therewith potentially increasing their albedo (Schwartz and Slingo, 1996).
Seawater could provide the material for their manipulation and compared to the stratosphere,
they are easily accessible. These arguments led to the proposal by Latham and colleagues
to inject sea salt into these clouds on a large scale (Latham, 1990; Latham et al., 2008) for
climate engineering purposes. The main argument for the effectiveness of this method is ship
tracks: A whitening of marine stratocumuli that has been observed from space since the first
satellites in the 1960ies (Conover, 1966). The efficacy of this approach has been questioned
after its simulation with a global aerosol transport model (Korhonen et al., 2010). In their
230        J. Heintzenberg

model the added sea salt particles suppress the in-cloud super saturation and prevent existing
aerosol particles from forming cloud drops. A model scenario with considerably higher
sea salt additions than previously assumed still yields lower cloud drop numbers than in
previous studies. An inadvertent side effect of the spray emissions in their simulation is that
sulfur dioxide and sulfuric acid concentrations are suppressed due to chemical reactions on
the additional salt particles. Negative effects on the global water cycle have been suspected
by Bala et al. (2008, 2010). In their model the global-mean precipitation and evaporation
decreases but runoff over land increases, primarily due to increases over tropical land. It
should be noted though, that present simulations are unable to cover all possible side effects
on marine chemical and biological processes.
    The most realistic ‘climate engineering’ approach involving aerosols concerns the
elimination of anthropogenic soot in the atmosphere (Bond and Sun, 2005; Hansen et al.,
2000; Hansen and Sato, 2001; Jacobson, 2002, 2005). Besides mineral dust soot is the
only particulate substance in the atmosphere with significant absorption of solar radiation.
After its uptake into cloud drops this absorption can even be amplified (Heintzenberg and
Wendisch, 1996). Opposite to the major anthropogenic greenhouse gases the atmospheric
residence time of soot is with 10 to 30 days rather short (Ogren and Charlson, 1983), so
that its elimination from the atmosphere can be left to natural processes after reduction of
its human sources. It is suspected that in deposited form soot can still cause some heating
though its reduction of the albedo of snow and ice covered surfaces (Hansen and Nazarenko,
2003). The present total influence of soot on Earth’s energy balance has been modeled to be
on the order of the CO2 effect (Hansen and Sato, 2001; Hansen et al., 2002; Hansen, 2002).
A study of the United Nations Environmental Program reported that combined with the
elimination of anthropogenic methane (a ‘short-lived’ greenhouse gas) this approach “would
greatly improve the chances of keeping Earth’s temperature increase to less than 2°C relative
to pre-industrial levels” (UNEP, 2011).
    Positive side effects with the elimination of soot would be the reduction of carcinogenic
substances carried by soot particles and an increased ventilation of highly polluted urban
areas (Wendisch et al., 2008). A possible negative side effect would be the concurrent
elimination of all other particles emitted by the same combustion sources, which presently
increase Earth’s albedo (Chen et al., 2010).
    The only climate-engineering approach to increase Fl by manipulating aerosols and
clouds concerns cold ice clouds in the upper troposphere and lower stratosphere. Mitchell
and Finnegan, (2009) proposed to seed these clouds by means of additions to commercial
aircraft fuel producing ice nuclei that cause the formation of larger ice crystals, which would
increase their emission of thermal radiation. The proponents state an ensuing compensation
corresponding to a doubling of CO2 in the atmosphere. However, presently cloud physics has
no mechanistic connection between the properties of ice nuclei and those of atmospheric ice
clouds (Anderson et al., 2009).


5     Conclusions and outlook
A review of the different facets of the aerosol-cloud-climate conundrum yields two major
findings. One, a full understanding of Earth’s climate system and any predictive capability
concerning the climate in the Anthropocene (Crutzen and Stoermer, 2000) is dependent on
an understanding of the characteristics and mechanisms governing the role of aerosols and
       The aerosol–cloud–climate conundrum                                                231

clouds in Earth’s climate. Two, presently we are far from the necessary level of understanding
of the aerosol/cloud system.
    Even with a full understanding of aerosols and clouds we face the additional problem that
mechanistically well understood inadvertent or deliberate anthropogenic climate changes
can only be quantified and monitored against a background of natural climate noise. As
an example, satellite data on an annual basis show a global natural noise in the shortwave
radiative flux of 0.3 W m–2 (one-sigma). In order to distinguish effects of greenhouse
mitigation or climate engineering of that order of magnitude the related measurements with
existing satellite system would have to be maintained for at least 10–15 years (Loeb et al.,
2007). However, the stability of existing satellite systems is by no means secured for this
length of time. Based on the case of stratospheric climate engineering (Barrie and Hoff,
1984) add more arguments that any proof-of-principle field test would need to be realised for
a long time on a global scale with the risk of all possible side effects.
    Bifurcation is another inherent characteristic of Earth’s climate system (Lenton, 2011;
Lenton et al., 2008) that needs to be considered in discussions of inadvertent or deliberate
anthropogenic climate change. Lowe et al. (2009) and Molina et al. (2009) and others warn for
evolution pathways of the Earth system that lead to tipping points beyond which the climate
system will go through long hysteresis loops before an initial state can be reached again
(Rahmstorf, 2001). Any climate engineering would have to be maintained for very long times
because the natural sink processes of CO2 are very slow. More that 20% of anthropogenic
CO2 will stay in the atmosphere for more than 1000 years (Clery, 2008; Solomon et al., 2009)
before slow geological process will equilibrate the rest. Brovkin et al. (2009) and Ross and
Matthews (2009) demonstrate with climate models how sensitive Earth’s climate would react
to a sudden stop of stratospheric climate engineering. Similarly catastrophic temperature
increases have been calculated by Oschlies et al. (2010) after a cessation of marine climate
engineering and we do not know on which paths the climate system would develop afterwards.
    The present state of weather modification as reviewed by Cotton (2009) and Levin et al.
(2010) illustrates well how far we are away from understanding all connections between
aerosols and clouds. A quote from Stevens and Feingold (Stevens and Feingold, 2010)
summarises the present state of resolving the aerosol-cloud-climate conundrum: “Despite
decades of research, it has proved frustratingly difficult to establish climatically meaningful
relationships among the aerosol, clouds and precipitation. As a result, the climatic effect of
the aerosol remains controversial. We propose that the difficulty in untangling relationships
among the aerosol, clouds and precipitation reflects the inadequacy of existing tools and
methodologies and a failure to account for processes that buffer cloud and precipitation
responses to aerosol perturbations”.
    One approach could be to request: Before any climate engineering might be considered
seriously the aerosol-cloud-climate conundrum has to be resolved to an extent that
leaves uncertainties in the understanding of related present and the projection of future
anthropogenic climate forcings that are comparable to those for greenhouse gases. Bearing
in mind the complexity of the aerosol-cloud system this may be a request that cannot be
fulfilled within the foreseeable future. We may never be able to reach that level and the
challenge of aerosol, cloud and climate research is to identify the necessary essential
knowledge and differentiate that from marginal details, focus the research efforts on
these essentials in order to simplify the complex aerosol-cloud system without loosing
indispensable features, following Einstein’s advice that everything should be as simple as
it can be, but not simpler.
232         J. Heintzenberg

Acknowledgements
This paper resulted from a kind invitation of the organisers of the 11th Global Conference
on Global Warming in Lisbon, Portugal, July 11–14, Ana Maria Silva and Heitor Reis.
I gratefully acknowledge the supportive material provided by Kjell Aleklett, Bernd Heinold,
Bjorn Stevens and Stephen E. Schwartz and comments and helpful suggestions by Albert
Ansmann, Bob Charlson and Heike Wex.

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Notes
1
 ERBE: Earth Radiation Budget Experiment.
2
 ENSO: El Niño-Southern Oscillation.

				
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