Earth Climate History Overview

Early Climate

Some of the oldest rocks on Earth contain water-laid sedimentary rocks. This shows that in the early history of the Earth, temperatures were within the stability range of liquid water (0 °C - 100 °C). The sun's energy output when the solar system formed was only about three quarters of its present intensity. But Earth didn't freeze solid because the early atmosphere was dominated by carbon dioxide which holds heat in the atmosphere. As solar intensity has increased progressively through its history the Earth did not eventually become unlivably hot like Venus because photosynthetic life evolved on Earth by 3.5 b.y. ago and has gradually removed CO₂ from the atmosphere.

Paleoproterozoic Glaciation: ~2.2 b.y.

The Gowganda Formation (north of Lake Huron) contains glacial deposits.

These indicate that earth was cool enough for glaciers to form by the early Proterozoic

Neoproterozoic Glaciation: ~750-630 m.y. "Snowball Earth"

Glacial tillites, striations, etc., are found on almost all continents. There were two major glacial episodes during this time. Many glacial sediments were apparently deposited at low latitude as shown by some paleomagnetic data. Was the earth locked in a global deep freeze?

Proposed mechanisms:

i) Bare land, lacking vegetation which had not yet evolved in the Proterozoic, reflects much more sunlight back into space and absorbs much less heat in comparison to the oceans. Most continental masses were lying at low and mid latitudes where the greatest amount of solar radiation strikes the Earth. Therefore, during this period with this unusual placement of land masses, more light was reflected from the earth's surface and less heat absorbed than is usual. So the earth cooled.

ii) The Precambrian supercontinent called Rodinia broke up around this time which would have allowed seaways to bring more humid air to baren land previously landlocked. This would have resulted in more rapid weathering in these previously dry continental interiors thereby drawing more CO₂ from the atmosphere.

Late Paleozoic Glaciation in Gondwana
Glacial deposits (unsorted silt, sand, gravel including boulders) are found in areas of the southern continents (Africa, South America, India, Australia, and Antarctica) that comprised the supercontinent, Gondwana, during much of the Paleozoic era.

Causes:

i) Gymnosperms, seed-bearing plants that reproduce by wind fertilization, evolved during the late Carboniferous and spread into drier upland and interior areas where the spore-bearing plants had more difficulty because they required water for fertilization to occur. CO₂ was removed from the atmosphere as forests spread across the continents and little of the wood decayed but was stored in coal because termites had not yet evolved.

ii) Mountain building as the supercontinent Pangea was assembling and resulting weathering of newly exposed rocks during the Carboniferous and Permian Periods would have resulted in a draw-down of CO₂ from the atmosphere, thereby help to cool global climate.

iii) Parts of Gondwana drifted over or near the south pole for most of the Paleozoic. During Permo-Carboniferous times the position of the south pole ran across Africa, Antarctica, and Australia yielding glacial deposits on those continents. Paleomagnetic data for Gondwana are in agreement with the paleoclimate indicators (think of Wegener's evidence) for the position of the south pole with respect to Gondwana during this time period.

Cretaceous Greenhouse Climate

During the Cretaceous Period the Earth was unusually warm (mean temperature 10 to 15 °C warmer than today). Forests spread to latitudes that today are tundra or ice covered. There were no polar ice caps. Reptiles, including dinosaurs, lived at high latitudes to the Arctic and Antarctic circles.
Cause: This period of warmth was coincidentally a time of rapid seafloor spreading as Pangea and Gondwana continued to break up. The increased rate of seafloor spreading released CO2 from the mantle at a higher rate than average. Additionally, massive flood basalts marked the points of breakup of Pangea and Gondwana (like the Parana-Etendeka flood basalts on the Atlantic coasts of South America and Africa). Under a proposed scenario this major period of flood basalt acitvity was the result of a major episode of plumes rising from deep in the mantle. For some period prior to the Cretaceous, the lower mantle heated up by heat transferred from the core and from radioactive decay in the mantle. Eventually, the lower mantle got so hot that it was less dense than the mantle above it. So blobs of hot, low density (solid-ductile) mantle began to slowly rise from the core-mantle boundary region up toward the Earth's surface leaving trails or conduits through which heated mantle material could continue to rise for tens of millions of years. These are called mantle plumes. They leave hot spot tracks, such as the Hawaiian Islands, on the Earth's surface as the plates slowly glide over the plumes. When the mushroom-shaped plume heads neared the Earth's surface, the great heat in them caused widespread melting and the formation of the flood basalts. The excess heat coming to the surface also induced rapid seafloor spreading. This increase in volcanic activity released additional CO₂ from the mantle into the atmosphere which increased the greenhouse effect and caused the climate to warm.

Descent to the Cenozoic Icehouse

Rapid seafloor spreading and Pangea breakup and separation in the Cretaceous had tectonic consequences. It resulted in rapid convergence and the subduction of a midocean ridge under western North America during the Cenozoic which resulted in arc volcanism and compression that built the Rocky Mountains. The breakup of Gondwana led to the collisions of Africa, Arabia, and India with Eurasia forming the Alpine-Himalayan mountain belt which is still experiencing active uplift. These high, steep, young mountains are the sites of increased weathering and erosion. The principal agent of chemical weathering is carbonic acid which is formed when CO₂ dissolves in water. The global increase of chemical weathering resulting from the Cenozoic mountain building decreased the amount of CO₂ in the atmosphere. Also during the Cenozoic, seafloor spreading rates decreased and so the rate of outgassing of CO₂ also decreased. Eventually, the Cenozoic became much cooler than the Cretaceous.

The Ice Ages Cometh

Two tectonic events led to continental glaciation in Antarctica and the northern hemisphere:
Antarctica was largely ice free during the Cretaceous and early Cenozoic in spite of lying over the south pole. This is because Antarctica was still connected or at least lay close to South America and Australia. This allowed mild ocean currents to bathe the coast of Antarctica. But by 34 million years ago the last connections with Australia and South America were severed and deep water passages surrounded Antarctica allowing the formation of a continuous Circum-Antarctic cold current to thermally Antarctica. It is at this time that glacial dropstones are first found in deep marine sedimentary strata off the coast of Antarctica. This marks the beginning of glaciation in Antarctica.

Northern Hemisphere glaciation did not begin until around three million years ago. This is around the time that the Isthmus of Panama closed. Prior to closure the surface currents in the Atlantic connected with the Pacific between the Americas. After closure the Atlantic circulation stayed in the Atlantic; the Gulf Stream intensified. One leading hypothesis holds that an intensified Gulf Stream carried warm water (producing warm, moist air over it) to high latitude. This warm, moist air caused an increase in precipitation (snowfall) which resulted in the growth of glaciers. Another hypothesis considers that prior to the closure of the Isthmus of Panama the Gulf Stream carried warm water into the Arctic Ocean thereby keeping the Arctic relatively warm. After closure the Gulf Stream was saltier and denser and as it cooled it became dense enough to sink in the North Atlantic forming a deep ocean current (North Atlantic Deep Water), thus depriving the Arctic of a source of heat.

Glacial-Interglacial Cycles: Continental ice sheets have advanced and retreated many times during the past three million years. These advances and retreats can be correlated with variations in the Earth's orbit around the sun. The amount of solar radiation that the northern hemisphere receives in the summer (to melt the winter's snow) varies with changes in

1) the eccentricity (more round to more oval) of the Earth's orbit (100,000 year cycles)
2) the tilt of the axis from 21.5° to 24.5° (41,000 year cycles)
3) the precession of the Earth's axis like the wobbling of a child's spinning top (23,000 year cycles).

With each increase in the polar ice caps comes a decrease in sea level; with each reduction of the ice caps sea level rises. During the maximum advances of the ice sheets the entire continental shelves were exposed, including the Bearing land bridge between North America and Asia. The last glacial maximum occurred about 21,000 years ago. We have been in an interglacial period for the past 10,000 years or so.

Human Influence on Climate - "Early Anthropogenic Hypothesis" - William Ruddiman

During the previous 4 interglacials the concentrations of CO₂ and CH₄ (methane, a potent greenhouse gas) gradually fell after peaking early in the interglacial.

Carbon Dioxide: In the current interglacial CO₂ began falling but then started a steady rise beginning around 8000 years ago. The timing coincides with the rise of agriculture and clearing of forests for cropland. Growing forests pull CO₂ from the atmosphere and store it. Cutting them down returns CO₂ to the atmosphere.

Methane: CH₄, concentrations began falling but then started a steady rise beginning around 5000-6000 years ago coincident with the beginning of rice paddy irrigation. Vegetation decays in rice paddies under low oxygen conditions thereby producing CH₄. Methane is also produced in the digestive tracts of ruminants, such as cows. The rise of large-scale beef production has added to the addition of methane to the atmosphere.

Climate models indicate that if CO₂ and CH₄ concentrations had continued to drop as in the previous 4 interglacials, the Earth would be at the onset of the next glaciation. So, if Ruddiman is right, we may have prevented what should be the next pulse of the ice age.