Key Messages
• Aerosol loading over India has substantially increased
during the recent few decades. The annual mean 500 nm
aerosol optical depth (AOD) from ground-based observations shows an overall increasing trend of *2% year
−1
during the last 30 years (high confidence). This trend in
AOD is subject to seasonal variability. The rate of
increase in AOD is significantly high during the dry
months of December–March.
• The aerosol radiative forcing over India shows wide
spatiotemporal variability resulting from the non-uniform
distribution of aerosol burden over the region. Estimates
of aerosol radiative forcing from measurements range
from −49 to −31 W m
−2 at the surface (high confidence)
and from −15 to +8 W m
−2 at the top-of-atmosphere (low
confidence). The estimates at the top of the atmosphere
are highly sensitive to the single scattering albedo values.
• The understanding of the aerosol indirect effect and
aerosol impacts on precipitation has low confidence and
needs to be addressed with process studies in different
cloud systems and their environments.
• There is substantial spatiotemporal variability in the
concentration of ozone (O 3 ) and its precursors over the
Indian region. In general, there is an increasing trend in
the ozone mixing ratios in the troposphere (+0.7 to
+0.9% year
−1 during 1979–2005, medium confidence)
and a decreasing trend in the stratosphere (−0.05 to
−0.4% year
−1 during 1993–2015, medium confidence).
Trends are driven by precursor gases emitted by anthropogenic activities.
• Over the Indian region, the estimates of radiative forcing
(at the tropopause) due to tropospheric ozone increase
since pre-industrial times vary between *0.2 and
0.4 W m
−2 .
5.1 Introduction
Aerosols and trace gases are essential drivers of climate
change. They influence Earth’s energy budget leading to
climate change through various pathways. Their climatic
impacts are eventually manifested as precipitation changes,
increased evaporation, elevated temperatures, etc. Hence,
information on their ‘sources and sink,’ ‘physical and
chemical processes,’ and ‘distribution’ is important for an
accurate prediction of the climate.
The Indian subcontinent is directly influenced by different
aerosol species via changes in the insolation, atmospheric
temperature structure, and alteration of the regional hydrological cycle. Along with absorption and scattering of
incoming solar radiation, aerosols interact with clouds
modifying its radiative properties and precipitation efficiency
(Box 5.1). The aerosol concentrations over the subcontinent
are dominated by wind-driven desert dust, biomass burning,
industrialization, agricultural activities, etc. Rapid growth in
population, industrialization, and urbanization—over South,
East, and Southeast Asia—has contributed to the significant
rise in emissions producing different types of aerosol over
the region. The associated increase in anthropogenic aerosol
loading in recent decades (Satheesh et al. 2017) has led to
increased reduction of surface insolation, contributing to
solar dimming over the Indian landmass, affecting the
energy balance at the surface (Ramanathan et al. 2005; Soni
et al. 2012). The high aerosol burden has also been linked to
changes in the hydrological cycle of the region (Box 5.3).
The long-term decline in southwest monsoon precipitation
has been associated with anthropogenic aerosol forcing over
South Asia [(Krishnan et al. 2016), Box 5.3].
Box 5.1: How Aerosols Affect Regional Climate?
Atmospheric aerosols are tiny solid/liquid/mixed particles suspended in the air originating from natural or
anthropogenic sources. With a typical lifetime of days
to weeks in the troposphere and about a year in the
stratosphere, aerosol size ranges from a few nanometers to several tens of micrometers. Aerosol particles
influence the climate in different ways (Box 5.1,
Fig. 5.1).
Fundamentally, aerosol particles absorb and scatter
incoming solar radiation modifying the global and
regional radiative budget. Non-absorbing aerosols like
sulfate, nitrate, and sea spray scatter shortwave radiation back to space leading to a net cooling of the climate system while absorbing aerosols produce the
opposite effect. Carbonaceous aerosols (black carbon,
organic carbon) and mineral dust can absorb and
scatter sunlight producing either warming or cooling
effects determined by aerosol properties and environmental conditions. Absorbing aerosols also affect climate when present in surface snow by lowering
surface albedo, yielding a positive radiative forcing,
directly changing the melting of snow and ice.
Although depending on the local emissions and
transport processes, regionally, the anthropogenic
aerosol radiative forcing can be either negative or
positive; it is well established that globally, the
radiative effect of anthropogenic aerosols produces
cooling of the planet (IPCC 2013).
Additionally, aerosol particles act as cloud condensation nuclei (CCN) and ice nuclei (IN) and
therefore have a significant impact on cloud properties
94
S. Fadnavis et al.
• Aerosol loading over India has substantially increased
during the recent few decades. The annual mean 500 nm
aerosol optical depth (AOD) from ground-based observations shows an overall increasing trend of *2% year
−1
during the last 30 years (high confidence). This trend in
AOD is subject to seasonal variability. The rate of
increase in AOD is significantly high during the dry
months of December–March.
• The aerosol radiative forcing over India shows wide
spatiotemporal variability resulting from the non-uniform
distribution of aerosol burden over the region. Estimates
of aerosol radiative forcing from measurements range
from −49 to −31 W m
−2 at the surface (high confidence)
and from −15 to +8 W m
−2 at the top-of-atmosphere (low
confidence). The estimates at the top of the atmosphere
are highly sensitive to the single scattering albedo values.
• The understanding of the aerosol indirect effect and
aerosol impacts on precipitation has low confidence and
needs to be addressed with process studies in different
cloud systems and their environments.
• There is substantial spatiotemporal variability in the
concentration of ozone (O 3 ) and its precursors over the
Indian region. In general, there is an increasing trend in
the ozone mixing ratios in the troposphere (+0.7 to
+0.9% year
−1 during 1979–2005, medium confidence)
and a decreasing trend in the stratosphere (−0.05 to
−0.4% year
−1 during 1993–2015, medium confidence).
Trends are driven by precursor gases emitted by anthropogenic activities.
• Over the Indian region, the estimates of radiative forcing
(at the tropopause) due to tropospheric ozone increase
since pre-industrial times vary between *0.2 and
0.4 W m
−2 .
5.1 Introduction
Aerosols and trace gases are essential drivers of climate
change. They influence Earth’s energy budget leading to
climate change through various pathways. Their climatic
impacts are eventually manifested as precipitation changes,
increased evaporation, elevated temperatures, etc. Hence,
information on their ‘sources and sink,’ ‘physical and
chemical processes,’ and ‘distribution’ is important for an
accurate prediction of the climate.
The Indian subcontinent is directly influenced by different
aerosol species via changes in the insolation, atmospheric
temperature structure, and alteration of the regional hydrological cycle. Along with absorption and scattering of
incoming solar radiation, aerosols interact with clouds
modifying its radiative properties and precipitation efficiency
(Box 5.1). The aerosol concentrations over the subcontinent
are dominated by wind-driven desert dust, biomass burning,
industrialization, agricultural activities, etc. Rapid growth in
population, industrialization, and urbanization—over South,
East, and Southeast Asia—has contributed to the significant
rise in emissions producing different types of aerosol over
the region. The associated increase in anthropogenic aerosol
loading in recent decades (Satheesh et al. 2017) has led to
increased reduction of surface insolation, contributing to
solar dimming over the Indian landmass, affecting the
energy balance at the surface (Ramanathan et al. 2005; Soni
et al. 2012). The high aerosol burden has also been linked to
changes in the hydrological cycle of the region (Box 5.3).
The long-term decline in southwest monsoon precipitation
has been associated with anthropogenic aerosol forcing over
South Asia [(Krishnan et al. 2016), Box 5.3].
Box 5.1: How Aerosols Affect Regional Climate?
Atmospheric aerosols are tiny solid/liquid/mixed particles suspended in the air originating from natural or
anthropogenic sources. With a typical lifetime of days
to weeks in the troposphere and about a year in the
stratosphere, aerosol size ranges from a few nanometers to several tens of micrometers. Aerosol particles
influence the climate in different ways (Box 5.1,
Fig. 5.1).
Fundamentally, aerosol particles absorb and scatter
incoming solar radiation modifying the global and
regional radiative budget. Non-absorbing aerosols like
sulfate, nitrate, and sea spray scatter shortwave radiation back to space leading to a net cooling of the climate system while absorbing aerosols produce the
opposite effect. Carbonaceous aerosols (black carbon,
organic carbon) and mineral dust can absorb and
scatter sunlight producing either warming or cooling
effects determined by aerosol properties and environmental conditions. Absorbing aerosols also affect climate when present in surface snow by lowering
surface albedo, yielding a positive radiative forcing,
directly changing the melting of snow and ice.
Although depending on the local emissions and
transport processes, regionally, the anthropogenic
aerosol radiative forcing can be either negative or
positive; it is well established that globally, the
radiative effect of anthropogenic aerosols produces
cooling of the planet (IPCC 2013).
Additionally, aerosol particles act as cloud condensation nuclei (CCN) and ice nuclei (IN) and
therefore have a significant impact on cloud properties
94
S. Fadnavis et al.