Earth's climate has always changed — ice ages, warm periods, mass extinctions driven by volcanic eruptions. What's different now is the speed, the cause, and the scale.
The greenhouse effect
Earth is kept warm by the greenhouse effect. Sunlight passes through the atmosphere and warms the surface. The surface radiates heat back upward as infrared radiation. Greenhouse gases — mainly water vapour, carbon dioxide, and methane — absorb some of this outgoing heat and radiate it back towards Earth, keeping it warmer than it would otherwise be. Without any greenhouse effect, Earth's average temperature would be around −18°C instead of the current +15°C. The greenhouse effect is not a problem; it's essential for life.
The problem is too much of it.
Imagine Earth's atmosphere is a duvet on a bed. A thin duvet keeps you comfortably warm on a cold night. A thicker duvet keeps you warmer still. Keep adding duvets and you start to overheat. Since the Industrial Revolution, humans have been adding CO₂ to the atmosphere at an unprecedented rate — thickening the duvet. The bed is warming, and the consequences of overheating ripple through everything the bed is connected to.
Where is the extra CO₂ coming from?
Primarily from burning fossil fuels — coal, oil, and natural gas — which releases CO₂ that was locked underground for millions of years. Deforestation releases CO₂ stored in trees. Agriculture produces methane (from livestock) and nitrous oxide (from fertilisers), both powerful greenhouse gases. Since 1750, atmospheric CO₂ concentration has risen from about 280 parts per million to over 420ppm — higher than at any point in the past 800,000 years, based on ice core evidence.
Why does 1.2°C matter?
Global averages hide local extremes. A 1.2°C rise in average temperature means much more frequent heatwaves, stronger storms, faster-melting ice, rising sea levels, and disrupted rainfall patterns that affect agriculture. The concern is feedback loops: melting Arctic ice (which reflects sunlight) exposes darker ocean (which absorbs it), causing more warming; melting permafrost releases stored methane, causing more warming. Each degree of additional warming makes subsequent degrees more likely and faster-arriving.