direct equivalent in the past climates of the Earth. The last
time we find a climate with the current level of atmospheric
CO 2 is several million years ago. The climate at this period
corresponds to a warm Earth, with a reduced cryosphere (no
Greenland) and a smaller Antarctic ice sheet yielding a much
higher sea level (15–30 m higher as reported by Haywood
et al. 2011, 2016).
Evolution of Greenhouse Gases
We have seen in previous chapters that the greenhouse effect
is above all a natural phenomenon, without which the surface temperature of the Earth would be about −18 °C,
making life, as we know it today, impossible. The main
greenhouse gases naturally present are water vapor (H 2 O),
carbon dioxide (CO 2 ) emitted by volcanic eruptions and
forest fires, methane (CH 4 ) produced by wetlands and various fermentation processes, ozone (O 3 ), and the nitrous
oxide (N 2 O) emitted by soils. Industrialization has led
societies to discharge massive amounts of these gases
through the combustion of fossil fuels (oil, gas, coal),
deforestation, agriculture, intensive livestock breeding and
fertilizer production. An inhabitant of an industrialized
country releases on average ten tons of carbon per year (or
CO 2 equivalent) compared with only two tons per year for
an inhabitant of most emerging countries. However, these
estimates mask large disparities from one country to another,
with differences ranging from 0.12 ton of CO 2 per year and
per inhabitant for Ethiopia to 49.3 tons of CO 2 per year and
per inhabitant for Qatar (from Global Carbon Atlas 2017). In
addition, human activities produce fluorinated gases (CFCs,
HFCs, PFCs, SF 6 ) used especially in refrigeration and air
conditioning systems, as well as in aerosol cans. In total,
more than forty of these gases have been identified by the
IPCC. The greenhouse effect produced by human activities
is called additional greenhouse effect. The contribution of
each gas to the additional greenhouse effect can be estimated
by taking into account the increase in their concentration and
their ‘radiative efficiency’. Between 1750 and 2011, the
variation in their concentration increased the greenhouse
effect by about 2.83 W/m
2 , with main contributions of 64%
for CO 2 , 17% for CH 4 , 12% for O 3 and 6% for N 2 O
(Fig. 31.1). Since 1750, in other words, since the beginning
of the industrial era, this anthropogenic phenomenon has
produced an energy imbalance of the Earth and has caused a
warming of the lower layers of the atmosphere. The additional radiative forcing corresponds to approximately 1% of
the total radiation received.
Analysis of air bubbles trapped in the ice of the Antarctic
ice sheet revealed that over the last 800,000 years, CO 2 levels
have changed by no more than 100 ppm, going from
180 ppm during glacial periods to 280 ppm in interglacial
periods. For periods prior to 1950, analysis of air bubbles in
the ice is the only reliable way to track the chemical composition of the atmosphere. In recent times, the first direct
measurements (i.e. in situ) were obtained in 1958 at the
Mauna Loa site in Hawaii (Keeling et al. 1995). These measurements revealed for the first time that not only was CO 2
increasing in the atmosphere, but that this increase was
modulated in line with seasonal variations due to photosynthesis of the terrestrial biosphere. This first measurement
campaign was then supplemented by campaigns covering
other sites in the northern and southern hemispheres. Currently, a wide range of direct and indirect measurements
confirm that atmospheric CO 2 levels have increased since the
beginning of the preindustrial era, rising from 275–285 ppm
between the years 1000 and 1750, to about 380 ppm in 2005
(Fig. 31.1) and to more than 410 ppm in 2018, that is a difference of more than 100 ppm compared to the pre-industrial
period. About 30% of the current atmospheric CO 2 has been
emitted by anthropogenic sources. In addition, the amount of
annual anthropogenic emissions has been continuously
increasing throughout the industrial era. In 1990, 2000, 2010
and 2017, global CO 2 emissions from human activities (fossil
fuel combustion, cement production, land-use change)
reached 22, 25, 32 and 37 Gt CO 2 /yr respectively. The
increase in annual CO 2 emissions accelerated from 1.1% per
year in the 1990s to 3.3% per year in the 2000s. This growth
rate of atmospheric CO 2 is ten times faster than the highest
rates recorded in ice cores, and is mainly due to the rapid
growth of developing countries and a drop in the efficiency of
fossil fuel use in the global economy. Between 65% and 80%
of CO 2 released in the atmosphere is trapped and/or dissolved
in the ocean and the terrestrial biosphere in 20–200 years,
depending on the various estimations. The rest is removed by
slower processes, including chemical weathering and rock
formation that take several thousands of years, indicating that
the effect of anthropogenic CO 2 will persist for hundreds to
thousands of years into the future (Archer et al. 2009).
Atmospheric methane is the third most important greenhouse gas after H 2 O and CO 2 in terms of atmospheric
concentration. Averaged over 100 years, the radiative efficiency of methane is estimated to be 28–36 times greater
than that of CO 2 but its lifetime in the atmosphere (i.e. the
time it takes for a CH 4 molecule to be removed from the
atmosphere by chemical reaction) is much less (* 9–
12 years) than that of CO 2 . Ice core records indicate that
CH 4 levels in the atmosphere also show variations from
about 350 ppb (during glacial periods) to 700 ppb (during
interglacial periods). In 2011, the level of methane in the
atmosphere, established from a network of measurements
covering both hemispheres, was at 1803 ppb, a level never
attained, at least throughout the last 800,000 years. Since the
pre-industrial era, CH 4 has increased by approximately
250%. Although the growth rate of methane was over 1%
444
S. Charbit et al.
time we find a climate with the current level of atmospheric
CO 2 is several million years ago. The climate at this period
corresponds to a warm Earth, with a reduced cryosphere (no
Greenland) and a smaller Antarctic ice sheet yielding a much
higher sea level (15–30 m higher as reported by Haywood
et al. 2011, 2016).
Evolution of Greenhouse Gases
We have seen in previous chapters that the greenhouse effect
is above all a natural phenomenon, without which the surface temperature of the Earth would be about −18 °C,
making life, as we know it today, impossible. The main
greenhouse gases naturally present are water vapor (H 2 O),
carbon dioxide (CO 2 ) emitted by volcanic eruptions and
forest fires, methane (CH 4 ) produced by wetlands and various fermentation processes, ozone (O 3 ), and the nitrous
oxide (N 2 O) emitted by soils. Industrialization has led
societies to discharge massive amounts of these gases
through the combustion of fossil fuels (oil, gas, coal),
deforestation, agriculture, intensive livestock breeding and
fertilizer production. An inhabitant of an industrialized
country releases on average ten tons of carbon per year (or
CO 2 equivalent) compared with only two tons per year for
an inhabitant of most emerging countries. However, these
estimates mask large disparities from one country to another,
with differences ranging from 0.12 ton of CO 2 per year and
per inhabitant for Ethiopia to 49.3 tons of CO 2 per year and
per inhabitant for Qatar (from Global Carbon Atlas 2017). In
addition, human activities produce fluorinated gases (CFCs,
HFCs, PFCs, SF 6 ) used especially in refrigeration and air
conditioning systems, as well as in aerosol cans. In total,
more than forty of these gases have been identified by the
IPCC. The greenhouse effect produced by human activities
is called additional greenhouse effect. The contribution of
each gas to the additional greenhouse effect can be estimated
by taking into account the increase in their concentration and
their ‘radiative efficiency’. Between 1750 and 2011, the
variation in their concentration increased the greenhouse
effect by about 2.83 W/m
2 , with main contributions of 64%
for CO 2 , 17% for CH 4 , 12% for O 3 and 6% for N 2 O
(Fig. 31.1). Since 1750, in other words, since the beginning
of the industrial era, this anthropogenic phenomenon has
produced an energy imbalance of the Earth and has caused a
warming of the lower layers of the atmosphere. The additional radiative forcing corresponds to approximately 1% of
the total radiation received.
Analysis of air bubbles trapped in the ice of the Antarctic
ice sheet revealed that over the last 800,000 years, CO 2 levels
have changed by no more than 100 ppm, going from
180 ppm during glacial periods to 280 ppm in interglacial
periods. For periods prior to 1950, analysis of air bubbles in
the ice is the only reliable way to track the chemical composition of the atmosphere. In recent times, the first direct
measurements (i.e. in situ) were obtained in 1958 at the
Mauna Loa site in Hawaii (Keeling et al. 1995). These measurements revealed for the first time that not only was CO 2
increasing in the atmosphere, but that this increase was
modulated in line with seasonal variations due to photosynthesis of the terrestrial biosphere. This first measurement
campaign was then supplemented by campaigns covering
other sites in the northern and southern hemispheres. Currently, a wide range of direct and indirect measurements
confirm that atmospheric CO 2 levels have increased since the
beginning of the preindustrial era, rising from 275–285 ppm
between the years 1000 and 1750, to about 380 ppm in 2005
(Fig. 31.1) and to more than 410 ppm in 2018, that is a difference of more than 100 ppm compared to the pre-industrial
period. About 30% of the current atmospheric CO 2 has been
emitted by anthropogenic sources. In addition, the amount of
annual anthropogenic emissions has been continuously
increasing throughout the industrial era. In 1990, 2000, 2010
and 2017, global CO 2 emissions from human activities (fossil
fuel combustion, cement production, land-use change)
reached 22, 25, 32 and 37 Gt CO 2 /yr respectively. The
increase in annual CO 2 emissions accelerated from 1.1% per
year in the 1990s to 3.3% per year in the 2000s. This growth
rate of atmospheric CO 2 is ten times faster than the highest
rates recorded in ice cores, and is mainly due to the rapid
growth of developing countries and a drop in the efficiency of
fossil fuel use in the global economy. Between 65% and 80%
of CO 2 released in the atmosphere is trapped and/or dissolved
in the ocean and the terrestrial biosphere in 20–200 years,
depending on the various estimations. The rest is removed by
slower processes, including chemical weathering and rock
formation that take several thousands of years, indicating that
the effect of anthropogenic CO 2 will persist for hundreds to
thousands of years into the future (Archer et al. 2009).
Atmospheric methane is the third most important greenhouse gas after H 2 O and CO 2 in terms of atmospheric
concentration. Averaged over 100 years, the radiative efficiency of methane is estimated to be 28–36 times greater
than that of CO 2 but its lifetime in the atmosphere (i.e. the
time it takes for a CH 4 molecule to be removed from the
atmosphere by chemical reaction) is much less (* 9–
12 years) than that of CO 2 . Ice core records indicate that
CH 4 levels in the atmosphere also show variations from
about 350 ppb (during glacial periods) to 700 ppb (during
interglacial periods). In 2011, the level of methane in the
atmosphere, established from a network of measurements
covering both hemispheres, was at 1803 ppb, a level never
attained, at least throughout the last 800,000 years. Since the
pre-industrial era, CH 4 has increased by approximately
250%. Although the growth rate of methane was over 1%
444
S. Charbit et al.