Temperature record : How did temperature change in the past?

One can look at variations of the temperature record (see Fig. 8-14 in textbook) over various timescales.  Over the lifetime of the Earth (4.5 billion years), temperature has been remarkably stable and liquid water was present on the surface for most of the time.  In contrast to our closest neighbours Venus (450°C) and Mars (-50°C) which are very hot and very cold, Earth has been able to maintain relatively pleasant temperatures.  Over the last 4.5 billions years, global temperatures on Earth haven't varied by much more than 15-20°C.

Most of the time, the Earth had a warmer climate than today, with little or no polar ice. However there is evidence from geomorphology that during infrequent periods in Earth's history, the temperature was so cold that ice might have covered most of the surface (and sometimes the oceans).  There are 5 such periods of glaciations (sometimes referred to as the "Great Ice Ages"):

• 2.2-2.4 billion years ago: Huronian glaciation
• 0.8-0.6 billion years ago: Late Proterozoic glaciation
• 440 million years ago: Late Ordovician glaciation
• 280 millions years ago: Permian-Carboniferous glaciation
• 1.8 millions years ago: Pleistocene glaciation

What are the causes of climate change? 
Long term climate change (hundreds of millions of years)
The Earth managed to maintain liquid water and life on its surface for the last 3.5 billion years despite changes in solar luminosity over that time period because of the negative feedback in the carbonate-silicate cycle: as the sun grew brighter, CO2 levels decreased and temperature was maintained within a fairly stable range (see the Faint young Sun discussion on pages 159-161).
Chapter 8 in you textbook has a good discussion of the causes of long-term climate change (see in particular pages 164-170).  These causes involve changes in continental positions driven by plate tectonics and changes in the levels of atmospheric CO2 (driven by the carbonate-silicate cycle, and by the biosphere).

Causes of climate change over the last 2 million years (timescale of hundreds of thousands of years)
The regular shifts in Earth's climate over the past 2 millions years were initiated by small changes in the configuration of Earth's orbit around the sun (Milankovitch cycles).  Variations in the orbital parameters of the Earth include obliquity variations, eccentricity variations, and precession variations (see pages 216-219 in textbook). Each of these variations occur regularly and their respective time periods are 41,000, 100,000, and 26,000 years. These small changes have affected the warmth and length of northern hemisphere summers and have allowed ice sheets to grow (see textbook, Chapter 11).  The Milankovitch cycles in and by themselves cannot explain global variations in temperature of 5C are recorded by ice cores, as they represent only minute changes in the repartition of radiation over the surface of the Earth.  What the Milankovitch cycles do however is provide a regular trigger for the glacial/interglacial switches. The climate system has amplified these small changes through a number of positive feedback loops, such as the ice-albedo feedback, changes in cloud properties, changes in levels of atmospheric CO2 (through changes in marine productivity for example).

Causes of climate change over the last 2,000 years
The causes of short-term climate variability are presented in Chapter 12 of the textbook . These causes include solar variability and volcanic activity.

Evidence of past climates: how do we know what climate was like in the past?

Direct measurements of temperature, such as thermometer records, extend back about 150 years (see homework #1). Humans have also noted aspects of climate change for about 1000 years in historical records (for example looking at cherry blossoms in Japan and grape harvests in Europe). For older evidence of past climate a wide variety of records span different times and areas. The table below summarizes some of the records used to look at conditions on Earth in the past.  Tree rings tell us about the changes in growing conditions that a tree might have encountered over its lifetime (temperature and rainfall), pollen from different plant species indicate shifts in vegetation patterns that occured as a result of climate change, the shape of the landscape (geomorphology) tells us about the extent of glaciers and ice sheets and sea level in the past, ice cores record information about the conditions in which the ice was formed and trap ancient air, corals give us indications on sea surface temperature,  and the shells of marine organisms found in marine sediments tell us about past temperatures and atmospheric CO2. 
 
 

	Information	Time range	Areas
Tree rings	Temperature, rainfall, wild fires	hundreds of years ago - present (longest record extends 11,000 years)	Continents (mostly Northern Hemisphere)
Pollen	Temperature, rainfall	several million years	Lake sediments, wetlands (mostly Northern Hemisphere)
Geomorphology	Extent of glaciers and ice sheets, sea level	billions of years	Worldwide
Ice cores	Surface temperature, snow accumulation, volume of continental ice, sunspot cycle, CO2, CH4, volcanic eruptions, sea-salt, wind speed (dust)	hundreds of thousands of years (longest record, in Antactica extends from 440,000 years ago to present)	Greenland, Antarctica
Corals	Sea surface temperature, sea level	each coral gives us information on hundreds of years, oldest corals recovered date back to 130,000 years ago	Tropical oceans
Marine sediments	temperature, salinity, ice volume, atmospheric CO2, ocean circulation, iceberg calving	up to 180 million years ago	Oceans

 Last Updated:
11/08/2001