Climate Change: Evidence, Causes & Responses
What Is Climate Change?
The world's climate is always changing. The question is how much, how fast and why. Earth has cycled through ice ages and warm periods for millions of years — what is different today is the speed of the change and the role of human activity. The current geological period, the Quaternary, has had at least seven major ice ages. The last one ended about 11,500 years ago.
Evidence for Climate Change
Climate scientists piece together the past from many different sources.
- Ice cores. Cores drilled from Antarctic and Greenland ice contain trapped air bubbles that show what the atmosphere was like up to 800,000 years ago. CO₂ levels track temperature closely.
- Tree rings. Wide rings indicate warm wet years, narrow rings cool dry ones. Trees give a year-by-year record over thousands of years.
- Pollen analysis. Layers of pollen preserved in peat bogs reveal which plants grew when. Different plants need different climates.
- Historical records. Paintings of frozen Thames Fairs in the 1600s and harvest records from monasteries show colder periods like the Little Ice Age.
- Thermometer records. Direct measurements have been taken for around 150 years and show a global warming trend, especially since 1980.
Ice cores, tree rings, pollen analysis and historical records all show that climate has changed for thousands of years — but recent change is unusually fast and global.
Natural Causes
Three main natural processes have caused past climate changes. Orbital changes — the Milankovitch cycles — alter how much sunlight reaches different parts of the Earth over tens of thousands of years. They explain the timing of ice ages. Volcanic activity can cool the planet for years; the 1815 Tambora eruption produced the 'year without a summer' in 1816. Solar output varies in cycles; sunspot activity affects how much energy reaches Earth, but the changes are small compared to the warming we see today.
Human Causes
Human activity has rapidly increased greenhouse gases in the atmosphere — especially since the Industrial Revolution. Fossil fuels burned for electricity, transport and industry release carbon dioxide. Agriculture — particularly cattle, rice paddies and synthetic fertilisers — releases methane and nitrous oxide. Deforestation destroys forests that would otherwise absorb CO₂. Atmospheric CO₂ has risen from around 280 parts per million in 1750 to over 420 ppm today — the highest in 800,000 years. Methane and nitrous oxide have also risen sharply, even though their amounts are much smaller. Methane traps about 28 times more heat than CO₂ over a century, and nitrous oxide nearly 300 times more, so even modest increases matter. Most scientists agree that human emissions are now the dominant cause of warming, and the speed of the change is what makes it unprecedented in the geological record.
Sunlight passes through the atmosphere and warms the Earth. The Earth radiates that heat back as infrared. Greenhouse gases — carbon dioxide, methane, water vapour — absorb the infrared and re-emit it in all directions, including back down. The atmosphere acts like a blanket. Without it, the Earth would average about −18°C. Adding more greenhouse gases thickens the blanket, raising the temperature.
Effects on People and the Environment
Climate change has wide-ranging effects. People: heatwaves in southern Europe push deaths up; tropical farmers lose crops to drought; coastal cities face rising seas; tropical-disease ranges expand. Environment: glaciers retreat, sea ice shrinks, coral reefs bleach when ocean temperatures rise, species like polar bears lose habitat. The Maldives, low-lying Pacific islands and Bangladesh face existential risk from sea-level rise.
Managing Climate Change
Two strategies are used. Mitigation tackles the causes — replacing fossil fuels with renewables, planting trees, capturing carbon, signing international agreements like the Paris Agreement (2015), which committed to keeping warming below 2°C. Adaptation tackles the effects — building flood defences, breeding drought-resistant crops, raising buildings above projected sea levels, designing cities for higher temperatures. Both are needed; mitigation alone is not enough because some warming is already locked in.
Mitigation reduces the causes of climate change (e.g. switching to renewables). Adaptation manages the effects (e.g. flood defences). Both are needed.
Signed by 196 countries in 2015, the Paris Agreement aims to keep global warming well below 2°C above pre-industrial levels, with efforts to limit warming to 1.5°C. Each country sets its own targets (Nationally Determined Contributions) and updates them every five years. Critics argue the targets are not enforceable, but the agreement created the first global framework for cutting emissions.