Planetary Energy Balance
and
Biogeochemical Cycles
?
?
“Faint young Sun Paradox”
Log(Luminosity)
Post Main Sequence Stellar Evolution: 4 H --> 1 He
Blackbody radiation
Solar flux is today F = 3.7 x 107 W/m2
Earth’s temperature depends (mainly) on three things:
1. Solar flux at 1 AU: S = 1370 W/m2
2. Earth’s albedo A (reflectivity) = 0.3
3. Amount of warming due to atmosphere
(greenhouse effect)
A = fraction of sunlight reflected from a planet
Planetary energy balance
Energy emitted
=
by earth
Energy
absorbed by
earth
= energy intercepted - energy reflected
= REarth2 S - REarth2 S A
= REarth2 S (1-A)
Energy emitted
=
by earth
4 REarth2  Te4
 Te4
Energy
absorbed by
earth
REarth2 S (1-A)
=
= S (1-A) / 4
Solve for Te = 255 K
brrrrr!
Earth’s temperature is 288 K.
33 K of greenhouse warming!
But: geological evidence for warm climate in past!
One layer atmosphere approximation
Sunlight
reflected away
= SA/4
Globally
averaged solar
flux = S / 4
Te4
atmosphere
S(1-A)/4
gets through
Ts4
Earth
Te4
Formaninifera (forams): single-celled protists, some
secrete CaCO3 shells
All CO2 would be consumed in a few thousand years!
Coincidence???
Plate tectonics
Why does plate tectonics occur?
Mantle convection causes
continental drift:
• Subduction zones
• seafloor spreading at
midocean ridge
Volcanoes emit H2O and
CO2 with small amounts of
N2, H2S, CH4 and SO2
Subduction: one plate being driven under another (Cascadia
subduction zone)
Seafloor spreading: Creation of new seafloor crust as plates move
apart
Seafloor spreading
Stratovolcano
Composite volcano
H2O, CO2 gases
from volcanoes
mantle
Carbonate-silicate cycle: stable over very long periods of time…
but what if...?
Changes in T, CO2 or H2O vapor
What if the volcanism stopped?
What if rain stopped?
Early Earth: No oxygen, methanogens thrived!
Today: only anaerobic environments
Long-Range Evolution of planetary atmospheres
H lost preferentially over O to space: planets oxidize over time
Snowball Earth
The Earth experienced several
episodes of GLOBAL glaciation
750-580 million years ago, and 23 billion years ago
Geological evidence for glaciers
During the Snowball Earth episodes, the normal
carbon cycle stopped. CO2 built up in the
atmosphere. Catastrophic warming followed.
x
POSITIVE FEEDBACK:
Increasing glaciation caused even
more glaciation
As Earth got whiter, it got colder
(reflecting sunlight away)
How did we ever snap out of it?
Strange finding: evidence for a
very WARM climate directly on
top of the snowball earth
sediments!!
A glacial dropstone in a banded iron formation
During the Snowball Earth episodes, the normal
carbon cycle stopped. CO2 built up in the
atmosphere. Catastrophic warming followed.
x
The Earth emerged from the Snowball episodes as CO2 levels
built up in the atmosphere
The response of climate to increasing CO2 levels is not
immediate. It takes about 400,000 years to warm up.
Geological evidence shows that the Earth went into an extreme
warming trend after the Snowball episodes.
How does CO2 warm the Earth?:
It traps heat, like a greenhouse.
Carbon is either in the form of atmospheric gases (CO2 and
CH4), both of which are greenhouse gases, or in solid
materials (living things, fossil fuels, rocks)
Processes that take carbon out of solids and put it into the
atmosphere include:
• volcanism
• loss of vegatation (deforestation)
• burning of fossil fuels
What is fossil fuel?
Coal, oil, natural gas are the remains of plants that lived
millions of years ago.
Complex hydrocarbons that can be burned, which releases
CO2
CH4 + 2 O2 --> CO2 + 2 H2O
Positive and negative feedback loops