Our climate is dominated by two major factors, the sun and the atmosphere.
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 climate today
Since the last ice age ended 11,000 years ago, the Earth has moved into a relatively stable interglacial climate. Global temperatures have been largely stable, averaging about 14 °C. This stability has allowed complex ecosystems to thrive, supporting a wide range of life on Earth. In the last century, however, our climate has started to change rapidly. This isn't just a blip in the climate system - the evidence all points to a long-term change in our climate that's happening quickly.