Earth Climate Part 2 Chapter 4 and 5 Notes. Notes below are the things I highlighted in my book.

Chapter 4: CO2 and Long Term Climate

• greenhouse eras (times when no ice sheets are present) and to cool off but not freeze solid during icehouse eras (times when ice sheets are present)
• The average amount of solar radiation sent to each planet varies inversely with the square of its distance from the Sun (1/d2 ).
• Venus is closer to the sun, atmosphere block more solar radiation and traps more solar radiation
• Venus has an atmosphere 90 times as dense as that of Earth, and 96% of its atmosphere is composed of carbon dioxide (CO2), a greenhouse gas that is very effective in trapping radiation
• Venus Carbon is mainly in the atmosphere, while earth is locked away in oil, coal, and natural gas
• Faint Young Sun Paradox story of the early earth and greenhouse gas temperature regulation
• Understanding Carbon reservoirs and exchange rates are important for earth temperature

4-1 Volcanic Input of Carbon from Rocks to the Atmosphere

• Carbon cycle is constant and slow between Earth’s interior and its surface
• The present rate of natural carbon input to the atmosphere from the rock reservoir is estimated at approximately 0.15 gigatons of carbon per year.
• This value is uncertain by a factor of at least 2 because volcanic explosions are irregular in time and because the amount of CO2 released varies with each eruption. As we will see later, this natural rate of carbon input is roughly balanced by a similar rate of natural removal.
• This balance between natural input and removal rates helped to keep the size of the “natural” (preindustrial) atmospheric carbon reservoir at ,600 gigatons.
• The largest reservoir of carbon on Earth lies in its rocks.
• changes in volcanic input persisting over that relatively “small” span of time could have had a drastic effect on the CO2 content of our atmosphere.
• There are many complex interactions in the carbon cycle
• amazing that over this immense span of time its volcanoes have somehow managed to keep delivering just enough carbon from Earth’s interior to keep the atmosphere from running out of CO2 and freezing the planet, but not so much as to overheat and boil it.
• The basic operating principle of a thermostat is that it first reacts to external changes and then acts to moderate their effects: a thermostat detects the chill of a cold night and sends a signal that turns on the heat.
  • Volcanic activity does not operate this way

4-2 Removal of CO2 from the Atmosphere by Chemical Weathering

• The major long-term process of CO2 removal is tied to chemical weathering of continental rocks
• Two major types of chemical weathering occur on continents: hydrolysis and dissolution.
• Hydrolysis is the main mechanism for removing CO2 from the atmosphere.
  • key ingredients in the process of hydrolysis are the minerals that make up typical continental rocks, water derived from rain, and CO2 derived from the atmosphere.
• The chemical weathering removes CO2 from the atmosphere and buries it in ocean sediments.
• Dissolution is the process that eats away at limestone bedrock and in some areas forms caves
• rainwater and CO2 combine in soils to form carbonic acid (H2CO3) and attack limestone bedrock (CaCO3), and the dissolved ions created by dissolution again flow to the ocean in rivers. Dissolution of limestone proceeds at much faster rates than hydrolysis of silicates.
• Earth’s long-term habitability requires that the rates of input and output must have always remained fairly closely balanced even though they varied
• rates of chemical weathering are influenced by three environmental factors: temperature, precipitation, and vegetation.
• higher temperatures cause faster weathering of individual silicate minerals
• Increased rainfall boosts the level of groundwater held in soils, and the water combines with CO2 to form carbonic acid and enhance the weathering process
• Vegetation also enhances chemical weathering. Plants extract CO2 from the atmosphere through the process of photosynthesis, and deliver it to soils, where it combines with groundwater to form carbonic acid. Vegetation is closely linked to precipitation and temperature
• The global rate of chemical weathering is analogous to a thermostat because it reacts to (depends on) the average state of Earth’s climate and then alters that state by regulating the rate at which CO2 is removed from the atmosphere

4-3 Was Methane Part of the Thermostat

• Carl Sagan suggested that higher concentrations of methane (CH4) warmed the early Earth.
• Earth’s early atmosphere held much less oxygen than it does today. As a result, methane could have stayed in the atmosphere longer, attained higher concentrations, and helped to warm the early Earth.
• The subsequent long-term increase in atmospheric oxygen over several billion years would then have reduced the methane content of the atmosphere and weakened its greenhouse effects.
• It is not so obvious why methane would have acted like a thermostat by reacting to long-term climate changes. It seems to have acted more as an independent (and coincidentally opposing) climatic factor

4-4 The Gaia Hypothesis

• The biologists James Lovelock and Lynn Margulis proposed in the 1980s that life itself has been responsible for regulating Earth’s climate.
  • They called their idea the Gaia hypothesis and claims that life regulates climate on Earth for its own good.
• Modern biological processes are, without question, important components of the processes of chemical weathering and carbon cycling
• Critics of the Gaia hypothesis point out that early lifeforms were too primitive to have had much effect on chemical weathering, and the delicate climatic balance maintained through Earth’s history must have been achieved primarily by physical-chemical means rather than by biological intervention.
• Critics also note that the very late appearance of shell-bearing oceanic organisms near 540 million years ago means that life had played no obvious role in transferring the products of chemical weathering on land to the seafloor for the preceding 4 billion years
• Supporters of the Gaia hypothesis respond with several counterarguments. First, they claim that critics underestimate the role of primitive life-forms such as algae in the ocean and microbes on land in Earth’s earlier history.
• They point to recent discoveries that modern bacteria with similarities to early primitive life-forms are now thought to play a greater role in the weathering process than has generally been recognized
• Gaia hypothesis supporters also point out that the general path of biological evolution matches Earth’s need for progressively greater chemical weathering through time.
• The “maximum” claim by proponents of the Gaia hypothesis—that individual life-forms regulate their own evolution for the greater benefit of all life on the planet—is not accepted by most scientists. Somewhere in between lies the answer to the role of life in determining the presence of life on Earth.
• Ice sheets occur today at high latitudes, yet they coexist on the same planet with hot tropical regions where a strong overhead Sun heats the land and ocean
• For a continent-sized ice sheet to have extended to sea level near the equator, temperatures in the normally hot tropics would have to be near or below freezing through most of the year
• scientists have inferred that as few as two, or as many as four, major glacial eras occurred
• climate models indicate that CO2 concentrations would have to have been much lower than today to permit ice sheets to exist in tropical latitudes. The thermostat mechanism seems to have malfunctioned for a little bit
• The reason for the thermostat malfunction remains unresolved. One explanation for the onset of a snowball Earth is that continents were clustered near the equator, where they would initially have been subject to heavy rainfall.
• Paradoxically, unusually heavy tropical precipitation could have driven unusually strong chemical weathering that greatly reduced CO2 concentrations and chilled the planet.

Chapter 5: Plate Tectonics and Long-Term Climate

• This chapter will examine how plate tectonic processes work, explore the possibility that icehouse intervals occur when plate tectonic motions shift continents across cold polar regions.
• Will use climate models to investigate the factors that controlled climate 200 million years ago, a time when all landmasses on Earth existed as a single giant continent.
• We will evaluate two hypotheses that link changes in plate tectonic processes to changes in CO2 levels.
• In 1914, the German meteorologist Alfred Wegener proposed that continents have slowly moved across Earth’s surface for hundreds of millions of years.

5-1 Structure and Composition of Tectonic Plates

• Continental crust is 30–70 kilometers thick, has an average composition like that of granite, and is low in density (2.7 g/cm3 ).
• Ocean crust is 5–10 kilometers thick, has an average composition like that of basalt, and is higher in density (3.2 g/cm3 ).
• Below each of these crustal layers lies the mantle, which is richer in heavy elements like iron (Fe) and magnesium (Mg) and has an even higher density (>3.6 g/cm3 ). The mantle extends 2,890 kilometers into Earth’s interior, almost halfway to its center at a depth of 6,370 kilometers
• The outer layer, called the lithosphere, is 100 kilometers thick and generally behaves just the way the word “rock” implies: as a hard, rigid substance.
• Below the lithosphere is a layer of partly molten yet mostly solid rock called the asthenosphere. This layer lies entirely within the upper section of Earth’s mantle at depths of 100 to 350 kilometers.
• The lithosphere consists of a dozen tectonic plates, each drifting slowly across Earth’s surface
  • These plates move at rates ranging from less than 1 up to 10 centimeters per year and average about the same rate of growth as a fingernail.
• Tectonic plates can consist as a combination of continents and ocean basins
• These rigid tectonic plates have three basic types of edges, or margins. Most tectonic deformation on Earth (earthquakes, faulting, and volcanoes) occurs at these plate margins
  • Plates move apart at divergent margin
  • Plates come together at convergent margins
  • Plates also can slide past each other at transform fault margins

5-2 Evidence of Past Plate Motions

• Magnetic field is big evidence of the plate tectonics
• Compass needles today point to magnetic north, which is located a few degrees of latitude away from the geographic North Pole, the axis of Earth’s rotation.
• Fossil compasses give scientists studying paleomagnetism a way to reconstruct past positions of continents and ocean basins with respect to the pole of rotation
• Paleomagnetism is used to reconstruct changes in the configuration of Earth’s surface in two ways.
  • Back to about 500 million years ago paleomagnetic compasses recorded in continental basalts can be used to track movements of landmasses with respect to latitude.
  • Over the last 175 million years, paleomagnetic changes recorded in basaltic oceanic crust can be used to reconstruct movements of plates and rates of spreading of the seafloor.
• For older intervals, paleomagnetism must rely on basalts deposited on the continents. The orientations of the magnetic compasses frozen in these basalt layers are used to determine the past latitude of that rock in relation to the magnetic poles.
• marine geophysicists found stripe-like magnetic patterns called magnetic lineations on the ocean floor
• An early hypothesis of long-term climate change focused on latitudinal position as a likely cause of glaciation of continents.
• The polar position hypothesis made two key predictions that can best be tested over the younger part of Earth’s history:
  • ice sheets should appear on continents that were located at polar or near-polar latitudes
  • no ice should appear at times when continents were located outside of polar regions.
    • Rather than explaining icehouse intervals as caused by worldwide climate changes, this hypothesis simply calls on the movements of continents and tectonic plates across Earth’s surface.
  • The fact that modern ice sheets occur on the polar continent of Antarctica and the near-polar landmass of Greenland makes this hypothesis seem plausible

5-3 Glaciation and Continental Positions since 500 Myr Ago

• We can directly test the polar position hypothesis against evidence in the younger geologic record.
• Near 420 million years ago, small landmasses that were later to form modern North America and the northern part of Eurasia lay scattered across a wide range of latitudes.
• The other land areas, equivalent to modern Africa, Arabia, Antarctica, Australia, South America, and India, were combined in a much larger southern supercontinent called Gondwana.
• Gondwana was located on the opposite side of the globe from North America, but it had begun a long trip that would carry it across the South Pole and then northward to a collision with the northern landmasses, creating the giant supercontinent Pangaea, meaning “All Earth”
• The polar position hypothesis accounts for part of Earth’s glaciation history during the last half-billion years.
• During that interval, ice sheets developed only on landmasses that were at polar or near-polar positions, consistent with the presence of ice at the South Pole today.
• It cannot explain the absence of ice during some intervals of the last 500 million years.
• The geologic record tells us that the presence of continents in a polar position is favorable to the formation of ice sheets but does not guarantee that they actually will form.
• Because the locations of continents are accurately known for the past 300 million years, climate scientists can use general circulation models (GCMs) to evaluate the impact of these geographic factors on climate.
• Pangaea provides climate scientists with a very different and yet a real Earth for testing the performance of climate models.

5-4 Input to a Model Simulation of the Climate on Pangaea

• GCM runs require the major physical aspects of a past world to be specified in advance as boundary-condition inputs in order to run simulations of past climates.
• The most basic physical constraint is the distribution of land and sea.
• Pangaea remained intact from the time it had formed (250 Myr ago) until it broke up after 180 million years ago. Modelers have simplified Pangaea for use as input to climate simulations by making the land distribution symmetrical around the equator
• A second important decision on input to the model is global sea level.
• A third important decision is the distribution of elevated topography on the continents, but this aspect of Pangaea is not as well known and has to be approximated
• Sun had not yet reached its present strength and was still about 1% weaker than it is today. By itself, this weaker Sun should have made Pangaea significantly colder than the modern world, with snow and ice closer to the equator than today, but we see no evidence of that, which means higher CO2. 
• Pangaea’s climate was somewhat warmer than today’s climate
• The model experiment examined here assumed a CO2 level of 1,650 parts per million, almost six times the recent preindustrial value of 280 parts per million.
  • produced temperature distributions consistent with the evidence from the lack of permanent ice and frost-sensitive vegetation, but it also simulated other climatic features that match independent evidence from the Pangaean geologic record.

5-5 Output from the Model Simulation of Climate on Pangaea

• The model simulates widespread aridity at lower latitudes, especially in the Pangaean interior.
• Simulated mean annual precipitation and soil moisture levels are very low across vast expanses of interior and western Pangaea between 40S and 40N .
• This pervasive aridity reflects two factors: the large amount of land at subtropical latitudes beneath the dry, downward-moving limb of the Hadley cell, and the large amount of land in the tropics, causing trade winds to lose most of their ocean-derived water vapor before reaching the continental interior.
• In contrast, the simulated ocean around Pangaea received far more rainfall than the land, and considerably more than it does today
• Geologic evidence supports the model simulation of widespread Pangaean aridity.
• Because the moderating effects of ocean moisture failed to reach much of Pangaea’s interior, the continent was left vulnerable to seasonal extremes of solar heating in summer and cooling during winter.
• This wide range of seasonal temperatures could explain the lack of ice sheets on Pangaea. Inconsistent results with the models and ground truth evidence for ice extent
• Another fundamental characteristic of the climate of Pangaea was the strong reversal between summer and winter monsoon circulations
• Geologic evidence of seasonal moisture contrasts on Pangaea comes from the common occurrence of red beds, sandy or silty sedimentary rocks stained various shades of red by oxidation of iron minerals.
• Red beds were more widespread on Pangaea, consistent with the models of highly seasonal changes in moisture between wet summer monsoons and dry winter monsoons.
• The polar position hypothesis and the climate of Pangaea suggest that changes in Earth’s geography alone cannot explain the variations between warm greenhouse climates and cold icehouse climates during the last 500 million years and that variations in the CO2 concentration in the atmosphere played a role.

5-6 Control of CO2 Input by Seafloor Spreading

• In 1983, a new hypothesis proposed that climate changes during the last several hundred million years were driven mainly by changes in the rate of CO2 input to the atmosphere and ocean by plate tectonic processes.
• This hypothesis is named BLAG after its authors, the geochemists Robert Berner, Antonio Lasaga, And Robert Garrels). It is also referred to as the spreading rate hypothesis
• Most CO2 is expelled to the atmosphere by volcanic activity at two kinds of locations: 
  • margins of converging plates, in mountain belt and island arc volcanoes, delivering CO2 and other gases from Earth’s interior
  • margins of divergent plates (ocean ridges), where hot magma carrying CO2 erupts directly into ocean water.
• The central feature of the BLAG hypothesis is the concept that changes in the average rate of seafloor spreading over millions of years have controlled the rate of delivery of CO2 to the atmosphere from the large subsurface rock reservoir of carbon and that the resulting changes in atmospheric CO2 concentrations have had an impact on Earth’s climate
• Changes in the mean rate of spreading should alter the transfer of CO2 from Earth’s rock reservoirs to its atmosphere at ocean ridges and subduction zone volcanoes
• Faster rates of spreading at ridge crests create larger amounts of new ocean crust and more frequent releases of magma, which deliver greater amounts of CO2 to the ocean.
• Faster spreading also causes more rapid subduction of crust and sediment in ocean trenches and delivers larger volumes of carbon-rich sediment and rock for subsequent melting and CO2 release through volcanoes.
• Conversely, slower spreading reduces both kinds of CO2 input to the atmosphere.
• The BLAG hypothesis also calls on chemical weathering for negative feedback to moderate these changes. Increased volcanic emissions caused by faster seafloor spreading lead to higher atmospheric CO2 levels and a warmer climate.
• Chemical weathering feedbacks work to offset some of the impact of cooling caused by slower volcanic input of CO2.
• The BLAG hypothesis relies on chemical weathering to moderate any fluctuations in climate driven by changes in volcanic CO2 input.
• The BLAG hypothesis further proposes that much of the cycling of carbon between the deeper Earth and the atmosphere occurs in a long, slow-acting loop.
• The BLAG hypothesis proposes that this cycling of carbon provides long-term stability to the climate system by moving a roughly constant amount of total carbon back and forth between the rocks and the atmosphere over long intervals of time.
• As a result, atmospheric CO2 levels are constrained to vary only within moderate limits. But the long delays between carbon weathering and burial permit small imbalances to occur between the rate of carbon burial and the return of CO2 to the atmosphere. These imbalances could drive climate changes over intervals of tens of millions of years

5-7 Initial Evaluation of the BLAG (Spreading Rate) Hypothesis

• Unfortunately, the predictions of the BLAG hypothesis cannot be directly tested over most of the geologic past because no undeformed ocean crust older than 175 million years exists to use for calculating past spreading rates.
• Most reconstructions have inferred that the global mean spreading rate was faster near 100 million years ago than it is at present.
• This prediction would agree with geologic evidence of a warmer climate 100 million years ago, and with the absence of large polar ice sheets.
• A second hypothesis that attempts to explain how plate tectonic processes control atmospheric CO2 levels emerged from work by the marine geologist Maureen Raymo and colleagues in the late 1980s.
• The uplift weathering hypothesis proposes that chemical weathering is an active driver of climate change, rather than passive negative feedback that moderates climate.

5-8 Rock Exposure and Chemical Weathering

• The BLAG hypothesis emphasizes changes in CO2 delivery to the atmosphere by seafloor spreading, and it assumes that removal of CO2 by chemical weathering responds only to climate-related changes in temperature, precipitation, and vegetation.
  • They are not the only processes that affect chemical weathering.
• The uplift weathering hypothesis starts from a different perspective. It asserts that the global mean rate of chemical weathering is mainly affected by the availability of fresh rock and mineral surfaces for weathering to attack, and it proposes that this exposure effect can override the combined effects of the climate-related factors on a global basis.
• Rock exposure is important because the more surface area the more weathering that can occur
• The huge increase of weathering far exceeds the combined changes estimated from changes in temperature, precipitation, and vegetation.
  • Based on this analysis, climate-related factors are not the only processes to consider in evaluating chemical weathering.

5-9 Case Study: The Wind River Basin of Wyoming

• Show importance of rock exposure because the rocks are similar to the continental crust
• The Wind River Mountains have been glaciated repeatedly over the last several hundred thousand years, and each glaciation has left deposits of unsorted debris
• These Wind River moraines provide an opportunity to quantify the amount of weathering of ground-up debris that is identical in composition but widely varying in age.
• The extent of weathering can be determined by analyzing the soils that have subsequently developed on the moraines.
• The cumulative amount of chemical weathering that has occurred since each moraine was deposited can be determined by measuring the total loss of these weathering-sensitive cations. Dividing the total amount of weathering since the moraine was deposited by the time elapsed yields the average rate of chemical weathering over that entire interval.
• The Wind River deposits show a rapid decrease in the mean rate of cation weathering with time of exposure .
  • The younger moraines have average rates of weathering that are at least a factor of 100 faster than the older ones.
  • The older moraines presumably also went through a phase of fast weathering during the tens of thousands of years soon after their deposition, and then weathered much more slowly later on.
    • The most likely explanation is that freshly ground rock has far more weatherable material—more of the kinds of fresh, unweathered silicate grains that are most vulnerable to the weathering process.
    • As these vulnerable minerals are preferentially removed through time, only more resistant minerals are left, and rates of weathering slow.
  • Another part of the uplift weathering hypothesis relates to the effect of grain sizes on weathering.
    • Finer grain sizes expose more mineral surface area and cause faster weathering early in the process, but the finer sizes of eroded material disappear earlier as weathering consumes them.
    • The coarser grain sizes that remain weather more slowly because they expose less surface area per unit of volume. Coarser fragments may also develop an outer coating, or “rind,” of weathering-resistant material that protects fresher material in their interiors and slows the weathering attack.

5-10 Uplift and Chemical Weathering

• The uplift weathering hypothesis focuses on exposure of fragmented and unweathered rock is a key factor in the intensity of chemical weathering.
  • It links this evidence to the fact that freshly fragmented rock is exposed mainly in regions of tectonic uplift.
• Several factors increase rates of exposure of fresh rock in uplifting areas.
  • Erosional processes known as mass wasting are common on Mountains and plateaus.
    • Mass-wasting processes include rock slides and falls, flows of water-saturated debris, and a host of other processes that dislodge everything from huge slabs of rock to loose boulders, pebbles, and soil. Every event that removes overlying debris exposes fresh bedrock and unweathered material.
  • Another important factor is earthquakes.
    • Mountains and high plateaus are built by tectonic forces that push together and stack huge slivers of faulted rock at the margins of converging plates.
    • This stacking process is accompanied by earthquakes that generate large amounts of energy, shake the ground, and dislodge debris. Even more fresh rock is exposed as a result.
  • A third important characteristic of steep slopes is that they are focal points for orographic precipitation. Heavy precipitation favors chemical weathering
  • Glacial ice also enhances chemical weathering in high terrain by pulverize blocks of underlying bedrock and deposit the debris in moraines at lower elevations.
  • The uplift weathering hypothesis proposes that uplift accelerates chemical weathering through the combined action of these processes.
    • Faster weathering draws more CO2 out of the atmosphere and cools global climate toward icehouse conditions.
    • uplift is less prevalent, chemical weathering is slower, and CO2 stays in the atmosphere and warms the climate, producing greenhouse conditions

• The two major kinds of plate tectonic processes that cause uplift have different implications for the uplift weathering hypothesis.
  • The first process, subduction of ocean crust underneath continental margins, is an integral part of plate movements and a process that is continually active in many regions on Earth. The second process that creates high terrain is the collision of continents, and these events are far less common.
•  The uplift weathering hypothesis focuses mainly on plateaus created by occasional collisions of continents, rather than on the ever-present mountain belts.
• Like the BLAG hypothesis, the uplift weathering hypothesis is consistent with the icehouse-greenhouse-icehouse climatic sequence.
• But if recent discoveries prove correct, neither the uplift weathering hypothesis nor the BLAG hypothesis nor the polar position of Gondwana is a complete explanation for the short glaciation in the Sahara near 445 million years ago

5-11 Case Study: Weathering in the Amazon Basin

• The effect of uplift on chemical weathering can also be evaluated by examining the drainage basin of the Amazon River of South America.
  • This region can be divided into two major units:
    • the low-lying Amazon Basin, where trade winds blowing westward from the Atlantic Ocean bring frequent precipitation to the rain forests of Brazil
    • the high-elevation eastern slopes of the Andes Mountains, which collect most of the rest of the incoming Atlantic precipitation.
•  Scientists determined the regional distribution by sampling the amount of chemically weathered ions flowing down to the Amazon River in dissolved form.
  • upper tributaries of the Amazon emerging from the foothills of the Andes carry almost 80% of the total dissolved chemical load discharged
  • lower Amazon Basin adds only the remaining 20% of the total. Most of the chemical weathering in the Amazon drainage basin occurs in the Andes
•  The lower Amazon Basin is a place where chemical weathering does indeed dominate in percentage terms, but in which the fresh minerals have long since been “used up” in the weathering process.
  • The lower Basin has older clays at and near the surface that are the end products of slow bedrock weathering over many tens of millions of years, and they have little weatherable material left.  As a result, the average rate of chemical weathering in this region is extremely low.
•  In contrast, the physical impacts of active uplift in the Andes combine to generate a continual supply of fresh, finely ground rock debris for weathering.
• The lack of obvious visible chemical weathering in the Andes has two explanations.
  • chemical weathering products such as clays are continually overwhelmed by the much larger supply of physically fragmented debris cascading down the steep slopes.
  • the fine clays and other products of weathering are soon removed from the steep slopes by fast flowing streams and rivers and carried away to the Amazon lowlands or the ocean
• The Amazon Basin studies confirm that the rate of chemical weathering is rapid in the Andes, and presumably in many of Earth’s other high-elevation regions as well, even though the visible effects of chemical weathering are not apparent.
• These studies also show that some warm, wet, vegetated regions may be places of surprisingly slow chemical weathering.

5-12 Weathering: Both a Climate Forcing and a Feedback?

• The original uplift weathering hypothesis left an important issue unresolved.
  • It did not specify a negative feedback that would act as a thermostat and moderate the climatic effects that uplift and weathering produce.
  • Without such a thermostat, what would stop rapid uplift from accelerating chemical weathering to the point where Earth would freeze?
• One possible mechanism that could moderate the degree of uplift-induced climate change is the total amount of fresh rock exposed at Earth’s surface.
  • Plate tectonic processes that cause uplift only affect the relatively small areas actively involved in subductions and collisions.
  • In addition, the effects of the uplift weathering processes are probably opposed by the chemical weathering thermostat.
  • In the end, the uplift-induced weathering increase would succeed in causing a net global cooling, but not nearly so large a cooling as would have occurred without the negative weathering feedback.
• Both the BLAG (spreading rate) hypothesis and the uplift weathering hypothesis seem to provide plausible explanations of the icehouse-greenhouse changes of climate over the last 325 million years.