1.1 General Introduction
The Earth’s climate has varied considerably throughout its
history. Periodic and episodic natural changes caused by
natural climate forcings such as orbital variations and volcanic eruptions, and amplified by feedback processes
intrinsic to the climate system, have induced substantial
changes in planetary climate on a range of timescales.
Multiple independent lines of investigation have provided
increasingly compelling evidence that human activities have
significantly altered the Earth’s climate since the industrial
revolution (Stocker et al. 2013). A distinctive aspect of the
present-day climate change is the rapid pace at which it is
proceeding relative to that of natural variations alone; a pace
that is unprecedented in the history of modern civilization.
The Earth’s energy budget, which is the balance between
the energy that the Earth receives from the Sun and the
energy that it radiates back into space, is a key factor that
determines the Earth’s global mean climate. The composition of the atmosphere alters climate by modulating the
incoming and outgoing radiative fluxes at the surface. The
key drivers of present-day climate change are anthropogenic
(human-caused) emissions of greenhouse gases (GHGs),
aerosols and changes in land use and land cover (LULC).
GHGs warm the surface by reducing the amount of Earth’s
terrestrial radiation escaping directly to space. Atmospheric
concentrations of the key GHGs—carbon dioxide, methane,
and nitrous oxide—are now at higher levels than they have
been at any time over the last 800,000 years, according to ice
core records. In addition, their mean rates of increase over the
past century are, with high confidence, unprecedented in the
last 22,000 years (Stocker et al. 2013).
Aerosols are small particles or droplets suspended in the
atmosphere produced from both natural and anthropogenic
sources. Natural sources include mineral dust from soil
erosion, sea salt and volcanic eruptions. Key anthropogenic
sources are industrial air pollution, transport, and biomass
burning, which produce airborne sulfates, nitrates, ammonium and black carbon, and dust produced by land degradation processes such as desertification. Aerosols tend to
cool the surface by scattering or absorption of solar radiation
(direct effect), or by enhancing cloud formation (indirect
effect). Aerosol pollution, due to human activities, has thus
offset a part of the warming caused by anthropogenic GHG
emissions (Myhre et al. 2013).
Much of the Earth’s land surface has been affected by
considerable changes in land use and land cover (LULC)
over the past few centuries (and even earlier), mainly
because of deforestation and the expansion of agriculture.
Deforested areas have a diminished capacity to act as a
carbon dioxide sink and, if accompanied by biomass burning, are a direct source of GHGs. Conversion of land from
natural vegetation to agriculture or pasturage also alters the
terrestrial albedo, contributing to changes in the surface
radiative balance.
The net effect of human-induced climate forcing has been
an increase in the global average near-surface air temperature
1 by approximately 1 °C since pre-industrial times (Allen
et al. 2018). Each of the last three decades has been successively warmer at the Earth’s surface than any preceding
decade since 1850 (Stocker et al. 2013) and 2001–2018 have
been 18 of the 19 warmest years in the observational record.
Trends in other important global climate indicators such as
rate and patterns of precipitation, temperature and precipitation extremes, atmospheric water vapour concentration,
continental ice melt, sea-level rise, ocean heat content, ocean
acidification and the frequency of powerful cyclones are
consistent with the response expected from a warming planet
(Stocker et al. 2013). At the current rate of temperature rise, it
is likely
2 that global warming will reach 1.5 °C between 2030
and 2052 (high confidence) (Allen et al. 2018) and 3–5 °C by
the end of the century relative to pre-industrial times. Even if
warming is limited to 1.5 °C in the twenty-first century,
certain slowly evolving changes such as ocean thermal
expansion would persist well beyond 2100 causing sea levels
to continue rising (high confidence).
While climate change is global, changes in climate are not
expected to be uniform across the planet. For instance,
Arctic temperatures are rising much faster than the global
average (Stocker et al. 2013), and rates of sea-level rise vary
significantly across the world (Church et al. 2013). Changes
in climate at regional scales are not understood as robustly as
at the global scale due to insufficient local observational data
or understanding of physical phenomena specific to given
regions (Flato et al. 2013). Yet, knowledge of present and
expected changes in regional climate is critical to people and
policymakers to plan for disaster management, risk mitigation and for formulating locally relevant adaptation strategies
(Burkett et al. 2014).
The regional climate over the Indian subcontinent
involves complex interactions of the atmosphere–ocean–
land–cryosphere system on different space and time scales.
In addition, there is evidence that anthropogenic activities
have influenced the regional climate in recent decades.
Impacts associated with human-induced climate change such
as increasing heat extremes, changing monsoon patterns and
sea-level rise pose serious threats to lives and livelihoods on
the subcontinent. This makes it necessary to understand how
and why the climate is changing across India and how these
changes are expected to evolve in the future. The following
1
The ‘near surface air temperature’ is defined as the temperature 2 m
above the surface over land areas, and as the sea surface temperature
(SST) for oceanic areas.
2
Defined in Box 1.4.
2
R. Krishnan et al.
The Earth’s climate has varied considerably throughout its
history. Periodic and episodic natural changes caused by
natural climate forcings such as orbital variations and volcanic eruptions, and amplified by feedback processes
intrinsic to the climate system, have induced substantial
changes in planetary climate on a range of timescales.
Multiple independent lines of investigation have provided
increasingly compelling evidence that human activities have
significantly altered the Earth’s climate since the industrial
revolution (Stocker et al. 2013). A distinctive aspect of the
present-day climate change is the rapid pace at which it is
proceeding relative to that of natural variations alone; a pace
that is unprecedented in the history of modern civilization.
The Earth’s energy budget, which is the balance between
the energy that the Earth receives from the Sun and the
energy that it radiates back into space, is a key factor that
determines the Earth’s global mean climate. The composition of the atmosphere alters climate by modulating the
incoming and outgoing radiative fluxes at the surface. The
key drivers of present-day climate change are anthropogenic
(human-caused) emissions of greenhouse gases (GHGs),
aerosols and changes in land use and land cover (LULC).
GHGs warm the surface by reducing the amount of Earth’s
terrestrial radiation escaping directly to space. Atmospheric
concentrations of the key GHGs—carbon dioxide, methane,
and nitrous oxide—are now at higher levels than they have
been at any time over the last 800,000 years, according to ice
core records. In addition, their mean rates of increase over the
past century are, with high confidence, unprecedented in the
last 22,000 years (Stocker et al. 2013).
Aerosols are small particles or droplets suspended in the
atmosphere produced from both natural and anthropogenic
sources. Natural sources include mineral dust from soil
erosion, sea salt and volcanic eruptions. Key anthropogenic
sources are industrial air pollution, transport, and biomass
burning, which produce airborne sulfates, nitrates, ammonium and black carbon, and dust produced by land degradation processes such as desertification. Aerosols tend to
cool the surface by scattering or absorption of solar radiation
(direct effect), or by enhancing cloud formation (indirect
effect). Aerosol pollution, due to human activities, has thus
offset a part of the warming caused by anthropogenic GHG
emissions (Myhre et al. 2013).
Much of the Earth’s land surface has been affected by
considerable changes in land use and land cover (LULC)
over the past few centuries (and even earlier), mainly
because of deforestation and the expansion of agriculture.
Deforested areas have a diminished capacity to act as a
carbon dioxide sink and, if accompanied by biomass burning, are a direct source of GHGs. Conversion of land from
natural vegetation to agriculture or pasturage also alters the
terrestrial albedo, contributing to changes in the surface
radiative balance.
The net effect of human-induced climate forcing has been
an increase in the global average near-surface air temperature
1 by approximately 1 °C since pre-industrial times (Allen
et al. 2018). Each of the last three decades has been successively warmer at the Earth’s surface than any preceding
decade since 1850 (Stocker et al. 2013) and 2001–2018 have
been 18 of the 19 warmest years in the observational record.
Trends in other important global climate indicators such as
rate and patterns of precipitation, temperature and precipitation extremes, atmospheric water vapour concentration,
continental ice melt, sea-level rise, ocean heat content, ocean
acidification and the frequency of powerful cyclones are
consistent with the response expected from a warming planet
(Stocker et al. 2013). At the current rate of temperature rise, it
is likely
2 that global warming will reach 1.5 °C between 2030
and 2052 (high confidence) (Allen et al. 2018) and 3–5 °C by
the end of the century relative to pre-industrial times. Even if
warming is limited to 1.5 °C in the twenty-first century,
certain slowly evolving changes such as ocean thermal
expansion would persist well beyond 2100 causing sea levels
to continue rising (high confidence).
While climate change is global, changes in climate are not
expected to be uniform across the planet. For instance,
Arctic temperatures are rising much faster than the global
average (Stocker et al. 2013), and rates of sea-level rise vary
significantly across the world (Church et al. 2013). Changes
in climate at regional scales are not understood as robustly as
at the global scale due to insufficient local observational data
or understanding of physical phenomena specific to given
regions (Flato et al. 2013). Yet, knowledge of present and
expected changes in regional climate is critical to people and
policymakers to plan for disaster management, risk mitigation and for formulating locally relevant adaptation strategies
(Burkett et al. 2014).
The regional climate over the Indian subcontinent
involves complex interactions of the atmosphere–ocean–
land–cryosphere system on different space and time scales.
In addition, there is evidence that anthropogenic activities
have influenced the regional climate in recent decades.
Impacts associated with human-induced climate change such
as increasing heat extremes, changing monsoon patterns and
sea-level rise pose serious threats to lives and livelihoods on
the subcontinent. This makes it necessary to understand how
and why the climate is changing across India and how these
changes are expected to evolve in the future. The following
1
The ‘near surface air temperature’ is defined as the temperature 2 m
above the surface over land areas, and as the sea surface temperature
(SST) for oceanic areas.
2
Defined in Box 1.4.
2
R. Krishnan et al.