Theory that the temperature of the earth is being increased due to man-made, but also non-made made, emissions of GHGs (greenhouse gases).

The greenhouse effect is caused by molecules that absorb infrared radiation released from the Earth’s surface, trapping heat in the atmosphere.

It is, however, a long time scientific fact (not connected to man-made emissions) that greenhouse gases,  mainly CO2 but also methane, contribute to warming the Earth. Without the approximately 34°C temperature increase due to these gases, the Earth would be very cold instead of an average observed global temperature of ~15°C. Thus, the human contribution is merely changing the greenhouse gas warming from 34°C to say 36°C. But even a tiny extra 2°C will cause big problems.