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2005 radiative forcings and uncertainties as estimated by the IPCC.
Atmospheric aerosols affect the climate of the earth by changing the amount of incoming solar radiation
and outgoing terrestrial longwave radiation retained in the earth's
system. This occurs through several distinct mechanisms which are split
into direct, indirect
and semi-direct aerosol effects. The aerosol climate effects are the
biggest source of uncertainty in future climate predictions. The Intergovernmental Panel on Climate Change, Third Assessment Report, says: While the radiative forcing due to greenhouse gases
may be determined to a reasonably high degree of accuracy... the
uncertainties relating to aerosol radiative forcings remain large, and
rely to a large extent on the estimates from global modelling studies
that are difficult to verify at the present time.
Aerosol radiative effects
Global aerosol optical thickness. The aerosol scale (yellow to dark reddish-brown) indicates the relative amount of particles that absorb sunlight.
Direct effect
Particulates in the air causing shades of grey and pink in Mumbai during sunset
The direct aerosol effect consists of any direct interaction of
radiation with atmospheric aerosols, such as absorption or scattering.
It affects both short and longwave radiation to produce a net negative
radiative forcing. The magnitude of the resultant radiative forcing due to the direct effect of an aerosol is dependent on the albedo
of the underlying surface, as this affects the net amount of radiation
absorbed or scattered to space. e.g. if a highly scattering aerosol is
above a surface of low albedo it has a greater radiative forcing than if
it was above a surface of high albedo. The converse is true of
absorbing aerosol, with the greatest radiative forcing arising from a
highly absorbing aerosol over a surface of high albedo. The direct aerosol effect is a first order effect and is therefore classified as a radiative forcing by the IPCC. The interaction of an aerosol with radiation is quantified by the single-scattering albedo (SSA), the ratio of scattering alone to scattering plus absorption (extinction)
of radiation by a particle. The SSA tends to unity if scattering
dominates, with relatively little absorption, and decreases as
absorption increases, becoming zero for infinite absorption. For
example, the sea-salt aerosol has an SSA of 1, as a sea-salt particle
only scatters, whereas soot has an SSA of 0.23, showing that it is a
major atmospheric aerosol absorber.
Indirect effect
The
Indirect aerosol effect consists of any change to the earth's radiative
budget due to the modification of clouds by atmospheric aerosols, and
consists of several distinct effects. Cloud droplets form onto
pre-existing aerosol particles, known as cloud condensation nuclei (CCN).
For any given meteorological conditions, an increase in CCN leads
to an increase in the number of cloud droplets. This leads to more
scattering of shortwave radiation i.e. an increase in the albedo of the
cloud, known as the Cloud albedo effect, First indirect effect or Twomey effect. Evidence supporting the cloud albedo effect has been observed from the effects of ship exhaust plumes and biomass burning
on cloud albedo compared to ambient clouds. The Cloud albedo aerosol
effect is a first order effect and therefore classified as a radiative
forcing by the IPCC.
An increase in cloud droplet number due to the introduction of
aerosol acts to reduce the cloud droplet size, as the same amount of
water is divided into more droplets. This has the effect of suppressing
precipitation, increasing the cloud lifetime, known as the cloud
lifetime aerosol effect, second indirect effect or Albrecht effect. This has been observed as the suppression of drizzle in ship exhaust plume compared to ambient clouds, and inhibited precipitation in biomass burning plumes. This cloud lifetime effect is classified as a climate feedback (rather than a radiative forcing) by the IPCC due to the interdependence between it and the hydrological cycle. However, it has previously been classified as a negative radiative forcing.
Semi-direct effect
The
Semi-direct effect concerns any radiative effect caused by absorbing
atmospheric aerosol such as soot, apart from direct scattering and
absorption, which is classified as the direct effect. It encompasses
many individual mechanisms, and in general is more poorly defined and
understood than the direct and indirect aerosol effects. For instance,
if absorbing aerosols are present in a layer aloft in the atmosphere,
they can heat surrounding air which inhibits the condensation of water
vapour, resulting in less cloud formation.
Additionally, heating a layer of the atmosphere relative to the surface
results in a more stable atmosphere due to the inhibition of
atmospheric convection. This inhibits the convective uplift of moisture,
which in turn reduces cloud formation. The heating of the atmosphere
aloft also leads to a cooling of the surface, resulting in less
evaporation of surface water. The effects described here all lead to a
reduction in cloud cover i.e. an increase in planetary albedo. The
semi-direct effect classified as a climate feedback) by the IPCC due to the interdependence between it and the hydrological cycle. However, it has previously been classified as a negative radiative forcing.
Roles of different aerosol species
Sulfate aerosol
Sulfate aerosol has two main effects, direct and indirect. The direct effect, via albedo, is a cooling effect that slows the overall rate of global warming: the IPCC's best estimate of the radiative forcing is −0.4 watts per square meter with a range of −0.2 to −0.8 W/m²
but there are substantial uncertainties. The effect varies strongly
geographically, with most cooling believed to be at and downwind of
major industrial centres. Modern climate models addressing the attribution of recent climate change
take into account sulfate forcing, which appears to account (at least
partly) for the slight drop in global temperature in the middle of the
20th century. The indirect effect (via the aerosol acting as cloud
condensation nuclei, CCN, and thereby modifying the cloud properties -albedo and lifetime-) is more uncertain but is believed to be a cooling.
Black carbon
Black
carbon (BC), or carbon black, or elemental carbon (EC), often called
soot, is composed of pure carbon clusters, skeleton balls and buckyballs,
and is one of the most important absorbing aerosol species in the
atmosphere. It should be distinguished from organic carbon (OC):
clustered or aggregated organic molecules on their own or permeating an
EC buckyball. BC from fossil fuels is estimated by the IPCC in the
Fourth Assessment Report of the IPCC, 4AR, to contribute a global mean
radiative forcing of +0.2 W/m² (was +0.1 W/m² in the Second Assessment
Report of the IPCC, SAR), with a range +0.1 to +0.4 W/m². Bond et al.,
however, states that "the best estimate for the industrial-era (1750 to
2005) direct radiative forcing of atmospheric black carbon is +0.71 W/m²
with 90% uncertainty bounds of (+0.08, +1.27) W/m²" with "total direct
forcing by all black carbon sources, without subtracting the
preindustrial background, is estimated as +0.88 (+0.17, +1.48) W/m²"
Instances of aerosol affecting climate
Solar radiation reduction due to volcanic eruptions
Volcanoes are a large natural source of aerosol and have been linked
to changes in the earth's climate often with consequences for the human
population. Eruptions linked to changes in climate include the 1600
eruption of Huaynaputina which was linked to the Russian famine of 1601 - 1603, leading to the deaths of two million, and the 1991 eruption of Mount Pinatubo which caused a global cooling of approximately 0.5 °C lasting several years.
Research tracking the effect of light-scattering aerosols in the
stratosphere during 2000 and 2010 and comparing its pattern to volcanic
activity show a close correlation. Simulations of the effect of
anthropogenic particles showed little influence at present levels.
Aerosols are also thought to affect weather and climate on a
regional scale. The failure of the Indian Monsoon has been linked to the
suppression of evaporation of water from the Indian Ocean due to the
semi-direct effect of anthropogenic aerosol.
Recent studies of the Sahel drought and major increases since 1967 in rainfall over the Northern Territory, Kimberley, Pilbara and around the Nullarbor Plain have led some scientists to conclude that the aerosol haze over South and East Asia has been steadily shifting tropical rainfall in both hemispheres southward.
The latest studies of severe rainfall decline over southern Australia since 1997
have led climatologists there to consider the possibility that these
Asian aerosols have shifted not only tropical but also midlatitude
systems southward.
