Climate Change Causes & Evidence
A Climate That Has Always Changed
The Earth’s climate has shifted constantly throughout the Quaternary period — the last 2.6 million years. It has swung between long, cold ice ages and warmer gaps called interglacials. Today we live in an interglacial that began around 11,700 years ago. Within that period, smaller swings have happened too. The job of geographers and climate scientists is to work out how today’s warming compares with these natural ups and downs — and what is causing it.
Three named periods since 1000 AD are useful to remember: the medieval warming (roughly 950–1250), the Little Ice Age (about 1300–1850) and modern warming (since around 1900, accelerating after 1970).
Periods of Warming and Cooling Since 1000 AD
The medieval warming (around 950–1250) made parts of north-west Europe warm enough to grow vines in southern England and to support Norse farms in Greenland. Then came the Little Ice Age from about 1300 to 1850, when winters were so cold that the River Thames in London froze hard enough for ‘frost fairs’, and Alpine glaciers advanced down their valleys. Since around 1900, modern warming has set in — very mild at first, then much faster after about 1970.
Evidence for Climate Change
Scientists draw on several types of evidence, each useful over different time scales.
- Global temperature records — instruments have measured air temperatures since the 1850s. NASA and the Met Office show global average temperatures roughly 1.1°C higher than the 1850–1900 baseline.
- Ice cores — deep drills in the Antarctic and Greenland trap tiny air bubbles. By analysing them, scientists can read carbon dioxide levels and temperatures going back over 800,000 years.
- Tree rings (dendrochronology) — thick rings show warm, wet years, narrow rings show cold or dry ones. Tree-ring records can stretch back several thousand years.
- Paintings and diaries — works such as Hendrick Avercamp’s ice-skating scenes and pre-Industrial Revolution diaries describe weather conditions, frozen rivers and crop failures during the Little Ice Age.
Snow lays down in annual layers. As more snow falls on top, lower layers compress into ice and trap tiny air bubbles — a frozen sample of the atmosphere from the year the snow fell. Drilling out a long ice core (the Antarctic Vostok core is over 3.6km long) lets scientists measure ancient carbon dioxide and oxygen isotopes layer by layer. The result: a record of temperature and greenhouse gases stretching back over 800,000 years.
Natural Causes of Climate Change
There are three well-known natural causes:
- Variations in solar output — the Sun’s energy is not perfectly steady. Lower activity (such as the Maunder Minimum from 1645–1715) is linked with the coldest part of the Little Ice Age.
- Changes in the Earth’s orbit — over tens of thousands of years the orbit shape, axis tilt and wobble change in patterns called Milankovitch cycles. These cycles are the main pacemaker of the long ice ages.
- Volcanic activity — large eruptions, like Mount Pinatubo in 1991, throw sulphur particles into the upper atmosphere. These reflect sunlight back into space, so global temperatures dipped by around 0.5°C in the year following Pinatubo.
Human Causes: The Enhanced Greenhouse Effect
The natural greenhouse effect keeps the Earth warm enough for life. Sunlight passes through the atmosphere, heats the surface, and the warm surface re-emits longer-wave heat that gases like water vapour, carbon dioxide and methane partly trap. Without this effect the planet would average around −18°C.
The problem is the enhanced greenhouse effect. Since the Industrial Revolution, humans have raised the level of greenhouse gases far above natural ranges. Burning coal, oil and gas in power stations, factories and vehicles releases carbon dioxide. Deforestation in the Amazon, Congo and Indonesia removes trees that would absorb carbon. Cattle farming and rice paddies release methane. Atmospheric carbon dioxide has risen from around 280 parts per million in 1850 to over 420 ppm today — higher than at any point in the 800,000-year ice-core record.
Atmospheric carbon dioxide concentrations have risen from around 280 parts per million in 1850 to over 420 ppm today — the highest level for at least 800,000 years.
Average global temperatures have risen by about 1.1°C since 1850. Sea levels have risen roughly 21–24cm since 1900 because warmer water expands and ice sheets are melting. Heatwaves are more frequent — the UK passed 40°C for the first time on record in July 2022. Arctic sea-ice in summer has shrunk by around 40% since the 1980s. Coral reefs, including the Great Barrier Reef, have suffered repeated mass bleaching events.