c) Methane Sensitivity
Due
to
the
GWP-values
(global
warming
potential)
as
published
in
the
AR4
report
of
the
IPCC,
methane
is
classified
to
have
a
72x
greater
potential
for
global
warming
than
CO
2
over
a
time
horizon
of
20
years
and
25x
greater
potential
over
100
years,
this
dependent
on
the
time,
over
which
methane
exists
in
the
atmosphere
(IPCC
assumes
a
shorter
lifetime
of
12
years
compar
-
ed to CO
2
with 55 years). Therefore, CH
4
is stronger weighted.
Note,
these
lifetimes
are
absolutely
speculative,
from
own
actual
studies
of
the
carbon
cycle
(see sub-section d) the residence time of CO
2
is only about 4 yr.
The
radiative
efficiency
of
methane
with
3.7x10
-4
W/m
2
/ppb
is
also
classified
to
be
25x
larger
than
that
of
CO
2
with
1.4x10
-5
W/m
2
/ppb.
Such
values
are
derived
from
the
changing
absorp
-
tivity
or
emissivity
of
the
gases,
when
their
concentration
is
chan
ging
by
1
ppb
(parts
per
bil
-
lion).
But
what
people
are
doing
is
to
compare
cherries
with
pumpkins,
two
gases
under
com
-
plete
ly
dif
ferent
conditions:
CH
4
at
a
concentration
of
1.8
ppm
and
CO
2
at
a
200x
larger
con
-
centra
tion,
when
it
is
already
strongly
saturated.
Also
the
interference
with
the
other
green
-
house
gases,
particu
larly
with
WV
is
for
both
gases
completely
different.
Only
these
diverse
condi
tions
pretend
a
much
higher
radiative
efficiency
of
methane,
resp
a
too
low
value
for
CO
2
.
So,
at
a
concentration
of
400
ppm
the
radiative
efficiency
of
CH
4
drops
to
about
half
of
that
of
CO
2
,
and
at
a
concentration
of
1.8
ppm
the
efficiency
of
CO
2
is
about
twice
that
value
specified for CH
4
.
Therefore,
to
compare
what
CH
4
will
really
contribute
to
global
warming,
can
only
be
determin
-
ed
by
analogous
calculations
to
the
CO
2
case,
and
this
also
for
the
same
relative
concentration
changes, e.g. a doubling of the respective concentrations.
Calculations
of
the
absorptivities
as
a
function
of
the
CH
4
concentration
then
take
place
on
a
scale
two
hundred
times
smaller,
now
from
0
to
3.6
ppm.
They
were
performed
for
global
con
-
ditions
with
a
water
vapor
concentration
of
14,614
ppm,
CO
2
of
380
ppm,
a
tempe
rature
of
16
°C and a lapse rate of 6.5 °C/km.
The
variation
of
the
sw
absorptivity
over
this
scale
with
0.31
%
is
3x
smaller
than
the
respective
changes
in
the
CO
2
case
over
760
ppm,
and
within
this
range
we
only
see
little
saturation
ef
-
fects.
On
the
other
hand,
the
lw
absorptivity
already
suffers
from
medium
saturation
despite
of these low concentrations.
This
different
behavior
of
sw
and
lw
absorptivities
is
in
so
far
of
some
importance
as
the
domi
-
nating
lw
absorption
contributes
to
an
ascending
Earth-temperature
but
already
shows
clear
satu
ration
at
doubled
concentration,
while
an
increasing
sw
atmospheric
absorptivity
contri
-
butes
to
a
reduced
surface
absorption
and
thus
a
smaller
surface
heating
with
higher
concen
-
trations.
With
these
absorptivities
of
CH
4
and
otherwise
identical
parameters
as
used
for
CO
2
,
simula
-
tion
with
the
two-layer
model
gives
a
CH
4
sen
sitivity
as
temperature
increase
at
doubled
concentration of
only EMS = 0.016 °C, which is 44x smaller than CO
2
under these conditions.
Therefore,
methane
has
no
noticeable
effect
as
GH-gas
for
global
warming,
and
the
same
holds for nitrous oxide.
Physics & Climate