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Instruction 6-3

Weather and Climate | Climate and Topography | How Has Earth's Climate Changed Over Time? | Computer Models to Study Greenhouse Effect | Summary

How Has Earth's Climate  Changed Over Time?

CCSTD Earth Science 6.c.

Various types of evidence suggest that Earth's climate has changed greatly over time.
 
Meteorological records of temperature, precipitation, wind speed, wind direction and atmospheric pressure have only been kept for the last hundred years or so.
 
But scientists can still find out what the Earth's climate was like before there were scientific instruments and systematic record keeping.
 
The study of this pre-scientific evidence is called paleoclimatology.
 
 
Paleoclimatology / Proxy Data

Paleoclimatologists regularly examine written or graphic descriptions of weather phenomena in books, personal journals and so on -- accounts or
representations of events like floods, droughts, frosts and periods of bitter cold and heavy snowfall.
 
They also examine historical records of things like crop productivity that can imply the nature of past climates. These are called climate proxy records. But there can be problems with this material because of its subjective nature.
 
Physical and biological data are more effective, since they can provide fossil evidence of the fluctuations in Earth's climate.
 
Examples of such physical and biological proxy data include:
 
Glacial Ice Deposits

Gas bubbles trapped in glacial ice can reflect the state of the atmosphere at the time the gas was deposited and reveal fluctuations in climate.
 
They can also show the chemical and physical properties of the ice.
 
Biological Marine Sediments
 
Climate change can also be evaluated by analyzing temporal changes in fossilized marine fauna and flora, morphological changes in preserved organisms, coral deposits and the oxygen isotopic concentration of marine organisms.
 
The best indication of Pleistocene sea surface conditions, for example, comes from microfossil assemblages in cores taken from the floor of the sea.
 
Inorganic Marine Sediments
 
This type of proxy data includes clay mineralogy, ice-rafted debris and aeolian terrestrial dust (aeolian means transported by the wind).
 
Terrestrial Geomorphology Data & Geology Proxy Data
 
These types of proxy data include glacial deposits, glacial erosional features, shoreline features, aeolian deposits, lake deposits and cave features like stalactites and stalagmites.
 
Terrestrial Biological Data
 
This is an extremely useful category of data.
 
Variations in climate can be discovered by analyzing annual tree rings, fossilized pollen and other plant fossils. And by evaluating the amount and distribution of insects and other organisms in lake sediments.
 
What does this proxy evidence suggest about the history of Earth's climate?
 
 
Earth's Climate Over Time

Climatologists believe that during most of Earth's history, global temperatures were probably 8 to 15 degrees C. warmer than they are today.
 
However, there were times when Earth's global temperatures became very cold -- cold enough to form glaciers that extended into the higher, middle and
even the lower latitudes.
 
In the last billion years, glacial periods (Ice Ages) have occurred with some regularity.
 
The first glaciation probably occurred about 2.3 billion years ago. And the most severe began about 800 million years ago when glaciers came within 5 degrees of the Equator.
 
The last major glacial period began about 2 million years ago and is known as the Pleistocene Age or The Ice Age.


The Pleistocene (Ice) Age
 
During this period -- which lasted from 2 million to about 14,000 years ago -- large sheets of ice covered much of North America, Europe and Asia.
 
These glaciers advanced and retreated with cold and warm weather.
 
The most recent glacial retreat began about 14,000 years ago. It is still going on.
 
We call the present period the Holocene Epoch.
 
In North America, Pleistocene glaciers began to form in the higher altitudes of the Rocky Mountains and in high-latitude locations in Greenland and north central Canada.
 
From these locations, the ice spread in all directions. Eventually, an ice sheet stretched from the Pacific to the Atlantic oceans.
 
A similar pattern of glaciation has been scientifically documented in Europe and Asia.
 
At present, glacial ice covers only 10% of Earth's land surface -- but at the height of the Pleistocene Age it was 30%.
 
 
The Last 14,000 Years
 
As we said, we are now living in the Holocene Epoch -- which began about 14,000 years ago with a glacial retreat that is still going on.
 
This warming period was interrupted by a sudden cooling, known as the Younger-Dryas, from about 10,000 to 8500 BC. Scientists think it may have been caused by the release of fresh water trapped behind ice into the North Atlantic.
 
The warming resumed by 8500 BC.
 
Between 5000 and 3000 BC, average global temperatures were 1 to 2 °C warmer than they are now.

Scientists call this period The Climatic Optimum.
 
From 3000 to 2000 BC another cooling trend occurred, which caused large drops in sea level and the emergence of islands like the Bahamas and coastal areas that are still above sea level today.
 
 
2000 BC - The Present
 
A short warming trend took place from 2000 to 1500 BC, followed by colder temperatures from 1500 to 750 BC. This caused renewed growth of glaciers
and a sea level drop to about 2 to 3 meters below present-day levels.
 
The period from 750 BC to 800 AD saw warming up until about 150 BC.
 
During the time of the Roman Empire (150 BC - 300 AD), cooling began that lasted until about 900 AD. At its height, this cooling caused the Nile River (829 AD) and the Black Sea (800-801 AD) to freeze.
 
From 900 - 1200 AD, a Little Climatic Optimum occurred, with the warmest climate since the Climatic Optimum. This is when the Vikings established settlements in Greenland and Iceland. And the snow line in the Rocky Mountains was about 370 meters above current levels.
 
A period of cool, extreme weather followed -- and a great drought took place in the American Southwest between 1276 and 1299. There are records of floods, droughts and extreme seasonal fluctuations up until the 1400's.
 
From 1500 to 1859 AD, global temperatures were at their coldest since the beginning of the Holocene period. Scientists call this period The Little Ice Age.
 
 
The Little Ice Age

During The Little Ice Age, the average annual temperature of the Northern Hemisphere was about 1 degree C. lower than it is today.

During the period from 1580 to 1600, the western United States suffered a
severe drought. Cold weather in Iceland from 1753 to 1759 caused 25%
of the population to die as a result of crop failure and famine. Newspapers in New England called 1816 the year without a summer.

The period from 1850 to the present is one of general warming.

There are a number of excellent charts and graphs which illustrate this.
Go to Figures 7x-1 and 7x-2 at http://www.physicalgeography.net/fundamentals/7x.html 


Cyclical Variations in Earth's Orbit

As we told you in the preceding Instruction, climatologists believe that a limited number of factors are responsible for the changes in Earth's climate.
 
One of these factors is cyclical variation in Earth's orbit around the Sun.
 
This Astronomical Theory of Glacial Cycles was first put forth by British scientist  James Croll in the 1860's and further developed by Serbian mathematician Milutin Milankovitch in the 1910's.
 
The basic idea is that these cycles cause variations in the amount of solar radiation the Earth receives.
 
There are three cycles, which are called Eccentricity, Precession of the Equinox and Obliquity.
 
Eccentricity
 
Eccentricity controls the shape of Earth's orbit around the Sun. This orbit gradually changes from elliptical to circular and back again in a cycle that lasts 100,000 years.
The more elliptical the orbit, the greater the variation in the amount of solar energy the Earth receives at the top of the atmosphere between its closest and farthest approach to the Sun.
 
Currently, the Earth is in a period of low Eccentricity.
 
Precession of the Equinox
 
The second variation results from the fact that as the Earth rotates on its polar axis, it wobbles like a spinning top.
 
This changes the timing of the equinoxes and solstices.
 
This effect is known as the Precession of the Equinox. It has a cycle of approximately 26,000 years.
 
Currently, the Earth is closer to the sun in January and farther away in July. In approximately 13,000 years, the opposite will be true -- the Earth will be closer to the sun in July and farther away in January. This will lead to colder winters and warmer summers than we have now.
 
 Obliquity

The third cyclical variation -- Obliquity -- is related to the fact that the tilt of the Earth's axis of rotation changes from 22.1 degrees to 24.5 degrees over a 41,000-year cycle.
 
When the tilt is small, there is less climatic variation between summer and winter in the middle and high latitudes. Winters are warmer and summers cooler. This causes more snow to fall during winter in high-latitude regions and less to melt during the cool summers, which leads to the formation of glaciers in the polar latitudes.
 
When there is a larger tilt, there is greater seasonal variation -- resulting in less snow and ice and the retreat of glaciers.
 
Currently, the Earth's tilt is approximately 23.5% -- right in the middle.

Experiments

Designed as a teacher-led classroom activity, this "Climate Change Game" offers students an entertaining television-quiz-style "challenge" to help them learn about the causes and impacts of global climate change. Click: http://www.climatechangenorth.ca/section-LP/LP_12_H_BMS_quiz.html

This "historical" Greenhouse Effect classroom activity offers students the chance to analyze greenhouse gasses and global temperatures as far back as 1850. This over-all title for this activity is "Visit to an Ocean Planet." Click: http://sealevel.jpl.nasa.gov/education/activities/ts1pcac1.pdf

This activity, entitled "Abrupt Climate Change" offers students an interesting (if somewhat complicated) way to track climate change during various periods in Earth's history. Don't be put off by the word "paleoclimatology." It simply means "studying the history of the climate of the Earth." Click:
http://www.ncdc.noaa.gov/cgi-bin/paleo/accwist.pl