Transcript No Slide Title

Chapter 12
Long-Term Climate Regulation
Sun about 30% less luminous than today - Ts would have been
below freezing - Earth seems to have had liquid water nonetheless Faint Young Sun Paradox (FYSP)
Warm Ts maintained by effect of greenhouse gases - how?
Yet “Snowball Earth” episode about 2.3 b. y. ago - possible? (what
else happened around then?)
Role of plate tectonics and the carbonate-silicate cycle
FYSP - figure shows temperature variation (left vertical axis) and
relative solar luminosity (right vertical axis) with time (horizontal axis)
FYSP - Explanation of figure 12.2
The solid curve shows the change in solar luminosity(S) relative to
today’s value (S0) since the “beginning”, about 4.8 b. y. ago.
Dotted lines are for temperature: Ts = mean surface T, Te =
effective radiating T and, for reference, the horizontal dotted line
corresponds to the freezing T of water.
Ts is calculated with a model that assumes water vapor feedback, a
constant albedo and level of atmospheric CO2 (340ppm) and the
solar luminosity increasing by 30% over time.
Ts is calculated using the principle of planetary energy balance (see
Chapter 3 and lab/HW work)
The shaded region between each dotted line represents the
greenhouse effect on temperature. Increase of greenhouse effect
with time and solar luminosity is due to the water vapor feedback.
FYSP - Comments on figure 12.2
If we simply consider the luminosity of the sun- it appears that the
planet would be frozen until about 1,900,000 years ago. However,
evidence from the geologic record proves that this was not the case.
There was liquid water at 3.8 bya and life was certainly present by
approx 2.2 bya.
The diagram shows us in a visual manner that there must have been
other factors influencing the Ts throughout Earth’s history or the
planet would have been frozen. Other factors:
• geothermal flux? OK but not large enough,
• lower albedo? possible but it needed to be near zero,
• greenhouse effect much stronger in the past? OK. Which gases
and how?
• CO2-rich Early Atmosphere? (recall Ca-Si cycle is a sink of CO2)
possible if smaller continents or more volcanism.
CO2-rich Early Atmosphere? Figure 12-3 shows the time (horizontal
axis) variation of atmospheric concentrations of CO2 (on both
vertical axes, two different measures of the same variable!) needed
to compensate for changing (reduced) solar luminosity.
Figure 12-3 - Notes & Comments
The figure shows the CO2-partial pressure in bars on the left y-axis,
relative (to today’s, hence NO UNITS!) CO2 concentration and time
on the x-axis.
The shaded area indicate the range between upper and lower bound
on CO2 concentrations needed to keep the Earth’s Ts above freezing
as solar luminosity was reduced.
The vertical dark-solid lines at times of glaciations specified in the
figure show limits of the CO2 concentrations (needed) estimated from
model simulations of past climates. These estimates are consistent
with geological evidence.
This figure implies that a CO2 level required to keep the oceancovered Earth would raise the temperature to 80-90 C! And this has
implications for the origins of life - recall the presence of heat lovinghyperthemophiles.
Figure 12-3 - Notes & Comments
Question: according to these model estimates, what was the range of
atmospheric CO2concentrations that were needed to keep Earth (and
particularly early oceans!) from freezing at 4.5, 3.5, 2.5 and 1.5 b.y.
ago, respectively? Give your answers in terms of partial pressure
values AND in relative (to today’s) values of CO2 concentrations.
For example, for ‘time before present’ (x-axis) = 4.5 b.y. we look at values on both
y-axis to answer this question. In the left y-axis, we read that at that time, pCO2
ranged between 0.1 and 10 bars, and of course it could have been any of the values
in between according to the shaded part. Then we look at the right y-axis to
determine the corresponding values of CO2 concentration relative to today’s value,
and we read that these range between 500 and 15000 x present level of CO2
concentration! Knowing present atmospheric levels (in ppm, for example), this
answer could also be given in terms of the range of ‘ppm’ values that would have
been needed.
You should make sure you can follow this example ande answer the
question for the other times! and in ‘ppm’ values.
Figure 12-4: dependence of average Ts (the dependent variable on
the y-axis) on partial pressure of atmospheric CO2 (the independent
variable on the x-axis) for different values of the fractional
abundance (mixing ratio) of atmospheric CH4, for S/So = 0.8 (from
model simulations!)
1000 ppm
upper limit on
Late Archean
CO2 conc.
derived from
paleosols
Freezing point of water
Figure 12-4 - Notes & Comments
The solid curve displays the changes of average global surface
temperature as a function of atmospheric CO2 and CH4
concentrations.
These calculations were performed for an assumed solar luminosity of
80% of the present value, which according to figure 12-2 was the
estimated value 2.8 billion years ago.
The figure is about the role of Methane in keeping Ts warm enough.
Originally, methane could have been produced at sites of seafloor
spreading (this is:prior to the origination of methanogens). Once
methanogenic bacteria arise the rate of CH4 would have been
increased. Methane is a strong greenhouse gas, hence the figure
shows the possibility that methane had a large effect on the Archean
atmosphere.
Figure 12-4 - Notes & Comments
‘Reading’ the figure:
(1) for a CH4 mixing ratio of 10-4 (or 1/10000!) what pCO2 value is
necessary to keep Ts above freezing?
(2) and for a CH4 mixing ratio of approximate 3 x 10-3?
Proceed as in the example for Figure 12-3: read values on the
axis, keep in mind the freezing point of water (temperature of),
make sure you give answers with numbers AND units, be able to
relate measures of present values of atmospheric CO2
concentrations; namely: what’s today’s value of pCO2 and what
value it corresponds to in ppm?
Long-term climate record: five periods of glaciations 1. 2.5 b. y. ago - the rise of oxygen would have used up much of the
methane sending the Earth into freezing - evidence supports this
2. then climate gets warm again
(a) carbonate-silicate weathering cycle might explain this
(b) methane in the atmosphere recovered
3. climate gets cold again - around 0.8 b. y. ago - Snowball Earth
4. climate also varied over the past 0.5 b.y. (500 m. y) - 3 major
glacial periods occurred: one about 440 m.y. ago (brief), one
about 280 m.y. ago (longish) and the most recent Pleistocene
glaciation (see Fig. 12.11). This last period will be discussed
more in Chapter 14.
Major cold and warm periods
during Earth’s history
1. Paleozoic - 544 to 250 m.y.
period: mostly warm, ice-free except
for two periods, most important the
Permian-Carboniferous - this
cooling most likely due to decrease
in atmospheric CO2 concentration.
2. Mesozoic - age of the dinosaurs,
warm and dry and much smaller
equator-to-pole temperature
difference - 251 to 65 m.y. ago.
Reasons: CO2 concentrations with
ice-albedo feedback, backward
thermohaline circulation, larger
extent of Hadley cell.
3. Cenozoic - past 65 m. y. of Earth
history, cooling begins about 70 - 80
m. y. ago, cold over the past 2 m. y
approximately (Quaternary).
Reasons: carbonate-silicate
weathering disturbed by plate
tectonic causes - see Fig. 12.16.
Latitudinal temperature gradient during the Mesozoic (specifically during the midCretaceous, 80 m. y. ago approx.) compared to today’s values. Equator was between 2-6
deg C warmer than today and the poles 20 to 30 deg C warmer!
more 13C - lower CO2
Less 13C in
sediments from
the Mesozoic
than in younger
sediments
higher levels of
CO2 then than
now.
Inferring past CO2 concentrations based on carbon isotopes. Photosynthetic
organisms take up 12C faster than 13C, more so if CO2 is abundant. So, low
13C/12C, negative 13, corresponds to organic matter produced under high-CO
2
conditions. If CO2 concentration is low, then organisms use whatever is available
and the ratio is relatively unchanged. Data consistent with decrease in CO2 over
the past 100 m. y.
Collision between
India and Asia
around 40 m. y. ago
gave rise to
Himalayas,
providing large
areas for
weathering to
proceed efficiently
and rapidly, aided
by rainfall
(monsoons) silicate weathering
could have
decreased levels of
CO2 toward present
values