If you are feeling a sense of déjà vu about the forthcoming climate conference in Paris, it's because this is the 21st time nations of the world have met to seek agreement on how to stabilise the atmosphere and prevent dangerous climate change.

The possibility that rising levels of carbon dioxide could alter the Earth's temperature balance has been known for more than a century, but it wasn't until 1987 and the release of a major United Nations report on sustainable development, "Our Common Future", that global warming was identified as one of the most serious challenges facing humanity.

This explainer on CO2 and the greenhouse effect was produced by ABC TV's Quantum program the following year.

It's interesting to watch it again today, because even then, people had begun to suspect that something unusual was happening to the world's climate. Some of the questions posed in the story still seem very familiar. How much of the changes to the climate are due to natural variation and how much are to do with human activities?

In 1992, an international treaty was drawn up with the goal to stabilise atmospheric greenhouse gas concentrations at a level that would prevent dangerous effects on the world's climate systems.

The 196 parties who signed it have since met regularly to assess progress. The 2015 Conference of the Parties (COP 21) in Paris will run from November 30 to December 11.

The goal now is not to prevent global warming, but to limit it to less than 2 degrees Celsius. According to the OECD, in order to have a 50 per cent probability of reaching that goal, CO2 levels need to be limited to no more than 450 parts per million.

When this video explainer was made 26 years ago, CO2 levels were 351 parts per million. Today the level is just over 400 parts per million.