Notes

Outline

Orbital-Scale Climate Change Changes in solar heating driven by changes in Earth’s orbit are the major cause of cyclic climate changes over time scales of tens to hundreds of thousands of years (23k years, 41k years, and 100k years) . Earth’s orbit and its cyclic variations: tilt variations, eccentricity variations, and precession of the orbit. How do orbital variations drive the strength of tropical monsoons? How do orbital variations control the size of northern hemisphere ice sheets? What controls orbital-scale fluctuations of atmospheric greenhouse gases? What is the origin of the 100,000-year climate cycle of the last 0.9 Myr (ice sheets melt rapidly every 100,000 years)?

Earth’s Orbit and Its Variations First, Earth spins around on its axis once every day è The Tilt. Second,  Earth revolves around the Sun once a year è The shape of the Orbit. Both the tilt and the shape of the orbit have changed over time and produce three types of orbital variations:      (1) obliquity variations      (2) eccentricity variations      (3) precession of the spin axis.

Orbit and Insolation Variations in the eccentricity of the orbit  cause changes in the annually averaged amount of sunlight hitting Earth. Variations in the tilt (obliquity variations and the precession of the tilt) do not affect the averaged amount of solar radiation to the Earth. The tilt variations affect seasons.

How Does the Tilt Affect Climate? At present-day, the axis is tilted at an angle of 23.5°, referred to as Earth’s “obliquity”, or “tilt”. The Sun moves back and forth through the year between 23.5°N and 23.5°S. Earth’s 23.5° tilt also defines the 66.5° latitude of the Artic and Antarctic circles. No sunlight reaches latitudes higher than this in winter day. The tilt produces seasons!!

Tilt Creates Seasons Assume the Earth has a perfectly circular orbit around the Sun. With no tilt, incoming solar radiation is always directed straight at the equator throughout the year. With no tilt, no seasonal changes occur in solar radiation received at any latitude. As a result, solstices and equinoxes do not even exit è NO SEASONS!

How Does Orbit’s Shape Affect Climate Earth’s orbit is not a perfect circle: it has a slightly eccentric or elliptical shape. This noncircular shape is the result of the gravitational pull on Earth from the Sun, the moon, other planets and their moons. The distance to the Sun changes with Earth’s position in its orbit. This changing distance has a direct effect on the amount of solar energy Earth receives.

Perihelion and Aphelion The position in which the Earth is closest to the Sun is called “perihelion”. Perihelion means “near the Sun” in Greek. The position in which the Earth is farthest to the Sun is called “aphelion”. Aphelion means “away from the Sun” in Greek.

Seasons and the Elliptical Orbit Seasons       Solstices: mark the longest and shortest days of the years (June 21 and December 21 in the northern hemisphere, the reverse in the southern)       Equinoxes: the length of night and day become equal in each hemisphere. At the present-day orbit, the winter and summer solstices differ from the aphelion and perihelion by about 13 days.

Seasonal Temperature Contrast The seasonal temperature contrast is referred to the range of temperature extremes between summer and winter. The combination of an eccentric orbit and a tilted spin axis means that the seasonal temperature contrast is different in the Southern and Northern Hemispheres. The Southern Hemisphere experiences a larger seasonal temperature contrast than the Northern Hemisphere.

Tilt Change (Obliquity Variation) Over time, the tilt angle varies in a narrow range (22.2° ~ 24.5°). These variations are caused by the gravitational tug of large planets, such as Jupiter. The present-day value of the tile is decreasing. Cyclic changes in the tilt angle occur at a period of 41,000 years. Changes in tilt cause long-term variations in seasonal solar insolation received on Earth. The main effect of tilt changes is to amplify or suppress the seasons: increase tilt amplifies seasonal differences, decreased tilt reduces them.

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Eccentricity of The Elliptical Orbit The degree to which the orbit is elliptical is called the eccentricity. The eccentricity is defined as the ratio of the distance between the center of the ellipse to either focus to the distance from the center to the edge of the ellipse along the major axis. The smaller the eccentricity, the more circular the ellipse is.

Eccentricity Variations Today’s eccentricity is 0.0167, lies well toward the lower end of the variation range of Earth’s eccentricity (closer to circular). The long-term variations in orbital eccentricity are concentrated at two periods: 100,000 years and 413,000 years. The eccentricity variations differ from the other two orbital variations (obliquity variations and the precessional variations) in one important aspect: Eccentricity variations cause changes in the annually averaged amount of sunlight hitting Earth.

Precession of Axis There are two kinds of precession: (1) the precession of the spin axis and (2) the precession of the ellipse. Earth’s wobbling motion is called the axial precession.  It is caused by the gravitational pull of the Sun and Moon. Axial precession is a slow turning of Earth’s axis of rotation through a circular path, with a full turn every 25,700 years.

Precession of Ellipse The precession of the ellipse is known as the elliptical shape of Earth’s orbit rotates itself at a slower rate than the wobbling motion of the axial precession.

Time Scales of Precession The combined effects of these two precessions cause the solstices and equinoxes to move around Earth’s orbit, completing one full 360° orbit around the Sun every 23,000 years.

Eccentricity and Seasonal Insolation Small variations in received insolation do occur in connection with Earth’s eccentric orbit about the Sun, but these appear only as changes in the total energy received by the entire Earth, not as  seasonal variations. Compared to the changes in seasonal insolation of 10% or more that occur at the tilt and precession cycles, these annual eccentricity changes are smaller by a factor of 50 or more.

Tilt and Seasonal Insolation The hemispheric and seasonal patterns of insolation changes for tilt and precession are fundamentally different. Insolation changes at high latitudes caused by change in the tilt are in phase between the hemisphere from a seasonal perspective. For the case of increased tilt, summer insolation maxima in the northern hemisphere occur at the same time in the 41,000-year cycle as summer insolation maxima in the southern hemisphere.

Precession and Seasonal Insolation For the precessional changes that dominant low and middle latitudes, the relative sense phasing between seasons and between hemispheres is exactly reversed from of tilt. The Earth-Sun distance is the major control on precessional changes in insolation. A precessional-cycle insolation maximum occurring at June 21 (or December 21) will be simultaneous everywhere on the Earth, a summer season in one hemisphere and a winter season in another hemisphere.

Precession and Seasons

Impacts of Precession and Eccentricity on Insolation

Seasonal Insolation Changes The 23,000-year cycle of precissional change dominants the insolation changes at low and middle latitudes. The 41,000-year cycle of tilt change dominants the insolation changes at higher latitudes. Eccentricity changes (the 1000,000 or 413,000-year cycles) is not a significant influence on seasonal insolation chanes.

Insolation Control of Ice Sheets What controls the size of Ice Sheets in the Northern Hemisphere?       è The summer insolation (The Milankovitch Theory).

Milankovitch Theory Milankovitch suggested that the critical factor for Northern Hemisphere continental glaciation was the amount of summertime insolation at high northern latitudes. Low summer insolation occurs during times when Earth’s orbital tilt is small. Low summer insolation also results from the fact that the northern hemisphere’s summer solstice occurs when Earth is farthest from the Sun and when the orbit is highly eccentric.

Temperature and Ice Mass Balance Continental ice sheets exist because the overall rate at which snow and ice accumulates equals or exceeds the overall rate of ice loss or ablation. Temperature is the main factor that determine whether ice sheets are in a regime of net ablation (negative mass balance) or net accumulation (positive mass balance). The boundary between the upper area of positive mass balance and the lower area of net loss of ice mass is called the equilibrium line.

Seasons and the Size of Ice Sheet Continental ice sheets exist because the overall rate at which snow and ice accumulates equals or exceeds the overall rate of ice loss or ablation. It is the summer insolation  that determines the ice sheets grow and shrink. Low (cold) summer insolation is the critical factor that cools the climate enough to allow snow and ice to persist from one winter to the next. Winter insolation is not important to ice sheet sizes, because (1) the temperatures are always cold in winter and (2) the Sun at high latitudes are low in the sky, regardless of ongoing orbital changes.

Insolation Control of Ice Sheet Size The Milankovitch theory proposes that high summer isolation results in net melting of ice sheets, while low summer insolation results in net accumulation of ice sheets.

Three Factors that Control Ice Sheets (1) Insolation control of ice sheet size (2) Initial lag of ice volume behind insolation (3) Subsequent lag of bedrock depression and rebound behind ice sheet growth and decay

Insolation Control of Ice Sheet Size During high summer insolation, the equilibrium line is driven north. The continents lie in a regime of net albation. During low summer insolation, the equilibrium line is driven south. The continents lie in a regime of net accumulation.

Ice Sheet Lags Behind Summer Insolation Forcing Under the most favorable condition (ice accumulates 0.3 meters each year), a full-size ice sheet 3000 meters thick would take 10,000 years to form. At the 41,000-year cycle of orbital tilt, ice sheets lag behind changes in summer insolation by about 10,000 years. At the 23,000-year cycle of orbital precession, ice sheet lags behind summer insolation by about 6,000 years. The lag times are both about one quarter of the periods of the cycles.

Delayed Bedrock Response Beneath Ice Sheets As ice sheets grow, the pressure of their weight on the underlyingion bedrock becomes significant. Due to the different densities of ice and rock, a ice sheet of 3000m thickness is roughly equivalent to the weight of 1000m of solid rock. The initial sinking would be elastic and immediate. The bedrock later respond more slowly and sinks half of the remaining distance toward the eventual equilibrium every 3000 years. The total time to reach the final depression depth is about 15,000 years.

Full Cycle of Ice Growth and Decay A full cycle of ice sheet growth and decay incorporates both the delayed response of the ice sheets to summer insolation forcing as well as the delayed response of bedrock to the ice sheet load. As a results of the delays, the maximum size of ice sheets does not happen at the time of the lowest summer insolation.

Factors in Long-Term Evolution of Ice Sheets The long-term evolution of ice sheets reflects the interaction of two factors: changes in summer insolation that drive shorter-term changes in ice sheet mass balance and a much more gradual global cooling represented by a slowly changing glaciation threshold. Ice sheets accumulate when summer insolation falls below a critical glaciation threshold and melt when it rises above it.

Conceptual Phases of Ice Sheet Evolution

Evidence of Ice Sheet Evolution This figures shows a North Atlantic Ocean sediment core holds a 3 Myr d18O record of ice volume and deep-water temperature changes. There were no major ice sheets before 2.75 Myr ago. After that, small ice sheets grew and melted at cycles of 41,000 and 23,000 years until 0.9 Myr ago. After 0.9 Myr ago, large ice sheet grew and melted at a cycle of 100,000 years.

Ice Sheet Changes Over the Last 150,000 Years A closer look of the last 150,000 years of the d18O record shows 23,000-year and 41,000-year cycles.

Insolation and Ice Volume

Orbital-Scale Changes in CO2 and CH4

Orbital-Scale Changes in CO2 and CH4 Methane (CH4) levels have fluctuated mainly at the 23,000-year orbit rthym of precession.       è These changes may be linked to fluctuations in the strength of monsoon in Southeast Asia. During glaciations, atmospheric CO2 values have repeatedly dropped by 30% from the levels typical of warm interglaciations.        è Most of the explanation for lower glacial CO2 appears to be tied to a transfer of carbon into the deep ocean. WHY?

Trapping Gases in the Ice Air moves freely through snow and ice in the upper 15 m of an ice sheet. Flow is increasingly restricted below this level. Bubbles of old air are eventually sealed off completely in ice 50 to 100 m below the surface.

Ice Core Drilling The best place on an ice sheet to take ice cores is at the top. Ice cores can be dated by counting annually deposited layer (or ice flow model). Annual layering is recorded in several properties of ice cores, the most obvious of which are layers of dust easily visible to the eye. Dust is usually deposited at the end of cold, dry windy winters. One of the most famous ice core drilling is at a site high on the Antarctic ice sheet, which is called the Vostok ice record.

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Orbital-Scale Changes in Methane The Vostok ice record shows a series of cyclic variations in methane concentration, ranging between 350 to 700 ppb (part per billion). Each Ch4 cycle takes about 23,000 years. This cycle length points to a likely connection with changes in orbital procession. The orbital procession dominates insolation changes at lower latitudes.

Monsoon and Methane On the 23,000-year cycle, methane variations closely resemble the variations of monsoon strength. The peak values of methane match the expected peaks in monsoon intensity not only in timing but also in amplitude. This match suggests a close connection between CH4 concentrations and the monsoon on the 23,000-year climate cycle. By why?

Insolation Control of Monsoons Monsoon circulations exit on Earth because the land responds to seasonal changes in solar radiation more quickly than does the ocean. Changes in insolation over orbital time scales have driven major changes in the strength of the summer monsoons. Changes of 12% in the amount of insolation received at low latitudes have caused large changes in heating of tropical landmass and in the strength of summer monsoons at a cycle near 23,000  years in length.

The Orbital Monsoon Hypothesis The 23,000-year cycle of orbital procession increases (decreases) summer insolation and at the same time decreases (increases) winter insolation at low and middle latitudes. Departures from the modern seasonal cycle of solar radiation have driven stronger monsoon circulation in the past. Greater summer radiation intensified the wet summer monsoon. Decreased winter insolation intensified the dry winter monsoon.

How Did Monsoon Affect Methane? Orbital procession affects solar radiation at low latitudes       è solar radiation affects the strength of low-latitude monsoons       è monsoon fluctuations changes the precipitation amounts in Southeast Asia       è heavy rainfalls increase the amount of standing water in bogs       è decaying vegetation used up any oxygen in the water and creates the oxygen-free conditions needed to generate methane       è the extent of these boggy area must have expanded during wet monsoon maxima and shrunk during dry monsoon minima.

Orbital-Scale Changes in CO2 The longest orbital-scale record of CO2 changes comes from the Vostok ice core drilling site. The CO2 record shows a series of regular oscillation between CO2 values as high as 280-300 ppm and as low as 180-190 ppm over the last 400,000 years. Values at the high end of this range occur within the present interglacial interval. These relative high CO2 values lasted for several thousand years before the abrupt increase after 1800. The dominant period of CO2 variations is about 100,000 years.

Ice Sheet and CO2 We have learned that the dominant cycle of ice sheet variations over the last several hundred years has also been 100,000 years. This suggests that the 100,000-year variations in atmospheric CO2 match those of ice sheets. Over the 400,000-year length of the ice core record, the major cycles of CO2 change line up well with the ice volume changes (indicated by d18O). The two signals share not only the 100,000-year cycle but also its asymmetric shape: abrupt increases in CO2 during times of rapid ice melting (i.e., warming) and slower decreases during times of slower phases of ice volume buildup (cooling). WHY?

The Basic Questions How could CO2 vary by 30% or more over orbital time scale? What factors can explain the observed 90-ppm dip in CO2 level during glacial intervals from the levels observed in interglacial intervals?

Possibility 1: Physical Oceanographic Explanation of CO2 Changes Changes in the physical oceanographic characteristics of the surface ocean – its temperature and salinity – during glaciations might alter the chemical solubility of Co2. Because CO2 dissolves more readily in colder seawater, atmospheric CO2 will drop by 9 ppm for each 1°C of ocean cooling. Because CO2 dissolves more easily in seawater with a lower salinity, saltier glacial seawater will increase atmospheric CO2 level. The effects of seawater temperature and salinity work against each other! The net effect of the physical oceanographic influences on atmospheric CO2 is to decrease CO2 by 11 ppm during glaciations. This process only explains 10% of the CO2 changes on orbital scales.

Possibility 2: Deep Ocean Reservoir The large changes in atmospheric CO2 over intervals of a few thousand years must involve rapid exchanges (compared to the rate of exchange between the atmosphere and rock) among: the atmosphere, vegetation and soil, surface and deep ocean. Not vegetation and soil: because vegetation reduced during glaciations. Not surface ocean: because surface ocean exchanges CO2 quickly (in a few years) with the atmosphere. The glacial carbon removed from the atmosphere, from the vegetation, and from the surface ocean must have been stored in the deep ocean. Carbon was exported to the deep ocean by higher rates of photosynthesis and biological productivity.

Ocean Carbon Pumping Hypothesis Carbon was exported from surface oceans to the deep ocean during glaciations by higher rates of photosynthesis and biological productivity. Photosynthesis and organic productivity occur in the surface ocean because of sunlight and nutrients. Photosynthesis extracts CO2 from surface waters and incorporates it in organic tissue (CH2O), some of which sinks to the deep ocean.

Where Did the Pumping Occur? The pumping hypothesis asserts that greater downward removal of carbon occurs because more nutrients become available to stimulate greater removal of organic carbon from surface water by photosynthesis. Large-scale changes in biological pumping of carbon to the deep sea can occu only in relatively productive regions. The most productive regions in the world ocean are in areas of upwelling along costal margins and near the equator and in the high-latitude surface ocean around Antarctica. These are the two regions where significant more organic carbon could have been pumped down into the deep ocean during glaciations than today.

How Glaciations Increased Productions at Low Latitudes? Increased Upwelling       Stronger glacial winds greater wind-driven upwelling of nutrient-rich water along coast and near equator. Increased Nutrient Levels By Iron Fertilization       Stronger glacial winds Dust rich in iron is blow to the ocean from arid continental interiors Ions fertilize the surface ocean and stimulate greater glacial photosynthesis and productivity. This fertilization effect stimulates the productivity in mid-ocean regions far from costal upwelling.

How Glaciations Increased Productions Near Antarctica? The deep-ocean circulation in the Antarctica brings abundant nutrients up into the surface water, which is not fully utilized by photosynthesis due to the brief summer seasons. During glacial intervals in the Antarctic region, increased carbon pumping out of surface water could occur either because       (1) nutrient-rich waters remained at the surface longer, or       (2) higher glacial nutrient levels stimulated more productivity.

Changes in Ocean Chemistry One of the major factors that determines CO2 levels in surface water of the world oceans is the amount of carbonate ion CO3-2. These ions are produced when corrosive bottom water dissolve CaCO3 on the seafloor. When these ions are later returned to surface waters by ocean circulation, they can combine chemically with CO2 and produce the bicarbonate ion HCO3-. Therefore, increasing the amount of CO3-2 ions dissolved by and carried in deep water can have the effect of reducing CO2 concentrations in surface water.

Changes in North Atlantic Deep Water The carbon isotope trend provides clear evidence that there was a link between the size of northern hemisphere ice sheets and the formation of deep water in the North Atlantic nearby. Less deep water formed in the North Atlantic every time ice sheets grew larger at the 100,000-year cycle after 0.9 Myr ago. More deep water formed when ice sheets were small.

Changes in Circulation of Deep Water During Glaciations The percentage of deep water originating in the North Atlantic and flowing to the equator during the last 1.25 Myr has been consistently lower during glaciations than during interglaciations. In general, southern-source waters (deep water formed near Antarctica) dominated tropical Atlantic waters during glaciations. Northern-source water (deep water formed in the North Atlantic) dominated tropical Atlantic waters during interglaciations.

Carbon System Controls on CO2 in the Glacial Atmosphere Among the changes in the distribution of ocean carbon that could reduce glacial levels of atmospheric CO2 are:       (1) fast downward organic carbon pumping in areas of costal or tropical upwelling,       (2) similar processes in Antarctic,       (3) changes in the chemistry of Antarctic surface water toward higher CO3-2 content,       (4) changes in the chemistry of shallow subsurface water originating from southern latitudes.

The Mystery of the 100,000-Year Cycle After 0.9 Myr ago, the 41,000-year and 23,000-year cycles of ice volume change were overridden by larger and more dominant fluctuations of ice sheets at a period of 100,000 years. The large 100,000-year cycle is not related to any cycle of orbital variations. TWO QUESTIONS:       (1) Why did more ice accumulate after 0.9 Myr ago?       (2) Why did these large ice sheets melt rapidly every 100,000 years?

Why Have Ice Sheets Grown Larger Since 0.9 Myr Ago? Explanation 1: A result of the gradual (tectonic-scale) cooling.       Tectonic-scale cooling due to the decrease of CO2  è cooling reach a threshold value 2.75 Myr ago  è moderate-size ice sheets can form in  northern hemisphere ice during major summer insolation minima and disappeared completely during summer insolation maxima.  è the falling CO2 levels continued  è cooling reached another threshold value 0.9 Myr ago  è that ice sheets never completely disappeared during weak summer insolation maxima  è ice sheets began to grow larger and persist longer

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Why Have Ice Sheets Grown Larger Since 0.9 Myr Ago? Explanation 2: Ice Slipping Effect (nothing to do with climate changes)      During earlier glaciations (2.75 – 0.9 Myr ago), ice sheets may have been thin because they slid on water saturated soil toward lower elevations and warmer temperatures.       During later glaciations (beginning from 0.9 Myr ago), after ice sheets stripped off most of the underlying soil, their central regions could grow higher because they no longer slid.

What Causes Abrupt Deglaciations? Slow global cooling together with the less sliding beneath ice sheets helped to create thicker and more extensive ice sheets beginning 0.9 Myr ago. What cause the rapid melting every 100,000 years? Explanation: the 100,000-year eccentricity cycle      The 100,000-year eccentricity orbital cycle only produces a trivial amount of direct insolation changes.       It is the modulation of the 100,000-year cycle on the amplitude of the 23,000-year precession cycle that actually affect the rapid melting every 100,000 years.

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Roles of Internal Climate Interactions The pace of the 100,000-year variations in ice sheet size is set by the external forcing from the eccentricity orbital cycle. The amplitude of the 100,000-year cycle depends in part on the internal interactions of the climate system. The large ice sheets themselves produced internal interactions within the climate system that hastened their own destruction every 100,000 years. The possible positive feedbacks to accelerate ice sheet melting include: rising CO2 level during deglaciations, the delayed in bedrock rebound provided warmer temperatures in the depressed land surface for fast ice melting, and other ice-controlled processes in wind, sea level, and deep ocean circulation.