Tectonic-Scale Climate Change

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)?

Faint Young Sun Paradox

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.

Three Possible Solutions

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?

Chemical Weathering

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.

Negative Feedback From Chemical Weathering

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.

Earth’s Thermostat – Chemical Weathering

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 Changes in the Last 500 Myr

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.

Plate Tectonics and Climate

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

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.

Tectonic Control of CO₂ Input – The Seafloor Spreading Rate Hypothesis

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.

Negative Feedback in Seafloor Spreading Hypothesis

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.

Tectonic Control of CO₂ Removal – The Uplift Weathering Hypothesis

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.

Can These Two Hypotheses Explain Tectonic-Scale Climate Changes?

Greenhouse Earth – 100Myr Ago

Earth’s climate 100 Myr ago was warm enough at poles to keep ice sheets from forming.

Why?

One Reason: Higher CO₂ Level

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.

Ocean Heat Transport Factor

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.

Why the Cooling over the Last 50 Myr?

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.

Summary: Tectonic Control of Climate

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.


	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.