Climate change: evidence and causes

The Quaternary period is the last 2.6 million years of the Earth's history. During it, the climate has swung between cold glacial periods (ice ages) and warmer interglacial periods, like today's.

Evidence of past climate:

• Ice cores: air bubbles trapped in Antarctic ice show the temperature and carbon dioxide levels over the last 800,000 years.
• Tree rings: a tree grows a ring each year; wider rings mean warmer, wetter years.
• Historical records: diaries, paintings and harvest dates.

Evidence of recent change:

• Temperature records since about 1850 show the world warming fastest in recent decades.
• Sea level is rising, as water warms and expands and ice melts.
• Ice cover is shrinking: Arctic sea ice, glaciers and ice sheets.

The Intergovernmental Panel on Climate Change (IPCC), which brings together the work of thousands of scientists, reported in 2021 that:

• the world's average surface temperature in 2011–2020 was about 1.1 °C higher than in 1850–1900;
• average sea level rose by about 20 cm between 1901 and 2018;
• carbon dioxide in the atmosphere in 2019 was higher than at any time in at least 2 million years;
• it is unequivocal (beyond doubt) that human influence has warmed the atmosphere, ocean and land.

The climate has always changed naturally:

• Orbital changes: over tens of thousands of years, the shape of the Earth's orbit, the tilt of its axis and its wobble all change, altering how much sunlight reaches different parts of the Earth. These cycles explain the coming and going of ice ages.
• Volcanic activity: big eruptions throw ash and gas high into the atmosphere, reflecting sunlight and cooling the Earth for a year or two, as after Mount Pinatubo in 1991.
• Solar output: the energy the sun gives off varies. A period of few sunspots, and weaker sun, from about 1645 to 1715 coincided with very cold winters in Europe, during the Little Ice Age.

None of these explains the rapid warming since the mid-20th century.

Greenhouse gases in the atmosphere trap heat, keeping the Earth warm: the greenhouse effect. Human activities add more of these gases, strengthening it:

• Fossil fuels: burning coal, oil and gas in power stations, factories, homes and vehicles releases carbon dioxide.
• Agriculture: cattle and rice paddies release methane; fertilisers release nitrous oxide.
• Deforestation: trees store carbon. Cutting and burning them releases carbon dioxide, and fewer trees are left to absorb it.

On the environment:

• Melting ice and rising sea levels, flooding low-lying coasts and islands.
• Warmer, more acidic oceans bleach coral reefs.
• Plants and animals shift their ranges towards the poles or uphill; some can't move fast enough.
• More extreme weather: heatwaves, droughts and heavy rain.

On people:

• Coastal homes and farmland at risk from flooding.
• Crop yields fall in hot, dry regions, threatening food supplies; some cooler places, such as the UK, may get longer growing seasons.
• More water shortages where droughts increase.
• Heatwaves harm health, especially of older people.

The poorest countries, which have produced the least greenhouse gas, are often the most vulnerable.