Paleoclimatology (British spelling: palaeoclimatology) is the scientific study of climates from times before weather tools were invented, when no direct measurements were possible. Since weather records only cover a small part of Earth's history, studying ancient climates helps scientists understand natural changes and how today's climate has developed.

Paleoclimatology uses clues from nature, such as rocks, sediments, ice sheets, tree rings, and shells, to learn about past climates. These clues are dated using scientific methods, and the results help scientists understand how Earth's atmosphere was in the past.

The field of paleoclimatology became well-established in the 20th century. Scientists study events like Earth's past ice ages, sudden cooling periods such as the Younger Dryas, and rapid warming during the Paleocene–Eocene Thermal Maximum. Research on past environmental changes and life on Earth often helps scientists compare past and present conditions, including how climate affected mass extinctions and how life recovered after them.

Understanding paleoclimatology is important for predicting Earth's future climate.

History

People in ancient Egypt, Mesopotamia, the Indus Valley, and China probably first noticed changes in the climate because they experienced long periods of droughts and floods. In the 1700s, Robert Hooke suggested that giant turtle fossils found in Dorset showed that Earth was once warmer, which he thought might be caused by a shift in Earth's axis. At that time, some people believed fossils were the result of a biblical flood. In the early 1800s, amateur astronomer Heinrich Schwabe began tracking sunspots, which led to discussions about how the Sun affects Earth's climate.

In the early 1800s, scientists started studying past climates more formally, using discoveries about glaciers and Earth's climate history to learn about the greenhouse effect. It was not until the 1900s that paleoclimatology became a unified scientific field. Before this, different parts of Earth's climate history were studied by many different disciplines. By the end of the 20th century, research about Earth's ancient climates was combined with more complex computer models. During this time, scientists also aimed to find ancient climates similar to today's to better understand current climate changes.

Reconstructing ancient climates

Paleoclimatologists use many different methods to learn about ancient climates. The methods they choose depend on what they want to study, such as temperature or rainfall, and how long ago the climate they are interested in existed. For example, deep ocean sediments, which provide much of the data on isotopes, are only found on oceanic plates. These plates are eventually pushed down into the Earth's mantle, and the oldest remaining ocean sediments are about 200 million years old. Older sediments are often damaged over time by processes like pressure, tectonic movement, and fluid flow. These changes can reduce the quality and amount of data available, making it harder to study very old climates accurately.

Some specific methods used to study ancient climates include analyzing lake sediment cores and speleothems. These methods look at layers of sediment or rock formations in caves. Scientists also use element-dating techniques involving oxygen, carbon, and uranium to determine ages and conditions from the past.

The Direct Quantitative Measurements method is the most straightforward way to study climate changes. By comparing recent data with older data, researchers can understand how weather and climate have changed in a particular area. However, this method has a limitation: climate records only began being collected in the mid-1800s. This means scientists have only 150 years of recorded data, which is not enough to study climates from 10,000 years ago. For older time periods, more complex methods are needed.

Mountain glaciers and polar ice caps provide valuable information about ancient climates. Ice cores drilled from Greenland and Antarctica, such as those from the EPICA project, have given data going back more than 800,000 years.

• Air trapped in snowflakes becomes sealed in tiny bubbles as snow turns into ice under the weight of new snow. These bubbles contain air from the time the ice formed, allowing scientists to study the composition of ancient air.
• Layers in ice cores can be seen because ice forms differently in different seasons. These layers help scientists determine the age of each part of the core.
• Changes in the thickness of layers can show how much snow fell or how warm the temperature was at the time.
• The amount of a heavy type of oxygen (O-18) in ice layers can indicate past ocean temperatures. More O-18 means higher temperatures, but other factors like salt levels in the ocean and ice sheet size also affect this. Scientists have found patterns in these oxygen levels over time.
• Pollen found in ice cores helps scientists understand which plants lived in an area when the ice formed. By counting different types of pollen in a sample, scientists can track changes in plant life over time. This helps them learn about past temperatures, rainfall, and animal life.
• Volcanic ash in some layers can help scientists determine the age of the ice. Each volcanic eruption leaves ash with unique features, which can be matched to known eruptions to date the layer.

A group of scientists called the European Project for Ice Coring in Antarctica (EPICA) drilled an ice core in Antarctica that contains ice from about 800,000 years ago. Scientists worldwide, through the International Partnerships in Ice Core Sciences (IPICS), are working to find the oldest possible ice core record from Antarctica, aiming to reach back 1.5 million years.

Trees can also help scientists study past climates. Trees grow faster or slower depending on conditions like temperature and rainfall, which is reflected in the thickness of their growth rings. Scientists compare tree rings from many trees in the same area to create a record of climate changes.

The thickness of tree rings can show how good the growing conditions were for the trees. Different tree species react to climate changes in different ways. By studying many trees of the same species and comparing them to other species, scientists can better understand how climate changes affected the environment.

Old wood that has not decayed can extend tree ring records further back in time by matching ring patterns to modern trees. Some areas have tree ring records dating back thousands of years. Older wood not connected to modern trees can be dated using radiocarbon methods. Tree ring records help scientists learn about past rainfall, temperature, water levels, and fires in specific areas.

For very long time periods, geologists study sedimentary rock layers. These layers can contain remains of plants, animals, plankton, or pollen that lived in certain climates. Special molecules, like alkenones, can show the temperature when they formed. Chemical clues, such as the Mg/Ca ratio in tiny ocean creatures called Foraminifera, can help scientists reconstruct past ocean temperatures. Isotopic ratios, like δO and δC, can also provide information about temperature and ice levels, though δC is harder to interpret because it depends on many factors.

Over very long time scales, rock layers can show changes in sea levels and features like "fossilized" sand dunes. Scientists study sedimentary rocks that are billions of years old to understand long-term climate changes. The way Earth's history is divided into periods is based on visible changes in rock layers that mark major climate shifts.

Coral "rings" are similar to tree rings and can be studied in a similar way. Coral growth depends on conditions like water temperature, salt levels, pH, and wave activity. Scientists use tools like the Advanced Very High-Resolution Radiometer (AVHRR) to study past ocean temperatures and salt levels. The δO in coralline red algae helps scientists learn about ocean temperatures and salt levels in areas where other methods are not possible.

In climatic geomorphology, scientists study old landforms to learn about past climates. These landforms can provide clues about ancient weather patterns and environmental conditions.

Notable climate events in Earth history

As we look further back in time, our knowledge of exact climate events becomes less certain, but some important events are well known:

• Faint young Sun paradox (start)
• Huronian glaciation (~2,400 million years ago, Earth completely covered in ice likely caused by the Great Oxygenation Event)
• Later Neoproterozoic Snowball Earth (~600 million years ago, came before the Cambrian Explosion)
• Andean-Saharan glaciation (~450 million years ago)
• Carboniferous Rainforest Collapse (~300 million years ago)
• Permian–Triassic extinction event (251.9 million years ago)
• Oceanic anoxic events (~120 million years ago, 93 million years ago, and others)
• Cretaceous–Paleogene extinction event (66 million years ago)
• Paleocene–Eocene Thermal Maximum (Paleocene–Eocene, 55 million years ago)
• Last Glacial Maximum (~23,000 BCE)
• Younger Dryas / Big Freeze (~11,000 BCE)
• Holocene climatic optimum (~7,000–3,000 BCE)
• Extreme weather events of 535–536 (535–536 CE)
• Medieval Warm Period (900–1300)
• Little Ice Age (1300–1800)
• Year Without a Summer (1816)

History of the atmosphere

The first atmosphere was made mostly of hydrogen gas from the space around the Sun. It also likely contained simple compounds with hydrogen, such as water vapor, methane, and ammonia, which are found today in gas giants like Jupiter and Saturn. As the space around the Sun thinned over time, much of these gases escaped, partly due to the solar wind pushing them away.

The next atmosphere was mainly made of nitrogen, carbon dioxide, and non-reactive gases. This atmosphere formed when gases were released from Earth's interior through volcanic activity. Additional gases came from impacts by large asteroids during a period called the late heavy bombardment. Much of the carbon dioxide in this atmosphere dissolved in Earth's water, forming carbonate rocks.

Evidence of water-related rocks dates back to about 3.8 billion years ago. Around 3.4 billion years ago, nitrogen was the main gas in the stable second atmosphere. Early signs of life, such as simple organisms, appear in the geological record as far back as 3.5 to 4.3 billion years ago. Scientists note a puzzling problem called the "faint young Sun paradox" because Earth remained warm despite the early Sun being about 30% less bright than today.

The geological record shows Earth's surface was generally warm for most of its early history, except for a cold period around 2.4 billion years ago. During the late Archaean eon, oxygen began to appear in the atmosphere, likely due to cyanobacteria (simple plants that perform photosynthesis). Fossils of these organisms, called stromatolites, are found from about 2.7 billion years ago. The carbon isotope ratios in early rocks match those found today, suggesting Earth's carbon cycle was already functioning 4 billion years ago.

The movement of Earth's continents through plate tectonics affects the atmosphere by moving carbon dioxide between the atmosphere and large carbonate rock deposits. Free oxygen did not exist in the atmosphere until about 2.4 billion years ago, during the Great Oxygenation Event. This event is marked by the end of banded iron formations, which formed when oxygen reacted with iron in the ocean. Oxygen began to build up in the atmosphere when the amount of oxygen produced by photosynthesis exceeded the amount of materials that could absorb it, shifting Earth's atmosphere from one that lacked oxygen to one that contained it. Oxygen levels eventually stabilized at over 15% by the end of the Precambrian era. The following period, called the Phanerozoic eon, saw the rise of oxygen-breathing animals.

Oxygen levels in the atmosphere have changed over the past 600 million years, reaching as high as 35% during the Carboniferous period, much higher than today's 21%. Two main processes influence these changes: plants take in carbon dioxide and release oxygen, while volcanic eruptions and the breakdown of pyrite release sulfur into the atmosphere, which reduces oxygen levels. Volcanic eruptions also release carbon dioxide, which plants can use to create oxygen. The exact reasons for these changes are not fully understood. Periods with high oxygen levels are linked to the rapid growth of animal life. Today's atmosphere contains 21% oxygen, which is enough to support the development of complex animals.

Climate during geological ages

The Huronian glaciation is the first known ice age in Earth's history and lasted from 2400 to 2100 million years ago. The Cryogenian glaciation occurred from 720 to 635 million years ago. The Andean-Saharan glaciation took place from 450 to 420 million years ago. The Karoo glaciation lasted from 360 to 260 million years ago. The Quaternary glaciation is the current ice age and began 2.58 million years ago.

In 2020, scientists published a detailed record of Earth's climate changes over the past 66 million years. They identified four major climate states, separated by changes in greenhouse gas levels and polar ice sheet sizes. They combined data from many sources. The warmest climate state since the dinosaur extinction, called "Hothouse," lasted from 56 to 47 million years ago and was about 14 °C warmer than today's average temperatures.

The Precambrian period began when Earth formed 4.6 billion years ago and ended 542 million years ago. It includes two eons: the Archean and the Proterozoic, which can be divided into smaller time units. Reconstructing Precambrian climate is difficult because there are few reliable indicators and a limited fossil record compared to later periods. Despite these challenges, evidence shows major climate events. The Great Oxygenation Event, which started around 2.3 billion years ago, is linked to the rise of photosynthetic organisms. This event caused a drop in methane levels, cooling Earth and leading to the Huronian glaciation. For about 1 billion years after this ice age (2 to 0.8 billion years ago), Earth likely had warmer temperatures, with oxygen levels between 5% and 18% of today's levels. At the end of the Proterozoic, global glaciation events, known as "Snowball Earth," occurred. Evidence for this includes glacial deposits, large-scale erosion called the Great Unconformity, and sedimentary rocks called cap carbonates that form after ice ages.

Before the industrial era, major climate changes were driven by variations in the Sun, volcanic activity, Earth's position relative to the Sun, and tectonic effects on ocean currents and watersheds. In the early Phanerozoic, higher carbon dioxide levels were linked to increased global temperatures. A study by Royer et al. in 2004 found that Earth's climate sensitivity during this time was similar to today's range.

The difference in average global temperatures between a fully glacial Earth and an ice-free Earth is estimated to be about 10 °C, with larger changes at high latitudes and smaller changes near the equator. Large ice sheets often form when continents are positioned near the poles. However, the presence of land near the poles alone does not guarantee ice ages. Evidence shows that Earth has had warm periods when polar regions had forests instead of ice.

A warm period between the Jurassic and Cretaceous eras coincided with increased volcanic activity due to the breakup of the Pangea supercontinent.

Short-term climate changes, similar to or more extreme than today's glacial and interglacial cycles, have occurred throughout Earth's history. Events like the Paleocene-Eocene Thermal Maximum may have been caused by sudden releases of methane from ocean reservoirs.

A major climate change event caused by a meteorite impact is thought to have led to the Cretaceous–Paleogene extinction. Other significant climate thresholds include the Permian-Triassic and Ordovician-Silurian extinctions, with various causes proposed.

The Quaternary geological period includes the current climate. Ice age cycles have occurred for the past 2.2 to 2.1 million years, beginning in the late Neogene Period.

The graphic on the right shows a strong 120,000-year cycle in climate patterns, with an uneven shape. This asymmetry is believed to result from complex feedback mechanisms. Ice ages develop gradually, but recovery to warmer periods happens quickly.

The graph on the left shows temperature changes over the past 12,000 years, based on multiple sources. The thick black line represents an average of these changes.

Climate forcings

Climate forcing is the difference between the sunlight Earth receives and the heat it sends back into space. This difference is measured using the amount of carbon dioxide (CO₂) in the tropopause, with results shown in units of watts per square meter. When more energy comes into Earth than leaves, the planet warms. When more energy leaves than enters, the planet cools. Earth's energy balance changes due to variations in sunlight reaching Earth and the amounts of greenhouse gases and tiny particles in the air. Climate changes can occur from natural processes within Earth's systems or from outside influences.

One way scientists study climate is by examining how changes in CO₂ levels affect Earth's climate over time. They use tools to estimate past CO₂ levels and compare them to current levels. This helps researchers understand how CO₂ has influenced climate changes throughout history.

Earth's climate system includes the atmosphere, living organisms, ice, water, and land. Together, these systems shape the climate. Greenhouse gases act as internal forces that influence the climate. Scientists and researchers studying Earth's past and present climates focus on how sensitive Earth's climate is to these forces. By analyzing all these forces, scientists can make general predictions about Earth's climate. These predictions include evidence about long-term climate patterns, such as changes in Earth's orbit, feedback effects like the Ice-Albedo Effect, and the impact of human activities.

• Ocean currents driven by temperature and salt differences (Hydrosphere)
• Living organisms (Biosphere)

• The Milankovitch cycles describe changes in Earth's orbit and tilt, affecting how much sunlight Earth receives.
• Volcanic eruptions are considered external influences on climate.
• Human changes to the atmosphere or land use.
• Human activities that release greenhouse gases, causing global warming and climate changes.
• Large asteroids that impact Earth's climate are considered external influences.

Over millions of years, the rising of mountain ranges and the weathering of rocks and soil, along with the movement of tectonic plates, play a key role in the carbon cycle. Weathering removes CO₂ from the atmosphere by reacting with minerals, reducing the energy balance. Volcanic activity adds CO₂ to the atmosphere, contributing to natural climate changes like ice ages. Scientist Jim Hansen noted that humans release CO₂ into the atmosphere much faster than natural processes have in the past.

Changes in ice sheets and the positions of continents, along with related changes in plant life, have influenced Earth's long-term climate. There is a strong connection between CO₂ levels and Earth's temperature, with CO₂ playing a major role in controlling global temperatures throughout Earth's history.