Evidence for Climate Change

Climate changes over different timescales

Climate is the average pattern of weather over many years. Climate change means a long-term change in that pattern, rather than an unusually hot day or one cold winter. To identify it, scientists look for changes that persist across many observations.

The Quaternary period began about 2.6 million years ago and continues today. Throughout it, Earth's climate has repeatedly alternated between colder glacial periods, when ice sheets expanded, and warmer interglacial periods, when ice retreated. The last major glacial period ended approximately 12,000 years ago; we now live in an interglacial period.

This history shows that climate has changed before. However, it does not mean that every change has the same cause or happens at the same rate. Evidence allows scientists to compare recent changes with earlier patterns.

Reconstructing climate before thermometers

There are no thermometer records stretching back millions of years. Instead, scientists use proxy evidence: something preserved today whose characteristics reveal conditions in the past. Different proxies cover different timespans, so no single record describes the whole Quaternary.

Glacial landforms and deposits show where ice once existed. U-shaped valleys were carved by glaciers; moraines are accumulations of debris deposited by ice; erratics are rocks transported by glaciers and left away from their original location. Finding these features outside areas covered by glaciers today indicates that ice was once more extensive and conditions were colder. They provide strong evidence of past glaciation, but do not give an exact annual temperature record.

Pollen preserved in lake sediments reveals which plants grew nearby. Different plants favour different climatic conditions. For example, an increase in oak and beech pollen can indicate warmer conditions. Changes in pollen between dated sediment layers therefore help reconstruct changes in vegetation and climate. This is indirect evidence: vegetation also responds to factors other than temperature, so scientists compare it with other records.

Ocean sediments also contain the shells of microscopic organisms called foraminifera. Their chemical composition helps scientists estimate past ocean temperatures and ice volume. Sediment records can extend much further back than modern measurements, making them important for understanding the longer Quaternary history.

Ice cores preserve two kinds of evidence

Scientists drill ice cores from ice sheets in places such as Greenland and Antarctica. Snow accumulates and becomes compressed into ice, preserving a sequence of layers. Some ice-core records extend back about 800,000 years: a substantial part of the Quaternary, but not its whole length.

The cores contain two distinct clues:

• Air bubbles preserve samples of the past atmosphere. Scientists measure gases such as carbon dioxide and methane within them.
• The ice itself contains different forms of oxygen, called isotopes. Their relative proportions vary with the conditions in which the snow formed, allowing past temperatures to be estimated.

Ice-core records show repeated colder and warmer intervals. Lower greenhouse gas concentrations generally accompany glacial conditions, while higher concentrations accompany warmer interglacials. The gas measurements and temperature estimates are separate pieces of evidence that can be compared.

Ice cores provide a detailed sequence, but dating very old layers is more difficult than counting recent ones. Temperature estimates also reflect conditions around the ice-sheet location rather than directly measuring the global average. Comparing cores from different places and checking them against sediment records makes the reconstruction more reliable.

Trees and documents bridge the gap

Tree rings provide evidence over hundreds or sometimes thousands of years. A tree usually adds a growth ring each year. Wider rings generally indicate more favourable growing conditions; narrow rings can indicate cold or drought. Matching ring patterns between trees helps build longer records.

Ring width is not a simple thermometer: both temperature and water availability affect growth. Scientists must establish which factor limits growth in a particular location before interpreting its rings.

Historical documents include diaries describing severe winters, ships' logs recording sea ice, and farm records noting harvest dates. These help reconstruct local conditions before widespread instrumental recording. However, accounts may be incomplete or subjective. One description of a frozen river demonstrates a cold event, not a long-term global trend.

Measuring recent climate change

Widespread systematic thermometer records extend back to the mid-nineteenth century. They show an overall rise in global average temperature, particularly since the 1970s. Individual years still fluctuate: a long-term warming trend does not require every year to be warmer than the previous one.

To read a temperature graph, first check its dates, units and whether it shows actual temperatures or temperature anomalies. An anomaly measures the difference from a stated reference average. For example, an anomaly of $+0.5\,^{\circ}\mathrm{C}$ means half a degree warmer than that average. Compare the beginning and end, identify sustained changes, and distinguish short fluctuations from the overall trend.

Satellites, used since the late twentieth century, provide broad coverage of temperatures and ice extent. Their records are much shorter than proxy records, but they allow repeated observations across large areas. Declining Arctic summer sea ice and shrinking glaciers provide additional evidence consistent with warming.

Other recent indicators include rising sea levels and changes in the timing of flowering, bird migration and nesting. These support the temperature evidence, although any single local observation may also be influenced by other factors.

Judging the evidence together

The strongest conclusion comes from agreement between independent methods. Geological and sediment evidence establish that climate changed throughout the Quaternary. Ice cores provide detailed evidence for repeated glacial–interglacial changes, while thermometers and satellites show recent warming.

Each method has limitations: proxies require interpretation, historical accounts can be subjective, and instrumental records cover only a small part of the Quaternary. Nevertheless, their different strengths complement one another. When temperature measurements, ice observations and biological indicators show consistent changes across many locations and years, the case for climate change is much stronger than one isolated observation.

Quaternary pattern

• Began about 2.6 million years ago; continues today.
• Repeated glacial cold periods and interglacial warm periods.
• Last major glacial period ended approximately 12,000 years ago.

Evidence to recall