- GLACIERS AND ICE AGES:
Classic studies of glaciers in the Swiss Alps and elsewhere in Europe show that:
- The majority of the Pleistocene Epoch, from about one million
to about 15,000 years ago, was marked by a great
expansion of glaciers (an "Ice Age" that covered most of Europe).
- Glaciers advanced in four distinct Glacial Periods (called Gunz, Mindel, Riss, and Wurm), separated by
warmer Interglacial Periods.
Studies of glacial deposits in North America show a similar Ice Age, also with four periods of glacial advance (called Nebraskan, Kansan,
Illinoisan, and Wisconsin), also ending about 15,000 years ago.
Studies of African lakes show a series of four Pluvial Periods of high rainfall and lake expansion, separated by Interpluvial Periods of low rainfall.
Increased rainfall in Africa is thought to coincide with increased snowfall and glacial advance in more Northern latitudes.
From about 310 to 290 million years ago, during the Pennsylvanian and ealy Permian time periods, a series of glacial deposits and glacial striations
(scratches in rocks) show that a great Ice Age took place on several Southern continents, including South America, Africa, India, Antarctica,
and parts of Australia.
The Dwyka Formation in South Africa's Karoo region shows this most clearly.
- PAST TEMPERATURES:
Glacial and Interglacial periods can show relative warmth and cooling, but they cannot show temperatures in numerical form, even approximately.
The best way to estimate temperatures of the past is by oxygen isotope ratios.
- Different isotopes of oxygen have slightly different solubility levels in ocean water at different temperatures,
so the ratio of oxygen isotopes can be used as a measure of ocean temperature.
- Various single-celled organisms (especially Foraminifera and Radiolaria) and multicellular animals (such as clams) build shells of calcium carbonate (CaCO3), and the oxygen isotopes
in these shells reflect the oxygen isotopes in the sea water at the time the shells were secreted.
- Using oxygen isotope ratios, the Italian-American scientists Cesare Emiliani was able to document ancient ocean temperatures as far back as the Jurassic.
- PAST CO2 LEVELS:
Ice cores taken from large glaciers in Greenland and Antarctica contain tiny bubbles of trapped air from the time that the ice was formed.
Carbon dioxide levels can be measured in these bubbles and show similar values from glaciers in different parts of the world.
The record of CO2 levels from these ice cores goes back tens of thousands of years.
Over the last few thousand years, records from around the world are more numerous, and they show simultaneous
highs and lows everywhere,
meaning that the CO2 levels were worldwide and not local.
- Comparison of temperature and CO2 records:
Comparison of temperature and CO2 records from various sources shows clearly that higher CO2 levels always accompanied
higher temperatures,
and low CO2 levels always accompanied lower temperatures.
Both CO2 levels and temperatures increased very slowly over the last 8,000 years or so.
CO2 levels and temperatures both increased much more rapidly beginning at the start of the Industrial Revolution (around 1800).
The rate of increase accelerated
(it increased more steeply) throughout the twentieth century.
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