In reconstructing the history of the Earth and its climate, we have three fundamentally different types of records:

• Written records, going back hundreds to thousands of years
• Artifacts produced by man and other living creatures, dating from tens to thousands of years
• The geological record, going back billions of years

This presents a problem, in that those records do not always overlap, so there are inevitably gaps in the history of the Earth.  Moreover, it is not always straightforward to relate one type of record to another.  The reconstruction of climate for the the early Earth is therefore subject to profound uncertainties, except when some localized artifact, such as a glacial moraine, is found to indicate the presence of ice, and a cold climate.

The historical record over the past several thousand years contains many events that point to climate variability.  These are usually difficult to quantify, but, nonetheless they provide confirmatory evidence of climatic changes indicated by other scientific data.  In the historical record we find evidence of

• Poor harvests occurring over time spans of 20 or more years.
• Long sequences of cold winters; these are often noted in chronicles and can be inferred from travelers' reports and from paintings and illustrations.
• Advances of mountain glaciers.  In the Alps are many glaciers that have been observed for thousands of years; some of those glaciers have advanced and receded by many kilometers within recent centuries.

The historical records all point to two notable extended climatic disturbances that affected European history over the past thousand years:

• An exceptionally warm period from about 700 to 1100 AD
• Viking explorations and settlement of remote regions such as Iceland and Greenland.  Iceland, previous to the Viking settlements had been visited by Irish monks, but the climate in earlier years was too cold to encourage extensive settlement.  Since the end of the Viking era Greenland has again been too cold to sustain any but tiny settlements.

Scientists have begun in recent years to deal with climate change by attempting to model the circulation of the oceans and atmosphere, and their response to changes in external influences, such as the changing orbit of the earth.  They thus hope to correlate the climate with observational data relating to

• Changes in pollen samples taken from earth cores and lake sediments.  Pollen is hard and resists decay; the shapes are also characteristic of particular plant types.  The types of pollen give an indication of what types of plants were growing when the sediments were deposited.  The distribution of plants is very sensitive to the length of the growing season, the temperatures during the growing season, and the amount of moisture available.
• Isotopic compositions of ice cores from polar regions and mountain glaciers.  Some ratios, such as the relative proportions of the oxygen isotopes ¹⁸O and ¹⁶O are indirectly related to global temperatures as the ocean and atmosphere circulation responds to warming and cooling.  The tiny differences in the weights of different isotopes cause measurable differences in the way they are absorbed by water or taken up by living organisms.
• Tree rings give evidence of rainfall and moisture over a growing season.  Droughts can be defined as extended periods during which plants receive less moisture than they need for vigorous growth; tree rings are excellent indicators of droughts because they average the moisture over an entire year or growing season.  Isotopic analysis of tree rings can also provide evidence of temperatures and atmospheric circulation.
• Composition of sediments in rivers and lakes; the relative proportions of fine and coarse material provides evidence of abnormally high river flows, from which it may be possible to deduce either enhanced precipitation or unusually rapid melting of glaciers and snow.
• Composition and position of glacial moraines and other material deposited by glaciers; this may be used to infer glacial advances and retreats in the near and distant past.  Much of the evidence of past Ice Ages comes from accumulations of debris far from any present day glaciers.
• Isotope ratios (¹⁸O/¹⁶O) ratios in shells of small sea creatures provide indication of sea temperature—complicated by differential evaporation from cold oceans.
• Records of winds, precipitation, and temperature at weather stations.  There are large gaps in the records because organized collection of weather data is a fairly recent development; in many places well-calibrated weather data exist for less than 100 years.  Data from the oceans are particularly sparse.
• Plate tectonics and sea floor spreading provide clues to cooling episodes that began about 30 My BP; increased tectonic activity leads to increased emission of greenhouse gasses CO₂ and H₂O from volcanoes.

The geological record is full of relatively sudden events, in which compositions of species changed dramatically within a time too short to resolve.  In many cases the finest resolution is thousands of years; some of those events may point to extreme climate changes over hundreds to thousands of years.

The story of the annihilation of the dinosaurs by the collision of a comet or small asteroid with the Earth makes for exciting reading, but it should be kept in mind that there have been many extinction events, both large and small, since life first appeared.  The term "Mass Extinction" is appropriate when a significant fraction of all the forms of life in the fossil record suddenly disappear.  A Mass Extinction is something much more profound than the vanishing of one or two species.  The extinction of the dinosaurs wiped out hundreds of species; not just dinosaurs but many other animals on the land and in the sea.

The geological periods were established because rocks of a given period can be correlated by certain assemblages of fossils that appear in them.  It was found that there are often sharp boundaries between the periods, in which there was complete replacement of one group of fossils by another.  It is now known that some—but not all—the period boundaries mark major extinction events.  There have been many such events, some relatively restricted, but a few that were catastrophic.

The table below lists some of the extinction events.  The third column gives the approximate number of genera that disappeared.  (Note that the dates, relative to the present, do not always agree with the sequences given elsewhere.)  The associations are sometimes conjectural, based on observations that may not be exactly coincident.

By the end of the Permian period, about 250 Million years ago, life on Earth had reached a very high degree of complexity.  The animals and plants had left the oceans, and had spread over the land.  The reptiles had taken over, and some rather large and very advanced species had evolved.

The most remarkable development of the Permian was a group of very advanced animals that were well on their way to becoming mammals.  These therapsids and their relatives dominated the land.  They had specialized teeth, that have been retained and improved by their descendants, the mammals.  Notice especially the large canine teeth, a feature which is almost never seen in the dinosaurs.  Some of them had fast, trim bodies.  They may have had fur and warm blooded metabolisms.  Their internal and bone structure was very much like modern medium to large sized mammals.  The Permian experienced periods of glaciation, and some of the therapsids appear to have lived in rather rigorous climates.  The late Permian fellow shown below, Lycaenops, would have looked quite at home in our wild lands today.  He was clearly a quick, agile carnivore.  You might be alarmed to meet him, but you wouldn't feel he was out of place.

Most of those creatures, along with plants and animals everywhere disappeared at the end of the Permian.  To place the late-Permian extinction event in context, compare it with the extinction at the end of the Cretaceous, that wiped out the dinosaurs.  The percentage of survivors in the Cretaceous event may have been ten or more times the percentage surviving the Permian event.

At the beginning of the Mesozoic eon there was very little life on Earth.  In the early Triassic there was a rapid recovery, initiated by small reptiles.  There is, however, a profound break with the general composition of the fauna of the Permian.  Sometime in the Triassic, a gang of little creatures that went about on two legs evolved into the dinosaurs.  At about the same time some of their favorite food items, little furry creatures, were forced to hide in the forest floors and under the vegetation.  Pisanos, below, is a typical small early dinosaur.  He was built for speed and agility, but he looks quite unlike anything we know today, except for the birds.

By the Cretaceous era, the dinosaurs had been evolving for much longer than the time the mammals have been given to evolve after the dinosaurs were gone.  Though some of the dinosaurs were indeed large and stupid, there are also many large mammals today that have not developed especially active brains.  Some dinosaurs in the Cretaceous may have been quite intelligent; what we know of their behavior is that some of them may have been as intelligent as birds.  Some modern birds have been found to display far more intelligence than might be expected when compared with mammals having similar sized brains.  Only a catastrophic event could have halted the further development of the dinosaurs; the mammals had almost no advantages, not even in intelligence.

Iridium layers and other evidence of an asterorid or comet collision have been associated with the end of the Cretaceous period and possibly several other extinction events, but no trace of an Iridium layer has been found at the biggest extinction event, the end of the Permian.  So we have at least one case of an extinction event where climatic deterioration has not been ruled out as a cause.  This should give us cause to worry.

After the end of the Mezozoic era and the extinction of the dinosaurs, came the current or Cenozoic Era which has lasted for 65 Million years.  The Cenozoic was initially very warm and stable.  However the position of the continents was shifting; and this led to profound and relatively rapid changes in the climate and the onset of Ice Ages.  The rate of climate change throughout the Cenozoic has been probably as rapid as during the late Permian.

The climate appears to have warmed rapidly in the early Cenozoic, and then gradually cooled thoughout most of the the Cenozoic era.  The changing ¹⁸O/¹⁶O ratio of sediments on the sea floor, and in polar ice sheets show this well, with fully developed cyclic Ice Ages appearing in the Pleistocene somewhat more than a million years ago.  The calibration of the oxygen isotope ratio is not very precise because of preferential evaporation of ¹⁶O from warm water; nonetheless the average temperature must have been 5 – 10 degrees higher in the Mesozoic and early Tertiary.  We live during an interglacial interval in the midst of a long Ice Age.

A prime suspect for the agent of gradual climatic change is the greenhouse gas CO₂, which is released by volcanoes.  As the Atlantic Ocean began to open at the end of the Mesozoic, and the continents drifted apart there was a large increase in volcanic activity along the mid-ocean ridges.  As the oceans covered the volcanoes of the mid-Atlantic ridge, the gradual decrease in CO₂ injected into the atmosphere may explain a large part of the cooling over the past 100 Million years.  The only major interruptions in the gradual cooling were several brief disturbances due to cometary impacts and to volcanic eruptions; these induced brief downward kinks in the temperature trend.

It must be kept in mind that water vapor, H₂O is another important greenhouse gas.  If it were not for the atmospheric circulation, and transport of cold air from the polar regions, the water vapor would provide a strong feedback to the climate syste.  As the Earth warms, more water evaporates, thus increasing the greenhouse effect due to the water vapor in the atmosphere.  Such a run-away greenhouse warming is unlikely in the present atmospheric circulation pattern, but could have been possible at some earlier times in the Earth's history.  That mechanism might have partially responsible for the warming in the early Cenozoic and previous eras.

The opening of the Atlantic Ocean also allowed cold water from the polar regions to mix with mid-latitude water, thereby nullifying the greenhouse effect of water vapor.  The connection to the polar sea was complete by the beginning of the Cenozoic, with the opening of the Norwegian Sea.

Besides the opening of the Atlantic Ocean, several other things were happening that strongly affected the climate.  As the American continents drifted away from Europe and Africa, the American Cordillera began to rise.  This set up a long barrier that impeded the atmospheric circulation.  Air currents crossing that barrier were deflected equatorward by Coriolus Forces.  This slight perturbation led to a circulation dominated by the Rossby waves, with their familiar repeating sequences of high and low pressure cells.  The Cordillera eventually caused a dramatic alteration of the oceanic circulation when the gap at Panama closed about 3 Million years ago.

As the Cenozoic progressed, ice began to form in the polar regions.  Antarctica was free of permanent ice for a long time after it drifted to the south pole.  But during the Oligocene Period, which began 38 Million years, ago it acquired an immense ice cap that has persisted to the present time.  The growth of the Antarctic ice sheets may have been aided during the Oligocene by the opening of the Drake Passage between South America and Antarctica.

The climate continued to cool and the Antarctic ice cap continued to grow until the continent was fully covered with ice as recently as 6 to 8 Million years ago.  Other mountain-building forces were also at work, such as the formation of the Himalaya Mountains, but the principal geologic events of the Cenozoic were the opening of the Atlantic, the formation of the Cordillera, and the glaciation of Antarctica.  These all exacerbated the cooling trend until the Quaternary Period which began about 1.6 Million years ago.  Since that time the Earth has been in a prolonged Ice Age, with occasional warm Interglacial periods lasting up to 20,000 years.  The Quaternary has been conventionally associated with the beginning of the Ice Ages.  The Quaternery has been subdivided into the the Pleistocene (or time of Ice Ages) and the Holocene (recent), but it is now known that there is essentially no geological or climatic difference between them.  The only feature that distinquishes the Holocene is the presence of Man and his works (and waste products).

The coupling of geological processes and climate is extremely complex and poorly understood.  Because of the feedback in critical elements of the system, it can proceed almost indefinitely in certain ominous directions, as seems to have happened on Venus.