Climate Change

Determining past climate
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The methods used to study the past climate typically involve physical, chemical, biological processes sensitive to: temperature changes, the presence of water, solar activity, etc.
The records must have survived over a long period of time.
We must be able to date the records relatively accurately.
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Geological evidence – examples:
♦ Advancing and retreating glaciers (surface temperature)
♦ Ice cores from Antarctica and Greenland (the amount of snow accumulation, isotopic ratios, the crystal structure of the ice, trapped air bubbles, solar activity, volcanic eruptions, fine dust and plant pollen, microorganisms)
♦ Plant fossils in sedimentary rocks (plants are sensitive to longterm temperature changes)
♦ Core samples of the ocean floor sediments (contain shells and microorganisms that live within some temperature range)
Dendrochronology (the rate of growth of tree rings-for trees which are sensitive to temperature or drought stress)

Glaciers

Ice Cores from the Antarctic and Greenland

Sedimentary Rocks

Fossils in Sedimentary Rocks

Ocean Floor Cores

Climate through the Ages: overview
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Throughout the geological history of the Earth, the temperature has been warmer than today (8 to 15 deg C)
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There have been several periods of glaciations (ice ages)
In the interglacial (warm ) periods the glaciers retreat and the polar regions are ice free, the sea level is elevated. •
Some of the changes occur over thousands of years, some as quickly as a few years.

Earth Temperature through the Ages
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During the last 2 billion years the Earth's climate has alternated between
♦ A frigid "Ice House", like today's world
♦ A steaming "Hot House", like the world of the dinosaurs

Climate During the last 1000 years
Little Ice Age
Medieval Climatic Optimum
Eighteen hundred and froze-to-death

Temperature trend during the past 100-plus years

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Mid-chapter summary
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The Earth’s climate is constantly undergoing change. Evidence suggests that throughout much of the Earth’s history the Earth’s climate was much warmer than it is today.
The most recent glacial period (or Ice Age) began about 2 million
• years ago. During this time, glacial advances were interrupted by warmer periods ( interglacial periods ). In North America, glaciers reached their maximum thickness and extent about 18,000 to 22,000 years ago and disappeared completely from North America by about
6,000 years ago.
Over the last 100 years, the the Earth’s surface temperature has increased by about 0.6 o C (about 1 o F).

Possible Causes of Climatic Change
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External : changes in the
1.
incoming solar radiation
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2.
composition of the atmosphere
3.
Earth’s surface
Natural phenomena may contribute to all three (
Human activity may contribute to
Internal : changes in the circulation patterns of the ocean and the atmosphere, etc.
2 and 3
1-3 )

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Climate Change and Feedback Mechanisms
The climatic elements are intertwined: feedback
Negative feedback : counteracts changes, stabilizes climate
♦ Thermal (IR) radiation of the planet: increased temperature leads to increased IR emission from the planet -> the planet cools and the temperature drops.
♦ Cloud negative feedback : higher temperature leads to more water vapor and therefore more clouds. The clouds increase the planet’s albedo and less solar light reaches the surface -> planet cools and the temperature drops.
Positive feedback : enhances the changes, destabilizes the climate (runaway warming or cooling)
♦ Water vapor-greenhouse feedback: T increases, water evaporates -> greenhouse effect -> T increases more
♦ Snow-albedo feedback: T increases, snow melts -> albedo decreases -> less solar energy reflected at the surface -> T increases even more

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Climate and Plate Tectonics
The continents have changed their position during the
• geological history of the Earth.
From one large concentrated land mass (super continent)
• they broke into seven smaller land masses
Continental drift affects ocean circulation and with it
• the heat flow throughout the ocean.
As land masses move to higher latitudes, ice sheets are more likely to form -> snow-albedo feedback effect now
180 million years ago

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Climate change and plate tectonics
Tectonic and volcanic activity adds CO
2 and water vapor to
• the atmosphere
Mountain building affects global circulation patterns in the atmosphere degassing degassing volcano underwater volcano spreading subduction

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Climate Change and Variations in the
Earth’s Orbit
Milankovitch Theory (1930s):
♦ Variations in the Earth orbit produce changes in the amount of energy the
Earth receives from the Sun during the seasons
Milutin
Milanković
1879-1958
♦
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The eccentricity – 100,000 years cycle
Precession of the axis of rotation - 23,000 years
Obliquity – 41,000 year cycle
♦ These variations result in three Milankovitch cycles which have been observed in the Earth climate and geological records.

Variation in the Eccentricity
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The Earth orbit changes between being less elliptical and more elliptical
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Large eccentricity translates in a bigger difference in the amount of solar energy that the Earth receives at the extreme points of the orbit
Currently the eccentricity is small. The Earth is closest to the Sun in January and farthest from the Sun in July by 3%. This results in about 7% difference in the solar energy received on Earth
If the difference in the distance to the sun increases to 9%, the Earth will experience about 20% variation in the received solar energy
This cycle has a period of about 100,000 years

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Precession of the Earth axis
Currently the Earth gets closest to the sun in January
Due to the precession in ~11,000 years it will be closest to the sun in July: The NH winter will be colder than today and the SH winter will be milder
This cycle has a period of 23,000 years

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Variations in the Obliquity
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The obliquity (the tilt of the Earth with respect to the
• ecliptic) varies between 22 and 24.5 degrees
This variation has periodicity of 41,000 years
The obliquity determines the seasonal variations
♦ Large obliquity -> larger differences between the winter and the summer
♦ Smaller obliquity -> less pronounced seasonal changes

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Milankovitch Cycles and Glaciations
100,000 year period is observed in the advancement and the receding of the glaciers at high latitudes.

Milankovitch Cycles in the Atmosphere

Aerosols in the Troposphere
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Sources: factory and auto emissions, agricultural
• burning, wildland fires, dust storms …
Impact on the energy balance :
♦ Some particles reflect sunlight very efficiently, which results in cooling of the atmosphere.
♦ Others are selective absorbers and absorb some of the incoming solar light or outgoing IR emission of the Erath. These aerosols result in heating of the Earth and the atmosphere.
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Aerosols serve as condensation nuclei and change the characteristics of clouds.
Aerosols stay in the atmosphere typically for a few days before they settle down. They tend to accumulate over the sources (industrial cities, large fires …)

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Volcanic Eruptions and Aerosols in the
Stratosphere
Fine particles of ash and dust are ejected high in the atmosphere
(stratosphere).
Sulfur and other volcanic gasses combine with water vapor and form sulfuric acid droplets that are very reflective.
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This reflective haze can stay in the stratosphere for several years covering a big portion of the globe.
The volcanic haze in the stratosphere results in cooling the Earth.
El Chichon
Mt. Pinatubo

The effect of Mount Pinatubo Eruption
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Sulfur Dioxide plume can be seen three months after the eruption, at an altitude near 25 km
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The eruption of Mt.
Pinatubo is followed by a decrease in the average global temperatures

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Variations in the Solar Output
The total energy emitted by the sun varies with a
• period of about 11 years.
The variation in the emitted energy is related to the amount and the size of the dark spots on the surface of the sun. •
The sun spots are cold regions with strong
• magnetic storms.
The variation of the solar
• energy is about 0.1%.
It does not have a big impact on the climate.

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Global warming and the effect of greenhouse gases.
Most common greenhouse gases: H
2
O, CO
2
, CH
4.
The greenhouse effect
• results in heating the atmosphere.
The H
2
O greenhouse effect provides a strong positive feedback •
Other factors at play
♦ Clouds
♦ Jetstream
♦ Ocean currents
♦ …

Projections and uncertainties