Paris Climate Agreement Beacon of Hope

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important GHG, is responsible for the majority of radiation sent back to the surface.
The abundance of H 2 O in our atmosphere is controlled by thermodynamics: i.e., the
evaporation of H 2 O from the oceans, condensation of H 2 O in the atmosphere to
form clouds, and deposition of H 2 O back to the surface in the form of precipitation.
Atmospheric H 2 O varied prior to the onset of human influence; the effect of thermo-
dynamics on various isotopes of H 2 O preserved in ice cores is an important tool for
quantitative reconstruction of past climate (Jouzel et al. 1987 ).
The radiative forcing (RF) of climate refers to the increase in the amount of heat
directed to Earth’s lower atmosphere as the abundance of GHGs rise. Here and through-
out this book, we follow the convention established in the 2001 Intergovernmental
Panel on Climate Change (IPCC) Physical Science Basic Report (IPCC 2001 ) that RF
of climate is defined as the change in the net flow of energy (sunlight plus infrared heat)
at the tropopause (boundary between the lower atmosphere, or troposphere, and the
upper atmosphere, or stratosphere) relative to a particular start date, after allowing for
stratospheric temperatures to adjust to radiative equilibrium. This concept is explained
well in Sect. 2.2 of IPCC ( 2007 ).
Figure 1.3a shows several time series of the RF of climate over the Anthropocene.
The curves are set to zero in year 1765 and represent changes relative to this start time,

Anthropocene

∆RF of Climate, All Greenhouse Gases

∆RF of Climate, CO 2 , CH 4 , N 2 O

∆RF of Climate, Total Anthropogenic

∆RF of Climate, Anthropogenic Aerosols

a

1750 1800 1850 1900 1950 2000

0.2

0.0

0.2

0.4

0.6

0.8

1.0

∆

T (°C) from preindustrial

∆RF of Climate, Total Anthropogenic

∆T, HadCRUT4

∆T, Jones and Mann 2004

b

∆

RF (W m

2 )

1.0

0.0

1.0

2.0

3.0

∆

RF (W

m

2 )

0.0

0.5

1.0

1.5

2.0

2.5

Fig. 1.3 Radiative forcing of climate and temperature, Anthropocene. (a) change in the radiative
forcing of climate (ΔRF) relative to year 1765, from various factors (as indicated) for the RCP 4.5
scenario (Meinshausen et al. 2011 ); (b) total anthropogenic ΔRF (red) and the global mean surface
temperature anomaly (ΔT) relative to pre-industrial baseline (1850–1900 mean) from the modern
instrument record (HadCRUT4) (Jones et al. 2012 ) and from various proxies (Jones and Mann
2004 ). See Methods for further information

1.2 The Anthropocene

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