Satellites measure greenhouse gases in the atmosphere, while on Earth scientists collect air samples from all over the world. Water vapour and clouds make up the majority of greenhouse gases, with carbon dioxide and other gases comprising about 25 per cent. Current samples are then compared with previous ones, including those from air bubbles that were trapped in ice many thousands of years ago.
From this we’ve determined that the atmosphere contains nearly twice the amount of carbon dioxide (the main greenhouse gas) as there was in 700,000 BCE. With the Industrial Revolution, we began to burn fossil fuels at ever increasing rates, leading to huge jumps in greenhouse gas emissions. In 1750, the atmosphere had a carbon dioxide concentration of about 280 parts per million. By 2000, it was nearly 400 parts per million.