Use the right form of the verb to complete the sentences.

The climate of the Earth ... . It ... as a result of natural causes. Nowadays, however, the term climate change ... when referring to change in our climate which ... since the early part of the twentieth century. The change we ... over recent years and those which ... over the next 100 years ... by many to be largely as a result of human behaviour rather than due to natural changes in the atmosphere.
The greenhouse effect ... vey important when we ... about climate change as it ... to the gases which ... the Earth warm. Although the greenhouse effect ... a naturally occurring phenomenon, it ... that the effect could be intensified by human activity and the emission of gases into the atmosphere. These ... the extra greenhouse gases which humans ... .