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How do we know what the temperature of the Earth was like thousands and millions of years ago?

Reliable measurements of global temperatures began in 1856, when the British Meteorological Society began collecting records from around the world. Climate conditions before this date can be estimated from less reliable historical records. Conditions can also be estimated from a variety of natural environmental records going back to well before humans existed. Information of this kind is called “proxy data” because it substitutes for, or acts as a proxy for, the actual data we are seeking.

Clues from Historical Data

Farmers' logs, travelers’ diaries, newspapers, and other written records can provide information about the climate of the past. The length of the growing season in a given location will vary with temperature. In a cool year, with spring arriving late, planting will be delayed. If autumn comes early, so will the harvest. In modern times we have accurate temperature records along with records of the length of the growing season. Since we know how these relate to one another, we can reconstruct the missing historic temperature data from what we know about planting and harvesting times. For example, historical grape harvest dates have been used to reconstruct summer temperatures in Paris, France, from 1370 to 1879. This method is not perfect, but along with the use of other indirect measures it allows a reasonable reconstruction of climate over a long period of time.

Tree Rings

Many trees produce one ring per year, due to rapid growth in the spring and summer and little growth in autumn and winter. A warmer year results in a wider ring. Patterns in the width, wood density, and hydrogen and oxygen isotopic composition of tree rings can be used to estimate temperature.

Clues from Corals

Corals have hard calcium carbonate (CaCO₃) skeletons. Some corals form annual rings of calcium carbonate as they grow. Like tree rings these can be used to estimate temperatures. When sea temperature is warm the coral will grow faster than if the temperature is cold, so warmer years will make wider growth rings and colder years will create thinner rings. Isotopes of oxygen contained in the calcium carbonate can also be used to estimate the temperature of the water when the coral grew.

Clues from Ice Cores

Permanent ice in high mountains and polar ice caps is built up from snow that has fallen over hundreds and thousands of years. Cores have been drilled deep into several thick ice sheets around the world. A series of ice cores have been taken at the Vostok station in Antarctica, starting in 1970 and continuing even today. These ice cores allow us to look back in time. One of these cores reached 3,623 m (11,886 ft) into the ice sheet to bring up ice that is nearly 500,000 years old.

This ice, like all water and ice, is made of hydrogen and oxygen. Small amounts of the hydrogen are a special heavy form called deuterium. Scientists have observed a relationship between local temperature and deuterium concentration in ice collected during periods that temperature was also known. There is no reason to believe that this relationship has changed over time, so the levels of deuterium in ancient ice can be used to reconstruct past climate. The ice also contains dust, which sometimes occurs in layers that indicate major volcanic eruptions that have spread dust around the globe. Ice cores also contain small bubbles of ancient air, which can be analyzed to discover the concentrations of gases such as carbon dioxide (CO₂).

Scientists drill into coral and extract a core. It is carefully packaged and sent to a laboratory for analysis.

An ice core being removed from the drill. Above images courtesy of NOAA.

Sections of the ice core are labeled and stored at a temperature of -35°C (-31°F) .

Clues from Ocean and Lake Sediments

Rivers continuously wash mud and sand from the land into lakes, seas, and oceans, where it settles to form layers of sediment. Cores drilled into these sediments can reveal small fossils and chemicals that can help in the interpretation of past climates. Pollen grains are particularly useful. They are very tough, so are often well preserved in sediment layers. Each type of plant produces a different shape of pollen grain. Analysis of pollen grains can reveal what types of plants were growing nearby, and so indicate what the climate would have been like.