The Earth's atmosphere is made up of a blanket of gases, which trap enough heat to sustain life. However, by burning fossil fuels and cutting down forests humans pump billions of tonnes of carbon dioxide (CO2) into the atmosphere. We also add other gasses to the atmosphere in smaller quantities. These emissions increase the 'greenhouse effect', adding to the natural canopy of gases in the atmosphere and causing more heat to become trapped.
As a result, the global temperature is increasing, throwing the world's climate out of balance. The extra carbon dioxide gas we are adding to the atmosphere is mostly from burning large amounts of fossil fuels for energy production and transport. Changes in land use and deforestation also release more carbon dioxide into the environment. Trees, for example, are natural 'carbon sinks' - they absorb carbon dioxide - and when they are destroyed, carbon dioxide is released into the atmosphere.
While many greenhouse gases occur naturally, the rate humans are adding them to the atmosphere is far from natural. It is estimated that concentrations of carbon dioxide are now more than one third higher than before the industrial revolution, when the large scale burning of fossil fuels and modern industry and agricultural practices began, adding to the atmospheric levels of naturally occurring gases such as carbon dioxide, methane and nitrous oxide. Humans are also creating new, very potent greenhouse gases such as hydrofluorocarbons (HFCs) from industrial activities.
Even if all greenhouse gas emissions were stopped today, the effects from past activities will persist for many centuries, due to the long life of greenhouse gases in the atmosphere and the long time required for transfer of heat from the atmosphere to the deep oceans.
What this means is that every day the amount of warming gasses in the atmosphere increases with inevitable consequences - making it harder and harder for us to reign in climate change. For now, though, we still can largely choose our climate future.