The projected future heatwaves based on the extremes of
wet-bulb temperature, which includes the effect of humidity,
are found to be concentrated around densely populated
agricultural regions of the Ganges and Indus river basins (Im
et al. 2017). The human populations in these regions highly
vulnerable to heat stress are assessed to experience maximum daily wet-bulb temperatures exceeding 31 °C by 2100
under RCP4.5 scenario, considered dangerous levels for
most humans (Im et al. 2017). These extreme wet-bulb
temperatures are likely to approach and, in few locations,
exceed 35 °C by the 2070s under RCP8.5 scenario, considered as an upper limit on human survivability (Coffel
et al. 2018). The annual probability of occurrence of heat
waves with magnitude greater than the one in Russia in 2010
(the most severe of the present era) at 1.5 and 2 °C global
warming above pre-industrial levels is assessed using
CMIP5 ensemble to be different than zero in few parts of
India when measured by means of an heatwave index, which
takes into account both temperature and relative humidity
(Russo et al. 2017). The yearly probability of occurrence of a
heatwave at 4° global warming with magnitude greater than
this most severe event in the present period will be greater
than 10% in many parts of India, with the southern peninsula
expected to experience such type of humid heatwaves with
an annual probability greater than 50%, corresponding to an
average return period of two years (Russo et al. 2017).
2.4 Knowledge Gaps
To improve the assessment of India’s observed and projected
warming and its impacts, the following gaps would need to
be addressed:
• The uneven spatial distribution of temperature observation sites over India may lead to errors in the assessment
of present-day temperature changes, particularly over the
northern parts of the country with a very sparse network.
• Confidence in the assessed long-term temperature trends
may be constrained by the data inhomogeneity due to
changing observation site locations.
• There has been an increase in costly extreme temperature
events (e.g. heat waves) across India. Hence urgent
research studies are needed on event attribution that
evaluates how the probability or intensity of a heatwave
event, or more generally, a class of extreme temperature
events, has changed as a result of increases in atmospheric greenhouse gases from human activity.
• The CMIP5 multi-model ensemble members sampling
structural uncertainty and internal variability cannot be
treated as purely independent because some climate
models have been developed by sharing model components leading to shared biases. This implies a reduction in
the effective number of independent CMIP5 models.
The CORDEX South Asia ensemble consists of two
RCMs driven by a subset of CMIP5 AOGCMs, implying
a very little effective number of independent members in
this multi-RCM ensemble.
• The contribution of natural internally generated variability to the total uncertainty in the sub-regional/local temperature projections need to be quantitatively assessed
using an ensemble of high-resolution future climate projections for India. The existing ensemble of dynamically
downscaled temperature projections from CORDEX
South Asia multi-RCMs does not sample initial conditions, which are needed to quantify the contribution of
internal variability to the total uncertainty at smaller
spatial scales.
• More research is needed to understand whether the
increased water vapour under conditions of regional
warming is leading to significant positive feedback on
human-induced climate change, as water vapour is the
most important contributor to the natural greenhouse
effect.
• Assessment of joint projections of multiple variables over
India are needed to understand the key processes relevant
to future projected significant increases in temperature
variability and extremes, for example, projected changes
by combining mean temperature and precipitation; linking soil moisture, precipitation and temperature mean and
variability; combining temperature, humidity, etc.
2.5 Summary
In summary, the annual mean, maximum and minimum
temperatures averaged over India as a whole show significant warming trend of 0.15 °C, 0.15 °C and 0.13 °C per
decade respectively since 1986 (high confidence). The
maximum warming trend is seen during the pre-monsoon
season for the recent 30-year period 1986–2015. It is very
likely that all India averaged annual and seasonal
near-surface air specific humidity have increased since the
1980s. The significant increasing trend in specific humidity
assessed during the pre-monsoon season is consistent with
the largest surface warming trend found for this season.
The observed surface air temperature changes over India
are attributed to anthropogenic forcing (medium confidence).
The maximum temperature during the post-monsoon and
minimum temperature during the pre-monsoon and monsoon
seasons are attributed with confidence to climate change
induced by anthropogenic effects (medium confidence).
The all India averaged frequency of warm extremes has
increased since 1951 with accelerated warming trends during
the recent 30 year period 1986–2015. The annual increase in
2 Temperature Changes in India
41
wet-bulb temperature, which includes the effect of humidity,
are found to be concentrated around densely populated
agricultural regions of the Ganges and Indus river basins (Im
et al. 2017). The human populations in these regions highly
vulnerable to heat stress are assessed to experience maximum daily wet-bulb temperatures exceeding 31 °C by 2100
under RCP4.5 scenario, considered dangerous levels for
most humans (Im et al. 2017). These extreme wet-bulb
temperatures are likely to approach and, in few locations,
exceed 35 °C by the 2070s under RCP8.5 scenario, considered as an upper limit on human survivability (Coffel
et al. 2018). The annual probability of occurrence of heat
waves with magnitude greater than the one in Russia in 2010
(the most severe of the present era) at 1.5 and 2 °C global
warming above pre-industrial levels is assessed using
CMIP5 ensemble to be different than zero in few parts of
India when measured by means of an heatwave index, which
takes into account both temperature and relative humidity
(Russo et al. 2017). The yearly probability of occurrence of a
heatwave at 4° global warming with magnitude greater than
this most severe event in the present period will be greater
than 10% in many parts of India, with the southern peninsula
expected to experience such type of humid heatwaves with
an annual probability greater than 50%, corresponding to an
average return period of two years (Russo et al. 2017).
2.4 Knowledge Gaps
To improve the assessment of India’s observed and projected
warming and its impacts, the following gaps would need to
be addressed:
• The uneven spatial distribution of temperature observation sites over India may lead to errors in the assessment
of present-day temperature changes, particularly over the
northern parts of the country with a very sparse network.
• Confidence in the assessed long-term temperature trends
may be constrained by the data inhomogeneity due to
changing observation site locations.
• There has been an increase in costly extreme temperature
events (e.g. heat waves) across India. Hence urgent
research studies are needed on event attribution that
evaluates how the probability or intensity of a heatwave
event, or more generally, a class of extreme temperature
events, has changed as a result of increases in atmospheric greenhouse gases from human activity.
• The CMIP5 multi-model ensemble members sampling
structural uncertainty and internal variability cannot be
treated as purely independent because some climate
models have been developed by sharing model components leading to shared biases. This implies a reduction in
the effective number of independent CMIP5 models.
The CORDEX South Asia ensemble consists of two
RCMs driven by a subset of CMIP5 AOGCMs, implying
a very little effective number of independent members in
this multi-RCM ensemble.
• The contribution of natural internally generated variability to the total uncertainty in the sub-regional/local temperature projections need to be quantitatively assessed
using an ensemble of high-resolution future climate projections for India. The existing ensemble of dynamically
downscaled temperature projections from CORDEX
South Asia multi-RCMs does not sample initial conditions, which are needed to quantify the contribution of
internal variability to the total uncertainty at smaller
spatial scales.
• More research is needed to understand whether the
increased water vapour under conditions of regional
warming is leading to significant positive feedback on
human-induced climate change, as water vapour is the
most important contributor to the natural greenhouse
effect.
• Assessment of joint projections of multiple variables over
India are needed to understand the key processes relevant
to future projected significant increases in temperature
variability and extremes, for example, projected changes
by combining mean temperature and precipitation; linking soil moisture, precipitation and temperature mean and
variability; combining temperature, humidity, etc.
2.5 Summary
In summary, the annual mean, maximum and minimum
temperatures averaged over India as a whole show significant warming trend of 0.15 °C, 0.15 °C and 0.13 °C per
decade respectively since 1986 (high confidence). The
maximum warming trend is seen during the pre-monsoon
season for the recent 30-year period 1986–2015. It is very
likely that all India averaged annual and seasonal
near-surface air specific humidity have increased since the
1980s. The significant increasing trend in specific humidity
assessed during the pre-monsoon season is consistent with
the largest surface warming trend found for this season.
The observed surface air temperature changes over India
are attributed to anthropogenic forcing (medium confidence).
The maximum temperature during the post-monsoon and
minimum temperature during the pre-monsoon and monsoon
seasons are attributed with confidence to climate change
induced by anthropogenic effects (medium confidence).
The all India averaged frequency of warm extremes has
increased since 1951 with accelerated warming trends during
the recent 30 year period 1986–2015. The annual increase in
2 Temperature Changes in India
41