Paris Climate Agreement Beacon of Hope

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brighter (leading to cooling) and the deposition of dark, carbonaceous material on
snow that darkens the surface (leading to warming). Contrails from aircraft have led
to a slight warming, mainly because of induced clouds. A slight warming term is
also attributed to an increase in stratospheric humidity driven by rising tropospheric
CH 4 , which is converted to H 2 O when lost in the stratosphere. The depletion of
stratospheric O 3 has resulted in slight cooling. Finally and most importantly, scien-
tists have been able to estimate the time variation of total solar irradiance over the
course of the Anthropocene. The trend in solar irradiance over this two and a half
century time period is small. Even if the MWP discussed above turns out to be as
warm as the present decades, presumably due to an increase in solar irradiance dur-
ing the middle of the Common Era (Bard et al. 2000 ), scientists are nonetheless
confident the rise in temperature over the Anthropocene was driven by rising GHGs
and not a change in the luminosity of our Sun (Chap. 8, IPCC ( 2013 )).

1.2.2 Global Warming Potential

The global warming potential (GWP) metric was developed to guide public policy
decisions regarding trade-offs of release of various GHGs. The GWP of a particular
compound represents the ratio of the rise in global mean surface temperature
(GMST) due to the release of a particular amount (mass) of this compound, relative
to the rise in GMST resulting from the release of the same amount (mass) of CO 2.
Inherent in the computation of GWP is that the increase in GMST is found over a
particular time horizon.
The most commonly used time horizons are 20 and 100 years. The mathematical
expression for the GWP of CH 4 over a 100-year time horizon, which relies on the
use of a calculus concept called integration, is given by:

GWP

atdt

atdt

CH

CH

CO

yrs

CH

yrs

CO

4

0

100

4

0

100

2

4

2

()=

́ ()

()

ò

ò ́

(1.2)

where aCH 4 and aCO 2 are the radiative efficiencies (units W m−2 kg−1) of CH 4 and
CO 2 , respectively, and CH 4 (t) and CO 2 (t) represent the time dependent response
to the release into the atmosphere of the same mass of these two GHGs.^6 The
atmospheric lifetime of a GHG denotes the time it takes for a pulsed release of
the gas to decay by 1/e of the initial value, where e ≈ 2.718. The lifetimes of CH 4

(^6) Equation 1.2 represents a computer simulation of the cumulative radiative forcing of climate over
a 100 year time period due to release of a pulse of a certain amount (mass) of CH 4 , divided by the
cumulative radiative forcing over the same time period due to simulated release of the same amount
of CO 2. Since the pulse of CH 4 decays faster than the pulse of CO 2 , due to the shorter lifetime of
CH 4 , the GWP of CH 4 is larger when shorter time periods are considered.
1 Earth’s Climate System