Lecture 6: Climate Changes

Tectonic-Scale Climate Changes

Orbital-Scale Climate Changes

Deglacial and Millennial Climate Changes

Historical Climate Changes

Global Warming

Climate Change on Various Time Scales

Tectonic-Scale Climate Changes

Orbital-Scale Climate Changes

Millennial Climate Changes

Historical Climate Change

Anthropogenic Climate Changes

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 are the causes and climate effects of changes in sea level through time?

What caused Earth’s climate to cool over the last 55 Myr (the Cenozoic Era)?

Circulation of the Solid Earth

Twenty Rigid Plates

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.

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.

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.

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!!

Slide 14

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.

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.

Slide 17

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.

Slide 19

Historical Climate Changes

Climate changes over the last 1000 years have been smaller than those over tectonic, orbital, and glacial-age millennial time scales, never exceeding 1°C on a global basis.

Climate changes over the last several thousand years have been highly variable in pattern from region to region.

The Little Ice Age

Medieval Warming: A relatively warm climate near 1000 to 1300.

Little Ice Age: The cooling during 1400-1900 that seriously affect Europe.

Twentieth-Century Warming

IPCC AR4
Ch. 10: Global Climate Projections

IPCC Special Report on Emission Scenarios (SRES)

A1 Scenario

The A1 story line is split into 4 scenarios which each are modeled in different ways.

There is an energy intensive A1 group called A1T, an oil and gas resource focused A1 called A1G, a coal based A1 resource A1C, or a mix of resources which was called A1B.

AR4 Simulations

The AR4 scenarios were bases on three scenarios: A2 (high emission), A1B (medium emission), and B1 (low emission).

CO2 Emission for All Scenarios

http://www.grida.no/climate/ipcc/emission/5-2.htm

Slide 27

Projected Surface Warming
(relative to 1980-1999)

Warming Pattern / Ocean vs. Land
(during 2080-2099)

Vertical Distribution of Warming

Projected Temperature Extreme

Projected Changes in Cloud Cover

Projected Precipitation Changes

Projected Monsoon Precipitation Changes

Projected Precipitation Extreme

Projected Hurricane Activities

Projected Extratropical Storms

Projected ENSO Activity

Projected Sea Ice Cover

Projected Change in Atlantic
Meridional Overturning Circulation (MOC)

Positive Climate-Carbon Cycle Feedback

Projected Sea Level Rise

Global Warming and Sea-Level Change

IPCC AR4
Ch. 11: Regional Climate Projections

How to Make Regional Projections?

AOGCM simulations;

Downscaling of AOGCM-simulated data using techniques to enhance regional detail;

Physical understanding of the processes governing regional responses;

Recent historical climate change.

Table 11.1

Summary of Regional Responses - Temperature

Summary of Regional Responses - Precipitation

Projected Precipitation Changes

Africa

Key Processes to African Rainfalls

Hurricane Distribution

Europe and Mediterranean

North Atlantic Oscillation

The NAO is the dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and much into Northern Asia.

The NAO is a large scale seesaw in atmospheric mass between the subtropical high and the polar low.

The corresponding index varies from year to year, but also exhibits a tendency to remain in one phase for intervals lasting several years.

Asia

Projected Monsoon Precipitation Changes

North America

Central and South America

Slide 60

Key Processes to South America Rainfalls

Australia and New Zealand

Arctic and Antarctic

Antarctic Temperature Changes

ENSO and Antarctic Climate


	Tectonic-Scale Climate Changes
	Orbital-Scale Climate Changes
	Deglacial and Millennial Climate Changes
	Historical Climate Changes
	Global Warming


	Tectonic-Scale Climate Changes
	Orbital-Scale Climate Changes
	Millennial Climate Changes
	Historical Climate Change
	Anthropogenic 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 are the causes and climate effects of changes in sea level through time?
	What caused Earth’s climate to cool over the last 55 Myr (the Cenozoic Era)?


	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 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.


	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)?


	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.


	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!!


	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.


	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.


	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.


	Climate changes over the last 1000 years have been smaller than those over tectonic, orbital, and glacial-age millennial time scales, never exceeding 1°C on a global basis.
	Climate changes over the last several thousand years have been highly variable in pattern from region to region.


	Medieval Warming: A relatively warm climate near 1000 to 1300.
	Little Ice Age: The cooling during 1400-1900 that seriously affect Europe.
	Twentieth-Century Warming


	The A1 story line is split into 4 scenarios which each are modeled in different ways.
	There is an energy intensive A1 group called A1T, an oil and gas resource focused A1 called A1G, a coal based A1 resource A1C, or a mix of resources which was called A1B.


	The AR4 scenarios were bases on three scenarios: A2 (high emission), A1B (medium emission), and B1 (low emission).


	AOGCM simulations;
	Downscaling of AOGCM-simulated data using techniques to enhance regional detail;
	Physical understanding of the processes governing regional responses;
	Recent historical climate change.


	The NAO is the dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and much into Northern Asia.

	The NAO is a large scale seesaw in atmospheric mass between the subtropical high and the polar low.

	The corresponding index varies from year to year, but also exhibits a tendency to remain in one phase for intervals lasting several years.