Although climate change is a hot topic in the news, the Earth's climate has been changing throughout our planet's history.

There are many sources of evidence for climate change - fossils, for example.  When scientists look at core samples drilled into ocean floors, they find alternating layers of marine fossils. Layers with an abundance of warm weather animals alternate with layers in which these animals are very scarce. This indicates that the climate has changed periodically.

Fossils of large plant-eating dinosaurs of the Cretaceous (140 million to 65 million years ago) have been found in Antarctica and within the Arctic circle, evidence that at one time these regions were full of lush greenery and so must have been much warmer than they are today. Fossils of ferns and fig trees have been found in Greenland.

By studying ice cores in polar regions and measuring concentrations of gases, including oxygen, carbon dioxide and methane, scientists can discover how ocean temperatures have changed over time. They can determine what kinds of plants lived in the region at particular times, by examining pollen found in the cores. They type of plant life indicates what the environment was like.

Scientists can also find evidence of climate change by looking at shells and at coral reefs - the chemistry of the hard tissues changes in response to the environment.

Examining tree rings is another way of learning how climate has changed.  In temperate zones, a new tree ring will appear on a tree every year. The width of the tree ring reflects the length of the growing season, and provides information regarding what the climate was like each year. Carbon dating can be used to determine the age of the sea.

The most well known proof of climate change in the Earth's past is evidence of past glaciation. As glaciers advanced and retreated periodically throughout Earth's history, they left evidence in the landscape, including valleys that were carved out by the glaciers and remained after the glaciers retreated and ridges of accumulated debris that were carried by glaciers. These ridges are known as terminal moraines.

Factors that have contributed to climate change in the Earth's past have included the oxygenation of the Earth's atmosphere after the evolution of photosynthesis (which reduced the amount of methane - a greenhouse gas-  in the atmosphere and resulted in the formation of an ozone layer), continental drift (Antarctica was once much closer to the equator than it is now, for example) and in changes in the Earth's orbit and the tilt of its axis over long periods of time, which have affected the amount of solar radiation received by different parts of the Earth.

Volcanic activity has also affected climate, by releasing greenhouse gases in the air (contributing to warming) as well as by releasing dust particles that block sunlight (contributing to cooling).

When Krakatoa, a volcano in Indonesia, erupted in 1883, dust remained in the atmosphere for 3 years. During this time, a 10 percent decrease in solar radiation was recorded in southern France.

Sunspots may also have an effect on climate.

Climate Change Events

An episode of glaciation took place on Earth a little over 2 billion years ago. It is believed that the entire Earth was covered in ice during this time. This reduction in temperature was caused by a decrease in volcanic activity, which in turn caused a reduction in the amount of carbon dioxide in the atmosphere.

Another period of extreme cold was the Cryogenian Period - "cryo" is Greek for "ice cold"- which took place between 850 and 630 million years ago.  Glaciers advanced and retreated several times during this period, and the Earth may have again been covered entirely in ice. This change in climate is thought to have been related to the breakup of the supercontinent Rodinia and large one-celled and multicellular organisms sinking to the seabed, reducing the amount of carbon dioxide in the atmosphere.

The Andean-Saharan Glaciation took place between 460 and 430 million years ago, resulting in the Ordovician-Silurian Extinction, the second most severe mass extinction in the history of the Earth.

This period of glaciation is thought to have been related to the movement of the supercontinent of Gondwana away from the equator and toward the South Pole.

The evolution of land plants may have caused the Karoo Ice Age, which occurred between 360 and 260 million years ago. These plants would have absorbed carbon dioxide from the atmosphere.

The Earth's worst mass extinction event was the Permian-Triassic extinction, which occurred about 250 million years ago. Global warming caused by volcanic activity may have contributed to this extinction.

A period of global warming, known as the Paleocene-Eocene Thermal Maximum, took place about 55 million years ago and lasted for about 150,000 years. It may have been caused by a volcanic eruption.

Current Ice Age

About 34 million years ago, glaciers began to form in Antarctica, and an ice sheet formed there about 20 million years ago. This is the same ice sheet that exists in Antarctica today.

It is believed that the rise of the Himalayas during this time removed carbon dioxide from the atmosphere.

The glaciers that are now found in the Arctic began to appear only about 3 million years ago.

We have been living in an Ice Age known as the Quaternary, which began 2.6 million years ago, at the beginning of the Pleistocene Epoch, and continues to the present - the Holocene Epoch.

Although we may think that having glaciers at the poles is normal, in fact, in the Earth's long history, the poles have rarely been frozen.

During this time, glaciers have repeatedly advanced and retreated.

We are currently living in an interglacial period that began with the retreat of glaciers about 12,000 years ago.

Climate Change in Human History

There have been periods of climate change throughout human history.

Modern Homo sapiens first appeared on Earth about 200,000 years ago; our evolution was affected by the need to be able to survive through periods of glacial advance.

Between 900 and 1300, the Earth's northern hemisphere was warmer than it is today. During this time, known as the Medieval Warm Period, Vikings founded a settlement in Greenland.

A period known as the Little Ice Age began around 1450 and ended around 1850.  Evidence of this Little Ice Age includes photographs of frozen rivers - rivers that would never freeze today - as well as records of crop failures.

Since that time, the Earth's temperature has been increasing.  There has been a decrease in the number of glaciers and snow covered areas and a corresponding rise in sea level.

A large part of this rise in temperature has been attributed to human activity, particularly the use of fossil fuels.