To understand climate change, it's important to recognise the difference between weather and climate.
Weather is the temperature, precipitation (rain, hail, sleet and snow) and wind, which change hour by hour and day by day. Climate is the average weather we expect over a long period of time. So, while the weather brings different temperatures all over the world on a day to day basis, over a year we'd expect the global climate to bring an average temperature of about 14 °C.
You can also watch our YouTube video which looks what climate is.
Our climate is dominated by two major factors:
Sunlight provides the energy which heats the Earth, but that heat doesn't naturally stay here. Without an atmosphere, it would be given off as infrared radiation by the Earth and it would virtually all go back out to space - creating a very cold planet.
Our atmosphere stops that from happening. Certain gases in the atmosphere allow sunlight to pass through, but then stop the heat from escaping back out into space - much like glass in a greenhouse. That's why this is called the greenhouse effect, and why the gases responsible (water vapour, CO2, methane and others) are called greenhouse gases.
Scientists explained the heat-trapping effects of greenhouse gases more than 150 years ago. Research has shown that, without the greenhouse effect, the Earth would be about 30 °C cooler - making it uninhabitable to most forms of life. Because they're so effective in keeping the planet warm, we know that any changes in the amount of greenhouse gases in the atmosphere will affect the Earth's temperature.
The Earth's climate has changed before. We know this not only from human records of climate going back over hundreds of years, but also from evidence in ice-cores, tree rings and rocks that goes back over hundreds of thousands of years.
On long-term scales, a distinct cycle has been observed in our past climate -that of ice ages. This is a shift in the Earth's climate between glacial ages, when global temperature drops and large areas of planet get covered in snow and ice, and interglacial ages, which brings the less cold conditions, such as those most of us we experience now.
There's evidence to show these cycles are triggered by small changes in the Earth's orbit that affect the amount of energy the planet gets from the sun. It takes about 20,000 years for the change from one age to another to take place, with the global temperature change between each being about 4 °C. The full cycle, from glacial age to glacial age, takes about 100,000 years. Evidence suggests the last ice-age ended about 11,000 years ago.
Over shorter timescales, we know from tree ring records that there have been smaller changes in regional climates. There is evidence of a period of slightly warmer weather in Europe between tenth century and thirteenth century known as the Medieval Warm Period. It was followed by a period of colder weather, known as the Little Ice Age. Most of the information on these periods originates from Europe, and there's not enough evidence to say this was a global climate phenomenon. This means that there's not evidence to say these periods are like the kind of change we're seeing now - which is global and producing consistent warming.