8
Finally, changes in global mean surface temperature, to which RF of climate will be
related, are much more certain from 1765 to present than for times extending back
to the invention of agriculture (NAS 2006 ). Our choice is not meant to dismiss the
importance of human influence on climate prior to 1765. If, as suggested by
Ruddiman ( 2003 ), human activity 8000 ybp did indeed offset the onset of extensive
glaciation due to declining summer insolation at Northern high latitudes (driven by
Milankovitch orbital variations), this would be a fascinating benefit of human inge-
nuity, especially for indigenous peoples of high northerly latitudes.
Rather than wade deeper into the debate over the start of the Anthropocene, we
next describe a few figures that illustrate the human fingerprint on the global carbon
cycle and climate change over the past several centuries. Along the way, the math-
ematical principles needed to understand the material presented in Chaps. 2 and 3
are developed.
1.2.1 Radiative Forcing
In the absence of an atmosphere, the temperature of Earth would be governed by:
T
AlbedoS
EARTH=æ()-èç
çöø÷
÷
(^14)
1
4
s
(1.1)
Albedo, the Latin word for whiteness, refers to the fraction of incoming sunlight
reflected to space (commonly about 0.3 (or 30 %) for Earth), S is the luminosity of our
Sun at the distance of Earth’s orbit (1370 W m−2), and σ is the Stefan Boltzmann con-
stant (5.67 × 10−8 W m−2 K−4). A value of S/4 is used because the Earth intercepts
sunlight like a disk and radiates heat like a sphere; 1/4 is the ratio of the surface area
of a disk to that of a sphere (this concept as well as Eq. 1.1 are explained in many
introductory Earth Science textbooks). Solving for the putative temperature of planet
Earth without an atmosphere yields 255 K, which is −18° Celsius (°C) or 0° Fahrenheit
(°F). This is much colder than the average temperature of today’s Earth. If the tem-
perature found using Eq. 1.1 actually applied, our oceans would be frozen.
The greenhouse effect, the trapping of radiation by our planet’s atmosphere, is
responsible for the difference between the Earth’s actual temperature and that found
using Eq. 1.1. Earth’s mean surface temperature is about 15.5 °C or 60 °F. Earth’s
atmosphere is responsible for increasing the amount of energy the surface receives,
by several hundred W m−2, in comparison to an Earth devoid of an atmosphere. This
excess heat is driven by the abundance and molecular properties of GHGs such as
H 2 O, CO 2 , CH 4 , and N 2 O, as well as clouds (i.e., condensed H 2 O droplets).
Infrared radiation (or heat in the form of photons) emitted by Earth’s surface is
resonant with various vibrational modes of GHG molecules, inducing these photons
to be absorbed and re-emitted in all directions. Some of this absorbed and re- emitted
radiation is sent back to the surface. As such, GHGs in Earth’s atmosphere act as a
blanket, trapping heat that would otherwise escape to space. Water vapor, the most
1 Earth’s Climate System