Some of the gases in Earth’s atmosphere, such as water vapour and carbon dioxide, play an important role in influencing the Earth’s average temperature.

These gases are referred to as greenhouse gases (GHG), because they act like the glass on a greenhouse. They allow solar radiation in through the atmosphere, but prevent the escape of some of the counter radiation back out into space.

These greenhouse gases absorb counter radiation and emit it back towards the Earth, causing a warming of the lower atmosphere. (1)

Here’s an explanation by the US National Research Council:

Natural greenhouse effect

This naturally occurring process is an important part of the planet’s ability to support life on Earth as we know it. Without this process, the Earth’s average temperature would be approximately -18°C. Instead, the average temperature is approximately 15°C today. (2)
Greenhouse gases are naturally emitted by the oceans and land in an annual cycle.

• CO₂ is drawn into plants as they grow and sprout leaves
• CO₂ is released as dead vegetation rots. Volcanoes produce GHG. Animals release CO₂ as they breathe. There is methane in manure and from swamps.

What is not natural is the rate humans are adding greenhouse gases to the atmosphere. This process is referred to as the Enhanced Greenhouse Effect, and is responsible for triggering and enhancing the current warming trend.

Enhanced greenhouse effect

Humans are causing warming on a global scale by releasing additional greenhouse gases into the atmosphere. The largest source of additional GHG is from burning fossil fuels (e.g. coal, gasoline, and natural gas)

These additional greenhouse gases are responsible for altering the natural balance of the Greenhouse Effect.

As more greenhouse gases are produced, they accumulate in the atmosphere and absorb long wave counter radiation. (i.e. infra-red) More radiation is then radiated back towards Earth, causing enhanced warming.