Climate Archives

Much of climate history is recorded in four climate archives:

      (1) Sediments

      (2) Ice

      (3) Corals

      (4) Trees

How are those records dated?

Hoe much of Earth’s history each archive spans?

What is the resolution of climate history yielded by each?

Sediments

Sediments are the major climate archive on Earth for over 99% of geological time (and on all time scales), primarily as continuous sequences deposited by water.

Rainfall and the runoff it produces erode rocks exposed on the continents and transport the eroded sediments in streams and rivers.

The sediments are deposited in quieter waters where layer upon layer of sediments can be laid down in undisturbed succession.

For intervals before the last 170 million years, all surviving sedimentary records come from continents.

Dating Sediment Records

Sediments in the deeper parts of some lakes contain annual-layer couplet called varves.

Varve couplets usually result from seasonal alternations between deposition of light-hued mineral-rich debris and darker sediment rich in organic material.

Resolution of Sediment Records

The degree of resolution of climate records in sediment archives is related to the rate of deposition (and burial) of sediment and to the amount of activity of organisms burrowing into the sediments.

Glacial Ice

Ice cores retrieve climate records extending back thousands of years in small mountain glaciers to as much as hundreds of thousands of years in continental sized ice sheets.

The antarctic ice sheet has layers that extend back over 400,000 years.

The Greenland ice sheet has layers that extended back 100,000 years.

Dating Ice Records

The most visible forms of annual layering in ice are the alternations between darker layer, containing dust blown in from continental source regions during the dry windy season, and lighter layers from other seasons with little or no dust.

Resolution of Ice Records

Annual layer of snows are visible at the surface of many mountain glaciers and rapidly deposited ice sheets.

As snow is buried and slowly recrystallized into ice, annual layers remain resolable to a depth. Below this depth, the layering is lost.

Ice cores from mid-latitude ice sheets such as the one on Greenland, where deposition of snow is rapid, the annual layering may remain visible tens of thousands of year into the past.

Ice cores from Antartica, where only a small amount of snow accumulates each year, annual layering may not occur even at the ice surface.

Trees

Trees are climate archives for the interval of the last few tens and hundreds of years.

The outer softwood layers of many kinds of trees are deposited in millimeter-thick layers that turn into hardwood.

These annual layers are best developed in middle and high latitudes, where seasonal climate changes are larger.

Dating Tree Records

In regions of marked seasonal variations of climate, trees produce annual layers call tree rings.

These rings are alternations between layers of lighter, thicker wood tissue formed by rapid growth in spring and much thinner, darker layers marking cessation of growth in autumn and winter.

Corals

Corals form annual bands of CaCO3 that hold several kinds of geochemical information about climate.

Individual corals may live for time spans of years to tens or hundreds of years.

Coral archives are located at tropical and subtropical latitudes.

Dating Coral Records

In tropical oceans, corals record seasonal changes in the texture of the calcite (CaCO3) incorporated in their skeletons.

The lighter parts of these coral bands are laid down in summer, during intervals of fast growth, and the darker layers during winter, when grow slows.

Resolution of Climate Records

Climate System Change - Review

Slide 14

Three Major Factors

The major boundary conditions that have driven climate changes during the last 21,000 years have been the changes in:

      (1) size of ice sheet

      (2) seasonal insolation

      (3) level of greenhouse gases in the atmosphere.

The Last Glacial Maximum (21,000 Years Ago)

Seasonal insolation levels 21,000 years ago were nearly identical to those today.

The only factors that can explain the colder and drier glacial maximum climate 21,000 years ago are:

(1) the large ice sheets

(2) the lower values of greenhouse gases.

Cold and Dry Glacial Climate

Ocean Temperature Today

Climate Change During the Last Deglaciation
(between 17,000 and 6,000 years ago)

During the deglaciation interval between 17,000 and 6,000 years ago, climate changes were driven by rising summer insolation and by increased concentrations of CO2 in the atmosphere.

By 10,000 years ago, the angle of tilt of Earth’s axis had reached a maximum at the same time that Earth’s precessional motion moved it closest to the Sun on June 21. These combined orbital effects produced a summer insolation maximum at all latitudes of the northern hemisphere.

As the ice sheets shrank, their ability to influence climate diminished.

Retreat of the North American Ice Sheets

Radiocarbon dating of organic remains shows that the ice sheets in North America disappeared completely shortly after 6,000 years ago.

Deglacial Rise in Sea Level

Submerged corals off Barbados, in the Caribbean, show the deglacial history of the rise in sea level caused by the return of meltwater from the ice sheets to the ocean.

Mid-Deglacial Cooling: The Younger Dryas

The mid-deglacial pause in ice melting was accompanied by a brief climate osscilation in records near the subpolar North Atlantic Ocean.

Temperature in this region has warmed part of the way toward interglacial levels, but this reversal brought back almost full glacial cold.

Because an Arctic plant called “Dryas” arrived during this episode, this mid-deglacial cooling is called “the Younger Dryas” event.

Climate Changes Since Deglaciation
(the last 6,000 years)

During the last 6,000 years, with the ice sheets melt and CO2 level stabilized near interglacial levels, the gradual change in solar insolation is the main orbital-scale factor that affect the climate since deglacation.

Millennial-Scale Climate Oscillations

Millennial-scale climate oscillations are referred to climate variations with time scales as short as 1,000 years.

These climate variations are rapid enough to be relevant to human concern.

These changes were large when glacial ice sheets extended, but smaller during interglacial climate like today.

Millennial Changes in North Atlantic Ocean

The millennial-scale changes in the North Atlantic is characterized by short cooling cycle 1,500 years in length.

These oscillations gradually drift toward colder conditions and occasionally cumulate in major ice-rafting episodes, followed by an abrupt return to warmer conditions.

Other Millennial-Scale Oscillations

Millennial-scale oscillations are found in many places in the northern and southern hemisphere, such as those in

      (1) Greenland ice cores

      (2) North Atlantic sediments

      (3) European soil properties

      (4) CH4 concentration

      (5) amounts of sea slat, dust…..

What Causes Millennial Changes?

It is still not fully understood what caused the millennial-scale climate changes.

Any successful hypotheses have to explain

      (1) What initiated these oscillations?

      (2) How are they transmitted to various part of the climate system?

      (3) Why are they stronger during glaciations than during interglaciations?

Three possible hypotheses have been suggested:

      (1) The natural oscillations inherent in the internal behavior of northern hemisphere ice sheets

      (2) The result of internal interactions among subcomponents of the climate system

      (3) A response to solar variations external to the climate system.

Processes Within Ice Sheets

This hypothesis argues that northern hemisphere ice sheets are the source of millennial climate oscillations as a result of their own natural interannual variations.

This hypothesis emphasizes the marine margins of ice sheets, where ices are thin, to provide the rapid interactions with their surroundings.

Interactions Within Climate System

This hypothesis argues that millennial oscillations were produced by the internal interactions among various components of the climate system.

One most likely internal interaction is the one associated with the deep-water formation in the North Atlantic.

Millennial oscillations can be produced from changes in northward flow of warm, salty surface water along the conveyor belt.

Stronger conveyor flow releases heat that melts ice and lowers the salinity of the North Atlantic, eventually slowing or stopping the formation of deep water.

Weaker flow then causes salinity to rise, completing the cycle.

External Solar Variability

This hypothesis argues that the strength of Sun may have changed on the millennial scales.


	Much of climate history is recorded in four climate archives:
	(1) Sediments
	(2) Ice
	(3) Corals
	(4) Trees
	How are those records dated?
	Hoe much of Earth’s history each archive spans?
	What is the resolution of climate history yielded by each?


	Sediments are the major climate archive on Earth for over 99% of geological time (and on all time scales), primarily as continuous sequences deposited by water.
	Rainfall and the runoff it produces erode rocks exposed on the continents and transport the eroded sediments in streams and rivers.
	The sediments are deposited in quieter waters where layer upon layer of sediments can be laid down in undisturbed succession.
	For intervals before the last 170 million years, all surviving sedimentary records come from continents.


	Sediments in the deeper parts of some lakes contain annual-layer couplet called varves.
	Varve couplets usually result from seasonal alternations between deposition of light-hued mineral-rich debris and darker sediment rich in organic material.


	The degree of resolution of climate records in sediment archives is related to the rate of deposition (and burial) of sediment and to the amount of activity of organisms burrowing into the sediments.


	Ice cores retrieve climate records extending back thousands of years in small mountain glaciers to as much as hundreds of thousands of years in continental sized ice sheets.
	The antarctic ice sheet has layers that extend back over 400,000 years.
	The Greenland ice sheet has layers that extended back 100,000 years.


	The most visible forms of annual layering in ice are the alternations between darker layer, containing dust blown in from continental source regions during the dry windy season, and lighter layers from other seasons with little or no dust.


	Annual layer of snows are visible at the surface of many mountain glaciers and rapidly deposited ice sheets.
	As snow is buried and slowly recrystallized into ice, annual layers remain resolable to a depth. Below this depth, the layering is lost.
	Ice cores from mid-latitude ice sheets such as the one on Greenland, where deposition of snow is rapid, the annual layering may remain visible tens of thousands of year into the past.
	Ice cores from Antartica, where only a small amount of snow accumulates each year, annual layering may not occur even at the ice surface.


	Trees are climate archives for the interval of the last few tens and hundreds of years.
	The outer softwood layers of many kinds of trees are deposited in millimeter-thick layers that turn into hardwood.
	These annual layers are best developed in middle and high latitudes, where seasonal climate changes are larger.


	In regions of marked seasonal variations of climate, trees produce annual layers call tree rings.
	These rings are alternations between layers of lighter, thicker wood tissue formed by rapid growth in spring and much thinner, darker layers marking cessation of growth in autumn and winter.


	Corals form annual bands of CaCO3 that hold several kinds of geochemical information about climate.
	Individual corals may live for time spans of years to tens or hundreds of years.
	Coral archives are located at tropical and subtropical latitudes.


	In tropical oceans, corals record seasonal changes in the texture of the calcite (CaCO3) incorporated in their skeletons.
	The lighter parts of these coral bands are laid down in summer, during intervals of fast growth, and the darker layers during winter, when grow slows.


	The major boundary conditions that have driven climate changes during the last 21,000 years have been the changes in:
	(1) size of ice sheet
	(2) seasonal insolation
	(3) level of greenhouse gases in the atmosphere.


	Seasonal insolation levels 21,000 years ago were nearly identical to those today.
	The only factors that can explain the colder and drier glacial maximum climate 21,000 years ago are:
	(1) the large ice sheets
	(2) the lower values of greenhouse gases.


	During the deglaciation interval between 17,000 and 6,000 years ago, climate changes were driven by rising summer insolation and by increased concentrations of CO2 in the atmosphere.
	By 10,000 years ago, the angle of tilt of Earth’s axis had reached a maximum at the same time that Earth’s precessional motion moved it closest to the Sun on June 21. These combined orbital effects produced a summer insolation maximum at all latitudes of the northern hemisphere.
	As the ice sheets shrank, their ability to influence climate diminished.


	Radiocarbon dating of organic remains shows that the ice sheets in North America disappeared completely shortly after 6,000 years ago.


	Submerged corals off Barbados, in the Caribbean, show the deglacial history of the rise in sea level caused by the return of meltwater from the ice sheets to the ocean.


	The mid-deglacial pause in ice melting was accompanied by a brief climate osscilation in records near the subpolar North Atlantic Ocean.
	Temperature in this region has warmed part of the way toward interglacial levels, but this reversal brought back almost full glacial cold.
	Because an Arctic plant called “Dryas” arrived during this episode, this mid-deglacial cooling is called “the Younger Dryas” event.


	During the last 6,000 years, with the ice sheets melt and CO2 level stabilized near interglacial levels, the gradual change in solar insolation is the main orbital-scale factor that affect the climate since deglacation.


	Millennial-scale climate oscillations are referred to climate variations with time scales as short as 1,000 years.
	These climate variations are rapid enough to be relevant to human concern.
	These changes were large when glacial ice sheets extended, but smaller during interglacial climate like today.


	The millennial-scale changes in the North Atlantic is characterized by short cooling cycle 1,500 years in length.
	These oscillations gradually drift toward colder conditions and occasionally cumulate in major ice-rafting episodes, followed by an abrupt return to warmer conditions.


	Millennial-scale oscillations are found in many places in the northern and southern hemisphere, such as those in
	(1) Greenland ice cores
	(2) North Atlantic sediments
	(3) European soil properties
	(4) CH4 concentration
	(5) amounts of sea slat, dust…..


	It is still not fully understood what caused the millennial-scale climate changes.
	Any successful hypotheses have to explain
	(1) What initiated these oscillations?
	(2) How are they transmitted to various part of the climate system?
	(3) Why are they stronger during glaciations than during interglaciations?
	Three possible hypotheses have been suggested:
	(1) The natural oscillations inherent in the internal behavior of northern hemisphere ice sheets
	(2) The result of internal interactions among subcomponents of the climate system
	(3) A response to solar variations external to the climate system.


	This hypothesis argues that northern hemisphere ice sheets are the source of millennial climate oscillations as a result of their own natural interannual variations.
	This hypothesis emphasizes the marine margins of ice sheets, where ices are thin, to provide the rapid interactions with their surroundings.


	This hypothesis argues that millennial oscillations were produced by the internal interactions among various components of the climate system.
	One most likely internal interaction is the one associated with the deep-water formation in the North Atlantic.
	Millennial oscillations can be produced from changes in northward flow of warm, salty surface water along the conveyor belt.
	Stronger conveyor flow releases heat that melts ice and lowers the salinity of the North Atlantic, eventually slowing or stopping the formation of deep water.
	Weaker flow then causes salinity to rise, completing the cycle.


	This hypothesis argues that the strength of Sun may have changed on the millennial scales.