|
|
|
The faint young Sun paradox and its possible
explanation. |
|
Why was Earth ice-free even at the poles 100 Myr
ago (the Mesozoic Era)? |
|
What caused Earth’s climate to cool over the
last 55 Myr (the Cenozoic Era)? |
|
|
|
|
Solar luminosity was much weaker (~30%) in the
early part of Earth’s history (a
faint young Sun). |
|
If Earth’s albedo and greenhouse effect remained
unchanged at that time, Earth’s mean surface temperature would be well
below the freezing point of water during a large portion of its 4.5 Byr
history. |
|
That would result in a “snowball” Earth, which
was not evident in geologic record. |
|
|
|
|
Solution 1: Additional heat sources must have
been presented |
|
Unlikely:
The geothermal heat from the early Earth is sometimes suggested one such
additional heat source to warm Earth. However, the geothermal heat flux is
not big enough to supply the required energy. |
|
Solution 2: The planetary albedo must have been
lower in the past |
|
Unlikely:
It would require a zero albedo to keep the present-day surface temperature
with the 30% weaker solar luminosity in the early Earth. |
|
Solution 3: Greenhouse effect must have been
larger |
|
Most Likely: The most likely solution to the faint young Sun paradox is that Earth’s greenhouse
effect was larger in the past. |
|
But
(1) why and (2) why that stronger greenhouse effect reduced to the
present-day strength? |
|
|
|
|
The precipitation process in the atmosphere
dissolve and remove CO2 from the atmosphere. |
|
Rocks exposed at Earth’s surface undergo
chemical attack from this rain of dilute acid. |
|
This whole process is known as chemical
weathering. |
|
The rate of chemical weathering tend to increase
as temperature increases. |
|
Weathering requires water as a medium both for
the dissolution of minerals and for the transport of the dissolved
materials to the ocean |
|
è The rate
of chemical weathering increases as precipitation increases. |
|
|
|
|
The chemical weathering works as a negative
feedback that moderates long-term climate change. |
|
This negative feedback mechanism links CO2
level in the atmosphere to the temperature and precipitation of the
atmosphere. |
|
A warm and moist climate produces stronger
chemical weathering to remove CO2 out of the atmosphere è smaller
greenhouse effect and colder climate. |
|
|
|
|
Chemical weathering acts as Earth’s
thermostat and regulate its
long-term climate. |
|
This thermostat mechanism lies in two facts: |
|
(1)
the average global rate of chemical weathering depends on the state of
Earth’s climate, |
|
(2)
weathering also has the capacity to alter that state by regulating the rate
which CO2 is removed from the atmosphere. |
|
|
|
|
Climate in the past 500 million years have
alternated between long periods of warm climate and short periods of cold
climate. |
|
During the last 500 million years, major
continent-size ice sheets existed on Earth during three icehouse ear: (1) a
brief interval near 430 Myr ago, (2) a much longer interval from 325 to 240
Myr ago, and (3) the current icehouse era of the last 35 million year. |
|
|
|
|
|
How can one account for the alternating periods
of climatic warmth and coolness observed in the geologic record? |
|
è Part of the answer must lie in the tectonic
activity and the positions of the
continents. |
|
|
|
|
The polar position hypothesis focused on
latitudinal position as a cause of glaciation of continents. |
|
This hypothesis suggested that ice sheets should
appear on continents when they are located at polar or near-polar
latitudes. |
|
To explain the occurrence of icehouse intervals,
this hypothesis calls not on worldwide climate changes but simply on the
movements of continents on tectonic plates. |
|
This hypothesis can not explain the climate of
the Late Proterozoic Era, when both
continents and glaciers appear to have been situated at relatively
low latitudes. |
|
It can not explain the warm Mesozoic Era when
high-latitude continents were present but were almost completely ice-free. |
|
|
|
|
During active plate tectonic processes, carbon
cycles constantly between Earth’s interior and its surface. |
|
The carbon moves from deep rock reservoirs to
the surface mainly as CO2 gas associated with volcanic activity
along the margins of Earth’s tectonic plates. |
|
The centerpiece of the seafloor spreading
hypothesis is the concept that changes in the rate of seafloor spreading
over millions of years control the rate of delivery of CO2 to
the atmosphere from the large rock reservoir of carbon, with the resulting
changes in atmospheric CO2 concentrations controlling Earth’s
climate. |
|
|
|
|
The seafloor spreading hypothesis invokes
chemical weathering as a negative feedback that partially counters the
changes in atmospheric CO2 and global climate driven by changes
in rates of seafloor spreading. |
|
|
|
|
The uplifting weathering hypothesis asserts that
the global mean rate of chemical weathering is heavily affected by the
availability of fresh rock and mineral surfaces that the weathering process
can attack. |
|
This hypothesis suggests that tectonic uplifting
enhances the exposure of freshly fragmented rock which is an important
factor in the intensity of chemical weathering. |
|
This hypothesis looks at chemical weathering as
the active driver of climate change, rather than as a negative feedback
that moderates climate changes. |
|
|
|
|
|
Earth’s climate 100 Myr ago was warm enough at
poles to keep ice sheets from forming. |
|
Why? |
|
|
|
|
CO2 level was higher 100 Myr ago. |
|
Although higher atmospheric CO2 level
can account for the warm climate 100 Myr ago, this mechanism alone can not
explain the extremely small equator-to-pole temperature gradient at that time. |
|
|
|
|
The deep ocean 100 Myr ago was filled with warm
saline deep water formed in the tropics or subtropics, rather than with
cold water from high latitudes. |
|
A strong flow of warm deep water from the
tropics to the poles could have contributed to the poleward heat flux
needed to warm the poles. |
|
Also, the warm salty deep water could have
caused faster overturning of the deep ocean and greater poleward heat
transport. |
|
|
|
|
The collision of Indian and Asia happened around
40 Myr ago. |
|
The collision produced the Himalayas and a huge
area of uplifted terrain called the Tibetan Plateau. |
|
The Himalayas Mountains provided fresh, readily
erodable surfaces on which chemical weathering could proceed rapidly. |
|
At the same time, the uplifting of the Tibetan
Plateau create seasonal monsoon rainfalls, which provided the water needed
for chemical weathering. |
|
Therefore, the collision of India and Asia
enhanced the chemical weathering process and brought down the atmospheric
CO2 level to the relatively low values that prevail today. |
|
This reduced the greenhouse effect and cooled
down the climate over the last 50 Myr. |
|
|
|
|
Plate Tectonics probably does influence climate
over long time scales. |
|
The main influence of plate tectonics on climate
appears to be indirect: by modulating CO2 levels in the
atmosphere through the chemical weathering process. |
|
This, in turn, affects climate by way of the
greenhouse effect. |
|
Such change, in combination with the long-term
increase in solar luminosity, can account for the main features of the
long-term climate changes. |
|