Factors Affecting Past Climates

I. The Earth's Climate has changed many times, due to natural causes.

**There have been at least 4 Ice Ages in the last 3 million years - with only a 5oC cooler difference!

• 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.

Natural factors can affect the flow of energy into the climate system

• Decades to centuries-Solar intensity-Volcanic dust

• Millennia and longer-Changes in orbit

-Geophysical processes

Changes in the amount of energy coming from the Sun

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

Volcanic emissions

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.

What Are Some Natural Causes of Climate Change?

(A) Variations in the Earth's Orbit (tilt or shape) - cycles from 20,000 to 100,000 yrs.

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.

Collisions with comets or meteorites

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

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).

180 m.y.a. Present

Current Tectonic Plates

In 1915, Meteorologist Alfred Wegener (1880-1930) 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 sea-floor 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.

Natural Causes of Climate Change

Variations in Solar RadiationSolar IrradianceSunspot ActivityEarth-Sun Geometry (Milankovitch cycles)Atmospheric Dust and Volcanoes

Distribution of ContinentsPlate TectonicsMountain Building

Ocean VariationThermohaline Circulation

Feedback MechanismsPositive vs Negative Feedbacks

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.