Natural Causes of Climate Change
Feedback Mechanisms
Positive vs Negative Feedbacks
Variations in Solar Radiation
Solar Irradiance
Sunspot Activity
Earth-Sun Geometry (Milankovitch cycles)
Atmospheric Dust and Volcanoes
Distribution of Continents
Plate Tectonics
Mountain Building
Ocean Variation
Thermohaline Circulation
•Just 18,000 years ago, the Earth was in an ice age where continental glaciers reached as far
south as the Ohio River Valley over North America.
•Over Europe, the northern region and higher terrain were covered in thick sheet as well.
•The glacial advance/retreat occurred about 10 times over a 2 million year period.
Climatic Change associated with Feedback
Mechanisms
• Positive Feedback Mechanisms – A process whereby an initial change in an
atmospheric process will tend to reinforce the overall effect.
– Water vapor-greenhouse effect
• Increase in water vapor content will lead to warmer temperature which will lead to
more evaporation.
– Snow-albedo effect
• As the snow and ice melts from the polar regions, the albedo of the earth will be
reduced. Temperature will increase.
• Negative Feedback Mechanisms – A process whereby an initial change in an
atmospheric process will tend to weaken the overall effect.
– Increased warming of the earth surface will lead higher emission of IR
radiation
• The higher the IR surface output, the slower the rise in temperature in the
atmosphere.
– Increase in clouds
• Warmer surface will lead to more evaporation which leads to more clouds.
Solar Output Variation and Climate Change
• The sun usually experiences a 11-year cycle of maximum and minimum in sunspots.
•From 1645 to 1715, the sun went through a long duration of minimum sunspots
which coincided with a cooling spell called the Maunder Minimum.
•Recent researchers have found the 11-year sunspot cycle and the weather pattern in
North American can be related to:
•Warmer winter
•Reversal of the stratospheric wind pattern over the tropics
Milankovitch Theory of Climate Change
• Climate change may be driven by changes in earth's:
a) orbit (eccentricity) → from ellipse to circle at a 100,000 year cycle.
b) wobble (precession) → from the north pole pointing toward or away from
the sun in a 23,000 year cycle.
c) tilt (obliquity) → from 22° to 24.5° at a 41,000 year cycle.
Eccentricity is the major forcing factor for the timing of ice ages.
Milankovitch Cycles
Climate Change associated Tropospheric & Stratospheric Aerosols
•Car emissions and wild land fires are 2 major sources that emit aerosols into the
troposphere that reduce incoming radiation and have a net cooling effect on
earth's surface.
•Volcanic eruptions push aerosols into the stratosphere blocking out solar energy.
These aerosols in the stratosphere take a longer time to settle to the earth surface.
•Large eruptions, such as Mt. Pinatubo, have been linked to significant cooling
episodes.
Mt. Pinatubo Eruption & Impact
•Three months after the June 1991 eruption of this Philippine volcano, much of
the 20 million tons of ejected sulfur dioxide had been directed by zonal
stratospheric winds and inundated the equatorial region.
•Recorded changes in air temperature indicate the volcanic eruptions on climate.
•“The year without the summer” (1816) brought heavy snow in June and killing
frost with respect to the vegetation in July and August over the northeast United
States. These freakish events coincided with a massive volcanic eruption of Mount
Tambora.
Extinction of the Dinosaurs
• Billions of tons of dust and debris were injected into atmosphere around 65 m.y.a. from a
giant meteorite collision with earth.
• The reduction in sunlight from the dust and debris clouds caused photosynthesis to stop
and broke down the food chain.
• This catastrophic collision is evident around the world from a thin layer of sediment
deposit called the K-T boundary where it separates the end of the Cretaceous period, and
the beginning of the Tertiary Period.
• The K-T boundary is made up of iridium material which is commonly found in
meteorite.
Plate Tectonics
180 m.y.a.
Present
Plate tectonics and drift concentrated continents at higher latitudes allowed for
more ice cover, which reflected more sunlight and created a positive feedback to
cause greater cooling.
Periods of mountain building increase snow-covered high elevation areas
producing a similar snow-albedo positive feedback.
The position of continents, mountains, and oceans create complex changes in
atmospheric (and oceanic circulation).
Current Tectonic Plates
In 1915, Meteorologist Alfred Wegener (18801930) hypothesized the movement of
continents based on fossil evidence. Some
evidence he used was the presence of coal in
Antarctica, evidence for glaciation in Africa,
and similar fossils in South America and
Africa.
Geologists rejected Wegener's
theory, believing that the
Earth's crust is too rigid to
change positions. The theory of
continental drift reemerged
around 1960 based on
paleomagnetic evidence of seafloor spreading.
The Thermohaline and Associated Climate Change
• Cold salty water sinks drawing warmer water from the south into the Northern Atlantic.
• The warm water provides a transport of warm and moist air to the higher latitudes which
keeps the climate over northern Europe warmer than expected.
• When this ocean conveyor belt stops, temperature becomes much colder for northern
Europe.
• A massive influx of fresh water will stop the North America portion of the thermohaline
circulation which could lead to a possible increase in CO2.