Climate change, the essentials

What it covers

1. The greenhouse effect Sunlight warms the Earth's surface; the surface radiates heat back out as infrared. Some gases in the air (water vapour, carbon dioxide, methane, nitrous oxide) absorb that infrared and re-emit it in all directions, including back down. Without them the planet would average about minus 18 degrees Celsius instead of about plus 15. The effect itself is natural and was described in the 1800s; the problem is that human activity has raised the concentration of the long-lived gases, so more heat is trapped.
2. Where the carbon comes from Carbon dioxide in the air has risen from about 280 parts per million before industry to about 420 ppm, higher than at any time in at least 800,000 years of ice-core records. About 90% of today's emissions come from burning coal, oil and gas; most of the rest from deforestation and land use. Methane, from livestock, rice, landfill and gas leaks, is shorter-lived but traps far more heat per molecule. We know the extra carbon is from fossil fuels because its isotope mix matches ancient plant carbon, and because oxygen in the air is falling in step with combustion.
3. What has changed already The global average surface temperature is about 1.1 to 1.2 degrees Celsius above the late 1800s, with the last ten years the warmest on record. Sea level has risen about 20 centimetres since 1900 and is accelerating, from warming water expanding and from ice loss in Greenland and Antarctica. Oceans have absorbed most of the extra heat and about a quarter of the CO2, making them more acidic. Heatwaves, heavy rain and droughts have become more frequent and intense in most regions, and the IPCC states it is unequivocal that human influence has warmed the planet.
4. What is projected How warm it gets depends almost entirely on how much more carbon is emitted. Every additional 1,000 billion tonnes of CO2 adds about 0.45 degrees. Current policies point to roughly 2.5 to 3 degrees by 2100; the Paris Agreement goal is to hold warming well below 2 and aim for 1.5. The difference between 1.5 and 2 is large: twice as many people exposed to severe heat, most versus nearly all tropical coral reefs lost, and much higher risks to crops in Africa and South Asia. Reaching net zero CO2 stops warming within a couple of decades; the climate does not keep warming once emissions stop.
5. What actually works The big levers are known: replace coal and gas electricity with solar, wind, hydro and nuclear; electrify transport and heating so they run on that clean power; stop deforestation; cut methane leaks; and use energy more efficiently. Solar and wind are now the cheapest new electricity in most of the world, which is why they are growing fastest. Individual choices matter most where they add up: how you travel, what you eat, and what you vote and argue for. Adaptation, such as drought-resistant crops, flood defences and early-warning systems, is needed whatever happens.