The warming is caused because some of the heat is absorbed by the so-called greenhouse gases in the atmosphere, keeping the lower levels considerably warmer than the upper levels. The greenhouse effect is a natural phenomenon that keeps the Earth’s surface at a temperature suitable for life.

But scientists indicate that increasing levels of greenhouse gases emitted into the atmosphere from human activities are leading to an enhanced greenhouse effect which is causing global warming.  Global GHG emissions due to human activities have grown since pre-industrial times, with an increase of 70% between 1970 and 2004.