Climate change is a natural and ongoing process with the world’s climate delicately balanced, each year 'breathing' CO₂ in and out again. It does this by storing CO₂ in the oceans, soil, trees and other organisms. Some of the CO₂ is stored permanently under suitable conditions, such as peat bogs and ocean sediment and, over millions of years, forming coal, oil, shale and natural gas. Some is released again as a product of combustion from forest fires and decomposition of the animals and plants which absorbed it. This is called the carbon cycle. Under normal conditions the amount stored and released is about the same, approximately 222 billion tonnes a year.

Currently, humans are adding another 7 billion tonnes of carbon to the atmosphere each year through the burning of fossil fuels and land use changes such as deforestation. While only a fraction of the total natural release of greenhouse gases, this extra amount accumulates in the atmosphere and may cause natural processes to become unbalanced. The Earth’s carbon sinks, such as rainforests and the oceans, cannot remove CO₂ from the atmosphere at the same rate as it is being released, so the volume of CO₂ has become unbalanced and is rising at a rate never previously seen.

CO₂ is measured in the atmosphere in parts per million (ppm). Pre-industrial levels were ~280ppm and had remained stable for several thousand years. Current levels are ~380ppm. It is this rate of increase that is cause for concern because changes in atmospheric concentration of CO₂ have never before increased as rapidly. Scientists estimate that once over the threshold of ~440ppm global temperatures will increase with a possible runaway effect, but the many positive and negative feedback mechanisms at play in the climate system make it difficult to predict a point at which temperatures may stabilise again.