Weather and Climate | Climate and Topography | How Has Earth's Climate Changed Over Time? | Computer Models to Study Greenhouse Effect | Summary
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
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:
Stevens, William K.: The Change in the Weather: People, Weather, and the
Science of Climate
for Students, Parents and Teachers
Now let's do Practice Exercise 6-3 (top). Choose printer friendly or online exercises. Printer friendly version requires the Adobe Acrobat Reader 5. Click HERE to obtain a free copy.
Computer Models to Study Greenhouse Effect (top)