1. Earth’s Energy Balance

2. Earth’s Energy Balance
Earth’s Energy Balance
Outgoing
Heat Energy
Incoming
Solar Energy
The intensity of sunlight reaching the earth’s outer atmosphere has been very steady, ranging only slightly from its mean value of about 1370 watts of short-wave energy per square meter as it goes through its sunspot cycles. Although this value is called the solar constant, there is evidence that it can vary by larger amounts on century and millennial time scales.
When averaged around the entire planet, this incoming energy is about 342 watts of energy for each square meter of the earth’s surface.
About 31% of this radiation is reflected back to space by clouds, atmospheric dust and other aerosols in the atmosphere, and the earth’s surface.
Changes in the amount of dust in the atmosphere, the extent and type of clouds and the reflective properties of the earth surface can cause the amount of reflected energy to vary considerably by season, from year to year and over longer time scales.
The remaining 69% of the sun’s energy absorbed by the atmosphere and the earth’s surface heats the climate system and drives the dynamic behaviour of the atmosphere and the world’s oceans.
To maintain a stable climate, just as much energy must be returned to space by some means as is absorbed by the climate system. The climate system accomplishes this through the emission of long wave (infrared) heat energy from the earth’s surface and atmosphere towards space.
However, molecules of trace gases within the atmosphere, commonly known as greenhouse gases, as well as clouds and aerosols absorb most of this outgoing radiation, then re-radiate the absorbed energy in all directions, some back towards the surface. Much of the energy re-radiated upwards is also absorbed again by other greenhouse gas molecules and aerosols at higher elevations, and this process is repeated. While, for a balanced climate, the amount of energy that finally escapes into space is still, on average, equal to the absorbed solar energy, enough of the outgoing infrared is recycled to increase surface temperatures significantly.
References: Adapted from IPCC Climate Change 2001: The Scientific Basis Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton,J.T. et al. (eds.)]. Cambridge University Press, Cambridge, UK. Fig 1.2

3. Estimating Earth’s Expected Temperature using Stefan-Boltzmann law
Estimating Earth’s Expected Temperature
using Stefan-Boltzmann law
Net Incoming Solar Energy
Outgoing Heat Energy =
(S0 (1-A) R2)
(4R2kTe4)
where
S0 is the solar constant
A is average albedo, or reflectivity
R is the radius of the earth
k is Boltzmann’s constant
Te is earth’s apparent temperature (seen from space)
In the above equation, the climate is assumed to be in balance, with net loss of infrared energy released from the earth’s surface and atmosphere to space equal (on average) to the net solar energy absorbed by the earth’s climate system.
All parameters in the equation are known except the effective radiating temperature, which is the temperature the earth would appear to have if measured with a radiometer located on the moon.
S0 is w/m2; A is 0.31; R is 6371 km; k is 1.38 x10-16 erg/deg.
The 33ºC difference in average surface temperatures between an earth without an atmosphere and our current climate is the difference between a livable planet and a snowball earth.
Reference: IPCC Climate Change 2001: The Scientific Basis Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton,J.T. et al. (eds.)]. Cambridge University Press, Cambridge, UK. pp 89-90
Demonstrate calculations

4. Estimating Earth’s Expected Temperature using Stefan-Boltzmann law
Estimating Earth’s Expected Temperature
using Stefan-Boltzmann law
Net Incoming Solar Energy
Outgoing Heat Energy =
(S0 (1-A) R2)
(4R2kTe4)
where
S0 is the solar constant
A is average albedo, or reflectivity
R is the radius of the earth
k is Boltzmann’s constant
Te is earth’s apparent temperature (seen from space)
In the above equation, the climate is assumed to be in balance, with net loss of infrared energy released from the earth’s surface and atmosphere to space equal (on average) to the net solar energy absorbed by the earth’s climate system.
All parameters in the equation are known except the effective radiating temperature, which is the temperature the earth would appear to have if measured with a radiometer located on the moon.
S0 is w/m2; A is 0.31; R is 6371 km; k is 1.38 x10-16 erg/deg.
The 33ºC difference in average surface temperatures between an earth without an atmosphere and our current climate is the difference between a livable planet and a snowball earth.
Reference: IPCC Climate Change 2001: The Scientific Basis Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton,J.T. et al. (eds.)]. Cambridge University Press, Cambridge, UK. pp 89-90 Te
equals -19C 
However, average global surface T is + 14C
Natural greenhouse effect warms the
surface by 33C 

5. Present-day Atmospheric Composition of Planets

6. Evolution of Earth’s atmosphere
Tied to changes in development of organisms and their influence on gases.
James Lovelock has suggested that Earth’s temperature has remained relatively constant over billions of years, through the development of organisms which have changed atmospheric concentrations, while the Sun has increased radiation loading.
The earth as a superorganism?

7. The Gaia Hypothesis
“The theory that views the evolution of biota and their material environment as a single, tightly-coupled process, with the self-regulation of climate and chemistry as an emergent property”
--The geophysiology of the earth.
Margulis and Lovelock 1976

8. Is the Gaia Hypothesis reasonable?
Why or why not?
The Gaia hypothesis is an ecological hypothesis
that proposes that living and nonliving parts of the
earth are viewed as a complex interacting system
that can be thought of as a single organism.