Aerosols and Climate
Bjørn H. Samset, PhD
Senior Researcher, CICERO
Contents
Summary ................................................................................................................................................. 3
Introduction............................................................................................................................................. 4
What aerosols are and where they come from ...................................................................................... 5
Major aerosol types and sources ........................................................................................................ 5
Natural aerosol sources................................................................................................................... 5
Anthropogenic aerosol sources ....................................................................................................... 6
Observing and modelling the global aerosol abundance .................................................................... 7
Aerosol-climate interactions ................................................................................................................... 8
Direct interaction with incoming sunlight ........................................................................................... 9
Scattering of incoming sunlight by aerosols can cool the climate. ................................................. 9
Aerosol absorption of incoming sunlight can warm the climate .................................................. 10
Deposition of black carbon on snow ............................................................................................. 10
Aerosols affecting clouds .................................................................................................................. 10
Cloud albedo effect ....................................................................................................................... 11
Cloud lifetime effect ...................................................................................................................... 11
Semidirect effects .......................................................................................................................... 12
Aerosols in the context of anthropogenic climate change ................................................................... 12
Direct radiative forcing from anthropogenic emissions .................................................................... 13
Indirect effects .................................................................................................................................. 13
Historical evolution and global dimming........................................................................................... 14
Aerosols and climate sensitivity ........................................................................................................ 15
Aerosols and precipitation ................................................................................................................ 15
Aerosols as climate mitigation .......................................................................................................... 16
Expected trends in anthropogenic aerosol emissions....................................................................... 16
Large-scale climate impacts from aerosols ........................................................................................... 17
Volcanic eruptions ............................................................................................................................. 17
Nuclear winter ................................................................................................................................... 18
Climate engineering via aerosols ...................................................................................................... 18
The scientific study of aerosols ............................................................................................................. 19
Early ideas linking aerosols to climate evolution .............................................................................. 20
Aerosol science in the age of supercomputers ................................................................................. 21
Recent developments and outstanding issues .................................................................................. 21
Figures ................................................................................................................................................... 23
Figure sources and full resolution links ................................................................................................. 26
References / sources ............................................................................................................................. 26
Summary
Among the factors that affect the climate, few are as diverse and challenging to understand as
aerosols. Minute particles suspended in the atmosphere, aerosols are emitted through a wide range
of natural and industrial processes, and are transported around the globe by local weather
conditions. Once airborne, they affect the climate both directly, through scattering and absorption of
solar radiation, and indirectly through their impact on cloud properties. Combining all their effects,
anthropogenic changes to aerosol concentrations are estimated to have had a climate impact over
the industrial era that is second only to CO2.
Major aerosol types include sea salt, dust, sulfate compounds and black carbon - or soot - from
incomplete combustion. Of these, most act as scatterers of incoming sunlight, and thus mainly cool
the climate. Black carbon, however, is a solar absorber, and therefore acts as a heating agent much
like a greenhouse gas. Further, all aerosols can act as cloud condensation nuclei, causing clouds to
become whiter - and thus more reflecting, further cooling the surface. Black carbon is again a special
case, acting to change the stability of the atmosphere through local heating, and also changing the
albedo of the surface when it is deposited e.g. on snow and ice.
The wide range of climate interactions that aerosols have, and the fact that their distribution
depends on the weather at the time and location of emission, lead to large uncertainties in the
scientific assessment of their impact. This in turn leads to uncertainties in derivations of the present
climate sensitivity, since aerosols have predominantly acted to oppose 20th century global warming.
Finally, aerosols are important for large-scale climate events such as major volcanoes, or the threat
of nuclear winter. The relative ease with which they can be produced and distributed has led to
suggestions for using targeted aerosol emissions to counteract global warming - so-called climate
engineering.
Introduction
On June 15th 1991, Mt Pinatubo on the Philippines erupted, ejecting a massive mushroom cloud of
volcanic ash and dust. Billions of tonnes of fine grained magma were emitted to the atmosphere,
along with twenty million tonnes of sulfur compounds. The dust cloud gained altitudes of over
twenty kilometers, entering the stratosphere, and in the following months spread out to cover the
entire Northern Hemisphere.
At the same time, global mean temperature dropped by several tenths of a degree Kelvin, and stayed
low for around two years. The Mt Pinatubo eruption and its influence on global temperature is a
dramatic example of the impact that aerosols – minute particles suspended in the atmosphere – can
have on our climate. It also marked a turning point in the scientific study of aerosols as an important
and complex factor in climate evolution. A US research group, led by climate scientist James Hansen,
used a numerical climate model to correctly predict the broad features of the climate response to
the eruption.
Volcanoes are a natural, and unpredictable, source of atmospheric aerosols. Their effects are
potentially very strong, but always transient. Over the industrial era, however, anthropogenic
activities have also added sources of aerosol emissions. Major examples are soot from incomplete
combustion in diesel engines, cook stoves and large scale agricultural burning, and sulfate
compounds resulting from SO2 emissions, mainly from fossil fuel usage. Like volcanic ash, these
emissions also have an impact on our climate, and need to be studied in detail if we are to
understand the observed ongoing changes to global temperature and precipitation patterns.
Today, aerosol physics, chemistry and climate connections are among the most active subfields of
climate science. It is generally recognized that changing aerosol emissions, both natural and
anthropogenic, have played a key part in the climate evolution of the last century, obscuring the
impact of the strengthened greenhouse effect due to CO2 and other long lived greenhouse gases.
Due to the many ways aerosols can impact the climate, however, their role is far from certain. In the
summaries of anthropogenic radiative forcing regularly produced by the UN Intergovernmental Panel
on Climate Change (IPCC), increased CO2 concentration can be seen to dominate, but the main
uncertainty on our net impact on the global energy balance comes from the wide range of aerosolclimate interactions.
This review first gives an overview of what atmospheric aerosols are, and where they come from:
emissions and secondary production, and anthropogenic and natural processes. Then, we discuss the
various ways in which aerosols can affect the climate, before tackling their precise contributions to
ongoing climate change. We cover global concerns about aerosols, such as past and future major
volcanic eruptions, the specter of nuclear winter, and the possibility of using them for large-scale
geoengineering. Finally, we give some historical notes on the evolution of the scientific study of
aerosol s in the climate context, and list some scientific questions that are, at the time of writing, still
unanswered.
What aerosols are and where they come from
Aerosols, or particulate matter, are here used as collective names for airborne particles, suspended
in the Earth’s atmosphere. The particles can be liquid, solid or of mixed phase, and their diameters
typically range from 0.01µm to 100µm.
Aerosols can be either emitted directly to the atmosphere, as is the case for dust or sea spray swept
up by winds, and soot from incomplete combustion, or they can be produced as a result of
atmospheric chemical reactions from so-called precursor gases, as when SO2 emissions react with
water vapor to produce sulfate compounds.
Once airborne, aerosols may undergo further chemical reactions, or merge through coagulation.
Some may change their size through hygroscopic growth. They may get washed back out of the
atmosphere by encountering precipitation, or simply fall out through gravitational settling. The
average lifetimes of aerosols in the atmosphere, which depends strongly on their size, emission
location and chemical properties and is hard to measure precisely, ranges from days to weeks.
Major aerosol types and sources
Aerosols present in the atmosphere today are commonly categorized as either primary (emitted) or
secondary (produced), and natural (resulting from a natural process) or anthropogenic (the product
of industrial or other human activities).
Natural aerosol sources
The most common natural aerosols are salt particles from sea spray droplets, mineral dust, and
carbonaceous compounds from wildfires.
Sea spray aerosols are emitted to the air through the bursting of bubbles when waves break, and its
production is therefore dependent on the strength of surface winds. 2000-7000 Tg (teragrams) of sea
spray is estimated to be emitted annually, mainly from the midlatitude oceanic regions in both
hemispheres.
Mineral dust is produced by disintegration of larger rocks, and swept up by winds from arid land
regions. Major sources are desert regions of Northern Africa and the Middle East, Australia and parts
of Southern Asia. Estimates of annual dust emissions range from 1000 – 4000 Tg.
Emissions of carbonaceous aerosols, or soot, from natural wildfires are less well constrained, and
also vary more from year to year. Wildfires occur in regions that both support significant growth of
vegetation and are vulnerable to extended dry seasons. Examples are South America and many
regions of Central Africa. Aerosol emissions from such wildfires are commonly termed biomass
burning aerosols.
In addition to these primary sources, natural emissions of aerosol precursors such as
dimethylsulphides, monoterpenes, isoprene and biogenic volatile organic compounds are estimated
to induce formation of hundreds to thousands of Tg of aerosols annually.
Anthropogenic aerosol sources
Major types of anthropogenic aerosols are sulfate and nitrate compounds, organic aerosols and black
carbon. Of these, sulfate and black carbon have received the most attention in the context of
anthropogenic climate change.
Anthropogenic sulfate aerosols result from emissions of sulfur dioxide (SO2), which, once airborne,
react with water vapor to produce sulfate compounds. The main source is combustion of fossil fuels
and biomass. Global, annual emissions are estimated to be around 50 Tg of primary sulfur,
concentrated around the largest industrial regions, and along major shipping lanes. Sulfate aerosols
are highly hygroscopic, i.e. they can undergo significant growth if they are emitted or transported
into humid air.
Anthropogenic black carbon (BC), or soot, is a collective term for the products of incomplete
combustion of fossil fuels, biofuels and biomass. BC comes in a wide variety of shapes and sizes, and
can undergo significant changes after emission that alters its radiative and chemical properties (socalled BC ageing). Around 5 Tg is estimated to be emitted annually; primarily from industrial and
major crop burning regions, though there is at present considerable uncertainty surrounding this
number. A key feature of BC, distinguishing it from organic carbon (see below) and most other
species of aerosol, is its ability to absorb solar radiation and act essentially as a short lived
greenhouse gas.
Nitrate aerosols are produced from the oxidation of NOX emissions, primarily from combustion of
nitrogen-bearing fuels. As for sulfate, they originate mainly form industrial regions and along
shipping lanes, though reductions of nitrogen content in ship fuels from the late 2000s has reduced
the latter source.
Organic aerosols (OA) are carbonaceous compounds that do not absorb solar radiation. They come
from primary emission sources such as combustion of fossil fuels, biofuels and biomass (known then
as primary organic aerosols, or POA), or are produced e.g. from organic emissions from vegetation
(known as secondary organic aerosols, or SOA). Sometimes the term organic carbon (OC) is also used.
Recent research indicates that some OA may be weakly absorbing, placing them somewhere
between the established categories OA and BC. Such aerosols are becoming known as brown carbon.
Observing and modelling the global aerosol abundance
Knowing the main primary and secondary sources of aerosols is not sufficient to determine their
concentration and composition in the atmosphere at any given point and time. After emission or
production, aerosol transport and evolution will be subject to the prevailing weather, ambient
temperature and humidity. The particles may be washed out by rain more or less immediately, or
they may get transported to high altitudes and across vast distances. Global wind patterns are such
that aerosols rarely cross from one hemisphere to the other, but transport of aerosols from one
continent to another is not uncommon.
To determine global abundances and composition of aerosols, a combination of observations and
detailed modelling is used. Observations of aerosol properties are done both from satellites,
permanent ground based stations, and mobile stations on ships and aircraft. Such observations are
mainly able to discern the amount of sunlight either scattered or absorbed by the total amount of
aerosols in the atmospheric column, and to some extent the size distribution of particulate matter.
They are rarely able to divide aerosols into source categories, as done above.
Two common observable quantities are the aerosol optical depth (AOD), which quantifies the
amount of incoming sunlight removed by aerosols when traversing down to the surface, through
scattering or absorption, and the absorption aerosol optical depth (AAOD), which measures the
fraction of energy directly absorbed by the particles. By measuring AOD at several wavelengths,
information on aerosol size distributions can also be extracted.
Numerical models, on the other hand, use explicit emission inventories for each source category, and
attempt to treat the complex physical and chemical processes that transport and alter aerosols after
emission. They are however limited in resolution, and hampered by incomplete knowledge of
emissions, the physical and optical properties of the particles, and their treatment of weather and
clouds.
Figure 1 shows a 10-year averaged map of AOD, as measured by the MODIS instrument (Moderate
Resolution Imaging Spectroradiometer) on the NASA satellite Terra. Here, an optical depth of 1
means that the solar radiation reaching the surface is reduced to a fraction 1/e of its top-ofatmosphere value. The map indicates the present day major sources of aerosols: Biomass burning
emissions from some regions of Africa and South America, dust from arid regions around the Equator,
and anthropogenic emissions in the industrial regions of North America, Europe and - most notably South-Eastern Asia. The mid-latitude sea spray regions can also be seen.
Overlaid are cake diagrams showing decompositions into individual aerosol categories, as calculated
by a numerical model (the OsloCTM, a detailed chemical transport model). They show how the local
composition varies from being dominated by anthropogenic species such as sulfate and BC close to
industrial regions, to dust and sea salt in remote areas. This difference in composition with region
matters significantly, both for the local and global climate impact of aerosols, as will be discussed
below. In addition, there is significant variability of species with altitude, which influences the climate
impact of absorbing aerosols such as BC.
In summary, while aerosols are present across the globe, throughout the troposphere and even in
the stratosphere, their abundances and composition vary greatly with region, season and altitude. In
the following, all of these factors will be shown to be of importance for the net climate impact of
aerosols – both in general, and as part of the ongoing anthropogenic climate changes.
Aerosol-climate interactions
This section reviews the various ways in which aerosols can affect the climate. See also Figure 3.
Broadly, the boundary conditions of the climate can be said to be determined by the balance and
flow of energy in the ocean/atmosphere system. The Earth is hit by short wave solar radiation, some
of which is instantly reflected back into space by clouds and high albedo land surfaces. A significant
fraction is however absorbed by the surface, notably the oceans, and later emitted back as longwave
heat radiation. Some of this radiation is kept in the atmosphere for a while, by absorption and reemission by greenhouse gases; i.e. the greenhouse effect.
Since the climate of the Earth is broadly quite stable, having until recently seen e.g. less than a
degree Kelvin of change over the last 10k years (the present interglacial hot period, the Holocene),
we know that the balance of incoming and outgoing energy from the Earth system must be
remarkably good. Interannual variations in temperature and precipitation are largely due to internal
rearrangement of energy in the climate system.
For aerosols to impact the climate system, they must in some way affect the incoming shortwave
energy, the outgoing longwave energy, or the internal rearrangements. The example in the
introduction of the drop in global temperatures after the Mt Pinatubo eruption, shows that such
aerosol-climate interactions exist, and can be significant. Over the past few decades, aerosol climate
science has uncovered a wide range of climate interconnections, and determined that some of them
have been significant enough to have had impacts on the climate, almost on par with those from
increased greenhouse gas concentrations.
Direct interaction with incoming sunlight
The most obvious aerosol-climate interactions are the ability of aerosols to scatter or absorb
incoming sunlight.
The first of these, scattering, is what was in play after the Mt Pinatubo eruption. The volcano
emitted large amounts of SO2, which transformed into sulfate compounds in the atmosphere. A
significant fraction of the sulfate ended up in the stratosphere, where there is no precipitation to
wash the aerosol back out and atmospheric dynamics acts to keep the aerosol airborne.
Over the course of a few months the sulfate spread round most of the globe. This layer of particles
scattered back so much additional sunlight to space that surface temperatures dropped in response.
The global energy balance had, albeit temporarily, been offset.
Scattering of incoming sunlight by aerosols can cool the climate.
All species of aerosol are, to various degrees, able to scatter sunlight. To determine their precise
scattering power, however, we need to know their sizes and optical properties. Optical properties
vary between species, and are usually determined through lab experiments. Aerosol size
distributions are more challenging, as they are strongly modified by hygroscopic growth. E.g. sulfate
is highly hygroscopic, meaning that it very readily grows if the ambient humidity is high; a process
which strongly modifies its scattering properties. Nitrate is also hygroscopic, but to a lesser degree
than sulfate. Black carbon is not hygroscopic when it is emitted, but can become so if it gets coated
with sulfate.
Optical processes, size distributions, growth and ageing factors are at present well represented in
global climate models. Based on Mie theory, numerical radiative transfer libraries solve the
differential equations describing the interactions of incoming shortwave sunlight with both aerosols
and other gases in the atmosphere. Of all the aerosol-climate interactions, this scattering effect –
part of what is commonly termed the aerosol direct radiative effect – is the one best constrained.
Aerosol absorption of incoming sunlight can warm the climate
Some aerosol species, notably black carbon, can also absorb incoming shortwave radiation. This
heats the aerosol particles, which in turn emit longwave heat radiation to the ambient air. Increasing
concentrations of absorbing aerosols may therefore warm the climate, similar to the effect of
greenhouse gases.
As was the case for scattering, the ability of an aerosol particle to absorb sunlight depends on size,
optical properties and morphology. “Black carbon” is today understood to include a wide range of
products of incomplete combustion, from small, single spheres to long chains with greatly varying
shapes. Understanding the time evolution of black carbon optical properties after emission is
presently an active field of research.
Location also matters, as does altitude. When located above a highly reflective surface, such as ice or
clouds, absorbing aerosols may interact both with downwelling and reflected radiation. As a result,
averaged aerosol absorption efficiency has been found to increase dramatically with altitude, up to
the tropopause above which there are no more clouds. This means that the impact of black carbon
on the global energy balance gets stronger the higher up in the atmosphere it is transported. It is also
elevated above ice sheets such as at the Poles or over Greenland. Hence, thorough knowledge of
transport mechanisms, removal rates and atmospheric lifetime is crucial to determine the total
perturbation to the global energy balance from absorbing aerosols.
Deposition of black carbon on snow
Black carbon, which, as the name implies, has a dark color, has an additional climate interaction after
it is removed from the atmosphere. If the surface it lands on originally has high albedo, black carbon
will darken it, causing absorption of sunlight and hence heating of the surface.
This process is especially important on snow and ice. In the Arctic, long range transported black
carbon deposition is thought to contribute both to an increased melting rate of sea ice, and to the
observed heightened rate of Arctic warming relative to the global mean (Arctic amplification).
Deposition on glaciers is also thought to contribute to the significant reduction in volume of
Himalayan glaciers.
Aerosols affecting clouds
In addition to direct interaction with sunlight, aerosols may also affect the formation, albedo and
lifetime of clouds. These processes are usually termed the aerosol indirect and semidirect effects
(see below for alternate names).
Cloud albedo effect
Clouds form when supersaturated water vapor condenses into droplets. To get started, this process
needs physical objects for the first water molecules to attach to – so-called cloud condensation
nuclei (CCN). Many aerosol species are highly efficient as CCN, meaning that a change in aerosol
concentration will alter the rate of cloud droplet formation.
Natural aerosols, i.e. dust or sea spray, are present almost everywhere throughout the troposphere.
Hence a shortage of CCN is usually not an inhibiting factor for cloud formation. However, the
concentration of CCN may still determine the number of droplets formed for a given amount of
atmospheric water.
Due to multiple scattering, a cloud consisting of many small droplets will appear whiter than one
made up of fewer, larger drops, for the same amount of water. Whiter clouds reflect more sunlight.
Hence, an increase in the concentration of aerosols in a cloud forming layer will lead to increased
reflection of incoming radiation – i.e. a cooling effect on the Earth’s atmosphere.
The idea that anthropogenic aerosol emissions could alter cloud properties in this way was first
introduced by Sean Twomey, in a series of papers in 1977. When satellite imagery of ocean regions
became readily available, through the late 1980s, the concept was shown to be highly relevant as
ship exhaust was clearly seen to modify clouds above and downwind of the emission tracks. See
Figure 2 for a recent example.
The cloud albedo effect is sometimes termed the Twomey effect in the literature, or the 1st indirect
effect.
Cloud lifetime effect
Next, altering the number concentration and size distribution of cloud droplets will also affect the
atmospheric lifetime of a cloud. Clouds dissipate through evaporation of droplets, or through droplet
growth via collisions that eventually leads to precipitation. A larger concentration of CCN, and
subsequent change in droplet concentration and size, will act to reduce the rate of precipitation from
a given cloud, but may also enhance evaporation.
Whether the cloud lifetime effect from an increase in aerosols will heat or cool the climate depends
both on the altitude of the modified cloud, and on the sign of the lifetime change. Low clouds
normally cool the surface by blocking incoming sunlight. Conversely, High altitude clouds heat the
surface by reflecting back outgoing longwave radiation. Hence, the magnitude and even the sign of
the cloud lifetime effect from present anthropogenic aerosol emissions are poorly known.
The cloud lifetime effect is sometimes alternately termed the Albrecht effect in the literature, or the
2nd indirect effect.
Semidirect effects
Finally the presence of aerosols may affect the convective stability of the atmosphere, and the
ambient temperature within a cloud. Both of these effects may indirectly alter cloud formation rates,
and hence influence the climate. Collectively, these effects are termed the aerosol semidirect effects.
Convective cloud formation begins with updraft from the heated planetary surface. The strength of
this updraft is dependent on the change in temperature with altitude, or lapse rate, through the
troposphere. If an absorbing aerosol, like black carbon, is inserted at a high altitude, it will heat the
ambient air and reduce the lapse rate. Hence, cloud formation below the aerosol layer may be
reduced. At the altitude of the aerosol, however, the increased temperature may inhibit cloud
formation. Further, absorbing aerosols embedded in droplets within an already existing cloud may
contribute to its evaporation, so-called cloud burnoff.
The semidirect aerosol effect is at present poorly constrained, both by observations and modelling.
In summary, atmospheric aerosols have a range of direct and indirect interactions with the climate.
They can both scatter and absorb incoming, or reflected, shortwave radiation, while remaining
mostly inert to emitted longwave heat radiation. By acting as cloud condensation nuclei they alter
both the formation and evaporation rates of clouds, and hence also their lifetimes and precipitation
rates. By altering atmospheric stability, they also affect convective cloud formation. All of these
interactions are potentially significant enough to have global impacts on the Earth’s climate, should
the long term concentration of aerosols change.
Aerosols in the context of anthropogenic climate change
Over the industrial era, anthropogenic activities have introduced sources of atmospheric aerosols in
addition to their natural formation processes. While aerosols only stay in the atmosphere for some
days, the anthropogenic sources are still large enough to alter their global mean concentrations.
Today, aerosols are counted as one of the major factors that have altered the global energy balance
since preindustrial times.
In this section, we review present estimates of the radiative forcing exerted by changes in aerosol
abundances, through the climate interactions discussed above. Radiative forcing (RF) is defined as
the perturbation to the global energy balance due to a change in the climate system, before any
response in temperature or other climate processes has occurred. Forcing estimates are primarily
taken from the IPCC 5th Assessment Report (AR5). We also discuss other topics relevant to
anthropogenic climate change, such as the link between aerosols and precipitation, and how they
hamper our ability to determine the transient climate sensitivity from historical data.
Direct radiative forcing from anthropogenic emissions
Aerosol radiative forcing due to their direct interaction with radiation, colloquially termed direct
radiative forcing, has been estimated both from models and observational studies. Since there are
multiple aerosol species, with varying scattering and absorption strength, even the net direct aerosol
RF over the industrial era (here taken as 1750-2010) is quite uncertain.
Figure 4, from the IPCC AR5, shows a recent best assessment of the direct RF of the major
anthropogenic aerosol species. Hatched boxes show the result from a recent multi-model
assessment, while the solid boxes combine these results with independent observational constraints
and expert assessment.
Sulfate can be seen to be the dominant scattering component, having had a cooling effect on the
climate over the industrial era. BC is found to be the major absorbing component, with a positive RF.
Organic aerosol (POA) and nitrates are also scattering, with RFs likely to be negative. For biomass
burning aerosol, secondary organic aerosol and mineral dust due to industrial activities, estimates
vary even in sign.
The AR5 assessed the net RF of the direct radiative effect from aerosols to be -0.27 Wm-2, with a 9-95%
confidence range of [-0.77,0.23] Wm-2; i.e. it is highly likely to have been negative, but with a small
possibility for a positive value.
Indirect effects
The aerosol indirect effects are harder to assess. Fewer observational constraints exist, meaning we
must rely on model information. Unfortunately, cloud microphysics is among the hardest processes
to treat in global climate models, as computations quickly become prohibitively expensive. Hence
model results on the indirect are limited in precision both by their internal representation of clouds,
and their parametrizations of the various aerosol indirect interactions.
However the indirect effects play out, they can be expected to scale with aerosol abundance and the
hygroscopic properties of the aerosol species. Sulfate is reckoned as the major contributor to the
cloud albedo and lifetime effects. Black carbon, as the only major absorbing aerosol, is counted as
the major species for the semidirect effect.
Figure 5 shows the IPCC AR5 summary of radiative forcing over the industrial era. The above estimate
of -0.27 Wm-2 from the direct effect can be seen in the column labeled "Aerosols and precursor".
Below this, the AR5 best estimate for the combined indirect effects, or cloud modifications due to
aerosols, is listed as -0.55 (-1.33, -0.02) Wm-2.
As Figure 5 shows, the IPCC AR5 estimate for industrial era RF from CO2 is 1.68 Wm-2, with a 90%
uncertainty range of 1.33 - 2.03 Wm-2. While the total aerosol contribution is assessed to be much
smaller in magnitude, its uncertainty range is relatively larger. Hence, while anthropogenic emissions
of greenhouse gases are the main drivers of the present climate energy imbalance, the main
component in the remaining uncertainty comes from aerosol processes. Specifically, the indirect
aerosol effect is, at the time of writing, the climate interaction that has the largest uncertainty
relative to its magnitude. Significant effort is being put into reducing this uncertainty, but the
limitations due to computational power and lack of good observations are hard to overcome.
Historical evolution of emissions, and global dimming
The above estimates for aerosol RF are taken over the whole industrial era, i.e. the conditions in
2010 relative to those estimated to have prevailed in 1750. Between these times, aerosol emissions
and concentrations have naturally had a non-linear, spatially heterogeneous evolution. This evolution
broadly follows the progress of industrialization.
In Europe and North America, emissions of black carbon from closed combustion sources is
estimated to have peaked around 1920, and then to have rapidly declined as combustion engines
improved and air quality concerns came to the fore. In the rest of the world, black carbon emissions
started to increase gradually around 1950, and are estimated to peak around the present time.
Sulfate emissions have followed similar trends, with European and US emissions peaking in the early
20th century, then declining e.g. with the introduction of the Clean Air Acts. South-East Asia, on the
other hand, has seen a strong increase in sulfate loading from SO2 emissions since the 1960s.
This shift, from European and American sources and over to East Asian sources, combined with the
relatively short atmospheric lifetime of aerosols, implies a shift in local radiative forcing. This shift has
been implicated in the observed behavior of regional surface temperature, which shows a peak
around 1950 in Europe and the US, but not in Asia. The simplified picture is that aerosol scattering of
incoming sunlight, from sulfate in particular, held global warming in check over parts of the world for
some decades. The phenomenon itself, known as global dimming, is well established. Its connection
to the temperature evolution is however more tenuous, as volcanoes and natural variability of the
sun and oceans may also have played significant parts.
Aerosols and climate sensitivity
The perhaps most hotly burning question in all of climate science is the sensitivity of the global
temperature to a strengthened greenhouse effect - or the global climate sensitivity (CS). While both
observations and model estimates have improved greatly over the previous decades, estimates of
the climate sensitivity have stubbornly refused to improve. In fact, the best estimate from the IPCC
AR5 is virtually identical to that from the first assessment report that came out in 1990.
One challenge we face when trying to determine the climate sensitivity, is that there have been
several processes at work at the same time, altering the global energy balance in different ways.
While the well-mixed greenhouse gases have added energy to the system, aerosols, as we have seen
above, have most likely acted to cool the Earth. The situation is much like a tug-of-war, with
greenhouses gases and aerosols pulling in opposite directions. What we observe is the resulting
change in global mean temperature, and from this we must try to infer the two opposing forces.
For the climate sensitivity, it has been shown that the uncertainty interval from aerosol radiative
forcing matters strongly. The relationship between CS an the total aerosol RF is non-linear, in such a
way that while an overestimation of aerosol RF would not greatly affect CS estimates, an
underestimation could mean that the sensitivity is quite high. While recent research indicates that
climate sensitivities above 4K for a doubling of CO2 are highly unlikely, the remaining aerosol
uncertainty is one reason why this troubling possibility can still not be completely ruled out.
Aerosols and precipitation
As shown above, aerosols can perturb both cloud microphysics and atmospheric stability, both of
which may lead to changes in precipitation. In addition, since they can act to cool or heat the climate,
they also contribute to the changes in precipitation expected as part of global warming. Further, it
has been suggested that the recent shift in aerosol loading from Europe and the US, over to South
East Asia, could be linked to changes in the Asian monsoon and the pattern of pacific El Nino events.
Basic physics suggest that a change in aerosol concentration will affect precipitation on two time
scales: One set of fast effects, connected to the rapid response of clouds and atmospheric stability,
and another set connected to the long term change in surface temperature – and hence evaporation.
Recent modelling studies indicate that an isolated increase in black carbon would, at first, cause a
decrease in precipitation due to reduced convection. Over time, precipitation will again increase, as
the BC absorption acts to heat the surface. Overall, however, the precipitation change due to BC is
estimated to be negative. For sulfate, on the other hand, there is little initial precipitation change,
but a strong decrease over time as the aerosol cools the surface. In combination, increased aerosol
concentrations therefore reduce average precipitation.
As aerosol emissions and concentrations are local phenomena, the above general relations may
however break down due to regional competing effects, or for other measurable quantities such as
extreme precipitation. The precise links between observed and future changes in aerosol
concentrations and precipitation trends are presently under very active investigation.
Aerosols as climate mitigation
As aerosol emissions have significant climate impacts, and their atmospheric lifetimes are short, both
scientists and policy makers have argued for emission reductions as climate mitigation strategies. It is
commonly argued that, due to the link between aerosols, air quality and respiratory disease, such
reductions would be win-win scenarios.
Recently, however, several studies have noted that the climate impact that can be achieved through
aerosol emission mitigation is relatively modest, compared to the long term effects of reduced
greenhouse gas emissions. Aerosols are also rarely emitted in isolation, but are co-emitted from the
same sources. E.g. a coal fire will emit both black carbon and SO2, meaning that a reduction of such
fires would remove both a source of atmospheric heating and cooling.
There is no doubt that reductions in aerosol emissions would give health benefits. Their climate
mitigation potential is, however, presently a hot topic amongst scientists and policy makers.
Expected trends in anthropogenic aerosol emissions
As shown above, anthropogenic aerosol emissions have changed drastically over the previous
century, in magnitude, composition and geographical pattern. While it is hard to predict future
trends, a common expectation is that in the west, reductions in emissions will continue as technology
improves the use of combustion engines in road vehicles decreases and the composition of ship fuel
changes. In the East, where the majority of anthropogenic emissions can presently be found,
concerns about air quality and rapid technological development are both already thought to be
reducing emissions. Hence, it has been stated by experts that “the age of aerosols may soon be over”.
This view is reflected in the emission pathways considered for the IPCC 5th assessment report. All
pathways assume drastic reductions relative to year 2000 values, first in Europe and the US, then,
after 2020, also in Asia. While the details differ, all pathways project a reduction by half of
anthropogenic aerosol emissions by 2100. Globally, the climate impacts of such a reduction are
expected to be minor, due to the increasing dominance of radiative forcing from long lived
greenhouse gases. Locally, however, the reductions in aerosol burden may have significant impacts.
Large-scale climate impacts from aerosols
The previous section discussed the climate impact of anthropogenic activities, i.e. emissions from
industry, road transport, shipping, and large scale agricultural burning and land use change. There
are, however, several other areas – both manmade and natural - where aerosols may have even
larger impacts. This section discusses three of these: Volcanic eruptions, nuclear winter and climate
engineering by sulfate emissions.
Volcanic eruptions
Volcanic eruptions are among the most dramatic of natural events, both in visual splendor and in
their consequences for society. The introduction to this review showed how the 1991 eruption of Mt.
Pinatubo noticeably lowered global temperatures for several years. Larger eruptions, such as Mt.
Tambora in 1815 and Krakatoa in 1883, are well documented in history books, scientific records and
even art. The global dimming and cooling following the Mt. Tambora eruption lead to 1816 being
labeled “the year without summer”.
The large scale climate impact of volcanoes comes from aerosols. When large volcanoes erupt, they
can emit copious amounts of dust and SO2. If the eruption is of sufficient explosive strength, this dust
may reach tens of kilometers into the atmosphere, and end up in the stratosphere. There, well above
the precipitation processes that normally reduce aerosol concentrations, the particles can remain
suspended for a number of years. Due to aerosol transport with prevailing winds, the precise
impacts of a given eruption will depend both on its geographical location and the time of year, in
addition to the explosive power and volume of mass ejected.
Historically, the radiative forcing exerted by volcanic aerosols is the strongest short term
perturbation to the climate system. In the forcing time series included in the IPCC AR5, volcanic
eruptions can be seen as spikes with global mean values in excess of -4 Wm-2. The report further
observes that volcanic eruptions are stochastic in nature, meaning that their size and location cannot
easily be predicted. Hence, they constitute a major source of uncertainty in future climate
predictions. Should a major volcanic eruption occur in the near future, it might well offset global
warming due to long lived greenhouse gases for a short duration, or even cause the opposite
situation: Global cooling and precipitation changes sufficient to impact food production for a number
of years.
Presently, the duration of the climate perturbation after a major eruption is being discussed in
scientific literature. An example is the study of the so-called “Little ice age”, a period around the year
1500 where may temperature reconstructions show a colder climate, most notably in Europe and
North America. While several factors are thought to have been important for this cooler period,
including the Maunder grand solar minimum and changes to ocean circulation, several authors have
linked the effect to periods of heightened volcanic activity. This indicates that in some cases, volcanic
aerosols may have climate impacts that last beyond the 2-3 years estimated from the most recent
major eruptions.
Nuclear winter
While an armed conflict involving the use of nuclear weapons would be horrific in itself, scientists in
the 1980s started warning about an additional hazard. Were a large number of nuclear explosions to
take place over major cities, an additional result would be large, widespread and long lasting fires.
The smoke from these fires, i.e. a layer of aerosols, would spread across the globe and cause
significant global dimming. The resulting hypothetical cooling was termed nuclear winter, and
became part of the public debate on the nuclear arms race.
At the time, numerical climate models had several known shortcomings. In recent years, the topic
has however been thoroughly investigated using todays state of the art modelling tools and updated
estimates for emissions from major city fires.
The conclusions remain the same: A major, international nuclear war would have, as yet another
consequence, a global cooling of up to tens of degrees Celsius, lasting for several years. Such an
event would have devastating consequences for global agriculture, vegetation and wildlife, and
hence for society. Recent work also suggests that even a limited, regional conflict between two
neighboring nuclear powers could have subsequent climate consequences exceeding those of even
very strong volcanic eruptions. Such consequences would be global, even if the conflict that sparked
them was not.
Climate engineering via aerosols
As the consequences of the ongoing climate changes start being felt, and the difficulties of achieving
global agreements on emission mitigation or widespread adaptation, many have suggested that
there may be a third option: Could we, through technological means, engineer the climate back to
the state we have been used to?
One of the first suggestions to be made for such a large scale climate engineering scheme, is to
deliberately emit sulfates or other scattering aerosols. The idea is that their dimming of incoming
sunlight can balance the extra energy stored due to the enhanced greenhouse effect. In other words,
we would mimic the climate impact of volcanoes, using aerosols emitted from balloons or airplanes
in the stratosphere.
Another suggestion involving aerosols has been to increase the flux of sea spray droplets in regions
where marine stratocumulus clouds form. Here, we would utilize the aerosol indirect effect to make
the clouds whiter, again reflecting out more sunlight. A third aerosol based option is to seed high
altitude cirrus clouds, increasing their evaporation rate. This would reduce their heating of the
surface due to reflection of longwave radiation.
While such options may sound attractive in principle, they are all still very far from being ready for
deployment. Climate engineering in general is a contentious issue, both for practical and ethical
reasons. Research into the topic is presently mainly conducted with climate models, and even here,
under idealized conditions, results show that the climate impacts of large scale climate engineering
via aerosols would be highly unpredictable. At present, the technology to efficiently disperse sulfate
or sea spray aerosol at the right altitudes is also not available.
Experts on climate engineering presently hold a wide range of views on whether or not such a
strategy could ever be implemented. The main bulk of research however indicates that while the
global energy budget could indeed be brought back into balance through increased aerosol
scattering, an exact cancellation of the heightened greenhouse effect would never be possible.
Hence, we would anyway see changes to precipitation and large scale atmospheric circulation.
Further issues include the question of what would happen if the climate engineering measures were
ever to be terminated, or the inability of aerosol climate engineering to counter ocean acidification.
The scientific study of aerosols
While the notion that dust and other minute particles may influence the climate goes back several
centuries, the study of aerosol-climate interactions only became a fully-fledged scientific subfield
over the last few decades. Due to the multiple possible interactions, the large number of aerosol
types and their short atmospheric lifetimes, it was only with the advent of supercomputers that
scientists were able to make firm statements about the net role that aerosols in general - and
anthropogenic aerosols in particular - have had on the climate over the industrial era. Presently,
aerosol research is one of the most active parts of climate science, with regular publications
appearing in the most prestigious academic journals.
Early ideas linking aerosols to climate evolution
When Krakatoa exploded in the East Indies in 1883, ash and dust reduced incoming sunlight around
the world for months. Scientists at the time had no way to measure changes to global temperature,
but it was obvious that such major emissions did indeed cause at least regional cooling. As one of
nature’s most dramatic spectacles, volcanoes were in the early 1900s hypothesized to be the causes
both of the ice ages and more recent deviations found in early climate records. In the 1950s, when
the first reports of a gradually rising global temperature started showing up, many initially attributed
this to the long preceding period without major known eruptions.
At the same time, air pollution and urban smog were becoming major topics of concern.
Measurements from ships far out at sea showed that industrial byproducts were being transported
by air to the furthest reaches of the globe. Few, however, thought that this observations could have
any implications for weather and climate.
The link between local pollution, global climate and the basic physics and chemistry of aerosols was
only made in the 1960s, when the study of atmospheric aerosols gradually became a fully-fledged
scientific field. Early pioneers such as Robert McCormic, John Ludwig, Reid Bryson, Murray Mitchell,
and Hubert Lamb debated, in increasingly technical terms, whether aerosols had so far had a climate
impact. At the same time, they urged the relevant communities to form stronger ties to make a more
focused attempt at understanding the full role of aerosols.
In a pioneering study in 1971, Ichitaque Rasool and Stephen Schneider showed that it had become
possible to perform numerical studies of aerosols that were detailed enough to give realistic answers.
They discussed the distinction between aerosols that scatter sunlight, i.e. cooled the atmosphere,
and absorbing aerosols that could lead to warming. Their early, admittedly primitive results indicated
that, dependent on composition and amount, aerosols could on relatively short timescales cause
climate perturbations of several degrees Celsius.
The 1970s saw a debate among scientists as to whether aerosols were then primarily cooling or
heating, and whether we should worry about a possible global cooling catastrophe from industrial
pollution. At the same time numerical models were being continually improved and new links
between climate and aerosols were being described scientifically. E.g. the first indirect aerosol effect
was described by Sean Twomey in 1977.
Through the 1980s, the major players in the problem of anthropogenic aerosols emerged: Soot,
sulfate and their interactions with radiation and clouds. In 1982, Mother Nature gave a helping hand
through the eruption of El Chichon in Mexico. Measurements of the amount of sulfur emitted to the
atmosphere and the resulting reginal cooling could be compared to rudimentary climate models,
allowing for tuning of parameters that made the calculations far more realistic.
Aerosol science in the age of supercomputers
In spite of two decades of advances, the first IPCC report in 1990 noted that, regarding
anthropogenic aerosol emissions, "at this stage, neither the sign nor magnitude of the proposed
climatic feedback can be quantitatively estimated".
The real turning point came with the event described in the introduction to this review: The 1991
massive eruption of Mt. Pinatubo. Following the event, the group of James Hansen at NASA GISS
quickly published a model based prediction of the ensuing global cooling - a prediction that was,
broadly, borne out by observations over the subsequent years. Their results, amongst others, led to
the realization that previous calculations of past and future temperature change due to greenhouse
gas emissions might have been significantly off since they did not include the cooling effect of
sulfates.
A revised calculation from the UK Hadley Center, published in 1995, showed that including sulfates
indeed gave a significant improvement in the predictive skill of their numerical model. It became
clear that sulfate emissions from Western industry, from 1940 and onwards, had temporarily held
global warming in check. This insight was one key aspect that allowed the IPCC in 1995 to conclude
that anthropogenic climate change was by then discernible.
Recent developments and outstanding issues
Since the mid-1990s aerosol science has steadily attracted ever more attention. The aerosol
representation in the complex earth system models used to predict future climate is becoming ever
more complex and realistic. Observations of aerosol distribution, transport and loading are available
from the surface, from satellites and from instruments mounted on aircraft.
As shown above, however, the scientific uncertainty on the net climate impact of aerosol is still
relatively large. In the 2013 Working Group I part of the IPCC 5th Assessment Report, a whole
chapter was devoted to recent developments related to aerosols and their climate impacts, together
with the influence of - and by aerosols on - clouds. This combination of topics presently feels natural,
as it has become clear that the total aerosol climate impact cannot be estimated without a thorough
knowledge of cloud microphysics.
A wide range of topics are presently being discussed in the aerosol climate field. While it is believed
that most major interactions between particulate matter and climate evolution are identified, the
validation of computer modelling of these effects by observational data is a key issue. Issues remain
both with the numerical representations and with the remote sensing and in situ observations
available. A major setback to the field came with the loss of the GLORY satellite on launch in 2011.
GLORY was, amongst other things, to have observed in detail the spatial and temporal distributions,
physical and optical properties of aerosols with unprecedented details. It is presently unclear if and
when a replacement satellite will be launched.
Burning scientific aerosol-climate issues at the time of writing include:
•
What are the global annual aerosol emissions, from natural and anthropogenic processes?
•
How far and how high are aerosols transported after emission?
•
How significant is the warming effect of black carbon, and can mitigation of black carbon
emissions be used as an effective short term strategy to counter global warming?
•
How can we sufficiently model the microphysical interactions of aerosols with cloud droplets
in global earth system models?
•
Do the cyclic variations in the flux of galactic cosmic rays, as modulated by the change in the
solar magnetic field over a sunspot cycle, have a noticeable impact on aerosol formation and
hence on clouds?
•
To what degree have scattering aerosols masked global warming from long lived greenhouse
gases over the 20th century?
•
What, if any, is the link between aerosol emissions and extreme precipitation?
Presently, atmospheric aerosols are part of a wide range of climate discussions. As an example of this,
the World Climate Research Programme (WCRP, a research promoting organization sponsored by the
World Meteorological Organization (WMO), the International Council for Science (ICSU) and the
Intergovernmental Oceanographic Commission (IOC) of UNESCO), recently defined a set of five
"grand challenges" that the international climate research community faces. Of these five, improved
knowledge about aerosols is crucial for four of them: "Clouds, circulation and climate sensitivity",
"Melting ice and global consequences", "Climate extremes" and "Water availability".
While our understanding of aerosols, their basic physics and chemistry, formation, transport and
various climate interactions has improved tremendously over the past decades, there is still
significant work to be done. Aerosols will remain a topic at the forefront of climate research for the
foreseeable future.
Figures
Figure 1: Aerosol optical depth from the NASA MODIS instrument, showing the annual average aerosol loading for
different regions. The pie charts show the species composition within selected boxes, as calculated by the chemical
transport model OsloCTM2.
Figure 2: Tracks of clouds appearing in the wake of passing ships, imaged from space. These so-called ship tracks are a
visual example of the ability of aerosols to act as could droplet condensation nuclei.
Figure 3: Summary of the major aerosol-climate interactions, and the nomenclature adopted in the IPCC AR5. See the
main text for additional mechanisms.
Figure 4: Overview of the radiative forcing from the direct radiative effect for the main aerosol species. The solid boxes
show the recent expert assessment presented in the IPCC AR5. Note the large spread on the total RF.
Figure 5: Summary of anthropogenic radiative forcing per 2011, from the IPCC AR5. Note how the aerosols and
precursors have both positive and negative impacts on the global energy balance, and the size of the uncertainty bar on
the cloud adjustments due to aerosols.
Figure sources and full resolution links
1. Myhre, G., Myhre, C. E.L., Samset, B. H. & Storelvmo, T. (2013) Aerosols and their Relation to
Global Climate and Climate Sensitivity. Nature Education Knowledge 4(5):7
http://www.nature.com/scitable/knowledge/library/aerosols-and-their-relation-to-globalclimate-102215345
2. NASA Earth Observatory
http://earthobservatory.nasa.gov/IOTD/view.php?id=37455
3. IPCC AR5, WG1, Chapter 7
http://www.climatechange2013.org/images/figures/WGI_AR5_Fig7-3.jpg
4. IPCC AR5, WG1, Chapter 7
http://www.climatechange2013.org/images/figures/WGI_AR5_Fig7-18.jpg
5. IPCC AR5, WG1, Summary for Policymakers
http://www.climatechange2013.org/images/figures/WGI_AR5_FigSPM-5.jpg
Selected references / sources
[Bond et al., 2013; Boucher et al., 2013; Editorial, 2009; Hansen et al., 1992; Isaksen et al., 2009; Liou,
2002; G. Myhre et al., 2013a; G. Myhre et al., 2013b; Robock and Toon, 2012; Shepherd, 2012;
Twomey, 1977; Weart, 2003]
Bond, T. C., et al. (2013), Bounding the role of black carbon in the climate system: A scientific
assessment, Journal of Geophysical Research: Atmospheres, 118(11), 5380-5552 doi:
10.1002/jgrd.50171.
Boucher, O., et al. (2013), Clouds and Aerosols. In: Climate Change 2013: The Physical Science Basis.
Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on
Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y.
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Editorial, N. (2009), Time for early action, Nature, 460(7251), 12-12 doi: Doi 10.1038/460012a.
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Liou, K. N. (2002), Chapter 8 Radiation and climate, in An Introduction to Atmospheric Radiation, in
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Myhre, G., C. Myhre, B. H. Samset, and T. Storelvmo (2013a), Aerosols and their Relation to Global
Climate and Climate Sensitivity, Nature Knowledge Project, 4(5).
Myhre, G., et al. (2013b), Anthropogenic and Natural Radiative Forcing. In: Climate Change 2013: The
Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen,
J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)], Cambridge University Press,
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Robock, A., and O. B. Toon (2012), Self-assured destruction: The climate impacts of nuclear war,
Bulletin of the Atomic Scientists, 68(5), 66-74 doi: 10.1177/0096340212459127.
Shepherd, J. G. (2012), Geoengineering the climate: an overview and update, Philos T R Soc A,
370(1974), 4166-4175 doi: DOI 10.1098/rsta.2012.0186.
Twomey, S. (1977), The Influence of Pollution on the Shortwave Albedo of Clouds, J Atmos Sci, 34(7),
1149-1152 doi: 10.1175/1520-0469(1977)034<1149:TIOPOT>2.0.CO;2.
Weart, S. R. (2003), The Discovery of Global Warming, Harvard University Press.